When the photo transistor is placed in the dark and a voltage is applied from the collector to the transmitter, a certain amount of current flows. This current is called dark current (ID). This current consists of the leakage current of the collector base junction multiplied by the DC gain of the transistor. The presence of this current prevents the photo transistor from being considered completely “off” or is an ideal “open” switch.
The dark current is specified as the maximum collector current that can flow to a certain test voltage of the collector-emitter. Dark current is a function of the applied collector-transmitter voltage and ambient temperature.
EG & G Optoelectronics photos and phototransistors are tested with an applied VCE voltage of 5 V, 10 V or 20 V depending on the device. Phototransistors are tested at dark current limits ranging from 10 nA to 100 nA.
The dark current depends on the temperature, increasing with increasing temperature. It is usually specified at 25 ° C.
the dark current is relatively small electrical current flowing through photosensitive devices, such as a photomultiplier tube, a photodiode or a device with a load coupling even when no photon enters the device; is composed of the load generated by the detector when no external radiation enters the detector.  It refers to the inverse polarity dispersion current in non-optical devices and is present in all diodes. From a physical point of view, the dark current is due to the random generation of electrons and holes in the area of device exhaustion.
The load generation rate is related to the specific crystallographic defects in the exhaustion region. The dark current spectroscopy can be used to determine the defects present by monitoring the currents of the current dark-temperature histogram evolution.
Black current is one of the main sources of noise in image sensors, such as chargers. The pattern of different dark currents may cause a fixed type of noise; the dark fall of the frame may eliminate an estimate of the fixed average model, but still remains a temporal noise as the dark current has a sudden noise.