JPH03235972A - Laser scanning type image forming device - Google Patents

Abstract

PURPOSE:To form a high-resolution, high-gradation image without causing an increase in clock frequency by deflecting a beam by a subordinate deflecting element according to image data and making it incident selectively on plural beam selection varying means, and controlling the quantity of the laser beam. CONSTITUTION:The subordinate deflecting element 1 which is controlled by a subordinate deflecting element control means 2 deflects the laser beam according to the image data to select one of the beam section varying means 31 - 3n, which vary the sectional shape of the laser beam, thereby varying the light quantity of and spread of a spot formed on a photosensitive body 5 with the laser beam by light diffraction. A light quantity control means, on the other hand, controls the light quantity of the laser beam to hold the peak illuminance of the illuminance distribution of the spot nearly constant, and the spread, i.e. dot size of the spot varies according to the image data eventually to obtain a record on the photosensitive body 5. Consequently, the high-resolution, high- gradation image can be obtained without causing the clock frequency to increase.

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) 2 document creation devices.

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 (spot diameter by a laser beam) was determined according to the number of pixels per inch (I; 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, it is impossible to express the number of gradations in a number greater than the number of pixels included in the matrix area.

For example, if the size of the matrix area is 4 x 4, the number of gradations that can be expressed is limited to 16 (4 bits), and 8
If there are 8 or 16×16, it is possible to express up to 64 (6 bits) or 256 (8 bits) gradations, respectively.

4-dot gradation may be sufficient for illustrations that mainly use line drawings to assist with gradation, but for paintings and photographs that primarily use gradation, at least 6-dot gradation is required. It is said that a gradation of 7 dots or more is necessary for high-quality expression.

Therefore, in order to express a high gradation image with a large number of bits and smooth gradation changes, the size of the matrix becomes large, so there is a contradictory condition that the resolution decreases accordingly.

Generally, newspaper photographs are said to have a resolution of 68 to 75 dpi and 5 to 6 dots, but for example, the maximum pixel density is 480 dp.
Even if you have a high-resolution printer that can print with i, if you want to express 4-bit, 6-bit, or 8-bit gradation,
The effective pixel density is 120dpi and 60dp, respectively.
There was a problem that the resolution dropped to 30 dpi.

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 (multiple 3-4 gradations) for each pixel, or laser PAM (
A method such as pulse amplitude modulation (pulse amplitude modulation) has been proposed in which the length or area of a dot is varied in units of one dot, thereby achieving multi-gradation expression without causing a decrease in resolution.

[Problem to be solved by the invention]

However, PWM requires a higher frequency band in proportion to the increase in the number of gradations, so even if you selectively use electrical processing elements or devise circuit patterns, noise intrusion into other circuits cannot be avoided. 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, in PAM, the spot diameter is determined in an analog manner by the threshold value of the photoreceptor, so it goes without saying that care must be taken to control the light output and stabilize the photoreceptor characteristics against fluctuations in environmental conditions. There is a problem that the dot size (area) changes in response to changes in the output of the light source, that is, the range of gradation expression cannot be made very large.

The present invention has been made in consideration of the above points, and an object thereof is to obtain a high-quality image, that is, a high-resolution, high-gradation image without increasing the clock frequency.

[Means to solve the problem]

In order to achieve the above object, the present invention deflects a laser beam whose light intensity is modulated output from a laser light source according to image data in the main scanning direction using a main deflector, and relatively moves it in the sub-scanning direction. In a laser scanning image forming apparatus that forms an image by scanning a photoreceptor as a spot.

A plurality of beam cross-section changing means arranged in parallel in the optical path of the laser beam and changing the shape of the cross-section of the laser beam so that the spot diameters on the photoreceptor are different, and a sub-deflection element that deflects the laser beam dot by dot at high speed. a sub-deflection element control means for controlling the amount of deflection of the beam by the sub-deflection element so as to select one of a plurality of beam cross-section changing means according to image data; A light amount control means is provided for controlling the light amount of the laser beam so that the peak illuminance of the illuminance distribution of the spot formed on the body is substantially constant.

