Photosensors

A photosensor is a device that detects light and converts it into an electrical signal. It covers detectors that respond to electromagnetic radiation in the visible, near‑infrared, and sometimes ultraviolet ranges, and is used to measure light intensity or presence. Common photosensors include photodiodes, phototransistors, and photoresistors, as well as more specialized devices such as photomultiplier tubes and infrared detectors.

Most photosensors operate on light-induced generation of electrical signals. Photodiodes and phototransistors generate current when illuminated; many are reverse-biased to improve speed and linearity. Phototransistors provide current gain, producing larger signals from modest light levels. Photoresistors (or LDRs) change their resistance with light exposure but tend to be slower and less linear than semiconductor junction devices. Some sensors operate in photovoltaic mode, producing a voltage with little or no external bias, while others require biasing for operation.

Key performance parameters include spectral responsivity (A/W), response or rise/fall time, dark current, noise, dynamic range, and linearity. Temperature can affect sensitivity and speed. Common materials include silicon for visible light, gallium arsenide or indium gallide for infrared, and cadmium sulfide or cadmium sulfide/cadmium selenide for photoresistors.

Applications span automatic lighting and display backlighting, camera exposure metering, ambient light sensing in mobile devices, optical communication receivers, barcode scanners, safety and environmental monitoring, and autofocus and proximity sensing. Selection considerations include spectral range, speed, linearity, noise, packaging, environmental stability, and system integration with readout electronics.