1. Field of the Invention
The present invention relates to an image reading apparatus that reads an original image with a CCD line image sensor and outputs image data to a utility instrument, for example, an image forming apparatus such as a digital copier, a facsimile, etc., after converting to and correcting a digital signal. In particular, the present invention relates to an image reading and forming apparatus capable of correcting the image data in accordance with density adjusting data obtained by comparing a density of a reference white plate with that of a reference original even when the density of the reference white plate varies.
2. Discussion of the Background Art
In a current digital copier, a CCD sensor reads an image when an optical unit is moved in a sub scanning direction perpendicular to a main scanning direction while an original image receives exposure from a light source, namely, an original surface is read. An analog image signal is obtained from the original and is then converted to a digital signal. The digital signal is then corrected in various manners. There exists a so-called shading correction that is a process for eliminating unevenness (existing in a widthwise direction of a photo acceptance unit) and for making an output from each element of the photo acceptance unit substantially the same. Such widthwise unevenness is generally caused by one of unevenness of sensitivity of each element in a CCD line image sensor and that of an amount of a transmission light through an imaging lens toward the CCD. Unevenness and degradation of an illumination optical unit can also be a cause thereof.
The shading correction process illuminates a reference white plate 101 (see FIG. 1) arranged in the vicinity of an original document setting and reading section prior to an actual reading of the original, and then reads a reflected light from the reference white plate. The shading correction process then obtains white level reference data to be used in a shading correction process for respective pixels. Then, a prescribed correction calculation is performed. Specifically, data read from the original is corrected by the correction data and also receives shading correction.
To precisely perform shading correction, a density of the reference white plate should be normalized within a standard original reading density range even if there exists density unevenness among reference white plates. Specifically, when unevenness of a density inherently existing in a reference white plate attached to an apparatus or caused by a time elapse is large, image data indicating a density is affected. As a result, a density varies even after the shading process is performed.
To resolve such a problem, the density unevenness of a reference white plate is normalized by setting a different reference potential to the A/D converter (which generally performs A/D conversion based upon a reference potential). Specifically, even if unevenness exits, outputs after the A/D conversion are adjusted to be constant (i.e., adjusted within a standard original reading density range).
FIG. 7 illustrates an exemplary background circuit employing a system for adjusting density unevenness caused when a conventional reference white plate is used. As shown, an analog image signal read by a one-dimensional line state CCD 201 is processed by all of wave-fairing, sample holding, and amplifying signal processing circuits 202, and is then converted into digital data by the A/D converter 203. Subsequently, the digital data is input to the shading correction circuit 207.
Different reference voltages Ref1 202 and Ref2 205 are used and applied to the A/D converter 203 respectively when the reference white plate and an original are read. Specifically, a reference original is used so as to define a readable density limit as a standard that is referred to when unevenness existing in a reference white plate is adjusted and its output is normalized. A prescribed reference potential is obtained by adjustment of an adjustment device (not shown) so that an output generated when the density uneven reference white plate is read at the prescribed reference potential substantially equals that obtained when a reference original is read at Ref1. Then, such a prescribed reference potential is used as a reference potential “Ref2” of the A/D conversion when an original is actually read. Thus, different reference potentials Ref1 and Ref2 are prepared respectively when the reference white plate and original are read, and a selector 206 is required so as to alternately set one of these potentials to the A/D converter 203 when respective reading operations are performed.
However, because different reference potentials and a device for changing and selecting one of such potentials are required, a circuit is complex, parts vary, and noise is generated. In addition, the costs of the device increase (e.g., expensive parts are required when highly precise adjustment is performed).