IMPROVED SCREEN FOR REAR PROJECTION VIEWERS

• Document Type: CREST
• Collection: NGA Records (Formerly NIMA)
• Document Number (FOIA) /ESDN (CREST): CIA-RDP79B00873A002000010070-5
• Release Decision: RIPPUB
• Original Classification: C
• Document Page Count: 32
• Document Creation Date: December 28, 2016
• Document Release Date: November 8, 2012
• Sequence Number: 70
• Publication Date: March 3, 1967
• Content Type: REPORT

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CORNING GLASS WORKS
ELECTRO-OPTICS LABORATORY
RALEIGH, NORTH CAROLINA
IMPROVED SCREEN FOR REAR-PROJECTION VIEWERS
Technical Reports No. 18 and 19
Date - March 3, 1967
Period Covered - January 6, 1967
to
March 3, 1967
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ABSTRACT
This report summarizes the optical properties of additional samples of
Corning Glass Works materials. Some of the high-index glasses with low-
index scattering centers have been measured and appear promising.
Prototype rear projection screens of Fotoforg)glass measuring 8" x 10"
are being fabricated. Work is continuing on making lenticular screens
from Fotoforg)glass, however problems have been encountered in attempting
to redissolve the crystalline phase after the lenticules have been formed
and etched.
Three techniques for fabricating large rear projection screens from
glass-ceramic materials are discussed along with a novel lenticular screen
which is particularly insensitive to ambient light.
Declassified in Part - Sanitized Copy Approved for Release 2012/11/08: CIA-RDP79B00873A002000010070-5
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TECHNICAL REPORTS NO. 18 AND 19
I. Materials
A. Glass-Ceramics
1. New Materials
We have measured additional samples of glass-ceramic materials.
The relevant optical data is summarized in Table A-1 and curves
for T45 vs. axial gain are given in Figure A-1. The data and
curves describing the scattering functions appear in data
appendix 3.
Samples of the AW and AX series are of a high-index glass and
are expected to be . good screen materials. For the most part,
these samples either did not have the optimum particle size or
the particle concentration was not within acceptable limits.
Electron micrographs are being made of these materials to
determine their physical properties and remelts will be made
to improve these materials.
Electron micrographs have been prepared of samples AS-4 and AS-9
and are shown as Figures A-2 and A-3; the white bar represents one
micron. The average particle size is .14 microns for sample AS-4
and .28 microns for sample AS-9, the latter being very close to
desired value. Figure A-4 shows a micrograph of high-index
particles (n 2.0) in a transparent glass (n C=7.1.5). The oblique
illumination is apparent, and measurements must be scaled to the
ellipse representing 4 microns,depending on the diagonal along
which the measurements are made. Particle size in this sample is
around 0.46 microns.
2. Screen Fabrication
Because of the impossibility of fabricating screens larger than
6" x 6" by mechanical grinding and polishing, other techniques
are being investigated. The most straightforward approach is to
grind the glass-ceramic into a powder, mix this with a plastic
resin, and spray or otherwise deposit the mixture onto a ,clear
,
,
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substrate. Modification of the scattering properties of the
glass-ceramic materials will occur; the degree, depending upon
the relative refractive index of the ceramic powder relative
to the embedding resin.
Investigations using an ordinary glass powder and a resin are
being conducted to determine the suitability of this approach.
Following this work, a glass-ceramic powder will be used and
screens measuring 12" x 12" will be made using different sizes
of the ceramic particles provided this technique is suitable.
A second approach which should give even better screens and be
easier to fabricate has been initiated. It consists of forming
very small spherical beads from a "green" glass-ceramic. The
beads are then cerammed and sized. As before, a particular size
can be mixed with the resin and sprayed onto a clear substrate
forming a thin layer. They can also be directly applied to a
treated surface which will bond the bead to it. At present, the
glass beads are being made and sized.
A third approach is to redraw a sheet of green glass-ceramic
into ribbon. This technique would give very thin ribbon up to
1" to 2" in width. The strips could then be laid side by side
on a substrate to give the necessary screen size. An attempt
has been made to redraw a plate of the green glass-ceramic AS-9.
Because of the redrawing temperature, cooling rate, and basic
glass properties, the glass devitrified during the redraw; making
it impossible to control the crystal characteristics. Other glass-
ceramics will be tried;however, this is a typical problem
associated with the reforming of glass-ceramics and will most
probably be encountered with many of the other glasses.
B. Fotoform Glass
1, Thin-Layer Material
Optical and physical data for these samples have been measured.
