New Meters Employing Light-Sensitive Cells for the Measurement of Erythemal Energy

The applications of some well-known principles to the measurement of erythemal energy with light-sensitive cells are described. In one meter described the differential current of two cells is registered, and by selection of the proper filter for each cell, this current can be made a measure of the intensity of radiant energy weighted approximately according to its erythemal efficiency. In the other meter, the radiant energy to be measured is incident on a filter which transmits ultraviolet energy but absorbs visible energy almost completely. A phosphor having a suitable spectral sensitivity is then activated, and the resulting fluorescent light is transmitted to the cell through filters which totally absorb ultraviolet energy. The spectral response of the device is then almost identical to that of the phosphor. Quantitative data are presented on the spectral and absolute sensitivities of the meters.