Abstract: Values of measured optical density are corrected by measuring a high and a low density which are known as standard densities to obtain their measured value by an optical densitometer subjected to a calibration; providing data for zero point correction based on the difference between the measured and known optical densities; storing the zero point correction data in a memory; and correcting the measured density of a subject by using the zero point correction data read out from the memory. For providing the zero point correction data, the known standard optical densities are previously set in the densitometer.

Abstract: A method of measuring optical density is characterized by the steps of measuring reflected light from a subject being measured a plurality of times during a predetermined interval, and calculating a mean value of the plurality of measured values. For providing the zero point correction data, the known standard optical densities are previously set in the densitometer. A densitometer for practicing the calibrating method is formed compact and handy and comprises a housing in which measuring elements such as a light projecting member, a light receiving member and a reference light detecting member are incorporated. In the housing are further provided circuits for calculating a density based on outputs from the light receiving and reference light detecting members and for battery power checking. The housing includes a transparent panel formed with a measuring aperture by which a measuring area of a subject is defined.