Light is applied by a laser diode 121 onto a photosensitive roll 110 formed with a developed toner image 161, the surface potential of the photosensitive roll 110 to which the light is applied is measured with a surface potential sensor 122, and the toner amount of the developed toner image 161 on the photosensitive roll 110 is derived based on the measured surface potential.

La luz es aplicada por un diodo 121 del laser sobre un rodillo fotosensible 110 formado con una imagen de toner revelada 161, el potencial superficial del rodillo fotosensible 110 a el cual se aplica la luz se mide con un sensor potencial superficial 122, y la cantidad del toner de la imagen de toner revelada 161 en el rodillo fotosensible 110 se deriva basó en el potencial superficial medido.