4 
surface, the comfort and convenience of viewing remain primary design 
considerations. For these reasons the flat film-supporting platform 
of the T-64 Orthophotoscope is tilted toward the operator approximately 
40 degrees from the horizontal plane. The instrument's projection system 
is, of course, tilted corresponding. 
Preparation of Orthophotographs 
As in all of the earlier instruments, the orthonegative is exposed 
through the scanning aperture to small differential areas of the stereoscopic 
model surface as the platen carriage moves along its track. Although 
the film emulsion is exposed to both the red and the blue projective rays 
which create the anaglyphic model, only the blue rays have an actinic 
effect on the blue-sensitive film. The orthonegative, the immediate product 
of the instrument, is correct-reading but has the reversed tones of a 
negative. It is exposed at the scale of the anaglyphic model. 
The positive-print orthophotograph is processed by means of a copy 
camera used as a projection printer. The orthonegative is mounted on a 
transparent easel, backlighted, and photographed through the base material 
to produce a correct-reading positive. As the orthophotograph is usually 
desired at a scale smaller than that of the stereomodel, the copy camera 
also provides a convenient means of changing the scale. 
Using the copy camera to bring the orthophotograph to final scale 
eliminates the need to adjust the stereomodel to a specified scale. As 
model scale is not critical, the projection distance used is that which 
results in optimum image definition on the orthonegative. Tests have 
established a value of 430 millimeters as the optimum projection distance 
for exposing orthonegatives with ER-55 (Balplex) projectors. This 
distance differs considerably from this projector's nominal optimum 
projection distance of 525 millimeters, the latter value being based on 
lens measurements made with white light. The difference is caused by 
the longitudinal chromatic aberration inherent in the Hypergon lenses 
used in ER-55 projectors. Because of this aberration, the effective focal 
length of these lenses is reduced by 2 percent when blue light is the 
only segment of the spectrum being considered. The monochromatic 
sensitivity of the film not only reduces the distance at which an ortho 
negative is best exposed but, more significantly, makes it possible for 
the images on an orthonegative to be better defined than those observed 
visually in the anaglyphic model formed with present ER-55 projection 
lenses. 
Preparation of Orthophotomosaics 
Whenever the area of interest extends beyond the coverage provided 
by a single orthophotograph, it becomes necessary to prepare orthophoto 
mosaics. Scale-stable materials must be used in preparing these mosaics 
in order to retain the metric accuracy of the product. Because the 
materials are not stretched or shrunk to obtain agreement between the 
adjacent orthophotographs, the accuracy of the aerotriangulation upon 
which the mosaics are based is critical. The peripheral imagery on each 
segment of the orthophotomosaic must join with that of adjacent segments. 
In this process there is no opportunity, as there is in map compilation, 
to graphically adjust planimetric features that otherwise would fail to 
join. As gaps, overlaps, and mismatches are immediately apparent, the 
orthophotomosaic contains a built-in means by which its fidelity can be 
judged.