this invention relates to a system for the calibration of an optical particle - size analyzer of the light - intercepting type for spherical particles , wherein a rotary wheel or disc is provided with radially - extending wires of differing diameters , each wire corresponding to a particular equivalent spherical particle diameter . these wires are passed at an appropriate frequency between the light source and the light detector of the analyzer . the reduction of light as received at the detector is a measure of the size of the wire , and the electronic signal may then be adjusted to provide the desired signal for corresponding spherical particles . this calibrator may be operated at any time without interrupting other processing .