Photosite

A photosite is the light-sensitive element of a digital image sensor—the smallest unit that can detect light. It is the light-detecting site within a pixel, and the term is often used interchangeably with pixel, though some sources distinguish the raw photodetector from the complete pixel cell.

Each photosite comprises a photodiode that converts incoming photons into electrical charge. In CCD sensors, the charge is stored in the photosite until it is transferred to a readout register. In CMOS sensors, the photodiode is paired with switching transistors and often amplifiers within the same pixel, enabling parallel readout and sometimes per-pixel processing.

Color imaging commonly uses a color filter array to separate light into red, green, and blue channels, with micro-lenses focusing light onto the photosites. Back-illuminated designs place the diode closer to the light-facing surface to improve quantum efficiency by reducing front-side obstruction.

Photosite size, also called pixel pitch, influences sensitivity and noise performance. Larger photosites collect more photons, offering better dynamic range and low-light performance but yield lower spatial resolution for a given sensor area. Smaller photosites increase resolution but can reduce full-well capacity and raise noise. Sensor designers optimize fill factor, microlensing, and architecture to balance these factors.