Quantum yield is a term used in fluorescence spectroscopy to define the effiency with which a compound takes light that it absorbs and converts it to emitted light. If a compound absorbs light and emits nothing (i.e. dark), then the quantum yield is zero. If every photon absorbed results in photon emitted, the quantum yield is one. Tryptophan in water, for example, has a quantum yield of 0.14, the highest of all the amino acids.

Compounds with a high quantum yield are often useful to spectroscopists because it is much easier to detect their fluorescence.