Image reader unit and image forming apparatus including same

An image reader unit comprising:

• a platen on which a document is placed and including a document reference line at an edge of the platen;
• a plurality of split light sources aligned in a main scanning direction along a surface of the platen, the plurality of split light sources configured to emit light beams toward the platen;
• a movable exposure unit configured to move in a sub-scanning direction perpendicular to the main scanning direction while obtaining a reflected light reflected from the surface of the document;
• an image sensor configured to receive the reflected light;
• and
• a reader processor configured to
• identify a length of the document in the main scanning direction based on a received light quantity from the image sensor obtained by activating fewer than all of the split light sources, the activated light sources being a plurality of blocks that are each a distance greater than zero millimeters from the document reference line, and
• perform a lighting and light-receiving process to simultaneously light only split light sources used for identifying the length of the document in the main scanning direction among all of the plurality of split light sources and obtain a received light quantity of the image sensor as to each of the split light sources lit;
• and
• identify the length of the document based on the received light quantity.