JPH03206478A - Laser scanning type image forming device - Google Patents

Abstract

PURPOSE:To obtain a high-definition image, that is, a high-resolution and high- gradation image without causing a clock frequency band to be higher by providing a sub deflecting element, a sub deflecting element control means, a beam cross section modulation means and a laser output control means. CONSTITUTION:The sub deflecting element 4 controlled by the sub deflecting element control means 5 deflects a laser beam in accordance with image data, and the beam cross section modulation means 6 cross-sectionally modulates the shape of the cross section of the laser beam or the light quantity distribution thereof in accordance with the deflection quantity. Therefore, the light quantity and the spread of a spot formed by the laser beam on a photosensitive body are changed by the diffraction of the light. Meanwhile, the laser output control means 2 controls the output light quantity of a laser light source 1 to keep the peak illuminance in the illuminance distribution of the spot nearly constant, so that the spread of the spot, that is, a dot size is changed in accordance with the image data and the image is recorded on the photosensitive body. Thus, the high-definition image, that is, the high-resolution and high-gradation image is formed without causing the clock frequency band to be higher.

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]

Image forming devices are external or built-in output devices for office automation equipment such as personal computers, EWS (engineering workstations), document processing devices, digital copying machines, and high-speed facsimiles. Laser scanning type image forming devices with excellent printing speed and resolution, such as laser printers (laser beam printers), are used as image forming devices that meet the demands for improved processing speed and high quality images of OA equipment, that is, host machines, as described above. ) is increasingly being used.

This type of laser printer simply prints characters and line drawings (2
In addition to forming gradation images), it is also used to form multi-gradation images such as photographs and paintings using area gradation methods that take advantage of its high resolution. In other words, conventional laser printers have a high pixel density (DPI; number of pixels per inch).
Since the dot diameter (spot diameter by laser beam) was determined according to
! For example, an image is divided into small matrix regions each consisting of NXN 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 together. The dither matrix method was used as a practical means. However, since the area gradation method is a digital multi-gradation expression, it is impossible to express the number of gradations greater than the number of pixels included in the matrix area. For example, if the size of the matrix area is 4×4, the gradation that can be expressed is limited to 6 (4 bit) gradations, and 8×
If there are 8 or 16x16, each can express up to 64 (6 bits) or 256 (8 bits) gradations. 4-bit WtrI4 may be sufficient for line drawings that mainly use gradation as shown in the illustrations, but for pictures and photographs that mainly use gradation, at least 6-bit gradation is required. It is said that gradation of 7 bits 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, but there is a contradictory condition in which the resolution decreases accordingly.

Generally, newspaper photographs are said to have 68-75 dpi and 5-6 bit gradation, 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 l 2 0 dpi, 6
There was a problem that the image quality dropped to 0dpi and 30dpi. For this purpose, for example, PWM (Pulse Width Modulation or Time Width Modulation) is used to change the on-time of the laser according to the gradation data of the image data (because it has multiple gradations) for each pixel.
Alternatively, methods such as PAM (Pulse Amplitude Modulation), which changes the output of a laser light source, have been proposed in which the length or area of dots is varied in units of edots, thereby achieving multi-tone expression without reducing resolution. Ta.

[Problem to be solved by the invention]

however. PWM requires a high-frequency clock 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, making it necessary to more strictly correct aberrations in the optical system. There are optical problems such as the need to

In addition, since PAM uses analog processing, it goes without saying that it is necessary to pay attention to controlling the optical output and stabilizing the photoreceptor characteristics against changes in environmental conditions. There were problems such as the change in the gradation, that is, the range of gradation expression could not be made very large.

This invention has been made in view of the above-mentioned points, and it is an object of the present invention to obtain a high-quality image, that is, a high-resolution, high-gradation image without increasing the clock frequency. [Means for Solving the Problems] In order to achieve the above object, the present invention deflects a laser beam whose light intensity is modulated and is output from a laser light source according to image data in the main scanning direction using a main deflector. In a laser scanning image forming device that forms an image by scanning a photoreceptor as a spot, which moves relatively in the sub-scanning direction, the spot is placed at or near an optical intersection position of the photoreceptor surface scanned by the spot. 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 laser beam by the sub-deflection element according to image data; A beam cross-section modulating means cross-sectionally modulates the shape or its light intensity distribution according to the amount of deflection, and the peak illuminance of the illuminance distribution of the spot formed on the photoreceptor by the laser beam cross-sectionally modulated by the beam cross-section modulating means is approximately The device is equipped with a laser output control means that controls the output light amount of the laser light source so that it remains constant. The beam cross-section modulating means may be constituted by a light-shielding plate that blocks part of the cross-section according to the amount of deflection of the laser beam by the sub-deflection element. Alternatively, the beam cross-section modulating means may be constituted by an optical filter or a reflector having a distribution characteristic mainly in the sub-deflection direction such that the transmittance or reflectance changes within the effective diameter of the laser beam at the position where the beam cross-section modulating means is disposed. good. [Operation] With this structure, the sub-deflection element controlled by the sub-deflection element control means deflects the laser beam according to the image data, and the beam cross-section modulation means deflects the laser beam according to the amount of deflection. Since the shape of the cross section or its light intensity distribution is cross-sectionally modulated, 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 laser output control means controls the output light amount of the laser light source 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 the photoreceptor is is recorded in Further, since the sub-deflection element is arranged at or near the optically concentric position of the photoreceptor surface, the spot position does not change even if the laser beam is deflected.

