1. Field of the Invention
The present invention relates to improvements in correcting the misalignment of the three laser beams of three primary colors in a color laser image recording apparatus.
2. Description of the Prior Art
Laser printers record image information by modulating a laser beam according to image signals, and irradiating an image recording medium through an optical system by the modulated laser beam for exposure scanning.
FIGS. 4 and 5 show the respective basic constitutions of two conventional color laser printers. The color laser printer shown in FIG. 4 is of a three-tube type employing three lasers, namely, a laser 18 for red (hereinafter referred to as "R-laser"), a laser 19 for green (hereinafter referred to as "G-laser") and a laser 20 for blue (hereinafter referred to as "B-laser"), as the light source. The color laser printer shown in FIG. 5 is of a single-tube type employing a single He-Cd white-light laser 17 capable of emitting white laser light containing three primary color laser beams, namely, a R-laser beam, a G-laser beam and a B-laser beam.
In the three-tube type color laser printer, color laser beams emitted from the lasers are modulated by acoustooptic modulators (hereinafter referred to as "AOMs") 16, 16' and 16" for the three colors by color image signals, and then the modulated color laser beams, are reflected by dichroic mirrors 14, 14' and 14", respectively, disposed so as to collect the three color laser beams in a single laser beam. The single laser beam travels through an optical system and is deflected by a deflector for scanning operation to form a color image on the surface of a photosensitive material 13.
In the single-tube type color laser printer, the single laser emits R-laser beam, G-laser beam and B-laser beam in a single laser beam. However, the component color laser beams need individual intensity modulation by the corresponding color image signals. Accordingly, the laser beam is decomposed into a R-laser beam, a G-laser beam and a B-laser beam with dichroic mirrors 15, 15' and 15". The rest of the constitution is the same as that of the three-tube type color laser printer.
In either conventional color laser printer, the three modulated color laser beams need to be collected in a single beam and to fall on a focal plane in a single spot.
A sensor (beam detecting means) 1 which provides a horizontal synchronizing signal is provided before an image write starting point on the focal plane. Image write starting timing is determined by a synchronizing signal produced from the detection signal of the detection signal provided by the sensor 1.
FIGS. 6(a) to 6(c) are time charts showing a mode of synchronized scanning. FIG. 6(a) shows the waveform of the output signal of the horizontal synchronizing sensor 1, FIG. 6(b) shows a scanning synchronizing signal obtained by shaping the signal shown in FIG. 6(a), and FIG. 6(c) shows the timing and period of image write scanning.
In the conventional color laser printer, it has been difficult to collect three laser beams in a single laser beam so that the single laser beam will fall on the focal plane in a single spot. To make the three color laser beams fall on the focal plane in a perfect single spot, the three color laser beams need to be collected perfectly in a single laser beam at a very high accuracy by the dichroic mirrors 14, 14' and 14" (FIGS. 4 and 5). For example, when the diameter of the spot on the focal plane is 80 .mu.m, the focal length of the f.theta. lens is 200 mm and the allowable color beam deviation is a quarter of a dot, error in the respective angles of the dichroic mirrors 14, 14' and 14" (FIGS. 4 and 5) must be 1/180.degree. (20") or below, which requires a highly accurate dichroic mirror setting mechanism and highly skilled work and much time in setting and adjusting the dichroic mirrors.
Furthermore, in the color laser printer capable of multicolor recording through exposure scanning by means of a plurality of laser beams of different wavelengths such as a R-laser beam, a G-laser beam and a B-laser beam, the deviation of the spots of the laser beams on the focal plane attributable to chromatic aberration caused by the focusing lens, namely, the f.theta. lens, is a problem. Suppose that a laser beam B including laser rays of .lambda..sub.1 and .lambda..sub.2 in wavelength, respectively, is reflected by a polygonal rotating mirror 8 and is projected through the f.theta. lens 9 on a focal plane in a recording medium or a photosensitive material 13 as illustrated in FIG. 14. Then, the respective spots of the laser beam of .lambda..sub.1 and the laser beam of .lambda..sub.2 in wavelength are discrepant from each other due to color aberration caused by the f.theta. lens 9 as indicated continuous lines for the laser beam of .lambda..sub.1 and by broken lines for the laser beam of .lambda..sub.2 in wavelength in FIG. 14. Such discrepancy between the spots of the laser beams causes color spot deviation on the recording medium 13, which deteriorates the quality of the image, and the expansion of the spot reduces the resolution of the color laser printer. FIG. 15 illustrates a mode of color spot deviation diagrammatically. For example, a region A is irradiated by the laser beam of .lambda..sub.1 in wavelength while a region B is irradiated by the laser beam of .lambda..sub.2 in wavelength. Then, regions A.sub.1 and B.sub.1 are color deviation regions increasing the apparent width of a line scanned by the laser beam.
To obviate such a problem attributable to chromatic aberration, the following measures are taken.
(1) Employment of a perfect achromatic lens as the f.theta. lens (focusing lens).
(2) Correction of chromatic aberration by delaying the picture element clock. The correction is achieved through every-clock correction and within-clock correction. That is, data is delayed for every-clock correction while a clock for a D/A (digital-to-analog) converter is delayed for within-clock correction.
Referring to FIG. 16 showing a chromatic aberration correcting circuit, image data is given through a every-clock correcting circuit 21 to the D/A converter 22, and a picture element clock is given as a conversion clock through a within-clock correcting circuit 23 to the D/A converter 22. The every-clock correcting circuit 21 and the within-clock correcting circuit 23 are controlled by a control circuit 24. The control circuit 24 delays the data by controlling a clock to be given to the every-clock correcting circuit 21 for every-clock correction. On the other hand, the control circuit 24 controls the within-clock correcting circuit 23 to delay the conversion clock to be given from the within-clock correcting circuit 23 to the D/A converter 22 for within-clock correction.
The measure described in article (1) is impossible because no perfectly achromatized lens for three of more light rays of different wavelengths is available. Even an achromatized lens for two light rays of different wavelengths requires special materials for forming the same and highly accurate assembling. Accordingly, the yield of a process for manufacturing such an f.theta. lens is low and hence such an f.theta. lens is costly.
To execute the measure described in article (2), a beam deviated from a correct position due to chromatic aberration needs to be moved to the correct position through fine adjustment because the picture element clock is fixed. Referring to FIG. 17, suppose that a unit delay time of a delayed picture element clock from a basic picture element clock is .DELTA.t, then the respective delay times of the successive blocks are 2.DELTA.t, 3.DELTA.t, . . . , respectively, sequentially increasing by an increment of .DELTA.t. Accordingly, the correcting circuit 23 needs elements capable of increasing the delay time up to a period T by an increment of .DELTA.t such as, for example, a delay line and a selector 26 capable of sequentially selecting delayed clocks delayed by the delay line.
Referring to FIG. 18 showing an exemplary delay circuit, clocks given to a delay line 25 is delayed sequentially up to a delay time T by an increment of .DELTA.t. The delayed picture element clocks are selected by a selector 26 and an optimum delayed picture element clock is provided for every block. However, the delay line is expensive, for example, 4000 yen for a piece of 8-tap delay line, and the accuracy of the delay line, for example, .DELTA.t.+-.5%, is not satisfactory. Furthermore, this method is able to shift the focal position backward by delaying the picture element clock, but is unable to shift the focal position forward.