An action spectrum (also called a response spectrum) is the amount of light or radiation of a specific wavelength required to produce a defined biological response. Action spectra are very important. From them, the risk of a particular response can be predicted for exposure to any source of light or radiation. The most common example in photomedicine probably is the erythemal action spectrum (Figure 1). Normalization of action spectrum data, as indicated by the vertical axis being relative effectiveness, allows comparison of various types of responses which may require vastly different levels of exposure to elicit. This action spectrum is a composite of all available human erythemal action spectra; it was collated by Alastair McKinlay and Brian Diffey from more than 12 erythemal action spectra measured from 1929 through 1985.
An action spectrum is usually obtained by determining the responses of individuals to monochromatic radiation. While only a finite number of specific wavelengths can be tested, these provide a basis for estimating the energy required to produce the response at nearby wavelengths. Action spectra can also be obtained using polychromatic sources, but these require mathematical fitting of data to possible action spectra.
Why should an erythemal action spectrum be important to our basic understanding of human responses to sunlight? The erythemal action spectrum is significant not only because erythema is an important acute event, but because the action spectra of other responses of human skin are similar. These other responses are delayed melanogenesis, photo-induced elastosis, and photocarcinogenesis. Figure 2 shows the various action spectra plotted together. It is important to note that both erythema and melanogenesis are acute responses to sunlight injury; data were obtained using human volunteers. Both photo-induced elastosis and photo carcinogenesis data are responses obtained after chronic radiation exposure of hairless mice.
An action (or response) spectrum allows prediction of risk of injury from exposure to …