A plot of a relative biological or chemical photoresponse (= \(\Delta y\)) per number of incident photons against @[email protected] or energy of radiation under the same @[email protected] of light. This form of presentation is frequently used in the studies of biological or solid state systems, where the nature of the absorbing species is unknown. This type of action spectrum is sometimes called @[email protected] or sensitivity spectrum. The precise action spectrum is a plot of the @[email protected] By contrast, a plot of the biological or chemical change or response per absorbed photon (@[email protected]) versus @[email protected] is the @[email protected]