(Sanitized)

• Document Type: CREST
• Collection: NGA Records (Formerly NIMA)
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78B05171A000400020017-3
• Release Decision: RIPPUB
• Original Classification: K
• Document Page Count: 4
• Document Creation Date: December 28, 2016
• Document Release Date: February 1, 2005
• Sequence Number: 17
• Publication Date: June 8, 1972
• Content Type: LETTER

Off ice r-#.r harge
U.S.A. munication, vice Group
Post Office Box 72 No Air Station
Ott Field, CaIlfo `o 94035
Subject:
Gentleme !`
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1 fT 2Q2&-1.-6597
8 June 1972
Puont to General Provision Article 26, your permission to
pre$ont the attached report to a meeting of the Optical Society
of America in September 1972 at Boulder, Colorado, is respect-
l6liy requested. The text of the report will appear in the con-
lnnce summary publication and has been reviewed by your
t t nical representative.
Should further information on this subject be required, kindly
5X1
contact
ndersigned.
member of technical staff, or the
Very truly yours,
25X1
25X1
25X1
25X1
Supervisor, Contract ministration
WGB:eh
Attachments as stated
Declassification Review by NGA/DoD
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Application of Tungsten Halogen Lamps in a Wide Field Microscope Illuminator
25X1
A six-inch long tungsten-halogen microscope illuminator produces a three-
inch diameter, variable intensity, 25,000 ft. lambert, daylight spot.
Large photographic film formats used in aerial photography are assessed for detail over
large illuminated viewing surfaces with the aid of a scanning zoom microscope. At low
magnifications the microscope field covers only a small portion of the total illuminated
surface. We therefore sought a more compatible means which could use the lamp input
power more effectively where it is needed.
Typical viewing surfaces contain fluorescent lamps which supply 3000-3500 ft. lamberts
of daylight quality. However it is desirable to visually penetrate into the deeper shadows
or dense film regions. This can be accomplished by increasing the viewer luminance if ex-
tremes of heat or electric power input and loss of daylight quality can be avoided. In
our search for a practical solution, we found that,a highly concentrated spot of light
covering only the instantaneous microscope field can be generated with a standard but
efficient tungsten halogen lamp supported by appropriate optica I components. Such a
spot could be positioned beneath the microscope, beneath any portion of the format.
An illuminator generating the high intense spot can in most instances substitute for a
large area source because it can fulfill several important requirements:
1. Illuminate the maximum field of the zoom microscope at its lowest
magnification.
2. Fill the numerical aperture of the zoom microscope objective.
3. Have a daylight quality for color film application.
4. Have intensity control but maintain daylight quality.
5. Be adaptable for tracking the position of the microscope field over the
viewing area.
6. Operate at a reasonable power input relative to the fluorescent lamps.
7. Be simple to maintain.
8. Be compact.
Two types of light sources are available for creating such a spot of light - one is the
highly intense small arc source which can produce a well collimated beam, the other is
a larger tungsten halogen source. We chose to try the new ELH tungsten halogen lamp
because by comparison with an arc, it is easily available and replacable, requires no
adjustment, and lends itself to a compact illuminator design requiring no special power
supply.
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Figure I. Optical Layout of Illuminator
The experimental illuminator optics configuration is illustrated in Figure I. A 300
watt ELH lamp produces about a 3/4" diameter bundle of high intensity through the heat
absorber. Two simple lenses located near the intense bundle project an image of the
reflector aperture and lamp at the Fresnel lens. An iris for intensity control is mounted
adjacent to the lenses. A mixer is inserted close to the principal focus of the Fresnel
lens. The Fresnel lens projects the image of the mixer at about 5x. The projected
Fresnel beam is intercepted by a 3x gain Polacoat rear projection viewing screen
material, in this case about 3" from the Fresnel, to provide a highly intense spot of
about 3" diameter. The 45? mirror directs the beam vertically and under the micro-
scope viewing area. A sealed Wratten filter placed next to the Fresnel lens alters the
tungsten color temperature to approximately 5000?K.
The two important components in this approach are the new ELH lamp and the mixer.
An ELH lamp is excellent in this application. It has a deep dichroic reflector which col-
lects a high percentage of the total visible filament radiation needed to compensate for
light loss in the color filter. The collected light is condensed into an intense bundle near
the iris diaphragm which introduces severe spot non-uniformity when the diameter is altered
for intensity control. A 20x20/square inch lens array mixer redistributes the light into an
acceptable uniformity.
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The following table gives the characteristics of the illuminator.
Table of Characteristics
Lamp ELH 300 watt dichroic reflector
IR Control KG-1 filter + dichroic IR transmission
Spot Size 3" maximum, 2-3/4" effective
Uniformity 25% fall-off at 2-3/4" diameter
Color 5000? ? 500?K
Luminance 1,200 to 25,000 f.1. at 5000?K
Maximum 140,000 f.l. without color correction
A high luminance viewing area should fill the numerical aperture of the microscope
objectives. The selection of a 3x gain screen and the angular field subtended by the
wide open iris at maximum luminance combine to produce the brightness polar diagram.
We found the polar distribution adequately fills the numerical aperture and is shown in
Figure II.
.09 .2 .27
1 1 L
Polar Data
of Screen
Numerical Aperture
Measured
Illuminator
Output
2? Collection Cone
50 100 15? 20? From Norma I
Figure II. Polar Output of Illuminator as Numerical Aperture Increases
The significance of increasing luminance from 3000 f.l. is apparent when dense film is
viewed through high magnification stereo microscopes. The 8 to 9x increase reduces the
visual contrast threshold* increasing the visual resolution cut-off of the system by 20 to
25?,'0. In numerical values this corresponds to a change from 400 Ip/mm to approximately
500 Ip/mrn cut-off.
* W.E. K. Middleton, "Vision Through the Atmosphere" (University of Toronto Press, 1952)
p. 90.
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