Patent ID: 8861047

Claim:
A document size detector configured to detect a size of a document placed on a platen, comprising: an illumination unit configured to emit light onto the platen; a photodetection unit configured to output a signal in accordance with a received light amount; a determination unit configured to determine an amount of light emitted by the illumination unit that is switched on when a size of a document placed on the platen is detected, based on the signal that is output from the photodetection unit in a state which the photodetection unit receives light from an external light source, wherein the determination unit determines the amount of light emitted by the illumination unit that is switched on based on the signal that is output from the photodetection unit in a state in which the photodetection unit receives light from an external light source while the illumination unit does not emit the light; and a document size detection unit configured to detect the size of a document placed on the platen based on the signal that is output from the photodetection unit under the illumination unit that is emitting the determined amount of light.