(9 
  
In this equation, the focal plane illuminance E, and the output voit- 
age Ve are obtained by the illuminance measurement mentioned above. 
The value of the feed-back resistance and of the feed-back capaci- 
tance can previously and arbitrarily be determined; therefore the ex- 
posure H is obtained by measuring the output voltage V,. 
2.2.3 Method for Calculation 
2.2.3.1 Iiluminance Measurement 
The illuminance at a point in the film plane and at an aperture is 
measured by holding the camera shutter open and exposing the camera 
to the extended light source. The measured illuminance values are 
normalized by the one measured at the principal point and are ex- 
pressed as percentage, as shown in Equation (5) below: 
Relative illuminance at a point'á'in the film plane 
Illuminance at a point'd (1x) x 100 (Z) (5) 
Iliuminance at the principal point (1x) 
  
2.2.3.2 Exposure Measurement 
The exposure is measured in the same manner as above except the camera 
shutter is in operation. The measured exposure is normalized as above 
and is expressed by the following equation: 
Relative exposure at a point'd'in the film plane 
P 
_ _Exposure at a point 'd '(1x-s) x 100 C5 (6) 
Exposure at the principal point (1x-s 
2.2.3.3 Effective Exposure Time 
The diagram of the photo-current versus exposure time at an aperture 
is shown in Figure 7. In this figure, the exposure is proportional to 
the area ABCD and the illuminance at the same aperture measured by 
holding the camera shutter open is equal to the maximum illuminance 
during exposure, and is proportional to the height AB or CD of the 
trapezoid ABCD. The effective exposure time t, is calculated by di- 
viding the area ABCD by the height AB or CD. This corresponds to the 
width A'D' or B'C' of the rectangle A'B'C'D', the area and the height 
being the same as those of the trapezoid ABCD. 
The effective exposure time at a point 'd' can be calculated by di- 
viding the exposure at the point by the illuminance at the same point, 
as shown in Equation (7): 
(5)