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A photodetector converts incident light into a measurable signal, often an electric current or stored electric charge. A photodiode collects carriers produced by absorbed photons; a photomultiplier first releases photoelectrons and then amplifies their number.
A basic characterization measures the output with the beam blocked and with several known incident radiant powers. The slope gives spectral responsivity at the chosen wavelength; the blocked signal reveals offsets and dark current. Quantum efficiency, response speed and noise-equivalent power describe different limitations. Detecting a weak signal requires separating its contribution from both background photons and the detector's own fluctuations.