If a photocell is calibrated by means of a source of known spectral energy distribution, employment of the cell for the measurement of the illumination produced by another source of different energy distribution will entail correction of the initial calibration constant. The basic principles for obtaining such a correction factor, applicable for the general case, of wavelength selective receivers, are set out in this paper. As an example, the case is discussed in detail of deriving the natural illumination flux from radiometric measurements of solar shortwave radiation with the aid of current standard meteorological filters. Equations are presented from which the illumination can be computed from known values of the air mass (i.e., optical path length) and the atmospheric turbidity. These equations have been applied to a comprehensive series of measurements of illumination and shortwave radiation assembled in South Africa. It is shown that the equations are valid over very wide ranges of air mass and turbidity.
A. K. Ångström and A. J. Drummond, "Fundamental Principles and Methods for the Calibration of Radiometers for Photometric Use," Appl. Opt. 1, 455-464 (1962)