densitometer
A scientific instrument used to measure optical density, concentration of a substance, or color density in fields such as photography, printing, and chemistry.
この濃度計はフィルムの透過濃度を測定します。
This densitometer measures the transmission density of film.
Compound of 濃度 (のうど, 'concentration, density') and 計 (けい, 'meter, gauge').