Digital camera sensors and long exposures
The light conversion element in most image sensors as actually a reverse biased diode carefully crafted to retain charge until reset.  A photon enters the bulk of the photodiode and interacts with the Silicon lattice creating a electron/Hole pair.  The electric field within the diode from being reversed biased sweeps the minority carrier away while the majority carrier is kept within the photodiode.  As more photons are intercepted more carriers are collected.  With a lower rate of generation (i.e. with lower rate of photon arrival) one must accumulate for a longer period to have the same signal level.

In other words, photodiodes are integrating sensors.