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FIG. 8: FUNCTIONAL DIAGRAM FOR AN ILLUMINATION-TARGET SYSTEM V » 
PROVIDING HIGH TARGET-BACKGROUND CONTRAST 
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Given this design concept (and its technical assessment) defining the functional means by ; 
which a high target-background contrast could be obtained, a set of laboratory experiments were under- Ÿ 4 
taken to obtain performance data and to determine the other radio-metric engineering factors pertaining 
to this particular application. For photogrammetric applications, there are two basic requirements: 
1. When the photo co-ordinate is the centroid of the level discriminated video signal, the 
luminance of the target rays should be uniform over the defining area of the target (exit 
pupil) such that the principal point-level discriminated centroid defined ray passes through 
the geometrically defined centroid of the target area (exit pupil); 
2. The optical geometrics should be chosen to maintain a constant target signal over the 
required working distance (1 m to 4 m); this requires consideration of the location of the 
source, the size of its radiating area, the size of the corner cube, etc. 
As described in a report of these studies [Ayad (9? ], one basic factor noted from simple tests 
with a circular mask defining the target area with a single large corner cube was the existence of 
vignetting. Since such a problem is in proportion to the size of the cube corner, a possible solution 
would be to use an array of small cube corners in sheet form such as used in retro-reflectors for T 
motor vehicles and highway signs. This could also be advantageous for the second requirement since 
the entrance-exit pupil for each cube corner would then be considerably reduced with respect to the 
size of the source and stop of the lens. Even given this latter fact, the geometrical quality of small 
cube corners should still be such as to maintain a small divergence angle for the reflected rays, in 
order to achieve the required contrasts. 4 
Tests with such a material were carried out, both with a telephotometer in place of the 
camera and then with a video camera oscilloscope examination of the video signal. 
The results of these design development tests are described in the reference report‘$). Of 
importance to design feasibility was the result that, with a high level of simulated sun illumination 
on a white background, the measurements showed a ratio of target to background greater than five. 
Calculations indicated that by careful design an acceptable contrast could be obtained for the sun 
light illumination conditions of outer space. 
18 
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