Image reading device, pressure plate opening/closing detection method, and image forming apparatus

A method of detecting opening/closing of pressure-plate, used for an image reading device including

• a contact glass having a surface on which a document is disposed or along which the document moves,
• a reading unit including a light source configured to emit light to the document on the surface of the contact glass and an image sensor configured to receive the light reflected by the document, the reading unit configured to obtain image data according to the light received by the image sensor,
• a pressure plate moveable between a posture where the surface of the contact glass is closed and a posture where the surface of the contact glass is opened, and
• a pressure-plate opening/closing sensor configured to detect whether the pressure plate is closed,
• the method comprising:
• determining a presence or absence of external light input to a space between the pressure plate and the surface of the contact glass based on black reference image data obtained by the reading unit when the light source is turned off;
• comparing read white data of the light emitted from the light source and received by the image sensor when the pressure plate is closed, with reference white read data of the light emitted from the light source and received by the image sensor when the pressure plate is completely closed, to determine a difference between the read white data and the reference read white data;
• and
• determining whether the pressure plate is open or closed based on the determined presence or absence of the external light when the light source is turned off and the difference between the read white data and the reference read white data.