Abstract: A particle size analyzer having an optical assembly with a light source and a lens producing by reflected light an image of a field of particles carried on a moving conveyor. The image is cast upon two photo-transducers which produce substantially identical signals relatively displaced in time by an amount inversely proportional to the rate of conveyor movement. These two transducer signals are fed to discriminator means to develop respective sets of pulse signals identifying predetermined transition characteristics in the light transmitted to the transducers. These two sets of pulse signals are used to develop a control signal for adjusting the frequency of a clock-pulse generator such that a given number of clock pulses represents a corresponding distance moved by the conveyor, irrespective of its speed. One of the sets of pulse signals is used to gate the generated clock pulses to a counter and accumulator system which provides output signals representing certain particle size parameters.