A flicker type of photo-electric photometer giving high precision

There are numerous types of photometer, designed either to measure the relative intensities of two light sources or the “densities” of filters or photographic plates. In the bulk of these, an estimate has to be made by eye when two illuminated areas are equally bright, and this limits the accuracy which may be obtained to a probable error of about 1 per cent, for one observation. In recent years the photo-electric cell, with one of the alkali metals as cathode, has come into use. One great advantage is that all eye estimates of brightness are avoided, and the light is given by the reading of a galvanometer or electrometer. In the usual form, when employed to obtain the densities of filters or photographic plates, the image of a constant source of light is thrown on to a small aperture covered by the plate in question. The transmitted light falls on a photo-electric cell behind the aperture, and the current is measured by a galvanometer. To obtain the “density” of the plate from the galvanometer reading, the plate is removed and, say, a standard neutral optical wedge is placed over the aperture and adjusted until the same galvanometer reading is obtained.