Reading device and image forming apparatus

A reading device comprising:

• a carriage movable in a sub-scanning direction;
• an optical sensor mounted on the carriage, the optical sensor being configured to scan an object placed on a contact glass;
• a reference scale used as a reference when a dimension of the object is computed based on an image obtained as the optical sensor scans the object;
• a flat gauge to be scanned by the optical sensor to calculate a corrective value used to correct the image obtained by the optical sensor;
• and
• circuitry configured to
• calculate the corrective value based on a scanned image including the reference scale and the flat gauge obtained by the optical sensor, and
• correct, based on the corrective value, a measurement image including an image of the object and an image of the reference scale obtained by the optical sensor and compute the dimension of the object based on the corrected measurement image,
• wherein the reference scale extends in a main scanning direction orthogonal to the sub-scanning direction outside a range of image acquisition in which the optical sensor scans the object to obtain the image of 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
• wherein the flat gauge is arranged on the contact glass inside the range of image acquisition, where a plurality of first reference lines drawn on a face of the contact glass are oriented in the main scanning direction.