| 摘要 |
PURPOSE:To enable accurate photometry eliminating the dependence of the photometry value on the temperature, by employing a time constant element with a better temperature characteristic to perform a range compression of the photometry value based on a CR time constant curve. CONSTITUTION:A photometry circuit is made up of a light receiving element 1 comprising photodiode, a reference current generation circuit 6 comprising a series circuit of a resistance R and a capacitor C, an operation amplifier 7, a feedback circuit 8 comprising a parallel reverse connection of diodes D1 and D2, a comparator 9 and a Schmitt trigger 10. Then, while a reference current IR is larger than the photocurrent IL flowing through the light receiving element 1 according to extraneous light, the output of the comparator 9 moves to a high level and as the reference current IR decreases and becomes less than the photocurrent IL, the output of the comparator 9 goes to the low level. Thus, the pulse output T with the logarithmically compressed time base corresponding to the photometry value is obtained and converted into a digital photometry value thereby eliminating the dependence of the photometry value on the temperature. |
