JPS63147174A - Optical writing controller - Google Patents

Abstract

PURPOSE:To make dots formed on a photosensitive body uniform in size and to eliminate an irregularity in density and the inversion of the density by varying the quantity of a laser beam according to the main-scanning-direction position of the laser beam projected on the photosensitive body. CONSTITUTION:The projection beam of a laser diode 1 is converged on the photosensitive body 5 through an ftheta lens 4 and scanned in a main scanning direction. Further, this beam is photodetected by a photodiode 7 through a mirror 6 prior or the main scan and sent to a control circuit 8. A beam spot formed on the photosensitive body 5 varies in main-scanning-directional diameter with the angle of the laser beam incident on the photosensitive body 5 and a control circuit 8 adjusts the time of beam irradiation per clock so that the beam diameter is constant. Thus, the quantity of the laser beam is varied corresponding to the main-scanning-directional position of the laser beam to uniform the size of dots, thereby eliminating the density irregularity and the inversion of the density.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to an optical writing control device in a laser printer.

(Prior Art) In a conventional laser printer, an image is written by scanning a photoreceptor with a laser beam, but the incident angle of the laser beam on the photoreceptor changes depending on the main scanning position. Since no correction is made for this change in the incident angle of the laser beam, the shape of the spot formed by the laser beam on the photoreceptor differs between the scanning end and the center. When outputting a gradation image using a pseudo-halftone expression method that expresses shading by a small difference in the number of dots per fixed area (black area ratio), the change in the shape of the spot mentioned above causes a change in the size of the dot formed on the photoreceptor. Then, since the density level is changed, density unevenness and density reversal phenomenon (one-degree gradient is reversed between input image data and output image density) occur, making it impossible to express correct gradation.

As shown in FIG. 6, when the entire surface divided into a plurality of regions is outputted with a laser beam at the same concentration, and the average concentration for each region is plotted by opening a hole, the result is as shown in FIG. 7. Here, area numbers are sequentially assigned to each area from left to right, and this is done sequentially for each row from top to bottom. From FIG. 7, it can be seen that the average density of the area has periodic density fluctuations in the horizontal direction (main scanning direction). Further, as shown in FIG. 8, when looking at the average density of each area grouped by column, it is observed that the density decreases at both ends in the main scanning direction.

(Objective) It is an object of the present invention to provide an optical writing control device that can eliminate the above-mentioned drawbacks and prevent density unevenness and density reversal phenomena.

(Structure) The present invention is directed to a laser printer that outputs gradation images using a pseudo-halftone expression method, in which the light intensity of the laser beam irradiated onto a photoconductor is changed in response to the position in the main scanning direction of the laser beam. It is characterized by uniformizing the size of dots formed on the photoreceptor.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an example of a laser printer to which the present invention is applied.

The output beam of the laser diode l passes through the collimator lens 2
collimated by and deflected by the rotating polygon mirror 3, fθ
The light is focused on the photoreceptor 5 through the lens 4 and scanned in the main scanning direction. The photoreceptor 5 is rotationally driven, and the beam from the fθ lens 4 is applied to a mirror 6 before main scanning of the photoreceptor 5.
The light is received by the photodiode 7 via the photodiode 7. The signal output from the photodiode 7 at this time is used as a line synchronization signal and sent to the control circuit 8.

As shown in FIG. 2, the diameter a of the beam spot formed on the photoconductor 5 in the main scanning direction varies depending on the incident angle θ of the laser beam 9 on the photoconductor 5, and this diameter a is a function of 0 and is expressed as secθ. Proportional. Therefore, especially in a laser printer that uses a compact f.theta. lens 4 with a short focal length, the maximum value of O becomes large and fluctuations in the diameter a cannot be ignored.

Therefore, in this example, the beam irradiation time per clock is adjusted by the control circuit 8 so that the beam diameter a in the main scanning direction, which varies depending on θ, remains constant. In this case, as shown in Fig. 3, the clock CLK is counted based on the line synchronization signal S Y N C, and the pulse width PW corresponding to the laser irradiation time at which the beam diameter a becomes appropriate is set corresponding to that value. Then, a laser beam is generated for an irradiation time EXP according to the binary image data DATA.

FIG. 4 shows the configuration of the control circuit 8. As shown in FIG.

The write data DATA is input from a circuit not shown in synchronization with the write tarlock CLK.
ATA is for outputting gradation images using a pseudo-halftone expression method. The pseudo halftone expression method is a method of changing the black area ratio by changing the number of black dots per minute fixed area, and is generally represented by the dither method and the one-time pattern method.

