DIPOLES AND THEIR APPLICATION TO GRAPHIC ARTS

• Document Type: CREST
• Collection: NGA Records (Formerly NIMA)
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78B04770A002200060011-1
• Release Decision: RIPPUB
• Original Classification: K
• Document Page Count: 86
• Document Creation Date: December 28, 2016
• Document Release Date: March 24, 2005
• Sequence Number: 11
• Content Type: REPORT

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DIPOLES AND THEIR APPLICATION TO GRAPHIC ARTS
Abstract: Present pigments function as im-
perfect solid state devices to selectively ab-
sorb and reflect light. The need has existed
for ideal pigments for 3 color reproduction as
well as for other purposes. In terms of solid
state devices and getting away from chemistry
associated with pigments there is here presented
a new concept for obtaining the ideal pigment.
Dipole antenna theory which is applicable to
television reception is also applicable on a re-
duced scale to affect visible light in the form
of myriads of minute dipole particles each of
which reacts with visible light the same way that
a large scale antenna reacts with television sig-
nals. Control of color, reflectivity or absorp-
tivity in sharp or wide spectral bands are now
obtainable via this new concept. In addition,
orientation provides polarization effects as well
as control of reflectivity or absorptivity. All
these effects are obtainable by varying the
physical dimensions and orientation of dipolar
particles. The material will provide non-fading,
virtually indestructible colors. Only minute
quantities of the same material are required for
obtaining the entire gamut of characteristics.
Applications to new printing inks, 3-D printed
pictures are other fields in view.
In the graphic arts there has long been a
need for pigments possessing certain ideal color
characteristics which are required for better
color reproduction.
Figure 1 shows three different color pig-
ments employed in the graphic arts for color
reproduction. These curves show percent reflec-
tance versus wavelength. The real pigments are
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shown as the left hand curves which are the clos-
est approximation the art has been able to
achieve toward the desired ideal curves, which
are shown as rectangles on the right hand curves.
The transmittance and reflectance curves of
ordinary dyes and pigments are dependent upon
chemical structure.
Ordinary pigments and dyes produce colors
by the selective absorption and reflection of
various wavelengths of light at the electronic
level of the chemical molecules comprising the
pigments and dyes.
Electrons go from one quantum state to
another quantum state within the dye or pigment
molecules. These effects are strongly wave-
length dependent.
Stable chemical structures are relatively
rare. Most chemical structures are relatively
easily deteriorated by ultraviolet, visible and
infrared liOt, heat and chemical action. After
many years of research, chemical colorists have
found only a relatively few stable dyes and pig-
ments. These have fixed color responses and
must be chosen and blended with great skill in
order to achieve the desired end result.
A new approach was needed to enable the
specification and engineering of color producing
materials.
Light is an electromagnetic wave having
three fundamental attributes, which are; ampli-
tude or intensity; wavelength or color; and
polarization or the vibration direction at right
angles to the ray.
These three fundamental attributes of light
are shown in Figure 2.
A half-wave dipole antenna, which is nor-
mally used for television reception, has in-
teresting properties.
The half-wave dipole is capable of
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controlling all three attributes of light, by
varying its length, thickness, resistivity and
angular position.
The electric power absorbed from the radia-
tion by the hal-F-wave dipole depends upon two
orientation angles of the dipole. The first
angle, e, is that between the length of the di-
pole and the signal path. The second angle,0,
is that between the length of the dipole and the
direction of polarization of the signal.
Figure 3 shows a polar graph of radiant
power absorbed versus angle 8.
In Figure 4 the radiation ray path is nor-
mal to the plane of the diagram, and there is
shown the angle 0 versus the power absorbed by
the dipole.
A maximum response is obtained when the
antenna is aligned parallel to the polarized
electric vector of the radiation and at right
angles to the signal path (0 = 0, and 6 = 90 ).
The antenna absorbs no power when it is placed
at right angles to the polarized electric vector
of the radiation; or arranged parallel to the
ray path.
When adjusted for a maximum response, a
half-wave or X/2 antenna is then said to become
resonant to the particular wavelength X.
The power absorbed by the dipole from the
radiant energy may be re-radiated, or absorbed
and dissipated as heat, depending on the elec-
trical resistance of the half-wave dipole an-
tenna.
If power is to be absorbed from the dipole
antenna and utilized in an outside electric cir-
cuit, as for example in a television set, a
matched or characteristit resistance of 73 ohms
must be inserted at its center of the half-wave
dipole antenna, as shown in Figure 5.
An antenna may be made of such material,
thickness and length as to achieve full power
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absorption, or total reflection.
In Figure 5 there is also shown a half-wave
(X/2) antenna 2, in which the central resistor
is replaced with a single rod having a distribu-
ted resistance of approximately 80 ohms, which
results in total absorption of radiation in the
wavelength range X.
Now, if instead of a half-wave antenna with
a central resistor or an equivalent distributed
resistance, a half-wave antenna of low resist-
ance is employed, then the half-wave dipole an-
tenna becomes reflective for the full wavelength.
The radiant power may be said to be absorbed by
the half-wave dipole and then re-radiated in all
directions, with the intensity direction pattern
shown in Figure 3. Thus the resistivity charac-
teristics of the materials, together with the
length and width controls the distributed re-
sistance of the half-wave antenna. These factors
may be adjusted so that the half-wave dipole an-
tenna has high absorptivity or high reflectivity
for incident radiation of a given wavelength
band.
Figure 6 shows another very important pro-
perty of the half-wave dipole antenna, the "ef-
fective cross section".
Figure 6 shows a half-wave dipole antenna
having a thickness of (1/25) its length. its
length is X/2 and its thickness X/50. The
physical cross section of this half-wave dipole
at right angles to the light ray is:
(X/2)(X/50) = X2/100,
However, it is known that the effective cross
section of a half-wave dipole antenna is much
larger. The cross section from which the half-
wave dipcle appears to absorb power is approxi-
mately X4/8. A rectangle of this size is shown
in dotted lines surrounding the antenna rod, the
radiant power actually funnelling into the di-
pole. In this example the effective area of the
agtenna has been increased by a factor of
X'/100 divided by X4/8 or 12.5 times.
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Dipole antennas have been employed for the
electro-magnetic spectrum all the way from long
wave radio down through the television range in-
to the microwave and millimeter wave spectrum.
We have observed dipoles which are resonant
in the range of the wavelength of visible light.
Yellow light at the peak sensitivity of the
human eye has a wavelength of 0.565 microns
(yellow). Elongated metal rods of submicron
dimensions in colloidal suspension in a trans-
parent plastic solution results in myriads of
light responsive dipoles. The dissolved polymer
in the solvent acts as a protective colloid to
keep the dipoles in suspension.
The index of refraction n ofa given medium
may be defined as the ratio of the speed of
light in free space, to the speed of light in
the medium. Since the speed of light in all
substances is less than in free space, n is al-
ways greater than 1. The wavelength of light in
a given medium is inversely proportional to the
index of refraction n of the medium.
Because the index of refraction of most
plastics and solvents is approximately 1.5, the
dimensions of a half-wave dipole must be de-
creased in inverse proportion; that is for n =
1.5 the actual resonant length of a half-wave
dipole in such a medium becomes (1/2)X/1.5 = X/3.
For example, in a medium having an index of
refraction of 1.5, a half-wave dipole should
have a length of (0.565/3) = 0.188 microns of
yellow light for 0.565 microns wavelength.
The A/3 dimension, of course, is correct
only for n = 1.5 and will vary with the index of
refraction of the medium.
Another interesting property of the dipole
is that the sharpness of its tuning, or the
wavelength range over which it will absorb or
reflect, depends on the ratio of the length to
the thickness of the dipole; as well as on the
resistivity of the dipole material.
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Figure 7 refers to the reflection or ab-
sorption of radiant energy by a half-wave an-
tenna showing the relative power absorbed or re-
radiated, versus the ratio of length to thick-
ness of the antennae.
(A) For thin dipole antenna (25/1)
(B) For a thick dipole antenna (10/1)
We now come to the application of these
basic concepts to light control; that is, con-
trol of all three basic attributes of light,
intensity, color and polarization, by dipoles
dispersed to form colloidal suspensions in
plastic solutions.
Pigments formed from dipolar materials are
virtually indestructable. The reflectivity or
absorptivity characteristics of the colloidal
dipole suspensions are predetermined by the
appropriate selection of length, width and re-
sistivity of the dipoles, together with their
concentration and orientation.
Such a dipole suspension has the property
of absorbing or reflecting specified wavelength
ranges. Since a specific resonance characteris-
tic is obtainable from the same material merely
by changing its length to width ratio, very pure
colors can be obtained by transmission or re-
flection from coatings formed from such suspen-
sions.
The substances chosen to form the dipoles
are chemically stable materials, which remain
permanently within the suspension, and which are
not subject to chemical destruction by ordinary
atmospheric agents or by exposure to light.
The dipoles may be formed of metals such as
gold, platinum, palladium, chromium, tin and the
like, which are known to grow submicron crystal-
whiskers, under appropriate conditions, usually
from the vapor phase. Semi-metals such as
carbon are also known to form crystal-whiskers.
These crystal-whiskers may then be incorporated
into a plastic solution to form a dipole
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suspension.
A crystal-whisker "pigment" made of a
single substance of the utmost permanence, may
be predetermined in its properties; a perfect
black, a perfect white diffuse reflector, or
pigments having sharp reflectivity bands in the
yellow green, blue or other regions of the spec-
trum.
The effective cross section per particle in
a medium of index of refraction n is X2/8n4. In
a film, for complete light absorption or reflec-
tion, and assuming no aggregation of particles,
a suspension of submicron dipolar particles re-
quires
(8n2 /X2 ) = 8 x (1.5)2/(.565 x 10-4)2
-
= 6.25 x 109 particles/cm2.
Assuming a square cross section, the mass
per particle is
m = 6 (X/2n)3b2 where b = width to length ratio
and 6 = density in gms/cm3. For gold 6 = 19.
Hence for b = 1/25, m = 19 (0.565 x
252 = 2 x 10-14 gms/particle.
The mass of dipoles per unit area, required
to give complete coverage, ideally is:
(X/n)6b2 = (number of particles/cm2) x (mass/
particle) = 6.25 x 109x2x10-14=1.25x10-6gms/cm2.
Hence, very low concentrations of dipole
particles, of the order of 2 micrograms/cm2 are
sufficient to provide effective surface coverage.
