This paper considers the electrical noise originating in photodetector devices and associated amplifier equipment of photodensitometric instruments intended for quantitative assessment of thin media chromatograms. From a comparison of optical and electrical noise values the minimum intensity of illuminating light is calculated, which is necessary to meet a prescribed performance standard. Photomultiplier tubes and solid-state PIN-diodes are compared and it is shown that it is feasible to use these devices for high grade photodensitometric work. Advantages accrue both in the electrical and mechanical design of the instrument, and there is a resultant decrease in the expected costs plus an easier mode of operation.