Filtered pulsating light from a mercury vapor lamp operated on ac is directed upon a commercial high-vacuum phototube, and the photocurrent, which is thereby pulsating dc, is passed by an ac amplifier whose output is fed to a dc milliammeter through a copper oxide rectifier. Using different light filters a retarding potential of increasing magnitude is applied to the phototube until the milliammeter readings reach respective minimum values which are plotted as ordinate against optical frequency as abscissa. The slope of the resulting line is taken ash/e.