For a film of 10-3 cm (0.4 mil) thickness,
and density = 1 gm/cm2, this corresponds to a
dipole concentration of only 0.125% of the
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solid film.
Because their effective cross section is
much greater than the physical cross section,
the dipolar particles may be very sparsely dis-
tributed in space. The dipolar particles are
sufficiently far apart from each other so as to
have no physical interreaction. Each dipolar
particle acts independently of the other.
Figure 8 shows a film containing dipole
particles with their length oriented normal to
the surface. The film is transparent because
the cross section particles present to the radia-
tion is so small that substantially no light
scatter and no light absorption occurs.
Figure 9 shows a film in the XY plane in
which the dipole particles are aligned in the OX
direction. Light transmitted along the Z axis
into the surface emerges from the other side
plane polarized with the electric vector Ey in
the ZY plane. Reflected light is plane
polarized with the electric vector Ein the ZX
plane. Reflected light is polarized andscat-
tered.
Figure 10 shows a film having dipolar par-
ticles in random orientation. Reflected light
is symmetrically scattered in all directions.
The transmitted light and the reflected light
show no polarization. However, since the di-
poles are "tuned" to a particular wave band,
the transmitted and reflected rays are comple-
mentary in color. Consequently, in the random
orientation, the dipoles act as pigments. But
however, these dipolar pigments are subject to
control by variation of physical quantities of
dimension resistivity and orientation.
Dipoles may be oriented by electric or
magnetic fields and by viscous shear forces
produced in the suspending fluid.
Dipole particles tend to disorient rapidly
in suspending fluids of low viscosity. For low
viscosity fluids the disorientation of dipolar
particles may occur in milliseconds. The
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disorientation is due to Brownian movement or
the random impact of the fluid molecules on the
dipole particle.
However, if the suspending fluid viscosity
is high, dipole orientation will persist for a
longer time, from seconds to hours. A permanent
orientation of dipolar particles may be achieved
in a plastic solution by allowing the solvent to
evaporate while maintaining the orientation.
Birefringence is the property of a material
having different indices of refraction for light
of different color (wavelength), and wave direc-
tion (polarization). This phenomena is shown,
by transparent crystals, stressed glass, and
stressed plastics such as cellophane.
Birefringent effects may be obtained by
oriented dipoles which are transparent crystal-
line rods.
These effects may be achieved through the
use of oriented dipoles. With dipolar birefring-
ent materials, unique color characteristics may
be obtained.
The dipolar concepts presented here have
numerous important applications.
Dipoles tuned to different wave bands may be
mixed in various proportions, so as to obtain
specific color characteristics.
For example, in Figure 11, dipoles having
three different lengths and three corresponding
resonant peaks are intermixed to form a filter
(A), and similarly dip84-es having two other se-
lected lengths are mixed to form filter (B)
which has two resonant peaks intermediate to
those of filter (A), (solid lines) to produce a
filter (B), (dotted lines).
Filter (A) has peaks of absorption or re-
flection bands which lie between those of Filter
(B). We have termed such filters "multichrome
filters". These "A" and "B" multichrome filters
are capable of mutually extinguishing light in
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a manner similar to that observed with polarized
filters, but do not have the angular character-
istics of polarizing filters.
Multichrome filters and complementary
multichrome reflective pigments have important
applications for 3-0 printing inks, color sepa-
rations, and other applications. Complementary
white and black, or color pictures A and B may
be obtained.
Black inks result from absorptive dipole
particles having broad-banded response corres-
ponding to a relatively thick antenna, or ab-
sorptive dipolar particles having a selected
range of lengths. Utilizing dipoles having a
similar broad-band response but high reflecti-
vity, produces a white pigment, or ink.
A new type of ink having the property of
polarizing light may be developed and used for
3-0 printing applications. Alternatively,
multichrome inks may be used for 3-0 printing
applications.
3-D printed pictures must be separately
viewed by the right and left eyes. This may be
accomplished by dipolar ink oriented in two
directions of polarization 900 to each other,
viewed by the observer through polarized lenses
oriented at 900; or, in the case of multichrome
printing, via multichrome separation filters A
and B.
A dipole film is useful in electro-photo-
graphy for producing black and white or colored
images.
Locallized orientation and disorientation of
a dipolar surface coating is caused forming a
permanent or temporary image in the coating. An
electrostatic image is set up by the action of a
light image, as in xerography; but without the
dust particles.
A dipole film coating will provide a re-
production paper which is white when the dipolar
particles are all aligned normal to the surface.
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Pictures of printing on the surface are formed
by locally disorienting the particles.
Another application of dipolar coatings is
a carbonless copy paper for typewriters capable
of producing an original and many copies.
In addition to the important applications
of dipolar pigments in the graphic arts, they
are useful in most other applications, for the
dyeing of fabrics, surface paint, etc.
Rem--=- . -- =BE,
E0Selliml!
380 400 20 40 60 80 500 040 0 0 600 20 40 60 00 00) 380 400 0 40 60 BO 500 0 060 0 600 20
WAVELENGTH IN MILUMICRONS WAVELENGTH IN MILLIMICRONS
Figure 1. Real and ideal pigment colors.
40 60 BO 7 0
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DIRECTION OF
ELECTRIC VECTOR
EilOF LIGHT=
POLARIZATION
AMPLITUDE
(INTENSITY =A2)
L WAVELENGTH (COLOR)
Figure 2. The three fundamental attributes of
light are: ampl itude, wavelength
and polarization
Figure 3. Polar graph of relative response versus
angle of a dipole to a constant signal
intensity.
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cos2 0
0?
900
ANGLE
POLARIZATION DIRECTION
DIPOLE ANTENNA
1800
Figure 4. Relative response of a dipole antenna
versus polarization direction p.
N/2
i
A/2 DIPOLE
HALF WAVE DIPOLE WITH
I.CENTRAL LOAD RESISTOR
2. DISTRIBUTED
RESISTANCE
LOAD
RESISTOR
7 3 (&)
I
.,
Figure 5. Showing half-wave dipole with charac-
teristic load resistor tuned to absorb
maximum power.
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PHYSICAL
CROSS SECTION
Az/I00
X/2
EFFECTIVE A/50
CROSS SECTION THICKNESS
AREA X2 /8
Figure 6. Showing that the effective cross sec-
tion of an antennae may be many times
the physical cross section; in this
case 12.5X. Power is funnelled from
an effective cross section into the
smaller actual cross section of the
antennae.
WAVELENGTH
25/1,H 10/1
Figure 7. Showing the relative power absorbed
or re-radiated versus wavelength for
thick and thin half wave dipoles.
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COATING
DIPOLES
NORMAL
TO
SURFACE
SUPPORT
Figure 8. Particles aligned normal to the surface.
Coating is transparent.
DIPOLE
PARTICLESM
PARALLEL TO
SURFACE
UNPOLARIZED
LIGHT
RANDOM
VIBRATIONS
Iv
TRANSMITTED LIGHT
POLARIZED
VERTICALLY
EY
0 ?????? :
Ex
REFLECTED LIGHT
POLARIZED HORIZONTALLY
Figure 9. Shows the polarization effects of di-
pole particles oriented in the plane
of the surface.
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DIPOLES WITH
RANDOM DIRECTION
Figure 10. Showing dipoles having a random
orientation.
400
500
600
700
Figure 11. Showing complementary multichrome
filters A and B produced by mixed
dipoles of different lengths.
REPRINTED FROM TAGA PROCEEDINGS 1963
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5 ?.5-
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gq 7'0,3
A PROPOSAL FOR
IMAGE-INTENSIFIER SCREEN
By
19 March 1965
Proposal No. Q400-308-31
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STAT
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TABLE OF CONTENTS
SECTION
PAGE
1.0
Introduction
1
2.0
Technical Discussion
9
2.1
Discussion of Requirements
10
2.2
The Proposed IIS Configuration
16
2.3
VARAD Properties
21
2.4
Control-Grid PC Approach
28
2.5
Alternate PC Approaches
39
2.6
Anticipated Performance
61
3.0
Program Schedule and Related
Information
62
3.1
Schedule
62
3.2
Reports
62
3.3
Program - Technical Tasks
63
4.0
Program Management and Structure
67
4.1
Introduction
67
4.2
Management
71
4.3
?Project Personnel
73
4.4
Resumes
74
4.5
Related Experience
81
4.6
Facilities
83
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LIST OF ILLUSTRATIONS
FIGURE NUMBER TITLE PAGE
1.1. VARAD PC Screen 2
2.1 Exploded Schematic View of
VARAD-PC Flat Panel 115
2.2
2.3
2.4
2.5
2.6
2.7
Method of Display Illumination
Schematic of VARAD Panel
The
System
EL-PC Light Amplifier
Performance Characteristics
Brightness Variations with
Voltage
17
19
22
29
30
33
45
2.8 Lighting Geometry for IIS Top
View 47
2.9 Lighting Geometry for IIS Side
View 48
2.10 Artistrs conception of IIS
3.1 Image Intensifier Screen
Program Schedule 64
4.1 Organization Chart of 1
Information Sciences Center 68
4.2 Project Organization Chart 72
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1.0 INTRODUCTION
is pleased
to submit this proposal for the development of a flat panel image
intensifier screen (IIS). The system we propose is in full compli-
ance with "Development Objectives" Of 5 November 1964.
The system approach described herein is based on a new electro-
optical display material, together with the application of advanced
image intensifier techniques that will permit particularly advan-
tageous utilization of the new material. The approach is an outgrowth
of the intensive research and development program that has been
conducted
both with company? funds and on Government contract
work, in the fields of sophisticated display systems and photo-
interpretation.
The new material, called VARAD, has the property of varying its
transmittance, absorbence, or reflectance of radiation in response
to an applied electric field. VARAD, employed to replace the function
of electroluminescent (EL) material in an image intensifier screen,
would lead to certain fundamental advantages not achievable in the
conventional EL-PC light amplifier.
These advantages in an IIS, revealed by theoretical, experimental,
and system studies include:
a. Effective optical density of "black velvet", with a maximum
contrast ratio corresponding to a ratio of optical densities
of 15, and maximum brightness far exceeding the present
state-of-the-art capabilities of EL. This leads to image
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it
IMAGE ?
? INTENSIFIER
SCREEN
VARAD PC SCREEN
VARAD VIEWING
SCREEN
Figure 1.1 Transparency is imaged on rear surface of Image Intensifier
Screen, exciting photoconductive film. Light energy reflected"
from VARAD is many times greater than incident energy.