[For production]

With this configuration, the sub-deflection element controlled by the sub-deflection element control means deflects the laser beam according to the image data, selects one of the plurality of beam cross-section changing means, and selects one of the plural beam cross-section changing means. Since the beam cross-section modulating means changes the shape of the cross-section of the laser beam, the light intensity and spread of the spot formed by the laser beam on the photoreceptor change due to light diffraction.

On the other hand, since the light amount control means controls the light amount of the laser beam to keep the peak illuminance of the spot illuminance distribution approximately constant, as a result, the spread of the spot, that is, the dot size changes according to the image data and is recorded on the photoreceptor. Ru.

〔Example〕

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

Note that terms such as "amount of light" and "illuminance" used in this specification refer to terms such as "amount of light" and "illuminance" because, for example, if the laser light source is a laser diode, it outputs invisible infrared light.
It shall include meanings such as "irradiation energy density per unit area".

FIG. 2 is a schematic diagram of the internal mechanism of a laser printer showing an embodiment of the present invention.

According to this laser printer, a paper 11 is fed by a paper feed roller 12 from a paper stack 11a on one of the upper and lower paper feed cassettes 10a and 10b, for example, the upper paper feed cassette 10a. Registration roller pair 131: After timing is determined, the registration roller pair 131 is conveyed to the transfer position of the photosensitive drum 15.

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 a charging charger, and is scanned with a modulated spot from the writing unit 26 to generate an electrostatic charge on the surface. A 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 registration roller pair 13 by the action of the transfer charger 18, and the transferred paper is separated from the photosensitive drum 15 and transported 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. 3 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.

ROM' that stores character fonts, etc. ;2; - a RAM 33 for storing temporal data, dot patterns, etc.;
It consists of an I1034 that controls human output of commands and data, an operation panel 35 connected to the MPU 31 via the I1034, and an internal interface (1/F) 3B, which are connected to each other via a data bus, an address bus, etc. ing.

In addition, the host machine 38 that outputs print commands, character data, and image data is also connected to the MPU 31 via I1034.
connected to.

The engine driver 40 includes a sub microcomputer (hereinafter referred to as "CPU") 41 and a ROM 42 that stores programs and constant data required by the CPU 41.
, a RAM 43 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 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 60 receives print commands, character codes, and image data from the host machine 38, edits those codes and data, and if it is a character code, uses the character font stored in the ROM 3; Convert the dot patterns of those characters and images (hereinafter collectively referred to as "images") to RAM.
``Memory is stored in the VRAM (video RAM) area within iES.

In the case of multi-N 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.

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 2B. At the same time, it inputs signals indicating the status of each part of the engine from the synchronous sensor and other sensors 47 and processes them, and outputs necessary information and error signals to the controller 30.

FIG. 1 is a basic configuration diagram for explaining the principle of multi-gradation image formation according to the present invention, and shows the main scanning relationship after the main deflector, and the parts related to the mirror and prism that change the optical axis direction. It is omitted.

In addition, Figure 1 shows the deflection surface by the sub-deflection element 1 including the optical axis.The optical system is considered to have a component parallel to the plane of the paper of the beam, and a component of the beam perpendicular to the plane of the paper is ultimately focused on the spot imaging plane. The optical system and light amount control means are also omitted since it is sufficient to form an image.

A laser beam (hereinafter also simply referred to as "beam") incident from a laser light source (not shown) is deflected by the sub-deflection element 1.

Since the deflection angle is controlled by a signal output from the sub-deflection element control means 2 to the sub-deflection element 1 according to the image data (gradation data) of each dot, the deflected beam is divided into a plurality of beam cross sections. Deflection means 3i (i=1 to n)
Of these, the beam is selectively incident on, for example, the beam cross-section changing means 31 corresponding to the image data.

The beam whose cross section has been changed by the beam cross section changing means 31 is passed through the imaging lens 4 to a point on the imaging surface S (which is ultimately the same as or co-curved with the surface of the photoreceptor drum 15).
It is imaged into a point, forming a spot.

For example, if the diameter of a spot formed by a laser beam having a beam diameter φ and a wavelength λ on the imaging plane S by the imaging lens 4 having a focal length f is ω, then the spot diameter ω is given by φ. Here, k is a constant determined by the light intensity distribution including the shape of the beam cross section.

As is clear from equation (1), in order to change the spot diameter ω, it is sufficient to make the beam diameter φ incident on the imaging lens 4 variable; if φ is made large, ω becomes small; Then ω becomes larger.

