Patent ID: 7194214

Claim:
A density measuring apparatus that measures density of an image formed on an image carrying body, comprising: a light-emitting device that irradiates light to an image area where the image is formed on the image carrying body and a non-image area where the image is not formed; at least one light receiving device that receives a reflected light of the light irradiated from the light-emitting device to one of the image area and the non-image area and outputs a voltage signal; an A/D converting device that converts the voltage signal from the light receiving devices into a digital value using first reference voltage and a second reference voltage as reference voltages; an electric charge accumulating device disposed on a path from the light receiving devices to the A/D converting device; a switch that applies one of the first reference voltage and the second reference voltage to a terminal of the electric charge accumulating device on input side of the A/D converting device; and a switch control device that closes the switch when the reflected light from one of the image area and the non-image area is received by the light receiving devices, and opens the switch right before the reflected light from the other of one of the image area and the non-image area is received by the light receiving devices, wherein the A/D converting device outputs a digital voltage signal, which is a measure of the density, the digital voltage signal being output when a signal is input when the switch is opened.