Photo interpreter may examine magnified image on VARAD
screen under normal ambient light.
2
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contrast properties far superior to that achievable with EL
materials. No deterioration of output layer lifetime as a
function of high brightness.
b. No degradation of image contrast in high ambient light.
c. Wide-band excitation frequency response to electrical exci-
tation. This permits optimum design for sensitivity to low
light levels in the input image in terms of most effective
utilization of the photoconductive (PC) material, impedance
matching, and other considerations.
d. For a given brightness lower excitation voltage requirements
than those of high performance EL materials; greater gain.
e. Much wider dynamic range than is possible with an EL layer.
f. Several exciting new possibilities for limited or extensive
image manipulation when VARAD is used by itself or in
conjunction with EL material; the degree of image manipulation
achievable depending only on the degree-of complexity allowed.
g. Substantial improvement in time constant should be achievable.
Time constant build up limited only by photoconductive response
time.
With respect to other performance characteristics, an IIS
utilizing VARAD should be equal to or better than a conventional
EL-PC configuration. These characteristics include linearity,
resolution, signal/noise ratio, viewing angle, achievable size, life
expectancy, power requirements, monochromatic sensitivity, cost, and
size.
3
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In many cases, one of which is indicated below, it is expected
that because of the inherent design flexibility engineering trade-
offs can be made to significantly improve many of these performance
characteristics by using VARAD, beyond the requirements of the RFP.
It is anticipated that some of these additional performance
characteristics can be improved through the increased design flexi-
bility permitted by VARAD due to its w andi or by the
greater number of design parameters available on combining VARAD with
an EL layer. An example of this might be linearity. Here, a design,
problem would be to utilize natural non-linearities to best advantage.
Design considerations would involve thresholds that prevent excitation
signals from showing up in an unwanted way in the image. On the other
hand, non-linearities would be combined so as to achieve linearity
and contrast control over the range desired, as far as possible,
after reducing the non-linearities as appropriate, by solid state
and chemical methods. As more design parameters become available,
or as the allowed operating range becomes wider, more can be
accomplished along these lines.
Unlike EL material, VARAD is not self-luminous, but, rather, is
a modulator of light, being vastly more sensitive, and is a much
thinner layer (1.5 mils at the present time) than a Kerr cell or
ADP light switching arrangement.
VARAD, supplied
investigated extensively
precisely for the light amplifier and IIS application area.
has been
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STAT
STAT
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Other applications of VARAD have been explored
but STAT
apparently not the IIS applications. The proposed program involves
directed research and development, beyond the analysis and experi-
mental results presented in this proposal; but we have carried the
work far enough to be confident that the proposed approach offers
much greater promise than an attempt to improve on the EL-PC approach.
In addition to making use of the exceptional properties of VARAD,
we propose to further develop the control-grid approach developed by
Basically, our proposed IIS configuration will consist of substituting''
VARAD for the EL layer in a grid-controlled continuous layer panel.
The
grid-controlled PC layer eliminates the need for a
grooved PC configuration that degrades resolution, obtaining high
sensitivity without the grooves, and a resolution of 250 lines per
inch. The use of VARAD in this arrangement should lead to even.higher
sensitivity, and eliminate other design problems. This is discussed
in Section 2.
is staffed with an interdisciplinary group of scientists,
systems analysts, solid state physicists, and engineers engaged in
basic and applied research, system development, and production of
instruments tailored to needs of the Government and industry,
particularly in the image intensifier and display fields. This group
combines the "know-how" required for the utilization of VARAD and
other electro-optical materials in engineering applications with the
background in display and ITS techniques and research and development
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that is essential in the system development phase of the program. In
addition to notable achievements in the development of information
processing systems, and in the US area,
has been credited with STAT
innovations and scientific accomplishments in the development of
photochromic and video systems.
The technical discussion and analysis of the proposed image
intensifier screen is presented in Section 2. Section 2 includes
a description of VARAD in some detail, as well as setting forth
techniques for advantageously combining VARAD with PC material and
other IIS componensts. The proposed work tasks and program schedules
are then presented in Section 3. Section 4 outlines the
rganization and Management structure,
technical, personnel, and pertinent scientific and engineering back-
ground and facilities information.
Utilizing VARAD, together with the grid-controlled STAT
PC layer an
STAT
proprietary techniques outlined in Section 2, and
taking advantage of such conventional design practices in light
amplifier technology as may remain desirable in the
our proposed approach, it is the considered opinion of
can more than satisfy the requirements of the RFP.
that in performance of the proposed program, we can
new context of
hat we STAT
We also believe
achieve a major
breakthrough in the IIS field, in which VARAD will substantially
replace EL material as the output layer for future IIS equipment.
VARAD is an obvious answer to this problem, and this application
should, at this time, be pursued by a research group familiar with
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both the material and the application.
In addition to
experience in various large
screen displays, certain capabilities which are of particular use-
fulness in pursuing the IIS should be noted. In the course of
improving the sensitivity of our large screen electrophotochromic
display system,
has developed a very similar tech-
nique to that proposed for the
This technique also involves the use of a liquid light modulator
in thin film suspension, namely, a solution of organic spiropyran
photochromic dye in a toluene solvent suspended in a glass cell with
a thickness of liquid film of only .001". On this program, the
liquid photochromics were utilized in such a fashion both to obtain
high resolution, minimum molecular diffusion and the much higher
sensitivity attendant to using photochromic dyes in liquid form.
This development which was completed in the last two months has been
quite successful and large screen display images impressed on the
liquid photochromic film have been successfully demonstrated. In
addition, a feature not required per se for the IIS but important in
the construction of the IIS was developed; namely, a means of changing
the fluid in this very thin film almost instantly upon demand.
Certain other optical thin film technologies have also become
a matter of familiarity and knowledge of handling
The extensive use of special anti-reflective optical coatings both
multilayer dielectrics and single layer magnesium flouride coatings
had to be applied to bath plastics and mica for the first time known
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to us. This technology was instrumental in
successful development of its present second generation electro-
photochromic demonstrator which is being shown almost daily to
people in the display industry. The use of anti-reflection coatings
for the proposed system is of course important with regard to
preventing unwanted reduction from the illuminated front surface of
the IIS.
With regard to the opaque dielectric reflector utilized in the
proposed 'IS,
has utilized these quite successfully.
and has again utilized these successfully on very thin film substrates.
Such dielectric reflective layers were utilized in the form of so
called "dichroic" reflectors deposited on both .001" thick mica
sheets as well as on fiber optic plates. For the IIS such techniques
are important because it is necessary to obtain reflection of light
without using metallic reflectors which would tend to destroy the
electric charge distribution which controls the transparency of the
VARAD film which of course makes up the image to be viewed.
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2.0 TECHNICAL DISCUSSION
The following section covers area by area, the technical
considerations involved in the development of a VARAD/PC Image
Intensifier.
Since the VARAD/PC approach can make use of all technology
involved in the development of PC control structures for use with
EL/PC Image intensifiers, the discussion will generally be
related to work performed on EL/PC devices. Extrapolation to VARAD
instead of EL is, straightforward since the inherent characteristics
of VARAD are considerably less limited than EL.
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2.1 Discussion of Requirements
The prime objective of the proposed development is to
provide an Image Intensifier that will take a highly magnified dim
image projected onto its rear surface from a photo transparency and
provide on its front surface without degradation of an image of
greatly increased brightness.
The development of high acuity cameras and photographic
films of extreme resolution have increased the need for such an.
overall optical viewing system of high magnification, as an aid to
photointerpretation. To obtain such magnification ratios directly,
at brightness levels suitable for viewing under normal ambient
lighting, requires projection light sources of sufficient intensity
to cause physical damage to the film transparency, particularly at
high film density levels.
The proposed unit will provide high brightness levels at
the viewing screen with projection light sources of lower intensity. .
The viewplate is a two-dimensional VARAD "Light Amplifier," which
by photoelectric control will reproduce electronically on its front
surface an image projected on its rear surface, but at a greatly
increased illumination level. The Image Intensifier will preserve
the high resolution, dynamic range, and unity gamma associated with
existing optical projection techniques. It is also capable of
density expansion and compression, positive-negative viewing, optical
.contouring and other features which can greatly extend its usefulness.
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It is a prime development objective to obtain a screen
"package" which can be substituted for a diffusing glass screen
in existing viewers, and in new viewers to be produced. The visible
image on the viewplate is produced in a high efficiency VARAD screen.
The light sensor on the rear surface is a continuous photoconductive
layer followed by simulated electron multiplication. Power supplies
and control panels can be remotely situated.
The following tabulation lists the design objectives which
will guide the experimental and engineering effort on this program.
Some of the objectives are presently available in image intensifiers
while others will require intensive development. It is believed
that the following list represents realistic goals which can be
achieved during the course of the proposed program.
Light output up to 200 foot-lamberts at magnifications
up to 100X
Resolution of at least 10 lines/mm in both X- and Y-
directions
Completely flicker-free display
Dynamic tonal range of 20 gray shades
Screen size of 12" x 12" capable of expansion to 30" x 30"
Signal-to-noise ratio greater than 100
Unity gamma over a range of brightness of 100X
Positive-negative viewing
Minimal energy transmitted through film to preclude film
damage due to heating
Variable density compression and expansion
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Life expectancy in thousands of hours
Usable with illumination from laser monochromdtic light
source
Rear projection .
High rejection for incident ambient at viewing side
It is expected that the various design objectives will be
achieved through dramatic improvement of the presently available
devices.
The proposed approach is unique in that it substitutes a
Varad cell for the EL portion of a EL/PC type Image Intensifier.
This is advantageous for many reasons, but perhaps the prime considera-
tion is that since Varad approaches an ideal circuit, it permits
optimizing for photoconductor performance instead of having to
compromise photoconductor performance to match EL characteristics.
The other key consideration is that the Varad/PC Image Intensifier
is really a high efficiency light modulator which uses an external
light source. This permits independent control of the light output
level while the cells inherent scattering and absorbtion when
unexcited produces a true "black velvet" background even in high
ambients.
Historically, since 1946, a great deal of effort has gone
into the development of flat panel displays, with effort concen-
trated on photoconductive-electroluminescent panels until approxi-
mately 1960. Since then other means of driving electroluminescent
panels have been investigated, such as travelling wave piezoelectric
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devices, crossed-grid electric field devices, ferromagnetic, and
ferroelectric approaches.
