Abstract: A transmission densitometer determines the optical density of an electrically charged layer of toner material on a photoconductive layer by exposing the photoconductive layer to radiation through the toner layer such that the amount of exposure is characteristic of the density of the toner layer. The change in voltage on the surface of the toner layer caused by the exposure is detected, such that the change in detected voltage is characteristic of the density of the toner layer.The measurement of the change in the detected voltage is carried out by measuring the characteristic voltage on a toner layer developed upon a photoconductive layer using a first electrostatic voltmeter, discharging the characteristic voltage on the area of the photoconductor which was measured by the first electrostatic voltmeter using the exposing device, and measuring the remaining voltage in that area by using a second electrostatic voltmeter.