1. Technical Field
Aspects of the present invention relate to an image reading device having a function of updating correction data used to correct image data.
2. Related Art
Image reading devices configured to read an image formed on a document have been widely used. In general, such an image reading device is configured to illuminate a document, to read light reflected from the document through an image pick-up device (e.g., a CCD; Charge Coupled Device) converting the received light into a voltage value, and to convert the voltage value to digital data.
In order to correct unevenness of the intensity distribution of image data along a main scanning direction due to, for example, the difference in a light amount of the light source between a central portion and each edge portion along the main scanning direction, the variations in light amount of the light source with time or unevenness of performance of the CCD along the main scanning direction, the image reading device returns a reading unit to a home position to read a white reference plate through the CCD to obtain the distribution of the light amount in the main scanning direction so that the shading correction for correcting the unevenness of the image data in the main scanning direction can be executed.
In order to correct change of the light amount with time, it is required to return the reading unit to the home position each time a sheet of the document is read so that data of distribution of the light amount is obtained again. By thus updating the data of distribution of the light amount each time a sheet of the document is read, it is possible to properly correct the image data of each page of the document so that the unevenness of the image data can be suppressed. An example of such an image reading device is disclosed in Japanese Patent Provisional Publications Nos. 2002-330269 and HEI 11-46289. However, such a configuration requires a relatively long time to execute a reading operation for a document having a larger number of pages because in this case the reading unit reciprocates a plurality of times between a read position and the home position.
Japanese Patent Provisional Publication No. HEI 11-196266 (hereafter, JP HEI 11-196266A) discloses an image reading device configured to have a white reference plate at a position outside an image scanning range in a main scanning direction. According to a configuration disclosed in JP HEI 11-196266A, the image reading device reads the white reference plate through a part of a CCD situated outside the image scanning range to evaluate change of the light amount. Such a configuration makes it possible to evaluate change of the light amount without the need of returning the reading unit to the home position. That is, the image reading device returns the reading unit to the home position only when the update of the distribution of the light amount becomes necessary.