It should be emphasized that this entire technology can be
utilized in application to VARAD, and offers the chance for a quick?
breakthrough since VARAD is sensitive, flexible in driving require-
ments, and is a modulator of light rather than an emitter of light,
thus to a large extent minimizing trade-offs previously necessary
between speed of response, sensitivity, display brightness, and
resolution.
Sensitivity for present day EL/PC light amplifiers is on
the order of 0.01 foot candles or less, which is approximately 10-8
joules per cm2 for a 1 second exposure.
To our knowledge the best resolution to date for a
practical photoconductive controlled panel is that developed by
The panel was shown at the N.Y. World's
Fair where it demonstrated resolution of approximately 250 lines
per inch.
last year and is, therf ore, in an excellent position to combine it
with the use of VARAD.
Earlier continuous-layer EL/PC panels were made with a
vapor-deposited PC layer and exhibited an undesirably low impedance.
Our approach to solving this problem was to make PC layers of 'extremely
fine powders bonded with transparent plastics. This increased the
impedance but unfortunately resulted in reduced sensitivity to visible
radiation. Anther approach was the grooved panel developed by
has studied this approach during the
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which attempts to solve this problem by increasing
the surface area and, therefore, the sensitivity, through the use of
a grooved PC layer while retaining the desirable impedance charac-
teristics of a relatively thick layer. The poor resolution of this
device (1 to 2 lines/mm) is directly related to the grooving, and
no major improvements seem likely in the near future.
Limitations of brightness, resolution, lifetime versus
brightness, and sensitivity, as well as gamma transfer characteristics
have been serious problems in the past; but not all of these limita-
tions have been problems with all approaches. For example, resolu-
tion is the most serious limitation of the
yrooved STAT
PC layer approach, and is considered so serious as to rule out this
approach for the proposed application; except as a possible suppli-
mentary technique for alternative possibilities.
We believe that the grid controlled panels developed by
offer
the best approach because they use thin PC films permitt6d by their -
great sensitivity, but avoid the impedance problem by controlling
the panel impedance with a fine wire AC driven grid, phase-referenced
to the AC drive source.
The unique combination of this approach with VARAD
further enhance the overall efficiency of the system.
The characteristics of the two phase Image
Intensifier Screen are such as to provide a variety of controlled
gamma characteristics. The gamma characteristics can be varied by
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STAT
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adjusting the amplitude and phase of the control signal power supplies.
Using these characteristics to best advantage will yield unity
gamma over a 100:1 brightness variation or compression or expansion
if desired.
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2.2 The Proposed IIS configuration
The proposed image intensifier screen will consist of replacing
the EL layer in a conventional EL/PC HS with a VARAD cell and using a
grid-controlled continuous photoconductive layer panel for control.
Appropriate adjustment of parameters will be made to optimize the
resulting configuration, as well as providing for suitable illumin-
ation of the VARAD. The substitution of VARAD for EL will result
in greater brightness, and no resulting deterioration of lifetime.
VARAD also will permit a time constant of as short as 0.01 seconds,
contrasted with 0.06 seconds with EL material.
The manner of mating the fundamental input and output layers
that we propose is illustrated schematically in Figure 2.1.
Everything below the "opaque layer" in the figure is the same as in
the
EL/PC system. With respect to electric fields,
the geometry above the "opaque layer" is essentially the same as
required for EL excitation except that a VARAD cell replaces the
EL layer. The
PC portion, however, operates in the same
fashion. See Section 2.4 for description of the
system.
The dielectric constant will not be the same for VARAD as
for the EL layer, of course, and both layers in their respective
configurations would consume small amounts of energy from the
electric field, but this difference would not lead to significantly
altered field strengths. The properties of VARAD in responding
to the pattern of electric field strength, however, as contrasted
to the properties of an EL layer in responding, should provide a
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V
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A A
GLASS
TRANSPARENT ELECTRODE 1-2.p.
TRANSPARENT DIELECTRIC 10y.
VARAD. 41-0)A
WHITE LAYER 10}1.
OPAQUE LAYER 5-I0p.
PO LAYER 130)A
2.00)1
1,8xto-s
INCHES
1 POLYESTER DIELECTRIC %sou,
LAYER(S NE ILICO COATED)
= TRANSPARENT ELECT RODE
SCHEMATIC LEGEND
GLASS SIGNAL LIGHT INPUT
(NOT TO SCALE)
re-
f
POWER LIGHT INPUT
OUTPUT LIGHT
DARK PORTION OF VARAD
LIGHT PORTION OF VARAD
FIGURE 2.1: Exploded Schematic View of VARAD-PC Flat Panel 115
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significant improvement in the system; this was mentioned in the
INTRODUCTION and will be treated analytically in Section 2.6.
Thus electrically, Figure 2.1 does not differ
very much from the EL/PC system, but optically, it is
quite different above the opaque layer. As far as mechanical differ-
ences above the opaque layer in Figure 2.1 are concerned, large
panels of Varad have already been fabricated and very narrow separ-
ation has been achieved for small panels.
Some development.will be required to produce large
panels with narrow Varad layers, but this should not present undue
problems.
A white reflecting layer is used in the proposed
system, but an electrically non-conducting dichroic mirror may also
be useful as an alternate approach. The transparent dielectric
above the Varad layer in Figure 2.1 prevents electrolysis
of the Varad; the opaque and white layer below the Varad also performs
this function.
Figure 2.2 illustrates how the configuration in
Figure 2.1 would be front-lighted from the edge with the viewing
light input. This light actually comes into the display panel at a
shallow angle to the plane of the viewing surface, and then is
viewed at a steeper angle (up to 45? maximum). The viewing light
input illumination is shown coming only from one direction; but,
actually, it would come from all four edges of the square or rectan-
gular display frame, so as to provide roughly equal illumination at
all points. Resulting resolution, evenness of illumination, and
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wHtTE
SuRPACE
I W PUT* LAYERS \MAIM) DIELEGTR1G
AND GLASS ELEGTROOE
A140
(NOT TO SCALE)
> ????? .???? ???=Mo ????160.....7>(%)
LG HT SOUR.,CE
i/E- POWER LIGHT INPUT
ok.
OUTPUT LIGHT
DARK PORTION OF VARAD
LIGHT PORTION OF VARAD
FIGURE 2.2 Method of Display Illumination
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? other optical as well as electrical considerations are considered
quantitatively in Section 2.6 showing the improvement in many
performance measures that can be obtained with the Varad system
over the EL system.
Interference coatings may, of course, be added to
the system in Figure 2.1 in order to minimize unwanted reflections.
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2.3 VARAD Properties
VARAD is a suspension in fluid of organic molecular or metallic
dipoles "tuned" to optical frequencies (approximately 1014 cycles per
second). In its normal state, VARAD appears very dark brown or black
opaque, because the miniscule resonant dipoles scatter light in random
directions until it is finally absorbed. When an electrical field
strength of 10 to 100 kilovolts per centimeter (equivalent to 25 to
250 volts for a 1 mil layer) is applied in a direction parallel to the
viewing angle, VARAD becomes clear; VARAD is thus an electronically
controlled shutter with proportional control. In its present state
of development, it opens in about 10 microseconds, and closes in
10 milliseconds.
Up to February, 1965, the thinnest cell that had been made was
ove-.,?:?//
approximately 3 millimeters thick, and the largest area was on the
order of a few feet square. .Two VARAD panels,specially fabricated
for are approximately two inches on edge and contain a fluid
layer only 0.0015 inches thick. This is the first time a VARAD cell
has been made this thin.
The two panels are fabricated of glass that has been specially
coated with first a transparent conductive film and secondly a
protective dielectric film. Thus, each panel\is a 2 electrode
device with 2 wire leads. The two layers of dielectric film, and
AC voltage excitation, are necessary to keep the VARAD from polarizing
due to the migration of ions that would eventually drift to the
electrode and counteract the electrostatic field. The AC voltage
counteracts any such migration. Figure 2.3 is a schematic of the panels.
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ANTI ?REFLECTION
COP:TIN G
GLASS
NS Ut?AT114C,
TR 4.14GPARE.NT
DI ELECTRIC ,
'-'9.
?jjfiiih':
,
...
.,-
0,-,:'
zi:4
:..1...ii"
.::,50
:J..4.
4.':e,./
/.A.,4,..
?i?:-L4
..?'...?...i.t
-2:,
. ,,,
. .....
tl,,,-;f;
11:.43
. .-
.47
0t,-..,,,
'.e,..1
'''''.:
sk...A
44i."..f..:
;V.,..:,..
'XItfri,
;;
,
? -.1,
tr.21..!
41 ,":"
te' 4'
Pf.'
irl!,.'...1
;vi.,
VI
..,1),
:41.
i?ti,,.
, fr
1,11.7-
? ,=,,,,..$
,Iii
`'71-.'.4 i
4o "
..r ,

41
K'
14'
lk.sZh.
. s' ,
. 1
,
,.,i/
,
,
.../
\
2" SQUA
,
V
k 0
AR A
(.0015"
Tmictt)
TRANSPARENT
C?CoutouCTW
LAN ST:k
I/8" TOTAL
THICKNESS
FIGURE 2.3 Schematic of VARA) Panel
22
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The optical density for a single pass of light through the cells
in their unactivated states is 2, or 99% absorbing, in the un-excited
state. VARAD is a light modulator, unlike electrically excited
phosphors. Thus, when its optical transmission is controlled by a
PC light detector layer and the VARAD itself controls a strong
"readout" light beam that is passed through the VARAD once, reflected
by a white or mirror surface, and passed back through the VARAD a
second time, a very sensitive high brightness display system is
achieved. The use of a double pass of light through the VARAD squares
the sensitivity of the VARAD optical transmission as a function of
applied voltage.
Theoretically, photochromics could be used as light modulators
for IIS applications if they were about six orders of magnitude
more sensitive. Present day EL-PC light amplifier panels are
approximately 10 million times more sensitive than photochromic
materials. They are more sensitive because they use photoconductors
as a valve for controlling an external source of electric energy
instead of just passively responding to the image energy alone, as do
photochromic materials. These EL-PC panels have a high power gain
(100 might be typical) and the sensitivity is limited primarily by
the dark current of the photoconductor. The overall power gain of a
VARAD panel is much higher because VARAD itself is a second valve --
for light. When light is passed twice through a VARAD layer having
an optical density of 2, the system has an effective optical density
of 4. This "doubling" of the optical density would further increase
the system gain.
