JPH03222768A - Laser-scanning type image forming device - Google Patents

Abstract

PURPOSE:To obtain a high-resolving power and high-gradation image with a simple construction without raising a clock frequency by a method wherein pixels of a class corresponding to a spot of a set size are selected by the amount of one line to be outputted to an optically writing device, and multi-writing operations are performed by the number of classes while being successively changed over at every line writing. CONSTITUTION:When image data for one page is inputted, a pixel sorting means 2 sorts pixels into predetermined classes in accordance with the gradation data thereof. Under the control of a line multi-writing control means 5, a spot size setting means 3 sets a spot size (i) corresponding a certain class (i), a line pixel selecting means 4 selects the data of the class (i) for one line and outputs them to an optically writing device 1, and the data is optically written. A line multi-writing control means 5 performs the aforesaid operation successively n-times and, thereby, writes a multi-gradation image for one line, furthermore repeatedly writing a multi-gradation line image on a photosensitive body relatively traveling in a sub-scanning direction to form a multi-gradation image for one page.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a laser scanning image forming apparatus, and particularly to a laser scanning image forming apparatus capable of forming a multi-tone image.

[Conventional technology]

Personal computer, EWS (Engineering Workstation)9 document creation device.

2. Description of the Related Art Image forming apparatuses are used as external or built-in output devices for office automation equipment such as digital copying machines and high-speed facsimile machines.

As image forming apparatuses that meet the demands for improved processing speed of OA equipment, that is, host machines, and for high-quality images, such as laser scanning image forming apparatuses with excellent printing speed and resolution, such as laser printers (also known as laser beam printers) ) is increasingly being used.

This type of laser printer simply prints characters and line drawings (2
In addition to forming gradation images (gradation images), photographs are taken using area gradation methods that take advantage of its high resolution.

It is also used to form multi-gradation images such as paintings.

That is, conventional laser printers have a high pixel density (DP
■: Since the dot diameter (the spot diameter of the laser beam) was determined according to the number of pixels per inch, when expressing multiple gradations, the area gradation method was used, for example, to convert the image to NXN.
A method is used in which the dots are divided into small matrix regions each consisting of pixels, and a number of dots corresponding to the sum or average value of the gradation data of the pixels in each region are printed at once. etc. were used.

However, since the area gradation method is a digital multi-gradation expression, multi-gradation expression at several points of pixels included in the matrix area is not possible.

For example, the size of the matrix area is 2×2.

Theoretically, it is possible to express up to 4 (2-bit), 16 (4-bit), or 64 (6-bit) gradations with 4 x 4 or 8 x 8. ,
In order to express a high-gradation image with a large number of bits, the size of the matrix increases, so there is a contradictory condition that the resolution decreases accordingly.

For this purpose, for example, PWM (Pulse Width Modulation or Time Width Modulation), which changes the laser on time according to the gradation data of the image data (which has multiple gradations) for each pixel, or the output of the laser light source can be used. Methods such as PAM (Pulse Amplitude Modulation) have been proposed in which the length or area of a dot is varied in units of one dot, thereby achieving multi-gradation expression without reducing resolution.

[Problem to be solved by the invention]

However, since PWM requires a clock with a higher frequency in proportion to the increase in the number of gradations, noise intrusion into other circuits cannot be avoided even if electrical processing elements are selectively used or circuit patterns are devised. Furthermore, since the width of the laser spot in the main scanning direction must be reduced in approximately inverse proportion to the increase in the number of gradations, the beam diameter in the middle (in the main scanning direction) becomes thicker, which further reduces aberrations in the optical system. There are optical problems that require severe correction.

In addition, since PAM uses analog processing, it goes without saying that it is necessary to pay attention to controlling the photoreceptor and stabilizing the photoreceptor characteristics against changes in environmental conditions, but it is also necessary to pay attention to the dot size (area) due to changes in the output of the laser light source. There were problems such as the change in the gradation, that is, the range of gradation expression could not be made very large.

