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)