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Thin VARAD panels thus consist of the VARAD fluid contained
between two glass layers, very thin transparent conducting and
insulating layers being between the fluid and the glass plates. The
dipoles are submicroscopic (roughly 1 micron long) needle-like
particles that interact with light. The VARAD panel becomes light
transmissive when an electric field is applied between the transparent
conductors. This causes the dipoles to align at right angles to the
glass plates, so that the dipoles are more or less parallel with
the light rays. Upon removal of the electric field, the directions
of the needle-like particles randomize as a result of thermal
molecular impacts. Dipoles in random directions absorb or reflect
light.
By choosing the size and shape of the. dipoles, VARAD can be
made to act as a very selective color filter under certain circum-
stances, or to appear quite black by reflected light when the layer
is not in an electric field.
According to the manufacturer, the shelf life of VARAD should
be indefinitely long, and no noticeable deterioration of the material
has been noticed when cycling it electrically or by subjecting
it to high intensity light. The material may however be degraded
if excess voltage is inadvertently applied.
The resistivity of a VARAD panel, due primarily to the insulating
layers, is very high, of the order of 1000 megohms, so that its
electrical impedance is mainly capacitive. For instance, a cell 4
inches square and 3 millimeters thick, has a capacitance of 300
micro-micro-farads.
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The applied electric field for controlling VARAD is parallel
to the general viewing direction instead of perpendicular to it as
in the case of a Kerr cell, and no optical polarizing layers are
needed as in the case of the Kerr cell, APD, or KDP optical valves.
Between 100-200 volts will be sufficient for controlling the
VARAD for the proposed application, whereas tens of thousands of
volts would be needed to modulate a Kerr cell of equivalent viewing
area, or several thousand volts for the ADP arrangement. For comparison,
100 to 1000 volts are needed to produce EL luminosity of maximum
brightness. Voltage in the range of 100 to 200 volts for activating
the VARAD liquid can be directly controlled by state-of-the-art
photoconductive materials which would be put in an image plane similar
to that previously utilized in EL-PC light amplifying panels.
Compared to EL layers, VARAD operates over a large frequency
range up to at least 20 kc, whereas EL phosphors are usually
impractical at frequencies above 1 kc. Also, EL layers normally
require about 0.4 kc as a minimum for high light levels, although
of course, the familiar EL night lights operate on 60 cycle AC
voltage at very low light levels. Theoretically, VARAD could operate
all the way down to DC if a fluid could be found that would not
polarize. This wider frequency choice property of VARAD, as compared
with EL material, allows much greater flexibility in matching VARAD
to its image-controlling component such as a photoconductor, in terms
of impedance.
The spectral characteristics of the VARAD IIS can be tailored
to any desired portion, wide or narrow, of the IR, visible, or UV
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spectrum.
The VARAD panel requires very low power, since it is substantially
a capacitive load, with a very high resistance photoconductor
(approximately 107 ohms) causing the only power dissipation.
With a 10 cm x 10 cm VARAD cell, an electrodichroic ratio of
15 has been obtained at an applied frequency of 20 kc, the electro-
dichroic ratio being defined as the ratio of the optical density
closed to the optical density open.
For an electrodichroic ratio of 15, upper and lower ranges
of transmittance of
63% (D = 0.2) open
0.1% (D = 3) closed
or
or
50% (D = 0.3) open
0.0013% (D = 4.5) closed
31.5% (D = 0.5) open
1.3x10-5% (D = 7.5) closed
has already been achieved, according to the vendor.
The opening (or clearing) time is 10 microseconds with an applied
voltage pulse. To induce closing (darkening), the normal VARAD panel
depends upon Brownian motion for which closure time is of the order
of 10 milliseconds. Faster Closing times can be induced by the
application of a crossed electric field. Slower closing times can
be obtained with higher viscosity fluid, which is the case for the
thin 1 mil layers we have receivqd.
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VARAD panels are presently available up to 24 inches on a side.
Larger sizes are available on special order.
In the panels presently utilized, very low concentrations of,
dipole particles are adequate. The order of 2 micrograms per cm2 are
capable of producing the results described above.
is apparently the first to have a thin layer
VARAD cell, since the vendor has never fabricated one before, and the
material is only a few months old. VARAD has apparently not been
explored for display purposes by the vendor, and, as of this date,
probably not by others. It is a material conceived in crude form
by the manufacturer and perfected with Navy funds for flash blindness
prevention and similar applications.
has developed an optical
system and ffknow-how" for multiplying absorption effects and thus
greatly increasing sensitivity. This application was developed by
for the photochromic display systems but it is also applicable
to VARAD.
27
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, 2.4 Control-Grid PC Approach
The
PC films.
impedence
control-grid photoconductive panels use thin
They have great sensitivity, and control the panel
with a fine wire AC driven grid, phase-referenced to an
AC drive source (See Figure 2.4 ). The grid permits direct control
of gain, sensitivity, and type of imaging. The
system,
illustrated in Figure 2.4, uses al EL layer, where we propose to
use VARAD. In other sections in this proposal we are
interested in that portion of Figure2.4 0 below the "reflection
layer". We will, however, continue to describe the entire
Control-Grid approach in this section.
In addition to the unique control features inherent in the
approach, the gist of their approach is this: they
obtain use of lateral PC conductivity (large area activated-thin
layer) by using a grid feed, whereas such systems as the RCA grooved
panel approach use the surface of wedges to obtain the same effect.
However the grid system does not limit resolution to the grid
spacing, whereas the grooved approach does.
In Figure 2.5, drive and control signals are at the same
frequency; by varying their relative amplitude and phase the
following effects can be achieved:
Gamma Variation over the range -2 to +2
Positive Imaging
Negative Imaging
Mixed Positive and Negative Imaging
Gain Control Over the range from 0 to 100
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LIGHT OUT
REAR SLAV'S Va" THICK
t TRANSPARV4T ELECTRODE A
EL LAYER 50p.
REFLECTION LAYER I0).4
OPAQUE LA?MI:t
PG LAYER ElOp,
GRID WIRE%
POLYESTER D1E LEOTR so u.
LAYER (SILICONE. COATED) V
1 TRANSPARENT ELECTRODE Ei
t t
LIGHT IN
FIGURE 2 . 4
GLASS
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(A)
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LIGHT INPUT L1
1 1 1 1
: 11
1
1 PARALLEL FINE GRID ELECTRODE
' THIN FLAT PHOTOCONDUCTIVE LAYER
OPAQUE LAYER
? REFLECTIVE LAYER
Ii --\\... ELECTROLUMINESCENT LAYER
' ? TRANSPARENT ELECTRODE A
GLASS PLATE A
GLASS PLATE B
TRANSPARENT ELECTRODE B
TRANSPARENT DIELECTRIC LAYER
LIGHT OUTPUT L2 11
2
(B) TRANSPARENT ELECTRODE A
TRANSPARENT ELECTRODE: B
TRANSPARENT DIELECTRIC LAYER
THIN FLAT PHOTOCONDUCTIVE LAYER
PARALLEL FINE GRID ELECTRODE
ELECTROLUMINESCENT LAYER
Figure 2.5 EL-PC Light Amplifier
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? '_
4
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The panels can be made to have a dynamic range in excess of
100 to 1 at a gamma of unity. Their demonstrated resolution
is greater than 10 lines/mm, an order of magnitude better than
_
that achieved with any comparable device.
A cross-section of an experimental model of the STA1
light amplifying panel and its power supply connections is shown
in Figure 2.5. This structure, with its photoconductive layer sand-
wiched between an electroluminescent layer and transparent dielectric
layer, is similar to a dielectric panel triode.
The operating power comes from two AC driving voltages of the
same frequency but adjustable phase. A voltage V1 is applied
between the transparent electrode A and the grid electrode. A
voltage V2 is applied between transparent electrodes A and B. The
photoconductive layer is excited by the light input Li that passes
through the glass plate and the transparent electrode A.
Light output L2 is proportional to the current flowing through
the electroluminescent layer. Lateral photoconductive current
through the photoconductive layer from the grid electrode is dependent
upon light input Ll.
The use of lateral photoconductivity results in a low con-
centration of photoconductive currents, which enables resolutions
higher than might otherwise be expected from the'nominal spacing
of the fine, parallel-grid electrode.
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Light output is proportional to the amplitude of the vector
current 13 = I ? 12, as shown in detail A of Figure 2.5. Current
is the lateral photoconductive current varied by light input Ll;
12 is the vertical capacitive current through the photoconductive
layer. The amplitude of 13 and light output L2 can be made to.
increase, decrease, or exhibit a V shaped characteristic with increase
in light input Ll, by adjustment of amplitude and phase relationship
between V1 and V2.
Detail B of Figure 2.5 shows the simplest equivalent circuit
diagram of the light amplification panel.
The input image can be intensified and converted to a positive
image, a negative image, or a mixed output image with negative and
positive parts. The characteristics can be changed continuously by
adjustment of V1 and V2. This enables complete control of image
type, gamma and brightness over a wide range.
The performance characteristics shown in Figure 2.6 were obtained
with an experimental panel having a 10 micron diameter, tungsten-
wire grid wound with a 300 micon spacing The electroluminescent
layer is green ZnS, and the photoconductive layer is CdS. The curves (
"P.
show light output L2 as a fraction of light input Ll with the
amplitude and the phase of power supply voltages V1 and V2 as
parameters. A constant operating frequency of 800 cycles was used.
Curves A through D are experimental curves for 4 values of V2/
with V1 held constant at 300 volts. Curves E and F are similar
experimental curves for 2 values of V1, with V2 held constant at
1,500 volts.
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ELECTROLUMINESCENT LIGHT OUTPUT IN RAD-LUXES
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103
102
10
1
CURVE V1
A
B
C
D
E
F
V2 PHASE
300
0
300
300
SAME
300
450
OPPOSITE
300
1, 500
OPPOSITE
0
1, 500
90
500
SAME
11?.11
5
3
10-
2
107
1
TUNGSTEN LIGHT INPUT IN LUXES
Figure 2.6 Performance Characteristics
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Curve A is for V2 = 0. The photoconductive layer and electro-
luminescent layer are then effectively in series across the V1
power supply. The electroluminescent layer is excited with the
lateral photoconductive current Ii (=I3), which increases with
intensity of light input Li. Thus, the panel light output is a
positive intensified reproduction of the input image.
