= Photodetector
{wiki}
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.
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