Abstract: Spots representing quantities of deposited material in, for example, chromatograms are scanned optically and a video image is generated. The video image is, in turn, subjected to line by line sampling with a sampling and storing circuit which has a level control and which operates a gate transmitting oscillator pulses which are counted. The count represents the optical density at each scanned line. The count represents the surface area and density for each of the portion of the optical density of a particular level and hence represent the volume of the material deposited at the spot.