Curve B is for V2 in phase with Vi. Lateral photoconductive
current II and the vertical capactive current 12 are nearly in
phase. The amplitude of the total luminescent layer current
(13 = Ii ? 12) increases. Gamma and brightness range decreases
because part of the increased 13 is not under the control of the
incident radiation (input light Li).
Curve C is for V2 slightly more than 180 degrees out of phase
with V1. The literal dark current I is opposite in phase to the
vertical capacitive current 12. The structural form of the equivalent
circuit is similar to that of a bridge circuit. The vertical
capacitive dark current compensates the lateral capacitive dark
current 12, and the dark value of 13 is greatly decreased.
When light input Li is applied, II changes from lateral
capacitive dark current to lateral resistive photoconductive current.
Its amplitude also increases. This causes a sharp increase in the
amplitude of the luminescent layer current 13 = ? 12. The result
is positive image intensification with high gamma and large bright-
ness range.
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Curve D is for a much larger value of V2, applied 180 degrees
out of phase with V1. In the region where the input light Li is low,
is over-compensated by the excess L2. In this region, increase in
light input Li increases ii which tends to cancel the excess 12, and
the amplitude of 13 and dependent L2 decreases. The light output
in this region is a negative intensified reproduction of the input
image.
Where L1 is large enough to completely cancel the excess 12, and
beyond this point, the amplitude of 13 again becomes directly pro-
portional to Li. The curve of L2 as a function of Li is thus V
shaped. If the input image is relatively dark the output image is
the negative of the input. If the input image is adequately bright,
the output image will be a mixed negative and positive reproduction.
Curve E is for high V2 and zero V.
The electroluminescent
layer is effectively in parallel with the photoconductive layer, and
in series with the transparent dielectric layer. The layer is
excited by the vertical capacitive current L2 (=I3) that penetrates
the photoconductive layer.
As the intensity of L1 increases, the lateral photoconductivity of
the photoconductive layer increases and acts like an electrostatic
shield. Current 12 is bypassed to the parallel grid electrode. With
increasing light input, electroluminescent layer current 13 (=I2)
decreases monotonically. Light output decreases similarly. The
output image is an intensified reproduction of the input image
throughout the entire L1 region.
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Curve F is for high V2 and much smaller in-phase V'. When
light input is low, the electroluminescent layer is excited mainly
by the vertical capacitive current that penetrates through the
photoconductive layer. This current decreases with increasing light
input. Current Ll from the photoconductive layer is in phase with
current 12. It adds to 12 and tends to reduce the rate of decrease
of total 13 with increase in light input Ll. The output image is a
negative intensified reproduction of the input image, but gamma is
lower and brightness range smaller than when Vi is zero.
These examples describe performance with Vi and V2 of the same.
or opposite phase. Performance characteristics can also be varied
by continuous phase control, and interesting characteristics can be
obtained. Performance characteristics may also be varied (and
sensitivity improved) by direct current control of the lateral
conductivity.
Resolution of the intensified images is higher than that expected
from the spacing of the parallel grid electrode, because lateral
photoconductivity is used and photoconductive current is not converged.
Experiments which were made to check the resolution of the
panels showed that when the incident pattern is perpendicular to
the grid the resolution is almost independent of the grid pitch
(if the grid pitch is reasonably small) and is mainly dependent on
the uniformity of the photoconductive and electroluminescent layere,
and on their respective grain sizes. Resolution of over 10 lines/mm
is attained in this direction.
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When the incident pattern is parallel to the grid the benefits
of lateral conductivity are limited by the grid pitch. Resolution is
still higher than expected, and falls between the limits of one line/
pitch-length minimum and three lines/pitch-length maximum. Isotropy
of the resolution can be improved by using
instead of a parallel wire electrode.
As a result of discussions
a mesh grid electrode
we find that, with- STAT
out need for further experimentation, there are several significant
improvements which can immediately be made in the panel. A finer
precision mesh can be used in place of the present hand-wound grid.
At the outset of this program, an improved version of the present
panel will be built as quickly as possible for evaluation. As the
program proceeds subsequent versions will be built, each reflecting
the cumulative improvements attained in the previous models.
The research program will be oriented to improve all the
parameters of the grid-controlled PC layer, as well as to substitute
VARAD for the EL layer.
The image intensifier may be used to intensify greatly enlarged
images (100X) from high resolution photographs. If the film has a
resolution of 400 line pairs/mm and a magnification of 100X is
employed, the final resolution is 4 line pairs/mm and the present
EL-PC panel has more than adequate resolution. In practice the
resolution has been limited to about one third the control grid
wire spacing. Since the present panels use wound grids, a 250
micron spacing has been the best attainable. This can easily be
improved through use of an etched grid. In addition, the control
grids are wound as parallel structures rather than as meshes and a
37
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2:1 improvement in resolution uniformity can be expected through use
of a mesh grid. Appropriate grids are commercially available from
Buckbee-Mears and are used in storage tubes. The grids are in the
form of metal sheet about 1 mil thick and perforated with square holes,
1000 to the inch. This should give a resolution capability of at
least 40 lines/mm.
In subsequent parts of this proposal, we describe how the EL
layer is replaced with VARAD. There are other PC approaches, of
course such as the
approach (described elsewhere in
this proposal) that might also be advantageously
and, though we presently favor the
bination with VARAD, we will continue to examine
and will remain open to pursuing them.
combined with VARAD.
approach for corn-
other approaches
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31IkT
STAT
S AT
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2.5 Alternate PC Approaches
convinced that VARAD is a more promising
medium as the output layer of an image intensifier screen than
present electroluminescent materials. However, there are numerous
advanced photoconductive techniques that are of considerable interest
besides the
techniques.
In addition to the
grid-controlled layer, and some system
'where some of the first PC
image intensifiers, including the grooved system, were developed,
there are a number of other vendors of basic photoconductor powders
as well as a number of firms involved in manufacturing photoconductors
in large continuous sheets for application to image making. Examples
of such firms are
'who is presently
utilizing photoconductors in combination with their thermoplastic
films for use in data recording and displays;
who has recently announced a very high
resolution photoconductor control layer, and
which is working on PC-EL storage devices.
Although it is not the intent of this proposal to commit at
this point to a particular approach and its associated vendor, we
wish to point out that communication with various vendors has taken
place and some encouraging results with regard to feasibility of
mating a prefabricated photoconductor layer with
VARAD cell have developed.
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STKII
STN
STK
STA
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Of particular note is the Librascope approach only announced at
the Society for Information Display meeting on 26 February 1965.
This firm has developed a photoconductor control layer that has the
unusually high resolution of .003" spot size without utilizing the
more expensive structures such as grooving the photoconductor or
using embedded grids. Conversations with both their technical people
and production people since learning of this recent development have
been encouraging. Although a final decision with regard to feasi-
bility and performance will be one of the subjects to be further
developed in the first phase of the proposed program, at this point
no insurmountable barriers appear.
is presently further
investigating the feasibility of marrying their approach to our light
modulator panel and is presently making special measurements on their
photoconductor layer to this end. Another feature of this photo-
conductor approach is that it is amenable to deposition by silk
screen, settling, or vacuum evaporation; furthermore, is
STA
STA
in the process of tooling up production facilities to turn out a
hundred such panels by the end of the year. It might be pointed out,
however, that in their original intended application for use with the
EL phosphor, they are capable of achieving a brightness which is
inadequate for the IIS specification without utilizing the VARAD
system.
Other techniques for image intensification have been reported
in the literature but are not sufficiently developed for their
characteristics ,to be meaningfully evaluated. The field_ will be continually
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surveyed during the course of this program to determine whether any
of these techniques have developed into useable devices. In addition,
breadboard PC layers will be constructed at the Information Sciences
Center to test other candidate photoconductor material and associated
matrices.
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2.6 Anticipated performance
As indicated previously, the electrical field strengths exciting
the VARAD will be essentially the same as those exciting EL material.
Actually, of course, due to the lower voltage requirements on VARAD
and its wider choice of operating frequencies, different electrical
signals may be applied, and more sensitivity may thus be attained.
However, for purposes of an analysis, experimental and theoretical
data obtained for the EL-PC 'system can be used in the
present analysis if allowance is made for the differences betWeen
VARAD and EL material, and if allowance is made for the difference
in the visual readout technique.
The sub-optimum conditions considered below are still, in many
cases, far better than the c9nditions developed for previous EL-PC
systems and, in all cases, at least as good. Same indications will
be given as to how further improvements may be achieved.
The parameters will be analyzed in the following order:
Brightness of VARAD versus EL material
Uniformity of Output Illumination over Screen
Brightness Distribution Lobe
Gain
Resolution
Life Expectancy
Color Temperature
Recovery Time
Signal to Noise Ratio
Reflectance
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Brightness Levels and Tonal Rendition
Linearity and Gamma
Size
Power Requirements
Contrast Modulation, Image Reversal, Density Expansion and
Compression
Mbnochromatic Sensitivity
This order is selected for convenience in presenting our analysis.
After analyzing the parameters, conclusions will be presented
concerning the items listed, including:
a. The relative feasibility of achieving the stated
requirements of each of these parameters;
b. Compatibility, relationships, and trade-offs with other
parameters;
c. Our technical approach to achieving the design goals of
each parameter.
Brightness of VARAD versus EL Material
We will consider first EL material and then VARAD with respect
to brightness.
For EL, 220V across a 1 mil thick layer will produce about
130 foot lamberts with commercially available high performance
phosphors. For 110 volts, under the same conditions, approximately
42 foot lamberts are obtained, and for 55 volts under the same
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conditions, approximately 10 foot lamberts. This is roughly linear
after a threshold involving negligible brightness for some significant
voltage less than 55 volts. We have here a change of roughly 0.73
foot lamberts per volt over the more or less linear range
A field of 100 to 200V across a 0.001 inch layer of VARAD will
produce an electrodichroic ratio of 15. Starting with an optical
densitylD,of 2, when un-excited, this would lead to a D of 0.125
when excited. Assuming that a field of 200V across a 1 mil layer of
VARAD is necessary to produce the change in D from 2 to 0.125 (an
assumption on the conservative side with respect to the above figures'
but one which allows a direct comparison with the EL case) we may
compute a,corresponding change in the brightness associated with the
VARAD screen if we assume various illumination levels for the VARAD.
