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A scintillator is a substance that absorbs high energy (ionizing) electromagnetic or charged particle radiation then, in response, fluoresces photons at a characteristic Stokes-shifted (longer) wavelength, releasing the previously absorbed energy. See also Scintillation (physics). Scintillators are defined by their light output (number of emitted photons per unit absorbed energy), short fluorescence decay times, and optical transparency at wavelengths of their own specific emission energy. The latter two characteristics set them apart from phosphors. The lower the decay time of a scintillator, that is, the shorter the duration of its flashes of fluorescence are, the less so-called "dead time" the detector will have and the more ionizing events per unit of time it will be able to detect.