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DIFFERENCES BETWEEN VISUAL AND PHOTOGRAPHIC CALIBRATIONS 
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DISTORTION _ MICRONS 
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FrG. 3. Average radial 
magnitude of such a variation, the camera 
was tested on the photographic calibrator 
using two different filter arrangements 
with the usual "mean noon sunlight" 
light sources and flattened Aero Super 
XX plates.” The first filter was the com- 
monly used Wild 2X yellow filter. For 
the second calibration no filter was on the 
camera but all the collimator light sources 
were screened with Wratten 13 filters to 
transform the effective spectral sensitivity 
to about that of the human eye. Results of 
these calibrations are shown in Figure 3. 
The two upper curves show each set of 
distortion data plotted from its own best 
calibrated focal length, that is the focal 
length which makes distortion a minimum. 
The C.F.L. for normal photographic 
calibration exceeds that for the simulated 
visual calibration by 0.012 mm., agreeing 
measured distortion. 
with the differences of 0.01 to 0.02 mm. 
found between photographic and manu- 
facturer's calibrations. The third and low- 
est curve of Figure 3 shows the data from 
the simulated visual calibration plotted 
on the photographic calibrated focal 
length. Putting this curve on the same 
basis as the photographic distortion curve 
provides a direct indication of the differ- 
ences in image positions occurring between 
the two calibration methods. 
These actual differences in image posi- 
tion are replotted in the lower solid curve 
of Figure 4. To illustrate how they arise 
from the transverse colour, they are com- 
pared there with a curve computed from 
Figures 1 and 2. From Figure 1, a median 
wave-length of 605 mu was obtained from 
the curve for Super XX with minus blue 
filter and a median wave-length of 540 mp 
  
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DIFFERENCES IN IMAGE POSITION _ MICRONS 
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