This invention relates to an image scanning unit that is used for a stationary-manuscript reading type of device such as a facsimile, a copying machine and a digital image inputting apparatus.
FIG. 1 shows the basic composition of a conventional image scanning unit.
In FIG. 1, a full-speed scan reciprocating bed 2 is equipped with a light source 3 that supplies light to a manuscript laid on a glass template 1, and a mirror 4 that changes the optical path of its reflected light, and it moves under the glass template 1 to scan the manuscript length in the sub-scan direction. A half-speed scan reciprocating bed 7 is equipped with a pair of mirrors 5, 6 each of which changes 90xc2x0 the optical path of light reflected on the mirror 4, and it moves synchronizing at the half speed of the full-speed scan reciprocating bed 2. Light reflected on the mirrors 5, 6 forms an image onto a photoelectric converting element (CCD) 9 through a lens 8, converted into an electrical signal. The electrical signal is then converted into the image information of manuscript through an image processing circuit (not shown).
The full-speed scan reciprocating bed 2 is coupled with a wire rope 10 at its balance position, and the half-speed scan reciprocating bed 7 is coupled through at least one pair of sheaves 11 with the wire rope 10 at its balance position. By moving the wire rope 10 being winded around a drum 12 through the rotation of the drum 12, the full-speed scan reciprocating bed 2 and half-speed scan reciprocating bed 7 move in reciprocation. The drum 12 is driven by a motor 15, through a shaft 13 and a transmission gear system 14.
In the image scanning unit shown in FIG. 1, the scan-speed ratio between the mirror 4 and the mirrors 5, 6 is theoretically 2:1, therefore the distance between the manuscript laid on the glass template 1 and the lens 8 is kept constant and the plane of image formed on the photoelectric converting element 9 is made constant regardless of the scan. However, due to the mechanical variation of drive system such as a diameter tolerance of the drum 12 and a line-diameter of the wire rope 10, or due to an inclination of optical visual-line occurring when the full-speed scan reciprocating bed 2 and half-speed scan reciprocating bed 7 are scanned, there occurs a variation in magnification in the sub-scan direction (direction of the manuscript length).
Of the factors to cause such a variation in magnification in the sub-scan direction, the mechanical variation of drive system can be suppressed by enhancing the precision of parts. On the other hand, the inclination of optical visual-line occurring when the full-speed scan reciprocating bed 2 and half-speed scan reciprocating bed 7 are scanned is very difficult to adjust, since it results from the optical positional relationships among the mirrors 4, 5 and 6, lens 8 and photoelectric converting element 9.
A conventional method employed is to adjust the speed of motor driven so as to cancel a variation in magnification according to the enlarging or reducing of recording image or scan data obtained by scanning a manuscript. However, this method requires to pre-detect the variation in magnification in the sub-scan direction in the image scanning unit, therefore incurring a problem that the number of steps increases.
Meanwhile, Japanese patent application laid-open No.62-172338 (1987) discloses a magnification-controlling copying machine that a reference-size pattern is received by an image sensor, comparing its output signal with a designated copying magnification, moving a zoom lens in such a direction that the error is minimized, thereby enhancing the precision of copying magnification. However, the correction of copying magnification by the movement of the zoom lens is only for the main-scan direction and is difficult to apply for the sub-scan direction.
Also, Japanese patent application laid-open No.60-114082 (1985) discloses an image processing unit that the magnifications in the main-scan and sub-scan directions can be independently controlled. However, the correction of magnification in the sub-scan direction is conducted by detecting the reference coordinates on the template at previous scan, then controlling the DC servomotor. Therefore, it is necessary to position the reference points in the wide range on the template.
Accordingly, it is an object of the invention to provide an image scanning unit that can automatically correct the enlarging and reducing of magnification in the sub-scan direction to occur in response to an inclination of optical visual-line during the scanning.
According to the invention, an image scanning unit for correcting a variation in magnification caused by an inclination of optical visual-line between a manuscript and a photoelectric conversion element when a full-speed scan reciprocating bed and a half-speed scan reciprocating bed are scanned in the sub-scan direction, comprises:
an optical system that leads light reflected on the manuscript to the photoelectric conversion element;
a marking that is disposed on the optical visual-line of the full-speed scan reciprocating bed;
a light source that supplies light to the marking;
means for accumulating marking information that is input through the optical system to the photoelectric conversion element and detecting an inclination of optical visual-line by comparing the accumulated marking information and regular marking information at regular optical visual-line; and
means for canceling the variation in magnification in the sub-scan direction caused by the inclination of optical visual-line detected by the accumulating and detecting means.