Reading device and image forming apparatus

An image forming apparatus comprising:

• a conveyor configured to convey a sheet of recording medium to a contact glass;
• a reading device including
• a carriage configured to move in a sub-scanning direction with reference to an object placed on a contact glass,
• an optical sensor attached to the carriage, the optical sensor configured to scan the object placed on the contact glass, and
• a reference scale for computing a dimension of the object, the reference scale including a first reference scale extending in a main scanning direction and a second reference scale extending in a sub-scanning direction with reference to the object, the reference scale being outside a range of image acquisition in which the optical sensor scans the object as the carriage moves and inside a maximum movement range in which the carriage is movable and the optical sensor obtains the image of the object;
• and
• an image forming device configured to form the image of the object obtained by the optical sensor of the reading device on the sheet of recording medium.