The photographic method for rècording average illuminations

Abstract

(Text-figs. 1-3)The photo-electric cell is admirably suited for measuring instantaneous illuminations, but does not lend itself readily to the measurement of the total illumination received on a surface over a long period of time, or the average illumination received during that time, although it has been used for this purpose (Atkins & Poole, 1936a). The method used by these authors, however, would be difficult to use in the field and away from laboratory facilities. The photographic method, on the other hand, is admittedly not capable of yielding results of high precision, but the simplicity of operation, and absence of elaborate and bulky apparatus, make it suitable for use in the field.In principle, the amount of blackening produced in a photo-sensitive emulsion is used as a measure of the total illumination received; in order to increase the range over which measurements can be made, a neutral tint wedge, of suitable slope, is placed in front of the emulsion, and the distance along the wedge at which a standard amount of blackening is produced is taken as a measure of the illumination. The standard amount of blackening is best defined as that amount which is produced, in the absence of the wedge, by a known illumination acting for a known time. The method has inherent limitations, particularly when used for long-period observations, which result from (a) the failure of the reciprocity law, (b) the uncertainty in the value of the wedge constant, and (c) the varying sensitivity of the emulsion to lights of different wave-lengths.