On the other hand, there are many applications where line drawings are mainly used and gradations are used as an aid, such as explanatory drawings, in which case resolution is more important than the number of gradations, and a small number of gradations is used to improve the characteristics of photographs, especially face photographs. Image processing is also being carried out to extract and express.

This invention has been made in view of the above points, and its purpose is to obtain high-quality images, that is, high-resolution, multi-gradation images, with a simple configuration without increasing the clock frequency. do.

[Means to solve the problem]

In order to achieve the above object, as shown in the functional block diagrams of FIGS. 1 and 2, the present invention uses a deflector to direct a laser beam output from a laser light source that is turned on and off in accordance with image data in the main scanning direction. The optical writing device 1 is equipped with an optical writing device 1 that writes a line image by scanning a photoreceptor as a spot with a light beam deflected as a spot. In a laser scanning image forming apparatus that forms an image.

The first invention, as shown in FIG. 1, includes a pixel classification means 2 that classifies each pixel into classes according to the gradation data of each pixel of image data, and the pixel classification means 2 classifies each pixel. A spot size setting means 3 sets the spot size for each line written according to the class, and pixels of the class corresponding to the spot of the size set by the spot size setting means 3 are selected one line at a time and lighted. By performing multiple writing corresponding to the number of classes while sequentially switching the spot size set by the line pixel selection means 4 output to the writing device 1 and the spot size setting means 3 for each line writing, one line is A line multiple writing control means 5 is provided for forming a multi-gradation image of 1000 yen.

The second invention, as shown in FIG. 2, includes a pixel classification means 2 for classifying each pixel into classes according to the gradation data of each pixel of image data, and classification by the pixel classification means 2. A spot size setting means 6 sets the spot size for each page written according to the class, and pixels of the class corresponding to the spot of the size set by the spot size setting means 6 are selected one page at a time and light is emitted. By performing multiple writing corresponding to the number of classes while sequentially switching the spot size set by the page pixel selection means 7 output to the writing device 1 and the spot size setting means 6 for each page writing, one page is A page multiple writing control means 8 for forming a multi-gradation image of 1000 kHz is provided.

[For production]

According to the laser scanning image forming apparatus configured in this way, when one page of image data is input, the pixel classification means 2 first classifies each pixel into a predetermined class according to its gradation data.

Next, in the first invention shown in FIG. 1, under the control of the line multiplex writing control means 5, the spot size setting means 3 sets the spot size j corresponding to the class i, and the line pixel Optical writing is performed by the selection means 4 selecting one line of class i data and outputting it to the optical writing device 1.

The line multiple writing control means 5 writes a multi-tone image for one line by sequentially performing the above operation n times from i=1 to n, and further writes the multi-gradation image for one line on the photoreceptor that moves relatively in the sub-scanning direction. The writing of the multi-gradation line image is repeated to form a multi-gradation image for one page.

Furthermore, in the second invention shown in FIG. 2, under the control of the page multiplex writing control means 8, the spot size setting means 6 sets the spot size i corresponding to the class i, and selects the page pixels. Optical writing is performed by the means 7 selecting one page of class i data and outputting it to the optical writing bag M1.

The page multiple writing control means 8 performs the above operations from i=1 to n on the same area on the photoreceptor that moves relatively in the sub-scanning direction.
By sequentially repeating the steps up to n times, a multi-tone image for one page is formed.

〔Example〕

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

Note that terms such as "light amount" and "illuminance" used in this specification are used because, for example, if the laser light source is a laser diode, it will output invisible infrared light.

"Amount of energy""Irradiation energy density per unit area"
It shall include the following meanings.

FIG. 3 is a schematic diagram of the internal mechanism of a laser printer showing an embodiment of the present invention.

According to this laser printer, the paper 11 is fed by the paper feed roller 12 from one of the upper and lower paper feed cassettes 10a and 10b, for example, the paper stack lla on the upper paper feed cassette 10a. After being timed by the pair of registration rollers 13, the photoreceptor drum 15 is transported to a transfer position.

The surface of the photosensitive drum 15, which is rotationally driven in the direction of the arrow by the main motor 14, is charged by the charging charger 1B, and the surface is scanned with a modulated spot from the writing unit 26, which is the optical writing device 1. An electrostatic latent image is formed.

