GDRICUNICONSD REPTILES n m n pd 
exposure measurement (when opened). 
2.2.2.2 Illuminance Measurement 
Figure 5 shows the schematic diagram of the circuit. The illuminance 
E, is converted in to a photo-current I, by means of the silicon pho- 
todiode and the current is amplified and converted in to a voltage V, 
by an operational amplifier OP and its feed-back resistor R. The re- 
lationships between the illuminance and the photo-current, and the 
voltage is shown in Equation (1) beiow: 
Ve 
E, miki, = bury (1x) (1) (OQ 
  
Where k is a constant to be obtained experimentally. The instrument 
is calibrated by giving a known illuminance on the silicon photodiode 
and reading the output voltage. Since the value of the feed-back 
resistance can be arbitrarily determined, the value is chosen to suit 
the measuring conditions. 
2.2.2.3 Exposure Measurement 
The exposure is measured in a similar manner; however, in this case, 
the feed-back resistor in the measuring circuit is replaced by a feed- 
back capacitor C as shown in Figure 6. The charge upon the capacitor 
is equal to the integral with respect to the time of the photo-cur- 
rent flowing into it and is proportional to the voltage across it. 
Since the photo-current is proportional to the illuminance on the 
detector, the voltage across the capacitor is proportional to the 
exposure. These relationships are expressed by the following equa- 
tions. The voltage across the capacitor is: (@ 
VY, = ir we: (2) 
c C c 
referring to Equation (1) and Figures 4 and 5, we obtain the follow- 
3 ing: 
E_R : 
Ec(t) = KI. (t) = se en) (3) 
e 
From Equations (2) and (3), we obtain the exposure H by the following 
equation: 
EVe CR 
Rs Beitet rk Ea (1x-s) (4) 
e x 
(4)