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along a precise straight line. The distances are positioned so that performance 
of the pantograph, in both an extended and folded position, can be checked through 
out a range of reduction ratios varying from 6.5:1 to 3:2. 
AUXILIARY EQUIPMENT 
Improvements in auxiliary equipment also contribute greatly to better results 
in stereoplotting. Automatic-contrast-control devices with improved lens- 
distortion-compensation plates have been added to diapositive printers, resulting 
in better diapositive plates. Figure 13 illustrates an installation of electronic- 
10 
type contrast control in a 153/153 ratio diapositive printer. A large-size 
cathode ray tube is mounted directly beneath the stage plate of the printer and 
provides the modulated light for printing. The photo-multiplier pickup for the 
diapositive plate is contained in a housing above the stage plate. The velocity- 
modulation type of control is used in this installation. 
Contrast control may also be achieved by means of a Fluor-O-Dodge^ type of 
installation (see figure 14). As adapted to the 153/55 diapositive printer, a 
fluorescent screen is mounted on the stage plate of the printer and provides the 
modulated light for contrast control. The modulation is provided by the quenching 
action of infrared light on the screen by a bank of lamps installed above and to 
the side of the pressure piatei The lamp output is filtered to provide illumina 
tion peaked at 9,000 Angstrom units. 
Corrector plates are used to compensate for lens distortion and earth curva 
ture at specific flight heights. These distortion-compensation plates are used 
for the so-called "distortion-free" photography. The need for correction of small 
residuals of distortion in a photogrammetric system composed of aerial camera, 
printer, and projector is particularly necessary when the distortion residuals in 
^Electronic contrast control by LogEtronics, Alexandria, Virginia. 
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Fluor-O-Dodge control elements by Watson Electronics, Arlington, Virginia.