Quantum efficiency

Quantum efficiency is the fraction of incident photons that produce detected charge carriers or emitted photoelectrons. It measures photon-to-signal conversion and depends on wavelength and the device or material.

Photodiode: A semiconductor device that converts incident light into electrical current. Its photon-generated current provides a standard way to measure quantum efficiency.
Image sensor: A device that converts an optical image into an electrical signal. Its quantum efficiency determines how much of the incoming light contributes to an image.
External quantum efficiency: The ratio of collected charge carriers to photons incident on a device, including optical reflection and transmission losses. It uses incident photons as the denominator, matching the stated definition.
Photon: The quantum of electromagnetic radiation, carrying energy proportional to its frequency. Quantum efficiency counts photons rather than measuring optical power alone.
Photon shot noise: Statistical fluctuation in photon counts caused by the discrete, random arrival of photons. Finite detection efficiency reduces detected counts and shapes their shot-noise limit.
Photoelectric effect: The emission of electrons from a material after it absorbs electromagnetic radiation. Photoelectron yield is one form of quantum efficiency.
Charge-coupled device: An image sensor that transfers photo-generated charge between capacitive pixels for readout. Its wavelength-dependent efficiency affects sensitivity in astronomical and scientific imaging.
Internal quantum efficiency: The ratio of collected charge carriers to photons absorbed by a device. Unlike external efficiency, it excludes photons that were not absorbed.
Photocurrent: An electric current generated when light produces mobile charge carriers in a material. Photocurrent is the measurable output used to infer carrier yield.
Signal-to-noise ratio: The ratio of a measured signal's magnitude to the magnitude of its noise. Higher conversion efficiency can improve signal strength when other noise sources are fixed.

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