1. Field of the Invention
The present invention relates to an image reading apparatus including a reading unit configured to read an image on a recording medium and a calibration member for acquiring calibration correction data.
2. Description of the Related Art
Conventionally, a sheet through type scanner that conveys a document to be read by a fixed reading unit reads a white reference region disposed on an opposite surface of the reading unit to acquire white calibration correction data. As this white reference region, a surface of a roller-shaped calibration roller may be used.
However, in a conventional unit configured to acquire the white calibration correction data, there is a problem that, if a stain is adhered to the white reference region on the calibration roller, a sufficient correction effect cannot be obtained using the white calibration correction data acquired from the stained white reference region. Particularly, in the sheet through type scanner which combines the calibration roller that has the white reference region and a roller for conveying a document, a stain may be adhered to the white reference region due to conveyance of the document.
In an image reading apparatus discussed in Japanese Patent Application Laid-Open No. 5-319613, a control technique is provided in which a groove is formed on a part of the periphery of a reading roller and a bottom of the groove serves as a white reference region for calibration correction, and the white reference region is not stained due to conveyance of a document.
In an image reading apparatus discussed in Japanese Patent Application Laid-Open No. 2002-152510, a following control method is provided. Specifically, a fixed range on a surface of a reading roller which serves as a white reference for white calibration correction is read by a charge coupled device (CCD) image sensor. The read image data is divided into m pieces of blocks for each L line to calculate an average value of image data on each line of each block. The average value of each block is compared with each other and a peak value among all average values is determined as white calibration data. Accordingly, even if a stain is adhered to a white reference region, a correction effect can be obtained.
However, the above described prior art has the following problems.
In Japanese Patent Application Laid-Open No. 5-319613, since the white reference region is the bottom region of the groove on the calibration roller, a distance between an image sensor and a document read by the reading unit is different from a distance between the image sensor and the bottom of the groove on the calibration roller which is the white reference region. Thus, in the reading unit provided with the image sensor shallow in a focal depth, there is a problem that the white reference region is out of an in-focus position and the sufficient correction effect cannot be obtained in the white calibration correction data to be read. Further, in order to align the white reference region with the in-focus position, there is a problem that a mechanism to travel the calibration roller to align the groove portion on the calibration roller with a document reading position is required.
In Japanese Patent Application Laid-Open No. 2002-152510, since a distance from the center of the calibration roller to a conveyance path of the document and a radius of the white reference region for reading the white reference data for calibration correction on the calibration roller are the same, distances are consistent. However, when the document is conveyed, there is a problem that the calibration roller may be brought into contact with the document.