(1) Field of the Invention
The present invention relates to an image reader in a scanner, a unit scanner, a facsimile and the like installed in an image forming apparatus such as a digital copying machine and the like.
(2) Description of the Prior Art
In an image reader in a scanner, conventionally, shading correction is performed prior to the read operation of an original document image. Shading correction is intended for correcting the difference in sensitivity between pixels of a sensor (CCD) which reads an image based on the reflected light from the original document, and for correcting nonuniformity in the quantity of light of a light source comprising a plurality of lamps.
The difference in sensitivity between pixels of the readout sensor and nonuniformity in the quantity of light of the light source cause difference in the image density, and considerably decrease the reproducibility of the halftone image.
With the shading correction, a standard white board serving as a basis of correction is first read, then an image data including the difference in sensitivity between each pixel of the CCD and nonuniformity in the quantity of light of the light source is obtained as the shading data, and the correction value with respect to the image data is determined for each pixel based on this data, to thereby determine the threshold for the white/black judgement.
On the other hand, models installed with a sheet-through type automatic document feeder are recently increasing, and such an image forming apparatus includes a document fixing mode in which an original document is put on an original table, and the document image is read while an optical system is moved along the original table, and a document moving mode in which the original document is read by the optical system located in place, while the original document is moved by means of the sheet-through type automatic document feeder. For example, in Japanese Patent Publication Hei 10 No. 200711, as shown in FIG. 1, a standard white board 82 is set between a moving document read section for reading an original document moved by the automatic document feeder 80 and a fixed document read section for reading a fixed document put on the original table 81, and the shading correction is performed in the document fixing mode and the document moving mode, using the standard white board 82.
With this construction, however, at the time of continuous read in the document moving mode, problems described below occur.
In the case of the document fixing mode, the optical system 83 returns to the home position 84 of the optical system 83 every time when the optical system reads one sheet of document. Therefore, even in the case of continuous readout, white board 82 is irradiated to perform the shading correction every time each document is read, hence the threshold of the white/black judgement does not shift.
However, in the case of the document moving mode, when the optical system 83 initiates read in of the first document, the optical system 83 moves from the home position 84 to the moving document readout position, hence after the reflected light of the white board 82 has been read at the home position for the first document, and the shading correction has been performed, the correction must be performed always at the same correction value.
However, when dozens of documents are read continuously, for example, at the time of continuous copying, since the temperature of the light source increases, the quantity of light decreases, and with the same correction value as that of the first document, the threshold of the white/black judgement shifts. As a result, such problems are caused that the reproducibility of the halftone image decreases and that fogging is caused in the reproduced image.
In addition, even in the document moving mode, this problem can be solved by returning the optical system 83 to a predetermined home position 84 for each document to read the white board 82 and performing the shading correction, but such an operation not only conspicuously decreases the document read rate but also leads to a problem of precision of the document unit and complicated control.
Therefore, with the conventional image reader, in the document moving mode, the data for shading correction is not reacquired during continuous readout.