This latent image is visualized by applying toner by the developing unit 17.

This toner image is transferred onto the paper 11 transported by the pair of registration rollers 13 by the action of the transfer charger 18, and the transferred paper is separated from the photosensitive drum 15 and sent to the fixing unit 20 by the transport belt 19. , is pressed against the fixing roller 20b by the pressure roller 20a, and is fixed by the pressure and the temperature of the fixing roller 20b.

The paper that has left the fixing unit 20 is discharged by a paper discharge roller 21 to a paper discharge tray 22 provided on the side of the printer.

Toner remaining on the photosensitive drum 15 is removed and collected by the cleaning unit 23.

Also, at the upper part of the printer, there are printed circuit boards 24 that constitute a controller and an engine driver, respectively.
is installed.

FIG. 4 is a block diagram showing an example of a control section of this laser printer.

The controller 30 is a main microcomputer (
(hereinafter referred to as rMPUJ) 31, and the programs and constant data required by the MPU 31.

A ROM 52 that stores character fonts, etc., a RAM 33 that stores temporal data, dot patterns, etc., an l1054 that controls input and output of commands and data, and an operation panel 3 that is connected to the MPU 5i via the l1054.
5 and an internal interface (I/F) 3B, which are connected to each other via a data bus, an address bus, etc.

In addition, the host machine 38 that outputs print commands, character data, and image data also connects to the MPt via l1034.
connected to.

The engine driver 40 includes a sub microcomputer (hereinafter referred to as rcPUJ) 41 and a ROM 42 that stores programs and constant data required by the CPU 41.
, a RAM 45 for storing temporal data, and an I1044 for controlling data input/output, and are connected to each other by a data bus, an address bus, etc.

The l1044 is connected to the internal interface 36 of the controller 30, and inputs video signals from the controller 30 and the states of various switches on the operation panel 35, and outputs status signals such as 1-image clock and paper end to the controller 30.

The l1044 is also connected to the writing unit 26 and other sequence equipment groups 46 that constitute the printer engine 45, each of which is a printing means, and to various sensors 47 including a synchronization sensor.

The controller 30 receives print commands, character codes, and image data from the host machine 38, edits those codes and data, and in the case of character codes, creates a dot pattern necessary for image writing using the character font stored in the ROM 32. Convert the dot patterns of those characters and images (hereinafter collectively referred to as “images”) into RAM”
The data is stored in memory in a VRAM (video RAM) area in 55.

In the case of multi-tone image data, one dot of the dot pattern is composed of several bits, and the tone data for each dot (pixel) is stored in the VRAM area. Processing of this multi-tone image data will be described later.

When the image clock is input together with the ready signal from the engine driver 40, the controller 30 sends the dot pattern stored in the VRAM area to the engine driver 40 via the internal interface 36 serially or in parallel as a video signal synchronized with the image clock. Output.

The engine driver 40 controls the writing unit 26 and sequence equipment group 46 of the printer engine 45 using data from the controller 30, and inputs video signals necessary for image writing from the controller 30 and outputs them to the writing unit 26. At the same time, it inputs and processes signals indicating the status of each part of the engine from the synchronous sensor and other sensors 47, and outputs necessary information and error signals to the controller 30.

FIG. 5 shows the first part of the writing unit 26 which is the optical writing device 1.
FIG. 2 is a perspective view of essential parts showing an example.

This writing unit 26 includes an LD (laser diode) unit 50, a first cylinder lens 61° sub-deflection element 62, a first mirror 51, a light shielding plate 63, a second cylinder lens 64, and a disk type motor 52. Thereby, a rotating deflector 54 consisting of a polygon mirror 53 rotationally driven in the direction of arrow A, and an fθ lens 55 . Second mirror 56.
It is composed of an imaging cylinder lens 57.

The first cylinder lens 61. Sub deflection element 62, light shielding plate 63. The second cylinder lens 64 constitutes a spot size setting means 3 that can change the spot size at a relatively high speed each time one line is written.

