Document size detection device, image reading device, image forming apparatus, and document size detecting method

A document size detecting method, comprising:

• controlling switching on and off of a light source that irradiates a document with light;
• and
• controlling movement in a sub-scanning direction of the light source;
• and
• determining a size in a main-scanning direction of the document based on image data acquired by an imaging device that receives light reflected at the document,
• wherein the determining step includes
• acquiring first image data when the light source is switched off, while the light source is at a first reading position;
• acquiring second image data when the light source is switched on, while the light source is at the first reading position;
• acquiring third image data when the light source is switched on, while the light source is being moved from the first reading position to a second reading position;
• and
• determining the size in the main-scanning direction of the document based on the acquired first image data, the acquired second image data, and the acquired third image data.

A non-transitory recording medium which, when executed by one or more processors, cause the processors to perform a document size detecting method comprising:

• controlling a switching on and off of a light source that irradiates a document with light;
• controlling movement in a sub-scanning direction of the light source;
• and
• determining a size in a main-scanning direction of the document based on image data acquired by an imaging device that receives light reflected at the document,
• wherein the determining step includes:
• acquiring first image data when the light source is switched off, while the light source is at a first reading position;
• acquiring second image data when the light source is switched on, while the light source is at the first reading position;
• acquiring third image data when the light source is switched on, while the light source is being moved from the first reading position to a second reading position;
• and
• determining the size in the main-scanning direction of the document based on the acquired first image data, the acquired second image data, and the acquired third image data.