Dark current is not something from a Stephen King novel. If an optical sensor has a bias voltage applied to it, it will have a small leakage current, or dark current, as an output even when there is no light present at all. The devices are mostly diodes, which have a leakage resistance, or current, or they are light-sensitive resistors, which do not have infinite resistance with no impinging light. Optical sensors used for very sensitive measurements typically do not have a bias voltage applied because the dark current can be larger than that generated by the lowest light levels, and the dark current has a strong temperature dependence (doubles every 10C). A bias voltage is more commonly used for communications applications to improve response time, but the light levels are typically much higher.
Thanks for attending the seminar and asking the question. David Kress
