Please explain the dark current.

A light receiving device has a so-called dark current independent of an incident light, in addition to a photocurrent produced by it. This is treated as the noise N against the signal S, but its value is very important. It is generally called a signal to noise ratio. Its effect is more noticeable in a weaker incident light, giving more harms. The lower the value is, the better the effect is, but actually, it cannot be ignored.
The dark current fluctuates greatly as the temperature changes. Full care should be taken when using at high temperatures.