1. Field of the Invention
The present invention relates to a color image forming apparatus in which images for respective color components are superposed in succession on a recording medium.
2. Related Background Art
Recently color printers have come into commercial use and are being utilized to create various color presentations. Particularly color page printers are attracting attention because of their quietness, high quality printing, and high speed in printing.
Among such color page printers, light (or lower) beam printers form color images by effecting a first step of scanning a photosensitive member with a light beam in a main scanning direction, effecting a first development to the image on the photosensitive member transferring the developed image from the photosensitive member onto a recording medium such as a recording sheet, and effecting second, third and fourth steps in succession in similar manner but with different colors, thereby recording a full color image.
In in more detail such a following there will be described the recording method for a color image, with reference to FIGS. 8 and 9.
At first a photosensitive drum 1, rotated at a predetermined speed in a direction indicated by an arrow in FIG. 8, is charged to a predetermined voltage of a predetermined polarity by a charger 4. Then, recording sheets P are fed one by one by a feeding roller 14 from a sheet cassette 15. When the leading end of the sheet is detected by a detector 2, a laser beam L modulated by an image signal VDO is emitted from a semiconductor laser 5 toward a polygon mirror 7 for diversion into scanning motion, and is guided through a lens 8 and a mirror 9 onto the photosensitive drum 1. A signal (referred to as TOPSNS hereinafter) from a detector 2 positioned at an end of the light scanning path is supplied, as a vertical synchronization signal, to an image data generation apparatus 50 (FIG. 8). The image signal VDO is supplied in succession to the laser 5, utilizing a BD signal to be described later, as a horizontal synchronization signal, succeeding to the TOPSNS signal.
A beam detect signal (referred to as the BD signal), constituting the horizontal synchronization signal, is obtained by detecting the laser beam L with a detector 17. The polygon mirror 7 is rotated by a scanner motor 6, which is so controlled, according to a signal S2 from a motor control circuit 25 provided with a frequency divider for dividing the frequency of a signal S1 from a reference oscillator 20, as to rotate with a predetermined constant speed.
Thus, the photosensitive drum 1 is scan-exposed in synchronization with the BD signal, and a first electrostatic latent image is developed with a developing unit 3Y to form a first toner image of yellow color on the photosensitive drum 1.
On the other hand, immediately before the leading end of the recording sheet P, fed at the predetermined timing, reaches a transfer start position, a predetermined transfer bias voltage of a polarity opposite to that of the toner is applied to a main body 28 of a transfer drum 16, whereby the recording sheet P is electrostatically attracted onto a surface 27 of the transfer drum 16, simultaneously with the transfer of the first toner image onto the recording sheet P.
Then, the photosensitive drum 1 is scanned with the laser beam L to form a second electrostatic latent image, which is developed with a developing unit 3M to form a second toner image of magenta color on the photosensitive drum 1. The second toner image is transferred onto the recording sheet P, in alignment with the first toner image already transferred onto the recording sheet P. The leading end of the image of each color is defined by the TOPSNS signal.
Similarly, a third electrostatic latent image is formed and developed with a developing unit 3C to form a toner image of cyan color, which is transferred, in registration, onto the recording sheet P. Subsequently, a fourth electrostatic latent image is formed and developed with a developing unit 3BK to form a black toner image which is transferred, in registration, onto the recording sheet P.
In this manner, the VDO signal of a page is supplied in succession to the semiconductor laser 5 for each step. Also, the untransferred toner is scraped by a cleaner 10 after each transfer step.
Subsequently, when the leading end of the recording sheet P, carrying toner images of four colors thereon, approaches the position of a separating finger 12, the finger 12 moves closer and touches the surface 27 of the transfer drum 16, thereby separating the recording sheet P therefrom. The front end of the separating finger 12 continues to be in contact with the transfer drum 16 until the rear end of the recording sheet P is separated from the transfer drum 16, and returns to the original position thereafter. A charger 11 eliminates the charge accumulated on the recording sheet P, thereby facilitating the separation thereof by the separating finger 12 and reducing the discharge in the air at the separation. The separated recording sheet P is discharged, by fixing rollers 13, onto a discharge tray 29.
FIG. 10 is a timing chart showing the relation between the above-mentioned TOPSNS signal and the VDO signal, wherein A1 to A4 respectively indicate the printing operations of the first to fourth colors, and sections A1 to A4 constitute the color printing operations of a page.
FIG. 11 is a timing chart indicating the timing of the BD signal and the VDO signal for respective colors with respect to the TOPSNS signal.
It is noted, in the above-mentioned conventional example, that there is generated an aberration of almost one l between the BD signals of the first and second colors, as indicated by (t2-t1), though the BD signals thereof are mutually aberrated only by a little. The aberration in colors is generated within a cycle time T1 of the BD signal from the leading end A of the TOPSNS signal. The aberration between the first and third colors, and that between the first and fourth colors respectively correspond to (t3-t1) and (t4-t1). Also, the VDO signal from A1 is aberrated by T2 from the TOPSNS signal, and the VDO signals from A2 to A4 are aberrated from the TOPSNS signal by (t2+T2), (t3+T2) and (t4+T2), respectively.
The color image finally fixed on the recording sheet has to have precise registration of different colors, but is deteriorated in quality in case that a color is significantly aberrated as described above. In FIG. 5, C4 indicates the first line of a color showing aberration, while C5 indicates the first line of another color, and l indicates the pitch of lines.
In the color recording, the precision of alignment of respective colors is generally important, as described in "Imaging Part 2", pages 38 to 39 (published by Shashin Kogyo Shuppan Co.).
In the human visual system, the contrast sensitivity is highest in a spatial frequency region of 50-100 dpi, and the sensitivity becomes lower as the spatial frequency increases. However, the contrast sensitivity is still practically high even in a range of 400-800 dpi. In correlation of the precision of registration and the image quality, which is rated as 100 points for the perfect quality, a quality of 95 points requires a precision of about 90 .mu.m while a quality of 100 points requires a precision of 75 .mu.m or less. Thus, in an equipment of 300 dpi, since one dot corresponds to 85 .mu.m, an aberration of one dot deteriorates the quality rating. Also, in an equipment of higher resolution, the aberration of one dot is detectable as described above, so that the print quality is deteriorated.
The tolerance in registration is far narrower in the characters and line images (which are binary, or black-and-white images than in multi-level images, and the aberration in the registration results not only in the deterioration of resolution but also in the aberration in the hue of fine lines, thus giving rise to deteriorated print quality.
Aberration in color can be prevented by improving the individual accuracy of the rotation control of the driving systems for the scanning optical system and the photosensitive and transfer drums. However it is almost impossible to improve the accuracy of the rotation control so as to eliminate aberration smaller than one l as described above.