Photodiode
The semiconductor junction within a photosite that converts arriving photons into electrical charge.
Measured in
e⁻ (electrons)
In depth
The photodiode is where the physics happens: a photon of sufficient energy frees an electron, and the accumulated electrons are counted at the end of the exposure. Not every photon succeeds, and the proportion that do is the sensor's quantum efficiency. The response is linear, which is why raw data must be gamma-encoded before it looks correct.
Examples in practice
A photodiode with 60 per cent quantum efficiency converts roughly six of every ten arriving photons into signal.