Patent ID: 8873990

Claim:
A toner density sensor, comprising: a light emitting part that emits light in order to detect a toner density; a light receiving part that receives reflected light having been applied from the light emitting part and reflected at a detection target; a light path formed along the front surface of a substrate on which the light emitting part and the light receiving part are surface-mounted, the light path having an irradiation path that allows passage of light emitted from the light emitting part and a light receiving path that allows passage of the reflected light; and a diaphragm unit that is formed at the light path and partially narrows the light path, wherein an upper case that covers the light emitting part and the light receiving part, and forms the light path and a diaphragm unit upper portion, is provided on the front surface of the substrate, the diaphragm unit upper portion being one of upper and lower portions of the diaphragm unit, a lower case that is coupled with the upper case to hold the substrate therebetween is provided on the rear surface of the substrate, a protrusion part with its top end provided with a diaphragm unit lower portion is formed in the lower case, the diaphragm unit lower portion being one of upper and lower portions of the diaphragm unit, and a through hole is formed in the substrate, the hole allowing the diaphragm unit lower portion to protrude from the front surface of the substrate and penetrating in a thickness direction of the substrate.