Because of the double pass of light through the VARAD, the effective
values of D for the extremes considered are 4 and 0.25. If the VARAD
had no reflectance, only absorption, this means that, if the power
light input were enough to provide a brightness on the white surface
of, say 400 foot lamberts in the absence of the VARAD, it would
produce 0.04 foot lamberts with the VARAD un-excited and 224 foot
lamberts with the VARAD excited.
Figure 2.7 compares these data for EL material and VARAD. The
broken line represents the estimated characteristics of VARAD, and
the solid line represents high performance EL material. Use of a
higher intensity power light input than the one assumed would lead
to higher maximum brightness, for 200 volts excitation, than 224
foot lamberts shown in the figure.
44
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BRIGHTNESS IN FOOT LAMBERTS
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0 20 40 60 80 100 120 140 160 180
AC VOLTS EXCITATION
Figure 2.7 Brightness Variatipns with Voltage
45
200 220
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It has been assumed that there is no light reflected or scattered
from the VARAD, that is, that the light is absorbed to varying degrees
by the VARAD, and reflected only by the white layer behind the VARAD.
Any light seen by the observer that is not reflected from the white
layer can be kept negligible, using HEA interference films to minimize
reflected light. VARAD is effectively a pigment, and like ink on
white paper, would maintain contrast in high ambient light. Unlike
the case of the EL illumination, the VARAD image wculd be made brighter
by ambient light, like a half-tone magazine illustration or snapshot.
The above numbers assume that the illuminating light for
the VARAD screen comes in perpendicular to the plane of the VARAD
screen. Figure 2.8 shows how a side-lighted VARAD screen with a 45?
viewing angle would permit the light to come onto the screen at an
angle of about 300 to the plane of the screen. A screen lighted from
the top and bottom is illuminated from a larger angle, as seen in
Figure 2.9.
The fact that most of the light may come in at an angle between
about 30? and 45? with the plane of the screen and may be viewed
at an angle of from 450 to 900 with the plane of the screen, as
shown in Figures 2.8 and 2.9 , means that, on the average, the light
will pass through about 1.25 to 1.75 mils of VARAD rather than 1 mil
for a 1 mil thick layer. This will either reduce the maximum
brightness to about 170 foot lamberts, on the average, or require
increasing the total illumination of the screen by a factor of about
1.3. It will also slightly increase the picture contrast.
46
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'fp
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FIGURE t LIGHTING {.;EOMETRY FOR IIS TOP VIEW
47
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..2IGURE 2j1 LIGHTING GEOMETRY. FOR IIS SIDE. VIEW
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It is, of course, not mandatory that the housing for the lights
be as large as in Figures 2.8 and 2.9 . An alternative spotlight
say at S, in Figure 2.9 , would even more closely approximate a
situation where the screen is. lighted normal to the image plane, and,
in this case, the housing for the lights L would not be provided at
all. A smaller housing for the lights with more oblique lighting
can also be used with little loss in brightness. The situation
represented by Figures 2.8 and 2.9 represents a compromise between
the extremes of more oblique lighting and almost normal lighting
with a spotlight, and will be used as a basis for further calculations:
It was assumed in comparing VARAD to EL material that the same
voltage was available for each. Actually, because of the wider choice
of excitation frequency of the VARAD to electrical excitation when
compared with EL material, it would be possible, in design optimization
to utilize the PC layer more efficiently with VARAD than with EL
material. As an example, CdSe photoconductor layers can be made
to vary from 1012 ohm/square dark to 107 ohm/square at approximately
1 foot-candle. A 3 mil resolution spot on the VARAD layer has a
capacitor impedance of 107 ohms at 15 kc, but 108 ohms at 1.5 kc -
a good match. Hence, we would expect to be able to deliver more
voltage to VARAD than to an EL layer as a result of a given input
light level to the PC layer. In this sense, Figure 2.7 represents a
conservative comparison of VARAD to EL with respect to gain.
Figures 2.8 and 2.9 may obviously embrace more than the one
to four persons viewing the IIS, going beyond the requirements of
the RFP in this respect. It may also be seen that because of the
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anti-reflection coating and lamp design, no glare from the light
sources is reflected into the audience.
A vertical viewing angle 45? on either side of the horizontal
can easily be achieved as in Figure 2.9 in various ways. One isto
have an elevated screen with the side view resembling Figure 2.91
the screen being tilted down slightly. Another way is to use a
shallower side view configuration. The spotlight arrangement mentioned
above would be another way. Still another way would be to omit the
lights shown in Figure 2.9 , keeping the ones shown in Figure 2.8
Uniformity of Output Illumination Over Screen
There are two general factors in the VARAD system that would
contribute slightly to non-uniformity of output illumination
over the screen as a function of distance from the center of the screen.
These are (a) non-uniformity of illumination by lights L in Figures
2.8 and 2.9 , and (b) non-uniformity due to factors influencing the
electric field strength that excites the VARAD.
With respect to factor (a), note that the radiant energy from
a line-source of light falls off inversely as the distance rather
than as the square of the distance, so that at all points on the IIS,
one light linearly compensates for changes in illumination from the
other light as a function of distance from the lights,to a very good
first approximation. Furthermore, any second order variations due
to the vertical lights tend to be compensated by the horizontal
lights, and vice versa. Note, also, that the incidence angle of
the light from any one light source to any point on the IIS does?
not change very much in Figures 2.8 and 2.9 , nor does the distance
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from light source to points on the screen change very much. Variation
in output illumination over the screen due to factor (a) should thus
be entirely negligible.
There should be no electrical reason for non-uniformity of VARAD
itself due to factor (b) with proper design. The small edge effects
need cause no trouble if the IIS is made slightly larger than the
image projected upon it. The transparent conductor layers (tin oxide)
can be made low enough in resistance (the order of tens of ohms for
thin layers) with respect to the other impedances that there should
be no problem on this score. With proper optical design, there
should be no optical problem due to factor (b) in projecting the .
film image on the PC layer. However, obtaining uniformity of sensitivity
for large areas of PC still is a problem requiring further development.
Any non-uniformity of output as a function of distance from the
center that does occur can be compensated by making the white layer
in Figure 2.1 very slightly gray as a function of distance from the
center.
Non-uniformities of thickness at randomly located spots or
points should be held well within tolerances by proper manufacturing
and quality control techniques. The VARAD-PC system would have an
advantage, here, over an EL-PC system, because both the EL and PC
layers might tend to develop coincident "hot spots" but the VARAD
should not; being a continuous liquid film in constant thermal
diffusion equilibrium.
With respect to grid shadowing due to grids in the PC layer (if
this is the final approach taken), the wound wires or etched conductor
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paths of the proposed VARAD-PC image intensifier act as charge
distributors within the PC layer, and not in any real sense as
localizing structures. As has been demonstrated at the New York
World Pair exhibit
even the present widely spaced
? parallel wire distributors do not contribute significant visible
non-homogeneity in the output image at normal brightness levels. A
two-dimensional grid-mesh having 1000 lines/inch, which we propose
will similarly cause no grid-shadowing of the image and should
measureably improve both gain and resolution of the panel, as described
in our proposal. (Resolution is almost completely independent of the
grid pitch when this pitch is as small as or smaller than the 300
microns of the present panel. The much more closely
spaced distribution lattices to be used in the proposed new .panels
are for the purpose of greatly increasing total dynamic range - they
are not required for homogeneity of the output image.) The substi-
tution of VARAD for the EL layer in the system will not STAT
alter this matter.
Brightness Distribution Lobe
Viewing from the side will result in slightly lower intensity
and higher contrast, as pointed out above, due to looking through
slightly different distances in the VARAD. Within a viewing angle
45o from the normal to the IIS, maximum brightness should not change
by more than ? 10%, but portions of the VARAD that are not very
transparent would very possibly undergo a greater change in brightness.
We will attempt to improve this situation in the development phase of
the program.
Gain
The proposed system can easily exceed a brightness of 50 foot
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lamberts due to 1 foot candle highlights excitation. As seen in
Figure 2.7, substitution of VARAD
substantially.
Resolution
The resolution of EL-PCpanels developed by the
re 10 lines/mm in the Y
for the EL layer increases the gain
4 lines/mm in the X-direction. Discussions with
of the research institute have verified that the theoretical limitation
on resolution is far greater than obtained at present. In fact, the
limiting factor at present is the choice of grid structure, which
when replaced by a tectangular grid will provide a resolution capa-
bility in all directions exceeding the specified 10 lines/mm. The
VARAD system that we propose should have at least equal resolution.
A contrast of 100:1 will be a design objective that should be
achievable with VARAD since scattered light from unexcited VARAD
will be reduced to 0.3% by use of HEA multilayer anti-reflection
coating and the "black velvet" appearance of VARAD.. We anticipate that
the transfer function at 10 lines per mm can be at least 90% modulation,
20 lines per mm at 90% MTP will be the design goal.
Life Expectancy
With substitution of the VARAD layer for the EL layer, there is
no anticipated life expectancy problem whatever. The system could
easily operate at a maximum brightness in excess of 200 foot lamberts
for much more than 200 hours with far less than 10% degradation of
any performance parameter. The EL layer has been the source of
life expectancy problems, and, by substituting VARAD for the EL
direction and
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layer, the problem ceases to exist.
Color Temperature
Any color desired can be used. The color temperature range of
35000 to 55000 is particularly convenient because of the wide choice
of sources available.
Response Time
The VARAD layer will normally darken in 10 milliseconds and
clear in 10 milliseconds. The normal darkening time can be shortened
by providing for an electric field in the plane of the VARAD layer.
In terms of photoconductor time constants, the rise and decay times
of cadmium selenide type powder-sintered coatings deposited on glass
substrates are of the order of 8 milliseconds for rise time to 1/2
maximum current, or decay to 1/10 maximum current. These times
increase at 102 illumination level, but can be reduced by infrared
or heat quench. Even at the low illumination of 0.1 foot lamberts,
it appears that the 0.3 second rise time would not too critical
for step-and-repeat viewing of intensified images in still-picture
projection. Rise and decay time of these photoconductive materials
can be decreased by an order of magnitude (i.e., to 0.03 and 0.008
second) by presensitization with ambient light. Flash (or steady-
state) background light exposure will be investigated in the second
phase of development to reduce the time constants of the photo-
conductive layer to these relatively low values. Infrared quenching
of the photoconductive layer could also be used to still further
reduce decay time.
