ou __ 
————— us t " MAT = 
REN TL te | : eR RR m 
AC RTS du MS NE A A PME 
^ 
mr ke 
ls 
ITT ESTE 
The other characteristic is the blue glare spot which sometimes ap- 
pears at the center of a picture under bright conditions. We studied 
this in order to find the reason for this glare spot, and also to 
establish a practical testing method. 
Besides the above two characteristics, there are many other photo- 
metric properties of aerial survey cameras to be investigated, e.g. 
f number, T number, veiling glare, spectral transmittance of camera 
lenses and A.V. filters. These problems will be tested in the future 
by our group. 
2. Instrument for the Measurement of Exposure Distribution 
- 
Q 
2.1 Purpose of the Measurements 
Àn ever increasing demand for aerial color photography revealed some 
unfavorable phenomena which have been disregarded in black-and-white 
photography, for example, unusual non-uniformity in color and density 
distribution over the camera picture format due to the intrinsic op- 
tical characteristics and/or defects of the camera. 
A serious phenomenon is a significant under-exposure at the periphery 
and corners of the picture, due to the illuminance decline as the 
fourth power of the cosine of the (image) field angle and the vi- 
gnetting of the lens. These are unavoidable optical characteristics 
of the wide-angle lenses which are usually used with aerial survey 
cameras. Modern wide-angle lenses for aerial survey cameras are de-- 
signed so that they show almost no vignetting, and anti-vignetting 
filters (A.V. filters) are usually used with the lenses to obtain 
even illuminance over the picture format; however, the unevenness of 
the exposure distribution has still not been totally improved. 
To analyze the above phenomenon, we developed an instrument for the 
direct measurement of exposure distribution. Separate measurements 
been made of the exposure time and efficiency of the camera shutters, 
the vignetting and transmittance of the lenses, the transmittance 
distribution of A.V. filters, etc.; however, directly measuring the 
exposure distribution is by far the more accurate and reliable meas- 
urement as well as being, time-saving, less laborious and more direct- 
ly related to the actual picture density. The instrument is also able 
to measure the illuminance distribution and the effective exposure 
time distribution. 
2.2 Construction and Principles of the Instrument 
2.2.1 General 
Às shown in Figure 1, the shape and dimensions of the instrument are 
such that it fits the camera film gate. 15 silicon photodiodes have 
(2)