Image reading device, image forming apparatus, and image reading method

An image reading method for, in an image reading device including:

• a conveying portion configured to convey a sheet;
• a first reading portion disposed in the conveying portion and configured to read an image of a first surface of the sheet conveyed by the conveying portion;
• a second reading portion disposed below a contact glass in such a way as to be reciprocally movable along a scanning direction and configured to read an image of a second surface of the sheet;
• a first white reference plate disposed in the conveying portion to face the first reading portion;
• and a second white reference plate disposed in the conveying portion at a reference position that can face the second reading portion, reading the image, the image reading method comprising
• a control step of lighting a first light source included in the first reading portion and generating first white reference data by using reflected light reflected from the first white reference plate, and lighting a second light source included in the second reading portion and generating second white reference data by using reflected light reflected from the second white reference plate, wherein
• in the control step, the first light source and the second light source are controlled such that a lighting time of the first light source does not overlap with a lighting time of the second light source.