Method for setting black reference data used for excess exposure, and image reading apparatus

A black reference data setting method for excess exposure comprising:

• reading an outline of a document by an image pickup device having multiple pixels;
• setting a gamma curve that is consonant with the document based on a value of an electric signal outputted from the image pickup device at the reading step;
• determining whether the gamma curve is included in a predetermined area;
• determining a constant c constituting a magnification rate along an output axis of the gamma curve so that the gamma curve is included in the predetermined area;
• setting the exposure time for the image pickup device to constant c times the basic exposure time when determination that the gamma curve is outside the predetermined area is made;
• and
• reading a black reference in accordance with the exposure time set at the setting step, and preparing black reference data.

An image reading apparatus comprising:

• a light source for projecting light onto a document;
• an image pickup device for receiving light from said document and for outputting an electric signal that is consonant with the amount of light that is received;
• a document identification unit for determining whether the document is an excess exposure document requiring excess exposure based on the electric signal output from the image pickup device;
• an exposure time setting unit for, when the document identification unit determines that the document is an excess exposure document, setting an exposure time for the image pickup device that is a predetermined number times a basic exposure time;
• and
• a black reference data preparation unit for permitting the image pickup device to read a black reference so that an exposure time for reading the black reference is equal to the exposure time set by the exposure time setting unit, and for preparing black reference data.