Document reading unit, image forming apparatus incorporating same, and method of detecting original document using with same

A document reading unit comprising:

• an original document table on a surface of which an original document is placed;
• a moving irradiation unit having multiple block light sources aligned in a main scanning direction of a surface of the original document and to move in a sub scanning direction that is a direction along the surface of the original document table and perpendicular to the main scanning direction while irradiating light emitted by the multiple block light sources to the original document placed on the original document table and obtaining reflected light reflected on the original document;
• an image sensor to receive reflected light from the original document irradiated by the moving irradiation unit and read an image formed on the original document;
• and
• a controller configured to perform a length specifying operation to specify a length of the original document in the main scanning direction,
• wherein the controller starts the length specifying operation in a state in which the moving irradiation unit is located at a position facing the original document placed on the original document table, sequentially turns on the multiple block light sources one by one, obtains an amount of reflected light received by the image sensor under respective lighting conditions of the multiple block light sources, and determines the length in the main scanning direction of the original document based on the obtained amount of reflected light,
• wherein the controller causes the image sensor to receive the reflected light reflected on a region having a width in the sub scanning direction of the original document placed on the original document table under the multiple lighting conditions during the length specifying operation while causing the moving irradiation unit to move in the sub scanning direction.