The LD unit 50 has a laser diode (r
This device integrates an LDJ (LDJ), a collimator lens that converts the diverging beam emitted from the LD into a parallel beam, and an aperture that regulates the beam thickness.

The first cylinder lens 61 directs the sub-scanning direction component of the parallel light beam emitted from the LD unit 50 to the sub-deflection element 62.
In the first embodiment shown in FIG. 5, the first . The second cylinder lens 81.64 and the imaging cylinder lens 57 process only the component of the beam in the sub-scanning direction, and the main-scanning direction component is not affected by the cylinder lens.

The sub-deflection element 62 deflects the incident beam in the sub-scanning direction according to the spot size.

The deflected beam is reflected by the first mirror 51 and the light shielding plate 6
3, a portion of the beam cross section is blocked according to the amount of deflection, and the beam enters the second cylinder lens 64.

The second cylinder lens 64 forms an image of the sub-scanning direction component of the incident beam on the reflective surface 53a of the polygon mirror 53.

The beam reflected by the polygon mirror 53 and repeatedly deflected in the main scanning direction is deflected by the fθ lens 55 so that its parallel main scanning direction component forms an image on the surface of the photoreceptor drum 15 rotating in the direction of arrow B. After being refracted, the light is reflected by the second mirror 56, passes through the imaging cylinder lens 57, and reaches the photosensitive drum 15.

The imaging cylinder lens 57 forms an image of the sub-scanning direction component of the transmitted beam on the photoreceptor drum 15, so that the beam is imaged into a spot and forms the main scanning line 15a of the photoreceptor drum 15.
Main scan the top in the direction of arrow C.

The sub-scanning direction component of the beam focused onto the sub-deflection element 62 by the first cylinder lens 61 is focused again onto the reflective surface 53a of the polygon mirror 53 by the second cylinder lens 64, and is further focused by the fθ lens 55. Imaging cylinder lens 5
Since the image is formed three times on the main scanning line 15a of the photoreceptor drum 15 by the sub-deflection element 62 and the rotating polarizer 5,
4 (the reflective surface 53a) and (the main scanning surface of) the photosensitive drum 15 are at optically coextensive positions with each other.

In the normal print mode for characters, line drawings, etc. of binary images that are not multi-gradation images, the amount of deflection by the sub-deflection element 62 is fixed so that the beam passes through a position where it is not affected by the light shielding plate 63. , the LD unit 50 outputs a laser beam that is turned on and off according to binary image data to form an image.

The sub-deflection element 62 has the function of deflecting the optical axis of the transmitted laser beam according to the output from a sub-deflection element driver (not shown).

For example, as is already known, the sub-deflection element 62 is used to input ultrasonic waves into a transparent medium from one side at right angles to the optical axis, and the nodes and antinodes of standing waves generated by interfering with reflected waves from the opposite surface. If it is constructed from an acousto-optic element that deflects the optical axis of the beam using a diffraction grating with a pitch of 1/2 of the ultrasonic wavelength due to the difference in refractive index at Since the deflection angle decreases, the deflection angle can be arbitrarily controlled by changing the frequency.

Alternatively, the sub-deflection element 62 may be configured by combining a prism made of a transparent medium such as PLZT ceramics whose refractive index changes depending on the strength of the electric field and a prism made of optical glass whose refractive index is similar; By changing the voltage applied to the deflection angle, the deflection angle can be arbitrarily controlled.

As shown in FIG. 5, the lower half of the light shielding plate 63 is a transparent part,
If the upper half is used as a light shielding part, the sub-scanning direction component is once imaged on the sub-deflection element 62 by the first cylinder lens 61, and the laser beam that enters the light-shielding plate 63 while expanding again is directed upward by the sub-deflection element 62. As the beam is deflected, the upper part of the beam cross section is gradually blocked and the cross section becomes flat, so that the spot size on the photoreceptor drum 15 gradually increases. This tendency is particularly remarkable in the sub-scanning direction.

