Non-photochemical quenching is measured by the quenching of chlorophyll fluorescence and is distinguished from photochemical quenching by applying a bright light pulse under actinic light to transiently saturate photosystem II reaction center and compare the maximal yield of fluorescence emission under light and dark-adapted state. Non-photochemical quenching is not affected if the pulse of light is short. During this pulse, the fluorescence reaches the level reached in the absence of any photochemical quenching, known as maximum fluorescence, .
For further discussion, see Measuring chlorophyll fluorescence and Plant stress measurement.
Chlorophyll fluorescence can easily be measured with a chlorophyll fluorometer. Some fluorometers can calculate NPQ and photochemical quenching coefficients (including qP, qN, qE and NPQ), as well as light and dark adaptation parameters (including Fo, Fm, and Fv/Fm).