Therefore, changing the cross section by the beam cross section changing means is as follows:
For example, limiting (reducing) the beam diameter by using an aperture with a circular, square, oval, or rectangular aperture, or as shown in Figure 4 (A).
, (B) respectively show beam expanders and prisms to enlarge or reduce the beam diameter.

The beam expander 6 shown in FIG. 4(A) aligns two lenses Lt + L2, each having a focal length fl + f2 with a different absolute value, with their optical axes equal to the sum of their focal lengths. They are arranged at an interval d (=f1+fz), and when a parallel beam with a beam diameter φ1 enters the lens L1 along its tip axis, the beam diameter φ2
A parallel beam of is emitted.

At this time, the ratio of the beam diameter is φ1/φ2 = ft / fz (
2) is given by Here, the lens with the shorter focal length may be a negative lens, in which case the right side of equation (2) becomes lft/fzl.

Therefore, since the ratio of beam diameter is the ratio of focal length,
A beam that enters from the lens side with a short focal length (absolute value) has its beam diameter expanded, and a beam that enters in the opposite direction is reduced in diameter.

The prism set 7 shown in FIG. 4(B) includes, for example, two prisms P1 made of glass with a refractive index N and having the same apex angle.
+P2 are constructed by combining the directions of the apex angles in opposite directions.

When a parallel beam with a beam diameter φ1 is incident on the incident side prism P1 at an incident angle θ1, and the exit side prism P2 is tilted so that the output beam with a beam diameter φ2 is parallel to the incident beam, the prism P2 If the incident angle of incidence on the prism P1 is 02, the ratio of the beam diameter is given by θ2 is a dependent variable of θ1. is magnified 9 times.

Can be set arbitrarily in succession with reduction.

If the purpose is simply to change the beam diameter, it is possible to continuously expand or contract with a single prism depending on the angle of incidence, but this requires a large deflection of the optical axis. Since it changes according to the ratio of beam diameters, in order to keep the beam optical axes parallel or slightly deflected, the fourth
As shown in Figure (B), it is necessary to set at least two prisms.

However, in the method using a prism, the component of the beam parallel to the plane of the paper changes, but the component perpendicular to the plane of the paper does not change. On the other hand, in the method using a beam expander, components in all directions of the beam change at the same ratio, so it can be used depending on the purpose.

FIG. 5 is a perspective view of the main parts of an example of the writing unit 2, and FIG. 6 is a schematic configuration diagram along the sub-deflection plane showing the first embodiment of the beam section changing portion.

The writing unit 26 includes an LD (laser diode) unit 50, a first cylinder lens 51.degree. sub-deflection element IA, a first mirror 52. A diaphragm plate 53°, a second cylinder lens 4A, a rotary deflector 8 which is a main deflector consisting of a disk type motor 54 and a polygon mirror 55 which is rotationally driven in the direction of arrow A, and an fθ lens 56. Second
It is composed of a mirror 57 and a third cylinder lens 58.

The LD unit 50 integrally incorporates a laser diode (hereinafter referred to as rLDJ), a collimator lens that converts the diverging beam emitted from the LD into a parallel light beam, and an aperture that regulates the thickness of the beam. It is something.

The first cylinder lens 51 has the function of focusing the sub-scanning direction component of the parallel light beam emitted from the LD unit 50 onto the sub-deflection element 4, and in the example shown in FIG. The third cylinder lenses 4A and 58 also process only the sub-scanning direction component of the beam, and the main-scanning direction component is not affected by the cylinder lens.

The sub-deflection element 1A deflects the incident beam in the sub-scanning direction according to image data.

The diaphragm plate 53 is provided with a plurality of holes 591, 592, .

The deflected beam is reflected by the first mirror 52 while gradually expanding the beam diameter, and is reflected by the plurality of holes 59i (i
=z to n), the beam passes through a hole corresponding to the amount of deflection, and after the beam diameter is determined, the beam enters the second cylinder lens 4A.

The second cylinder lens 4A images the sub-scanning direction component of the incident beam onto one point on the reflective surface 55a of the polygon mirror 55.

The beam reflected by the polygon mirror 55 and repeatedly deflected in the main scanning direction is deflected by the fθ lens 56 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 57, passes through the third cylinder lens 58, and reaches the photoconductor tram 15.