In this case, the total light intensity of the beam decreases due to light shielding, and the spot size increases, so the illuminance of the spot on the photoreceptor (the amount of irradiated light per unit area) decreases, so the output of the LD unit 50 depends on the deflection angle. It is necessary to control the amount of light so that the illuminance (for example, peak illuminance) of the spot is constant.

FIG. 6 shows M which is the pixel classification means 2 of the controller 30.
7 is an explanatory diagram showing an example of a method in which the PU 31 classifies one page of image data consisting of pixels each having 2-bit gradation data. FIG.

R from the host machine 38 (Figure 4) via l1034
One page of image data stored in the area of VRAM 75 in AM3''l is classified into classes O to 3 according to its gradation data by data processing of MPU 51, and the corresponding pixels of VRAM 70 to 73 are classified. The image data is stored as 1-bit divided image data at each address.

For example, some gradation data in the j-th column and j+1-th column of the image data in the VRAM 75 are respectively (original data) j...
...r332210321023Jj+1...r
322100232102J, VRAM
In class 3 image data of 73, the corresponding address for gradation data of "3" is "1", and for all other cases it is "0" (Kura X3) j------... "1iooooxoo
oo1"j-H・rlooooooooooo", and the image data of class 2 in the VRAM 72 becomes "0" except for gradation data "2" (class 2) j...
...rooloooolooolo"j+1...ro
Similarly, the class 1 image data of VRAM71 is (Kura X1) j・-・-rooo
o10001000Jj+1...roooloooooo
It becomes olooJ.

The image data of class O in VRAM 70 will be all rQJ as a result, and the dots in the corresponding printed image will be white, so VRAM 7Q is actually not necessary, and three gradations of classes I to 3 will be used. It is possible to express four 2-bit gradations from "O to 3" by printing.

Even if the original data in the VRAM75 has more gradations than 2-bit gradations, it is possible to classify it into 4 gradations by setting appropriate divisions, and the number of classes (excluding 0) can be divided into 4 gradations. ) is not limited to 3.

FIG. 7 is a flowchart showing an example of each routine of line multiple writing in the first embodiment, and FIG. 7(A) shows the pixel classification means 2, line pixel selection means 4. The same diagram (B) of the MPU 31 which is also the line multiplex writing control means 5
(C) shows each routine of each device or unit surrounding the photosensitive drum 15, and (C) of the same figure shows each routine of a portion related to the paper 11.

When printing of the multi-tone image starts, the image shown in Fig. 7 (A)
As shown in , first, one page of image data is processed by VRA.
The image data is read into M75, and each pixel is classified into classes 1 to 3 according to the gradation of the image data, and stored in each VRAM 71 to 73 for each class (FIG. 6).

Next, a counter that counts class i (not shown) is cleared (i=o).

The counter is incremented by rlJ, and data for one line of the j-th column (starting from j = 0) of the classmi data is written to the optical writing device 1 (that is, the writing unit 26).
, and write for class 11 lines.

Next, it is checked whether the content i of the counter is i=3, and if not, it returns to incrementing the counter and i
=3, that is, if writing of the multi-gradation image for one line has been completed, the process proceeds to the next step and checks whether writing for one page has been completed.

If not, return to clearing the counter and continue writing, and if writing is completed, go to end.

On the other hand, as already explained and shown in FIG. 7(B), the photosensitive drum 15 is cleaned by the cleaning unit 23 and charged by the charger 16, and then the spot size setting means 5 (FIG. 5) The optical writing device 1, which is set to the spot size i corresponding to the class i, writes each line data inputted in the order of classes 1 to 3.

The electrostatic latent image formed by the writing is converted into a toner image by the developing unit 17, and is transferred onto the paper 11 by the transfer charger 18 as shown in FIG. 7(C).

After the transfer, the photosensitive drum 15 is neutralized and cleaned again.
charged. Further, a toner image is fixed on the paper 11 by a fixing unit 20, and a paper ejection tray is placed on the paper 11 by a paper ejection roller 21.
The paper is ejected at 2.

In this first embodiment, there is no need to wait for development, transfer, etc. until writing for one page is completed, and the photosensitive drum 1
Each of the peripheral devices No. 5 executes each routine one after another in an assembly line even while line writing continues.

