Definitions for "Quantum efficiency"
In an optical source or detector, the ratio of the number of output quanta to the number of input quanta.
The effectiveness of a CCD in generating electrons from the incident light falling on the CCD.
The ratio of effectiveness with which a CCD converts received photons (quanta of light) to measurable electrons. Amateur CCDs operate from about 15% up to about 75%, depending on the wavelength of light. High-end CCDs built for professional use frequently run 80% or higher.
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See QE.