Optical radiation can be characterized by its spectral power distribution. The mechanisms by which optical radiation is absorbed by chemical or biological systems are usually complicated, and are always wavelength (or frequency) dependent. For metrological purposes, however, the complexities of the absorption mechanisms can be ignored, and the actinic effect is characterized simply by an actinic action spectrum linking the photochemical or the photobiological response to the incident radiation. This actinic action spectrum describes the relative effectiveness of monochromatic optical radiation at wavelength to elicit a given actinic response. It is given in relative values, normalized to 1 for the maximum of efficacy. Usually actinic action spectra are defined and recommended by international scientific or standardizing organizations.

For vision, two action spectra have been defined by the CIE and endorsed by the CIPM: V() for photopic vision and V'() for scotopic vision. These are used in the measurement of photometric quantities and are an implicit part of the definition of the SI unit for photometry, the candela. Photopic vision is detected by the cones on the retina of the eye, which are sensitive to a high level of luminance (L > ca. 10 cd m^–2) and are used in daytime vision. Scotopic vision is detected by the rods of the retina, which are sensitive to low level luminance (L < ca. 10^–3 cd m^–2), used in night vision. In the domain between these levels of luminance both cones and rods are used, and this is described as mesopic vision.

Other action spectra for other actinic effects have also been defined by the CIE, such as the erythemal (skin reddening) action spectrum for ultraviolet radiation, but these have not been given any special status within the SI.