FIG. 8 is a partially enlarged view showing a part of the image on the paper 11 (part of the j and j+1st columns) printed by the routine described above, and its gradation data has already been explained. The example is also shown in FIG.

According to the first embodiment, when writing the data in the j-th column of class 1 and then writing the data in the j-th column of class 2, the photosensitive drum 15 changes the line pitch (in the sub-scanning direction). Since it has moved by the reciprocal of the pixel density, it can be seen in Figure 8 (
A deviation occurs as shown in A).

Therefore, one line of k-gradation data is n=
Each of the (k-1) lines will be expressed by scanning lines with different dot sizes, and the pixel density in the main scanning direction will not change, but the pixel density in the sub-scanning direction will be 1/n = 1.
/ (k −1), so it is not preferable to set the number of gradations or the number of classes n too large.

Generally, when forming a multi-tone image, a system that changes the size of each dot according to the gradation data is ideal because it does not reduce resolution, but conventional methods lead to a significant increase in cost. This is not practical, and in the dither method, for example, which expresses gradation by a collection of dots, a decrease in resolution is unavoidable.

In addition, if the pitch of each pixel in the original image data in the main and sub-scanning directions is equal, if the data is processed as is, the length of the image in the sub-scanning direction will be expanded by 0 times and image distortion will occur. Taking into account the aspect ratio of the pitch on the printed image and the aspect ratio of the pitch on the printed image, if pixels in the sub-scanning direction are grouped together and the average value of the gradation data is taken, for example, one image distortion can be prevented and the gradation can be reduced. Key noise can be reduced.

Furthermore, as shown in Figure 8(A), even if a single line multi-tone image is expressed as a row of n lines of dots shifted from each other, when viewed macroscopically without enlarging it, Although it is not a big problem, the sub-deflection element 62 as explained in FIG.
The reflective surface 53a of the rotating polarizer 54 and the photoreceptor drum 15 are arranged at the same optical width position with each other. If the spot is shifted slightly forward or backward, the spot will be aligned with the main scanning line 1 according to the deflection angle by the sub-deflection element 62.
5a in the sub-scanning direction, so by correcting the line deviation, one line of multi-gradation image can be expressed as one line of dots, as shown in FIG. 8(B). You can also do it.

FIG. 9 is a perspective view of the main parts of the second embodiment of the writing unit, and the same parts as those of the first embodiment shown in FIG. 5 are given the same reference numerals and the explanation thereof will be omitted.

The difference between the second embodiment shown in FIG. 9 and the first embodiment is as follows.
The difference is that instead of the spot size setting means 3 which can change the spot size for each line, a spot size setting means 6 which changes the spot size for each page is provided, but other than that, it is exactly the same.

FIG. 10 is a perspective view of essential parts showing an example of the spot size setting means 6.

Optically, the spot size setting means 6 is a main cylinder lens 66 fixed on the optical axis (X axis) of the laser beam.
2, which is fixed on a movable table 80 that moves in the Y-axis direction perpendicular to the optical axis and is inserted into and removed from the optical axis by the movement of the movable table 80.
Number 1. It is composed of second auxiliary cylinder lenses 87a and 87b, and all three cylinder lenses have power only for the sub-scanning direction component of the laser beam.

The moving table 80 is guided by two guide bars 828 and 82b provided between a pair of fixed yokes 81a and 81b, and is driven in the Y-axis direction by a screw 84 provided on the shaft of a reversible motor 83. .

In the first position shown in FIG. The diameter of the spot on the photoreceptor drum 15 is minimized.

In the second position where the movable table 80 moves in the Y-axis pressure direction and stops when it hits the stopper 85a, the first auxiliary cylinder lens 87a is inserted into the optical axis, and the laser beam has a combined power with the main cylinder lens 66. The reflective surface 53.
Since the image is formed slightly closer to you, the spot diameter becomes slightly larger.