Since the third cylinder lens 58 forms an image of the sub-scanning direction component of the transmitted beam on the photoreceptor drum 15, the beam is imaged into a spot with a diameter corresponding to the beam diameter regulated by the hole 59i of the aperture plate 53. The main scanning line 15 of the photoreceptor drum 15
A is main scanned in the direction of arrow C.

The sub-scanning direction component of the beam focused on the sub-deflection element 1A by the first cylinder lens 51 is transmitted through the imaging lens 4 (first
The second cylinder lens 4A corresponding to FIG. Third cylinder lens 5
Since the image is formed three times on the main scanning line 15a of the photoreceptor drum 15 by 8, the sub-deflection element IA and the rotating polarizer 8
(reflecting surface 55a of) and (main scanning surface of) photoreceptor drum 15
and are in optical conjugate position 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 1A is determined by using the hole 591 with the largest diameter on the aperture plate 53 so that the beam is not narrowed down. The LD unit 50 outputs a laser beam that is turned on and off according to binary image data to form an image.

In the case of a multi-gradation image, the beam is deflected for each pixel according to its gradation data, and the beam is deflected at each corresponding hole 59i (i=x to n) of the aperture plate 53. Since the diameter is regulated, spots with diameters corresponding to the gradation data are formed on the photoreceptor 15.

However, the spot with a large diameter has a small hole 59i.
This causes a decrease in the amount of transmitted light, and furthermore, since the light is dispersed over a relatively wide area, its illumination intensity is lower than that of a spot with a small diameter.

Therefore, in order to form a dot (electrostatic latent image) corresponding to the spot size, it is necessary to keep the illuminance, for example, the peak illuminance, approximately constant regardless of the spot size.

In this first embodiment, each hole 59i of the diaphragm plate 53 is provided as a light amount control means for this purpose, as shown in FIG.
After that, (optical) filters F 1 r F 2 . . . Fn having different transmittances corresponding to the diameters of the holes are provided to keep the illuminance of the spot substantially constant.

FIG. 7 is an explanatory diagram showing an example of the sub-deflection element 1 that controls the deflection angle of a beam at high speed according to an electric signal.
A) shows the acousto-optic element 61, and (B) shows the electro-optic element 6.
An example of each of 3 is shown below.

As is already known, when ultrasonic waves are input into a transparent medium from one side and interfere with reflected waves from the opposite side, the difference in refractive index between the nodes and antinodes of the standing waves causes the wavelength of the ultrasonic waves to change. A 1/2 pitch diffraction grating is generated.

The acousto-optic element 61 shown in FIG. 7(A) utilizes this diffraction grating for beam deflection.
Diffracted light of the second order, second order, etc., and -1st order, -2nd order, etc. are generated at different deflection angles, and the deflection angle of the higher-order diffracted light becomes larger, but the amount of light increases rapidly. Usually, either the positive or negative primary light is used.

Since the deflection angle is inversely proportional to the wavelength of the ultrasonic wave, that is, proportional to the frequency, the deflection angle can be controlled by changing the frequency.

Therefore, in order to prevent the mixing of zero-order and other higher-order diffraction lights, the reference optical axis is bent from the beginning, that is, a grating with a frequency fo that is also used when forming a normal binary image is created, and a multi-tone image is created. During formation, it is necessary to control the amount of deflection by shifting the frequency of the ultrasonic waves from fo.

The sub-deflection driver 62, which is the sub-deflection element control means 2, performs the above-described control on the acousto-optic element 61 according to image data.

Also, JJAP (Journal of the Japanese Society of Applied Physics) VOL.

24 (1985) P, 281-283 or JP-A-1-187535, the refractive index of PLZT ceramics placed in an electric field changes depending on the strength of the electric field. There is a transparent medium.

This transparent medium can be used as a prism to form a rectangular parallelepiped in combination with an ordinary optical glass prism with a similar refractive index, or a triangular electrode pair can be formed on one side of the transparent medium prism as shown in FIG. 7(B). 64 is provided to connect the electro-optical element 63
By applying a voltage perpendicular to the direction in which the light travels, the light can be deflected according to the voltage.

