According to Einstein, the electrons leave a photo-electric surface with an initial velocity proportional to the frequency of the impinging radiation. Consequently, when a sawtooth potential is applied to a photoelectric cell, illuminated by a monochromatic light, the current flow starts at a threshold potential corresponding to the wave-length of the light, and then gradually increases more or less linearly toward a saturation value. When light of assorted wave-lengths is applied, the total current is the summation of the currents for the individual wave-lengths, each of which currents has its threshold at a different potential. By double differentiation an indication of the spectral distribution can be observed directly on an oscilloscope which is deflected synchronously with the potential applied to the photoelectric cell. The accuracy of the indication is limited by the indefiniteness of the threshold values and non-linearity of the currents above the threshold potentials in available photoelectric cells. With S4 commercial surfaces, filters of the three primary colors can be easily differentiated, but with improved surfaces considerably better resolutions may be obtained and further improvement may be had by lowering the temperature of the photoelectric surface. The method lends itself readily to color matching, since two similar devices supplying their signal with opposite polarity will give zero output when the two colors match, while it provides a direct indication of the color and magnitude of mismatch when a signal is obtained. The output signal can be utilized for relays or other control devices in industrial processes.