In the third position where the movable table 80 moves in the Y-axis negative direction and stops when it hits the stopper 85b, the second auxiliary cylinder lens 67b is inserted into the optical axis, and the laser beam has a combined power with the main cylinder lens 66. The reflective surface 53.
Since the image is focused further in front of you, the spot diameter is maximized.

In addition, 1. Second auxiliary cylinder lens 67a.

67b is a toric surface having a weak power in the main scanning direction component in order to expand the diameter of the main scanning direction component in accordance with the enlargement of the spot diameter due to the deviation of the sub scanning direction component of each laser beam. Good too.

The first to third positions of the moving table 80 are as follows.

Each is controlled by a position sensor (not shown).

In this way, setting the spot diameter using a motor etc. takes time to switch, so it is impossible to switch lines as in the first embodiment, but it is sufficient for setting each page, and the setting accuracy and stability are high. In the second embodiment, when the image data classification is completed, the start point of the image writing area on the photoreceptor drum 15 is detected, the charger 16 starts discharging, and the optical writing device 1 blows. A certain writing unit 2
The spot size setting means 6 of 6 is set to the first position, and one page of class 1 data input from the MPU 31, which is the page pixel selection means 7, is written.
Developing unit 17. Transfer charger 18. The cleaning unit 23 does not operate, and the paper 11 is not yet conveyed.

When writing for one page is completed, the photosensitive drum 15 rotates once, and the start point is detected again, the charger 16 finishes discharging.

Next, the spot size setting means 6 is set to the second position, and the writing unit 26 writes one page of class 2 data.

When the photosensitive drum 15 rotates twice and the third start point is detected, the upper paper feed roller 12 is activated and the paper 11 is transferred from the paper feed cassette 10a to the registration roller pair 13.
will be sent to.

When the spot size setting means 6 is set to the third position and the writing unit 2 starts writing class 3 data, the developing unit 1
7 starts development, the registration roller pair 13 starts, and the paper 11 is conveyed to the transfer position.

Subsequently, the transfer charger 18 starts discharging, and a toner image in which a group of dot latent images formed by multiple exposure on the photosensitive drum 15 is developed by the developing unit 17 is transferred onto the paper 11.

Further, the photosensitive drum 15 is connected to a cleaning unit 23.
The remaining charge and toner are removed, and the paper 11 with the toner image transferred is fixed by the fixing unit 20 and discharged onto the paper discharge tray 22.

FIG. 11 is a partial enlarged view temporarily visualized to explain the formation process of the electrostatic latent image formed on the photoreceptor drum 15 according to the second embodiment, and FIG. The left and right images are shown in reverse to make it easier to understand the correspondence with the first embodiment shown in FIG. (Since the image actually formed on the photosensitive drum 15 is before transfer, the left and right sides are reversed from this figure.

) Figure 11 (A) shows part of the j-th and j+1-th columns of the image of class 1 data formed by the first writing, and Figure 11 (B) shows the part shown in Figure 11 (A). This shows a state in which class 2 data has been written with a slightly larger spot size on the image by the second writing.

FIG. 11(C) shows a state in which class 3 data is written at the maximum spot size by the third writing on the image shown in FIG. 11(B).

As explained above, according to the second embodiment, data for each class is written n times on a negatively charged photoreceptor without overlapping each pixel, resulting in a multi-gradation static image. After the electric image is formed, development, transfer, cleaning, and fixing are performed, so the circumference of the photoreceptor drum 15 must be at least longer than the maximum length of the paper, that is, as large as the drum diameter. Since the charged photoreceptor starts developing after (n-1) rotations, the photoreceptor is required to have an excellent charge retention ability, that is, a high dark resistance.

Further, the time required for printing itself is longer than in the normal case by (n-1) rotations of the drum, and as the number of gradations increases, it takes more time.

However, if the photoreceptor has excellent charge retention ability,
No matter how the number of gradations increases, the resolution does not decrease and the highest resolution can always be maintained.

In addition, since the spot size switching time does not require a very high speed, it is possible to use commercially available printers that have an MR (Multi Resolution) function that can change the resolution, that is, the pixel density (or spot size) for each page. It can be modified immediately by simply changing a part of the software for multiple write control.