In this case, there is no need to apply a voltage when there is no polarization, and the optical axis can be made approximately straight, but since the polarization angle in the direction of the electric field and the polarization component orthogonal to it are different, the polarizer In combination with 65, only the polarized light component with the larger (absolute value) of the deflection angle is used.

The sub-deflection element driver 66 applies a DC voltage to the electrode pair 64 according to the image data.

That is, the sub-deflection element drivers 62 and 64, which are the sub-deflection element control means 2, adjust the beam to a plurality of downstream beam cross-section changing means 3i (i=1 to n) so that the light is selectively incident on any one of the acousto-optic elements 61 . A control signal is output to the electro-optical element 63.

8 and 9 are schematic configuration diagrams along the sub-deflection plane showing the second and third embodiments of the portions related to changing the beam cross section, respectively, and FIG. 6 showing the first embodiment handle.

The second embodiment shown in FIG. 8 includes an acousto-optic element 61 and a sub-deflection element driver 62 as the sub-deflection element 13-24- and the sub-deflection element control means 2, and L as the light amount control means.
The D driver 70 uses imaging lenses 711, 712, and 713 having mutually different magnifications as the beam cross section changing means 3.

The beam output of the LD unit 50 is controlled by an LD driver 70, which controls the illuminance to be substantially constant even if the size of a spot formed according to image data changes.

The parallel beam output from the LD unit 50 is imaged onto the acousto-optic element 61 by the first cylinder lens 51, and is emitted from the acousto-optic element 61 as a diverging beam that is deflected according to image data.

The acousto-optic element 61 and the reflective surface 55a of the polygon mirror 55
In between, there are parts of coaxial lenses as shown by two-dot chain lines for re-forming the image on the acousto-optic element 61 onto the reflecting surface 55a at different magnifications. Since the imaging lens 71i (i=1 to 3) is disposed,
Due to sub-deflection, no matter which lens the beam passes through, it forms an image on one point on the reflecting surface 55a, and in the example shown in FIG. 8, the spot size formed by the imaging lens 711 closest to the reflecting surface 55a is the smallest. , the size of the farthest imaging lens 713 is maximized.

The effect is that the beam cross section is uniformly expanded or contracted, similar to the beam expander 6 shown in FIG. 4(A).

This method using the imaging lens 71 is based on the difference in the imaging magnification of each imaging lens 71i when the distance between the object point (acousto-optic element 61) and the image point (reflection surface 55a) by geometric optics is constant. Although it can be thought of as a change in size,
When considered as a diffraction image by physical optics, each imaging lens 7
The distance between 1i and the image point is the effective focal length f, and the incident beam diameter φ is the diameter when the diffused beam enters each imaging lens 71i, and is proportional to the distance from the object point to each imaging lens 71i. Therefore, ω is determined by equation (1), and is a change in spot size due to a change in beam cross section.

The third embodiment shown in FIG. 9 differs from the second embodiment (FIG. 8) in that an imaging lens 7 is used as the beam cross section changing means 3.
The prism set 7 shown in FIG. 4(B) was used instead of 1i, and the first cylinder lens 51 was thinned to change the beam cross section while remaining a parallel beam, and then reflected by the second cylinder lens 4B. This is because the image is formed on the surface 55a.

The beam when the sub-deflection angle by the acousto-optic element 61 is the smallest is the prism 72a which is the first prism set.
, 72b, the beam diameter is reduced, and then enters the second cylinder lens 4B.

The beam with the largest sub-deflection angle passes through prisms 73a and 7ESb, which are the second prism set, and has its beam diameter expanded, and then enters the second cylinder lens 4B.

A beam whose sub-deflection angle is in the middle is directly incident on the second cylinder lens 4B without passing through the prism.

The beams that have passed through any of the above paths are adjusted so that their optical axes are parallel to each other when they enter the second cylinder lens 4B. imaged at the same point.

Although two examples of the acousto-optic element 61 and the electro-optic element 63 have been described above as the sub-deflection element 1, the present invention is not limited thereto, and any element capable of high-speed deflection on the order of Ions may be used. .

In addition, as the beam cross section changing means 3, an example of the beam expander 6, prism set 7.72.7"S, hole 59, and imaging lens 71 has been explained, but a beam convergence rhofd lens with controlled refractive index distribution etc. It is also possible to configure it by a combination thereof.