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Signal-to-Noise Ratio
A modification to the conventional definition of signal-to-
noise ratio is needed to make the term meaningful for the Image
Intensifier Screen. For uniform illuminance of the Image Intensifier
Screen, the entire viewing screen should be of equal brightness with
no visible dark areas. The design goal for the Image Intensifier
Screen will be a S/N ratio of at least 100 for every square inch.
At reasonable illumination levels, "flicker" or "scintillation"
noise is negligible. The primary sources of interference in EL-PC
systems are to be found in phosphor granularity and blemishes in
deposition. Through substitution of VARAD which consists of a
suspension in liquid of millions of 1 micron length "needles" and
use of precision fabrication techniques to assure uniformity,
it is expected that effective S/N ratios of at least 100:1 can be
achieved or exceeded.
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Reflectance
Unexcited VARAD inherently presents the tone of "black velvet."
In addition, reflection minimizing interference films will be used to
reduce front surface reflection to 0.3%. The VARAD system will
operate in normal room light, or even bright sunlight.
Bright Levels and Tonal Rendition
The
ystem exhibited a tonal range of 10 gray shades.
The EL layer appeared to be the primary limiting component in the
number of gray shades available. We do not yet have full experimental
information on the number of gray shades that can be obtained with
VARAD, but because of its great dynamic range and freedom from the
problems that beset EL material, we feel confident that the proposed
system represents a significant improvement over the EL-PC grid-
controlled system in terms of the number of brightness levels that
Can be displayed.
Linearity and Gamma
The linearity characteristics of VARAD have not yet been fully
determined. However, there is no reason to expect VARAD to be
any less linear in its characteristics than EL material. In fact,
due to its wider excitation bandwidth, there should be greater
design flexibility in utilizing its most linear operating range or
in designing the system so that non-linearities of the VARAD and
photoconductor cancel and the overall result is linear operation.
In addition, the grid-controlled PC layer can be deliberately designed
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for non-linearities in order to partially cancel other system non-
linearities, within ?10%; gamma can actually be controlled in this
way.
By proper selection of power supply amplitude and phase the
gamma characteristics can be changed from +1 to -1 thereby producing
an exact photographic reversal, hence positive-negative viewing is
easily obtained. A wide range of lower and higher contrast ratios
may also be achieved, as well as partial reversal, for density
contouring.
Size
The thickness and weight-to-area ratio of the IIS that we pro-
pose can be made approximately that of a conventional screen. There
appears to be no theoretical limitation to screen size. The 12" x 12"
screen in question, does not appear to present any serious problems.
Fabrication of a 6" x 6" screen to prove feasibility will, upon
approval, be followed by fabrication of a 12" x le screen meeting
design requirements. One of our potential vendors is presently tooling
up for a 30" PC deposition chamber.
Power Requirements
The power requirements specified in the RFP can be easily met.
The oscillator driving the panel must only supply negligible wattage
dissipated in the PC layer, the VARAD layer being reactive rather
than resistive. The illuminating lamps will draw an estimated 200 watts
to achieve 200 foot lamberts if flourescent lamps are utilized, or
500 watts if tungsten lamps are selected.
?
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Contrast
Controllable
system. Complete
the option of the
contrast modulation is inherent in the proposed
linear intensity reversal would be available at
operator.
Expansion and compression of the bright-
ness range can also be achieved in modifications of the proposed system.
Monochromatic Sensitivity
The monochromatic sensitivity requirement of the RFP can be
easily met. Most photoconductors have a fairly broad range of
wavelengths in the region of their peak sensitivity, alloving a
choice of desired monochromatic exciting light.
Returning,now, to the three questions concerning (a) feasibility,
(b) trade-offs, and (c) the technical approach proposed by
submit the following:
(a) We believe that it
requirements of the RFP with
is feasible to achieve all of the
we STAT
respect to each of these parameters listed
in the RFP, to achieve the design objectives where noted in the RFP
together with minimum design requirements, and in many cases to far
exceed the specifications sought in the RFP, using VARAD and either the
grid-controlled PC layer or some other advanced PC system
system. Relatively speaking, we believe that
such as the
the easiest parameter to achieve with the VARAD approach (and the one
which goes the farthest beyond EL capabilities) is extremely high
brightness with absolutely no sacrifice of lifetime. High gain and
color flexibility are other easily achieved advantages, along with
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increased sensitivity. Also, some image manipulation capability is
an inherent property of the PC system proposed without having to put
it in ad hoc. The most difficult items may be brightness distribution
lobe, linearity, response-time, and uniformity, though we believe that ?
the VARAD system has an advantage over an EL system in most of these
respects. We foresee no particular difficulty regarding linearity, but
this is the parameter about which we have the least experimental infor-
mation at this time. Basically, we have more design flexibility than in
the case of an EL-PC system, and believe that we can exceed all design
specifications:
(b) The major trade-off for the proposed system is sensitivity
(or gain) vs. image resolution. This happens for two reasons. The
thinner the VARAD layer, the higher is both its resolution and its
voltage sensitivity, as it is for EL. However, the effects of stray
capacitive coupling causes resolution decrease, just as with EL.
Secondly, the inherent sensitivity of thePC layer, using any known
approach, must to some extent be traded off for resolution -- whether
grid, grooved, or the
approach is used. This holds true
for EL-PC, or any other PC layer approach. However, the significant
compatibility relationships are between high brightness, gain, sensitivity,
color flexibility, ability to operate in high ambient light, and high
life-expectancy as a result of the fact that VARAD modulates light
rather than emitting it. Also, limited image manipulation, capability
for gamma corrections for linearity, and high gain are complementary
properties of the PC layer that we favor because it
combines high resolution with high sensitivity.
(c) Our technical approach to achieve these design goals is
fundamentally based upon (1) replacing the EL layer with VARAD, and
(2) combining this with the most advanced PC technique available, along
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with other applicable techniques such as optical interference films.
The technical approach is answered in more detail by the entire
body of this proposal.
Figure 2.10 is an artists conception of an IIS such as we propose.
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TUBULAR
ILLUMINATING
LAMPS
- PROJECTOR
LAMP SHIELD AND REFLECTOR
FIGURE 2.10 ARTIST S CONCEPTION OF IIS
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3.0 PROGRAM SCHEDULE AND RELATED INFORMATION
3.1 Schedule
Figure
3.1 depiete the proposed schedule for accomplishing
the required analysis, design, fabrication, and test activities leading
to the completion of the Image Intensifier Screen.program. The delivery
of hardware is divided into three phases which are:
a. 6" x 6" Screen - 7 months ARO
b. 12" x 12" Screen - 9 months ARO
c. 30" x 30" Screen ?- 12 months ARO
The indicated tasks are described in detail in Section 3.3
of this proposal.
3.2 Reports
Although not called out in the proposal,
customer
proposes
Screens,
feels it would be most advantageous for the
to receive as much documentation as possible and therefore
the following reports be submitted.
In addition to delivery of the three Image Intensifier
will provide a compre-
hensive report concurrent with each screen. These reports will contain
information as to the approach taken, characteristics, and operating
parameters. Also, we will submit monthly letter type reports
relating progress for the month, any problem areas and contemplated
work for the next reporting period.
In addition,
will supply a final report, one month
after delivery of the 30" x 30" screen or thirteen (13) months after
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receipt of order. This report will encompass the entire project
period and will summarize discussions and technical problems and will
include overall conclusions derived from the work and any recommen-
dations for future activity that can be determined from the results
and conclusions of the program.
3.3 Program - Technical Tasks
During the first period, the following experiments and
determinations will be made on the VARAD panels presently in house:
'Modify an amplifier to be driven by a variable frequency
oscillator with voltage sufficient (up to approximately 300
volts) to completely activate the film panel.
Determine the light transmission as a function of applied
voltage.
Determine the spectral light absorption characteristics
of the VARAD light panel.
Determine the voltage-current relationships (complex
impedance) of the VARAD panel as a function of operating
point.
Determine the optical and electrical properties of the
VARAD panel as a function of applied electrical frequencies
(that is, per cent modulation and electrical loading).
Confirm resistance to fatigue of the panel from application
of high intensity light and round-the-clock activation of
the cell. Replace liquid if the per cent modulation of
light decreases under this test.
Determine the effects of temperature (high and low) on
VARAD panel opening and recovery time as well as on the
above mentioned optical and electrical parameters.
Determine the edge sharpness or resolution on the panel
that has only 1/2 of a back conductive electrode.
Apply point electrodes to the second cell to demonstrate
crude imaging.
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Demonstrate the VARAD panel in the electro-photochromic
projector to show modulation and projection under large
screen conditions. This will be with no image, only an
"off-on" effect in a measurement of dynamic range or
contrast at the screen.
Connect various discrete packaged commercial photoconductors
in a series circuit with the VARAD cell to simulate a
single element photoconductor-VARAD sandwich.
The result of the above work will be to provide a thorough
knowledge of the behavior of VARAD, electrically, optically, and as
a function of other physical parameters such as light intensity,
temperature, electrode spacing, etc. Using this information, a
mathematical design study will be performed to determine:
What parameters of the VARAD liquid (such as concentration,
viscosity, etc.) should be varied to optimize its perfor-
mance in a display device.
What parameters of the cell construction should be changed
to optimize its performance as a display performance
device (such as electrode spacing, optical coatings,
dielectric thickness, etc.).
Secondly, the mating of the optimized VARAD panels and
their predicted performance will be studied in conjunction with the
known properties of the various available photoconductive materials
so that optimized photoconductive material can be ordered. The
parameters involved include bulk resistivity, binder material, photo-
conductive film thickness, choice of the photoconductive component,
viz., CdS, . and CdSe, PbS, etc., mechanical structure such
as grooving and electrode configuration. Either the completed
photoconductive film or the basic material will be ordered for
coating here.
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Following these steps, the design of the first 6" x 6"
screen will be finalized and orders placed for the selected components.
While awaiting delivery of the new materials, experiments will be
continued on the commercial package photoconductive device with the
original VARAD panel. These experiments will determine response
curves for VARAD light transmission versus incident light on the
photoconductor most sensitive operating point as a function of bias
voltage and bias frequency, power gain when using incident light
transmitted through the VARAD cell, dynamic characterisitcs when the
incident light on the photoconductor is varied in intensity and apply
the knowledge gained to design the optimum VARAD cell-photoconductor
type combination for the various intended applications.
The final phase will commence with a careful analysis of
all work done to date to determine optimum parameters for the 30" x
30" panel together with cost trade-offs.
? Fabrication test of the panel will then proceed as shown
in Figure 3.1.
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