Figure A-5 shows the correlation between axial gain and the
crystal layer thickness. The thickness of the best samples lies
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between 150 and 300 microns. Some 3' x 3" samples have been
made for evaluation; however, to better evaluate these materials,
we are now preparing some 20" x 20" screens.. Delivery is
expected in 4 to 6 weeks.
2. Volume-Scattering Material
These samples are coded AV-1 through AV-11 and look quite
acceptable as screen materials. As can be seen from Figure 1,
these materials have the predicted efficiency. With values of
axial gain between 1.5 and 2.4,they exhibit very small variations
in brightness over the +45? viewing angle.
Samples of this glass have been prepared and are being ground into
powder. From this some 12" x 12" screens will be fabricated and
made available for further evaluation. Optical properties of the
bulk glass will be compared with those of the finished screens
to determine how much change the fabrication process has produced.
At the same time, screens made up of transparent glass powder will
be prepared and compared with screens containing ceramic powders
to determine the influence of the refractive-index difference between
the ceramics and the resin.
C. Lenticular Screens
1. Lenticular Plate
This is presently being redrawn into ribbon and, provided the
plate does not shatter during redraw, some preliminary results
should be available next period.
2. Fotoform Glass
We have had mixed success with this material. It was found that
lenticules could be formed quite easily;however, exposing and
developing the lenticular image in the glass produces crystals
about 50 a in size which are necessary if it is to be etched.
After the lenticular pattern is etched into the glass this
crystal structure must be removed. This is because the crystals
occur throughout the thickness of the screen and significantly
,
_
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degrade resolution. Several attempts to redissolve this
crystalline structure have been tried but without success. Work
on this problem is continuing.
II. Theoretical Investigations
A. A New Type of Lenticular Rear Projection Screen which is Insensitive
to Ambient Light
A very efficient lenticular screen is described in a 20-year-old
French patent (FP 959,731). The screen is constructed as shown in
Figure 1, and is made of a transparent substrate:on .Which,'a thin
Figure 1. Organization of the Lenticular Screen
layer of a transparent plastic material is poured. The glass beads
are then deposited such that they are half embedded in the plastic.
After the plastic has set, a second layer containing an absorbing
material is poured over the beads until they are "almost" covered
except for a very small region at the top; this plastic is then
allowed to harden and the screen is complete.
Light from the projector passes through the substrate and is focused
through the small openings at the ends of the glass beads and emerges
into the viewing area. Ambient light incident from the viewing side
is almost completely absorbed by the dark opaque layer and therefore
does not degrade the projected image. The resolution limit of such
a screen is given approximately by the reciprocal of the bead diameter;
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e.,a screen made from . 131Fr40-4-mm i111ameter).beads would have a limit
of resolution around 25 lines/mm.
The patent does not give the optimum refractive indices between the
glass beads and the embedding plastic for all of the light to be
focused exactly at the back of the bead. Such a theoretical analysis
follows.
Consider the glass bead shown in Figure 2. A ray of light is in.
Figure 2.
Geometry for Tracing Rays Through a Glass Bead
incident at a height y from the optical axis; and is focused near the
back of the bead emerging at a height h fromHthe optical axis. Ideally
h is zero or very near so for all light rays. We now wish to determine
L/R as a function of y/R and n1/n2.
From geometrical considerations,h/R is given by
h/R = y/R
X
2R - (X1
2
COAIFIBEJTIAL
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Without carrying through the mathematical details this has the
final form,
where
and
C - BD ,
=A I:1 - (1 + D) ? (
CD + A2 B)
A = y/R
B = n1/n2 '
C = - A2 B2
D =
(1)
Equation (1) has been evaluated for values of A from 0 to 1 and B
from .6 to 1.5. Figure 3 shows h/R as a function A for different
values of B and Figure 4 is h/R plotted as a function of B for
different values of A. Figure 3 is perhaps rMore useful. Fiom
this we can see that for B = .6 all of the incident energy will pass
through a hole of diameter .1 h/R. Thus 99% of the viewing side of
the screen will be covered by an absorbing material thereby making it
very insensitive to ambient light. As B increases,the spot size at
the back of the bead increases. Typical values of B lie between .8
and .95, and possibly as low as .7. For B greater than .6,the light
comes to focus slightly beyond the back surface; however for B less
than about .6,all rays come to focus inside the bead and spread apart
again before reaching the surface. Values of h/R for B less than .6
were computed but not shown,because of the impracticality of having
such high index beads.
Presently models of this particular screen are being fabricated for
preliminary evaluation. Realistic screens will then be fabricated
using 3 or 4 sizes of beads down to -150 mesh. The smallest beads
have a diameter of 65 microns, and will -have a resolution limit of
15 lines/mm.
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