Furthermore, although an example has been described in which a plurality of beam cross-section changing means 3 are arranged in a row, for example, two sets of sub-deflection elements are used with their sub-deflection surfaces orthogonal to each other, and a plurality of beam cross-section changing means 3 are arranged in a row. By dimensional arrangement, the number and range of tones that can be expressed can be greatly increased.

As explained above, according to the present invention, the beam is deflected according to the image data (gradation data) by the sub-deflection element that deflects the beam at high speed in units of dots, and the beam cross-section changing means is selected from a plurality of beam cross-section changing means. By making it incident on a target, the cross section is changed and the spread of the spot is changed.

Therefore, unlike the area gradation method, the resolution does not deteriorate as the gradation image becomes higher, and unlike PWM, there is no need to handle a signal with a higher frequency than the original image clock.

Furthermore, the decrease in light intensity due to this change in cross section or spot size can be avoided because the light intensity control means controls the light intensity of the laser beam so that the peak illuminance of the spot is approximately constant, so the spot is always approximately normalized to the peak illuminance. It is in a state of

Therefore, unlike PAM, it is not affected by changes in environmental conditions, and the dot size change, that is, the range of gradation expression can be made wider.

Further, although an example in which the present invention is applied to a laser printer has been described above, the present invention is not limited to a laser printer, but can also be applied to a digital copying machine.

Needless to say, the present invention can be applied to laser scanning image forming apparatuses built into office automation equipment such as high-speed facsimiles.

Further, the photoreceptor may be not only an OPC (organic photoreceptor) used in electrophotography, but also a silver salt photoreceptor such as a photographic film used in printing plate making.

〔Effect of the invention〕

As explained above, the laser scanning image forming apparatus according to the present invention can operate without increasing the clock frequency.
A high-quality image, that is, a high-resolution, high-gradation image can be formed.

[Brief explanation of drawings]

FIG. 1 is a basic configuration diagram for explaining the principle of multi-gradation image formation according to the present invention, FIG. 2 is a schematic configuration diagram of the internal mechanism of a laser printer showing an embodiment of the present invention, and FIG. Similarly, FIG. 4 is an explanatory diagram showing an example of the beam cross section changing means, FIG. 5 is a perspective view of the main part of the writing unit, and FIG. 6 is a cross section thereof. FIG. 7 is an explanatory diagram showing an example of the sub-deflection element, and FIGS. 8 and 9 are respectively the second and third embodiments of the changed section. FIG. 2 is a schematic configuration diagram showing an example. 1... Sub deflection element 2 - Sub deflection element control means 3... Beam cross section changing means 4... Imaging lenses 4A, 4B... Second cylinder lens 6... Beam expander (beam cross section changing means )7.
72.73... Prism set (beam cross section changing means) 8... Rotating deflector (main deflector) 15... Photosensitive tram 26... Writing unit 50... LD unit (laser light source) 51...・First cylinder lens 5:5...Aperture plate 5
Day... Hole (beam cross section changing means) 61... Acousto-optic element (sub-deflection element) 6i2.68...
...Sub deflection element driver (sub deflection element control means) 63...Electro-optical element (sub deflection element) 70...L
D driver (light amount control means) 71...imaging lens (beam cross section changing means) F...filter (light amount control means)

Claims (1)

[Claims] 1. A laser beam whose light intensity is modulated and is output from a laser light source according to image data is deflected in the main scanning direction by a main deflector, and is spotted on a photoreceptor that moves relatively in the sub-scanning direction. In a laser scanning image forming apparatus that forms an image by scanning, the laser beams are arranged in parallel in the optical path of the laser beams, and the shape of the cross section of the laser beams is changed so that the spot diameters on the photoreceptor are different. a sub-deflection element that deflects the laser beam in dot units at high speed; and a sub-deflection element that deflects the laser beam by the sub-deflection element according to image data among the plurality of beam cross-section change means. a sub-deflection element control means for controlling to select one of them; and a beam cross-section changing means for controlling the light intensity of the laser beam so that the peak illuminance of the illuminance distribution of the spot formed on the photoreceptor is approximately constant. What is claimed is: 1. A laser scanning image forming apparatus, comprising: a light amount control means for controlling the amount of light.