SUGGESTED PROCEDURE FOR DETERMINATION OF MICROSCOPE LUMINANCE

• Document Type: CREST
• Collection: NGA Records (Formerly NIMA)
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78B05171A000600070007-0
• Release Decision: RIPPUB
• Original Classification: K
• Document Page Count: 5
• Document Creation Date: December 28, 2016
• Document Release Date: April 4, 2003
• Sequence Number: 7
• Content Type: REGULATION

Approve or Release 2003/05/14: CIA-RDP78BW.171A000600070007-0
Suggested procedure for determination of microscope luminance
A critical specification of a microscope system is the luminous flux
per unit area in the image. Many visual functions such as resolution,
stereo-acuity. intensity discrimination, etc. . . are determined primarily
by luminance, making this specification one of the most valuable single
predictors of performance.
However, it is not ordinarily possible to utilize conventional photometric
techniques which are designed for use with a diffusing surface.e An alternative
is to make a binocular brightness match between the microscope image and a
conventional diffuse surface. With the system, one eye views a semicircle
(a square or rectangle may also be used) through the microscope while the
other views the other half of the circle directly. The two are juxtaposed in
binocular vision and matched in brightness (Fig. 1).
Because the accuracy of a photometric match falls off rapidly as the
fields to be compared, are separated, it is desirable-to bring the two halves'
as close to each other as possible in the binocular visual field. They should
also be the same size to avoid complications due to spatial summation and should:
as nearly as possible, beof'the same color temperature. Different sized semi
circles may be useful depending on the task.
A convenient device for providing the comparison. field is diagrammed in
Fig. 2.
The operator positions the comparator so that the two semicircles are
juxtaposed in the binocular field (it may be hand-held or a. jig provided),
adjusts the accommodation to be equal in the two eyes, and makes a brightness.
Two conditions must be satisfied: (1) The entrance -pupil .of the photometer
must be placed in precisely the same position as is normally occupied by the
entrance pupil of the eye and (2) the aperture stop. of the photometer must be
equal to or smaller than the exit pupil of the microscope.
Declass Review by NIMA/DOD
Approved For Release 2003/05/14: CIA-RDP78B05171A000600070007-0
 Approveor Release 2003/05/14: CIA-RDP78171A000600070007-0
match by rotating the polaroid. As a check, the eyes can be switched. When
the mean matching position of the polarizer has been determined, the lower half
of the comparator is unscrewed, the mask removed, and a conventional photometric
measurement taken from the ground glass surface.
if necessary, a color balancing filter can be inserted in the system.
It is also helpful to provide for voltage control of the light source as an
aid in color balance.
Since the spectral transmission of polaroid varies near the extinction
point, no settings within 10? to 12? of extinction should be allowed (the exact
position will depend upon the type of polaroid used). If greater attenuation
is required, fixed density filters are recommended.
An alternative procedure would be to maintain the ground glass surface
at a constant known luminance and take readings directly from a scale attached
to the polaroid (can be converted to density by the cosine2 function). If this
method is used, the voltage drawn by the bulb should be continuously monitored.
The effect of luminance depends, of course, on the size of the entrance
pupil of the eye. In ordinary photometry, this is rarely a problem because
the two fields to be compared are seen with the same eye. With the binocular
technique, it is imperative that the entrance pupils of the two eyes be the
same. If the size of .the natural pupil is less than either the exit pupil of
the microscope or the entrance pupil of the eye, this condition is satisfied.
However, pupil size fluctuates not only with general; luminance level, but varies
systematically as a function of age. Although the minimum pupil diameter is
2.0 mms., the maximum is strongly dependent on age, varying from approximately
8.0 mms. in the early twenties to 3.0 to 4.0 mms. in middle age. In addition,
the exit pupil of the microscope will depend on the magnification. A preferred
procedure is to first determine the size of the effective exit pupil of the
microscope and insert the same size pupil in the comparator. If the natural.
Approved For Release 2003/05/14: CIA-RDP78BO5171A000600070007-0
 Approved.Gor Release 2003/05/14: CIA-RDP78BOM71A000600070007-0
-3-
pupil of the eye should become smaller than the exit and artificial. pupils,
'no problem will arise since in a normal observer the two pupils are the same
size as a result of the consensual pupil reflex.
It is important that this procedure not be omitted or shortcutted by,
for example, the apparently logical and often used nothod'of.computing the
effective pupil area in the two eyes. Because of the Stiles-Crawford effect,
retinal illuminance is not a linear function of pupil area. In order to make
a meaningful photometric match, both the exit pupil of the microscope and the
entrance pupil of the photometer must be phyoically equal.
If it is desired to make measurements for different sized exit pupils
of the microscope, this procedure meat be regpeated since the effect of
luminous flux is not a linear function of pupil area.
STAT
16 April, 1970
See Also "The use and calibration of the 'Maxwellian View' in visual
instrumentation", American J. Psychol., 1954, 67, 530-532.
Approved For Release 2003/05/14: CIA-RDP78BO5171A000600070007-0
 Approve .For Release 2003/05/14: CIA-RDP78 171A000600070007-0
Figure 1. The subject's view of the visual field. The two components are
positioned as close together as possible.
-
Viewed in CID
binocular comparator
Variable pupil size
provided by series
of eye piece
inserts
I,ws mounted In 7
rotatable sleeve to permit
accommodation adjustment
Polaroid :analyser and rotatable
polarizer. Scale on outside
calibrated in degrees of rotation
from extinction point.
Viewed through microscope
optics
~,Hirror or prism
I
Ground or milk gLi i
Mesk - removable
Pigwre 2. Suggested design for photometer.
volt or
ammeter
Approved For Release 2003/05/14: CIA-RDP78BO5171A000600070007-0
 STAT Approved For Release 2003/05/14: CIA-RDP78BO5171A000600070007-0
Next 1 Page(s) In Document Exempt
Approved For Release 2003/05/14: CIA-RDP78BO5171A000600070007-0