Objects of class response_spct() can contain data for a response
spectrum or an action spectrum. Response spectra are measured using the
same photon (or energy) irradiance at each wavelength. Action spectra are
derived from dose response curves at each wavelength, and responsivity
at each wavelength is expressed as the reciprocal of the photon fluence
required to obtain a fixed level of response. In the case of biological
systems the action and response spectra frequently differ in their shape
and spectral values. This is a property inherent to a data set and not
subject to conversions, thus normally set when a response_spct object
is created and never modified.