In an optical source or detector, the ratio of the number of output quanta to the number of input quanta. Note: Input and output quanta need not both be photons.

percentage of input photons which contribute to a desired effect

Quantum efficiency is a measure of how well a detector generates electrons in response to incoming photons. A higher value for quantum efficiency means a more sensitive detector. Since the detector sensitivity changes for different wavelengths of photons, quantum efficiency is best expressed as a curve rather than a single value. For spectrometers, [..]

The intrinsic ability of a detector to convert photons into electrons. It is the ratio of generated electrons to incident photons expressed as a percentage.

Quantum efficiency (QE) is a figure given for a photosensitive device (charge-coupled device (CCD), for example) which is the percentage of photons hitting the photo reactive surface that will produce an electron-hole pair. It is an accurate measurement of the device’s sensitivity.

(When used with Antibodies) the amount of excitation necessary to give off the largest number of photons. Generally the higher the quantum efficiency, the better.

The ratio of hole-electron pairs or photoelectrons to the number of photons received by a photodetector.  The quantum efficiency can be as high as 80% for an APD and 25% for a PMT.

See QE.