Patent ID: 7435946

Claim:
A mark detector to detect a plurality of marks which are arranged in a predetermined cycle as a scale on a moving member using a light onto the scale, the mark detector comprising: a plurality of optical heads, each of the optical heads including an illuminator to irradiate the scale including a light source to irradiate a light beam, and a beam shaping device to shape the light beam into a predetermined beam, and a photo acceptor to accept the light from the scale and transfer the light into an electric signal having a phase that differs from electric signals generated by others of the plurality of optical heads; and a comparator to compare the plurality of electric signals received from the photo acceptors in the plurality of optical heads and to convert the electric signals to binary signals, wherein a plurality of the light beams irradiated from said plurality of illuminators have irradiated positions shifted on the scale and have different angles in a plane perpendicular to a moving direction of the scale.