been used as the detectors and are located on one of the diagonals of 
the camera picture format, one at the principal point and the others 
at 20 mm intervals (see Figure 2). All 15 detectors can be used for 
cameras of 230 mm X 230 mm format and 13 out of 15 also for cameras 
of 180 mm X 180 m» format. The shape of the detector is circular, 
and its diameter is 3. mm. Although a diffuser is placed in front of 
each detector, the deviation from the cosine law of incident angle 
becomes significant if the angle of incidence of the light rays fall- 
ing on the detector surface is increased. Measured illuminance must 
therefore be compensated for.. The surface of the instrument facing 
the camera lens is treated to simulate the film surface. 
An extended light source has been. used for the measurements. The 
correlated color temperature of the source is 2856 K. The uniformity 
of the luminance is such chat the luminance measured at any point 
normal to the surface is within +2 % of the average luminance over 
the surface. Figure 3 shows the relative spectral sensitivity of the 
detectors and the relative spectral power distribution of the light 
source. 
The estimated accuracy of the measurements is t4 % and. the precision, 
t5 Z. The instrument is able to measure the exposure distribution, 
the illuminance distribution and the effective exposure time distri- 
bution of any aerial survey camera that satisfies the following con- 
ditions: 
camera lens f-number- f/4 - £/16, 
focal length of the lens- 80 mm and longer (half angular field- 
65° and smaller), : 
camera shutter speed (exposure time)- 1/100 - 1/1000 s, 
illuminance in the film plane- 10 - 10600 1x. 
2.2.2 Principles of the Measuring Circuit 
2.2.2.1 General 
The measuring circuits consist basically of a silicon photodiode, a 
photo-current-voltage converting circuit and a photo-current inte- 
grating circuit. Fifteen (15) identical circuits, which correspond 
to the 15 measuring positions, are connected in parallel and each 
output signal is processed by a computer. 
Figure 4 shows the actual circuit. In this circuit, the photo-cur- 
rent is amplified by an amplifier OPl and is held by a sample hold 
circuit OP3. The other sample hold circuit OP2 is used for the com- 
pensation of input offset voltage. The switch 81 converts the cir- 
cuit to either the illuminance measurement (when closed) or to the 
(3)