The second embodiment has been described above as a laser printer equipped with a photoreceptor drum, but if the diameter of the photoreceptor drum becomes too large, a laser printer equipped with a photoreceptor belt will use a considerably large sheet of paper. However, it can be easily implemented.

Also, it is charged - times and developed and transferred after multiple writing.

Although the case where the toner image is fixed has been described, it is also possible to transfer the toner image to the paper every -times writing and fix the toner image after the writing is completed, as in a color printer.

〔Effect of the invention〕

As described above, according to the present invention, a high-quality image, that is, a high-resolution, multi-gradation image can be obtained with a simple configuration without increasing the clock frequency.

[Brief explanation of drawings]

FIG. 1 and FIG. 2 are functional block diagrams showing the first and second inventions, respectively. FIG. 3 is a schematic configuration diagram showing an example of the internal mechanism of a laser printer according to an embodiment of the present invention, FIG. 4 is a block diagram showing an example of the control section thereof, and FIG. 5 is also according to the first invention. FIG. 6 is an explanatory diagram showing an example of the classification of image data; FIG. 7 is an example of the line multiple writing routine of the first embodiment. FIG. 8 is a partially enlarged view showing a part of an image example according to the first embodiment. FIG. 9 is a perspective view of a main part showing a second embodiment of the writing unit according to the second invention, FIG. 10 is a perspective view of a main part showing an example of the spot size setting means, and FIG. FIG. 7 is a partially enlarged view for explaining an example of a latent image forming process according to the second embodiment. 1... Optical writing device 2... Pixel classification means 3.6
...Spot size setting means (line, page) 4.
... Line pixel selection means 5 ... Line multiple writing control means 7 ... Page pixel selection means 8 ... Page multiple writing control means 15 ... Photosensitive drum 15a ... Main scanning line 26
...Writing unit (optical writing bag M1) 30...Controller 31...MPU (main microcomputer) (
Pixel classification means 2, line and page pixel selection means 4, 7, line and page multiple writing control means 5, 8) 50...LD (laser diode) unit 53...
・Polygon mirror 54... Rotating deflector 55... f
θ lens 57...imaging cylinder lens 61, 64...first and second cylinder lens 62...
・Sub deflection element 63... Light shielding plate 66... Main cylinder lens

Claims (1)

[Claims] 1. Optical writing in which a line image is written by deflecting a laser beam output from a laser light source that is turned on and off according to image data in the main scanning direction using a deflector and scanning it as a spot on a photoreceptor. In a laser scanning image forming apparatus that forms an image for one page by relatively moving the photoreceptor in a sub-scanning direction perpendicular to the main-scanning direction, the level of each pixel of the image data is A pixel classification means that classifies each pixel into classes according to the tone data, and 1 according to the class classified by the pixel classification means.
A spot size setting means for setting the size of the spot for each line writing, and selecting pixels of a class corresponding to the spot of the size set by the spot size setting means one line at a time and outputting them to the optical writing device. and a line pixel selection means to perform multi-gradation for one line by sequentially switching the spot size set by the spot size setting means for each line writing and performing multiple writing corresponding to the number of classes. 1. A laser scanning image forming apparatus, comprising: a line multiple writing control means for forming an image. 2. An optical writing device that writes a line image by deflecting a laser beam output from a laser light source that is turned on and off according to image data in the main scanning direction by a deflector and scanning it as a spot on the photoreceptor; In a laser scanning image forming apparatus that forms an image for one page by relatively moving a body in a sub-scanning direction perpendicular to the main scanning direction, A pixel classification means that classifies pixels into classes, and 1 according to the class classified by the pixel classification means.
a spot size setting means for setting the size of the spot for each page writing, and selecting pixels of a class corresponding to the spot of the size set by the spot size setting means for each page and outputting them to the optical writing device. and a page pixel selection means for performing multi-gradation for one page by sequentially switching the spot size set by the spot size setting means for each page writing and performing multiple writing corresponding to the number of classes. What is claimed is: 1. A laser scanning image forming apparatus, comprising: page multiple writing control means for forming an image.