Dark current

One calls dark current

• the spontaneous formation of free charge carriers through heat in a light-sensitive semiconductor , for example a CCD image sensor, a photodiode , a photomultiplier or a photoresistor
• the release of electrons from the photocathode of a photomultiplier or a photocell due to the movement of heat in the material.

Explanation

Because of the third law of thermodynamics , the atoms or molecules that make up a photosensitive detector move . Inelastic collisions can trigger charge carriers in the detector, even if it is not exposed. This thermally caused conductivity in components that use the internal photoelectric effect or thermally emitted electrons in photocathodes ( external photoelectric effect ) generate the dark current.

The effect can be reduced by lowering the temperature of the photo receptors. Photo receivers for the mid- infrared must in principle be cooled (for example with liquid nitrogen ).

Noise

Systematic noise on digital images caused by the dark current can in part be removed later by image processing on the computer or already in the camera. The dark current itself also varies (apart from external factors such as chip temperature etc.) due to the Poisson statistics. This actual noise cannot be removed in this way, but is e.g. B. with CCDs in general very low compared to the noise of the actual signal.

Sensory physiology

In sensory physiology , the dark current describes the flow of sodium ions in the dark. In the dark, when the sensory cell is at rest, the Na channels in the membrane of the visual cell are open. The dark current depolarizes the membrane potential to about −30 mV to −40 mV.