Before valid data is input as write data DATA, a line synchronization signal 5YNC from the diode 7 is input, and the addition/subtraction counter 10 and flip-flop 11 are reset by this signal S'/NC. Mono multivibrators 121-128 are triggered by clock CLK, and capacitors 131-138
and generates eight types of pulses with different pulse widths set by resistors 141 to 148. On the other hand, the clock CLK
is input to the addition/subtraction counter 10 and counted, and the upper 3 bits of the count data cause the multiplexer 15 to select 8 from the mono multivibrators 121 to 128.
Select one of the types of pulses. The value of the counter 10 is compared with the value set in the digit register 16 consisting of dip inches by the comparator 17, and when they match, the flip-flop 11 is set by the output signal of the comparator 17, and the non-inverted output of the flip-flop 11 is set. 18 can be opened. The flip-flop 11 and the counter 10 are reset each time the line synchronization signal 5VNC is input, and the gate 19 is opened by the inverted output of the flip-flop 11. Clock CLK is line synchronization signal 5
Each time VNC is input and the flip-flop 11 is reset, it passes through the gate 19 and is incremented by the counter 10.

The flip-flop 11 is set by the output signal of the comparator 17, so that the output signal passes through the gate 18 to the counter 1.
Subtracted by 0. The clock set in the digitizer 16 corresponds to the clock number (the clock number after the line synchronization signal is input) when the laser beam from the fO lens 4 reaches the center of the photoreceptor 5 in the main scanning direction. The write data DATA is then amplified by the amplifier 21 through the gate 20 by the output pulse of the multiplexer 15, and is output to the semiconductor laser drive circuit as an exposure signal section xP to drive the semiconductor laser 1.

The output pulse widths of the mono-multivibrators 121 to 128 are set to pulse widths that sequentially change according to θ associated with the main scanning of the laser beam, and the beam diameter a is made constant.

In a laser printer, the intensity distribution of the laser beam from the laser diode 1 can be normalized to a Gaussian distribution curve, and as shown in Fig. 5, the intensity distribution of the laser beam from the laser diode 1 is 1/e2 of the maximum intensity.
The width dO corresponding to the level of is set as the beam diameter. Therefore, by changing the intensity of the laser beam, the effective beam diameter can be changed, and by changing the intensity of the laser beam from Io to E, the beam diameter can be changed to dO.
can be changed from to d. In this case, the shape of the laser spot will be similar, but the size (area) of the laser spot can be controlled to be constant6.In a normal laser printer, the incident angle O is Oo at the center point of the main scan on the photoreceptor.
Therefore, it is left and right symmetric. Therefore, the capacity of the counter for changing the power of the laser beam can be reduced to 1/2, and the counter can switch from addition to subtraction (or from subtraction to addition) with the center point (θ=0°) as the boundary.

FIG. 6 shows a control circuit in another example of a laser printer to which the present invention is applied. In this example, write data DATA is input directly to the amplifier 21 in the above-mentioned laser printer, and the power of the laser beam is controlled by a control circuit. In the control circuit, a counter 10. Flip-flop 11. Number register 16. The comparator 17 is similar to the above example, and the counter 10 enters the addition mode due to the inverted output of the flip-flop 11, and enters the subtraction mode due to the non-inverted output of the flip-flop 11, counting the clock CLK. The value of counter 10 is digital/
The analog converter 22 converts it into an analog quantity, and the power adjustment circuit adjusts the laser beam power of the laser diode 1 according to this analog quantity, thereby making the laser spot size constant.

(Effects) As described above, according to the present invention, in a laser printer that outputs a gradation image using a pseudo halftone expression method, by changing the light intensity of the laser beam in accordance with the position of the laser beam in the main scanning direction, Because it equalizes the size of the dots formed on top.

Density unevenness and density reversal phenomena can be prevented.

[Brief Description of the Drawings] Fig. 1 is a schematic diagram showing an example of a laser printer to which the present invention is applied, Fig. 2 is a diagram for explaining the laser printer, Fig. 3 is a timing chart of the laser printer, Figure 4 is a block diagram showing the control circuit of the laser printer.
FIG. 5 is a diagram for explaining another example of a laser printer to which the present invention is applied, and FIG. 6 is a block diagram showing a control circuit of the laser printer. lO...Counter, 11...Flip-flop, 121-128...Mono multivibrator,
15...multiplexer, 16...digitizer, 1
7...Comparator, 18-20...Gate. Ding υ Figure XP --- Crow 4 Figure b D Figure

Claims (1)

[Claims] In a laser printer that outputs gradation images using a pseudo-halftone expression method, dots are formed on a photoconductor by changing the light intensity of the laser beam in accordance with the position in the main scanning direction of the laser beam irradiated onto the photoconductor. An optical writing control device characterized by uniformizing the size of.