Note that even if the beam cross-section modulating means is constituted by a light shielding plate, a filter, or a reflector, the effect of changing the light amount and spread of the spot due to light diffraction remains the same.

〔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 are replaced by "amount of energy" 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 link showing an embodiment of the present invention. According to this laser printer, paper 11 is fed by the paper feed slot roller 12 from one of the upper and lower paper feed cassettes 10a and 10b, for example, the paper stack 11a on the upper paper feed cassette 10a, and the paper 11 is timed by a pair of registration rollers 13 and then conveyed to the transfer position of the photosensitive drum 15. The surface of the photoreceptor drum 15, which is rotationally driven in the direction of the arrow by the main motor 14, is charged by the charger 1 and scanned with a modulated spot from the writing unit 2, forming an electrostatic latent image on the surface. is expressed. This latent image is made into a visible image by applying toner by the developing unit 17. This toner image is transferred onto the paper 11 conveyed 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 conveyor belt. The toner is pressed against the fixing roller 20b by the pressure roller 20a, and 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. The toner remaining on the photosensitive drum 15 is removed and collected by the cleaning unit 23. Further, at the upper part of the printer, there are printed circuit boards 24 that respectively constitute a controller and an engine driver.
is installed.

FIG. 3 is a block diagram showing an example of the control section of this laser printer. The controller 30 is a main microcomputer (
rMPtJ") 31, a ROM 32 that stores programs, constant data, character fonts, etc. required by the MPU 51, a RAM 33 that stores temporary data, dot patterns, etc., and controls input/output of commands and data. MP via the I/034
It consists of an operation panel 35 connected to the tJ31 and an internal interface (I/F) 3B, which are connected to each other via a data bus, an address bus, etc. Further, a host machine 8 that outputs print commands, character data, and image data is also connected to the MPU 5 i via the I/034. The engine driver 40 includes a sub microcomputer (hereinafter referred to as r (::PUJ) 41 and its CPU 41
ROM that stores the programs and constant data required by
42, a RAM 43 for storing temporary data, and an I/044 for controlling data input/output, and are connected to each other by a data bus, an address path, etc. The I/044 is connected to the internal interface 3B of the three controllers, and inputs video signals from the controller 30 and the status of various switches on the operation panel 35, and outputs status signals such as image clock and paper end to the controller 30. do.

Further, this machine/044 is also connected to a writing unit 2 and other sequence equipment groups 46 each comprising a printer engine 45 serving as a printing means, and 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 dot patterns necessary for image writing using the character font stored in the RoM 32. , and store the dot patterns of those characters and images (hereinafter collectively referred to as "images") in RAM3.
It is stored in memory in the VRAM (video RAM) area of 3.

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. 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. 4 is a perspective view of essential parts of an example of the writing unit 26. This writing unit 26 includes an LD (laser diode) unit 50, a first cylinder lens 3, and a sub-deflection element 4.
, a first mirror 5'5, a beam cross-section modulating means 6, a second cylinder lens 7, a disk-type motor 54, and a polygon mirror 55 which is rotated in the direction of arrow A by the main deflector 8. A rotating deflector and an fθ lens 56
, a second mirror 57, and a third cylinder lens 58. The LD unit 50 incorporates therein 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 beam thickness. That's what it is. The first cylinder lens 3 has the function of imaging a certain portion of the parallel light beam emitted from the LD unit 50 in a certain direction onto the sub-deflection element 4, and in the example shown in FIG. 2. The third cylinder lens 3, 7.58 processes only the component of the beam in the sub-scanning direction, and the main scanning direction is not affected by the cylinder lens. The sub-deflection element 4 deflects the incident beam in the sub-scanning direction according to image data. The deflected beam is reflected by the first mirror 53, cross-sectionally modulated by the beam cross-section modulating means 6 according to the amount of deflection, and enters the second cylinder lens 7. The second cylinder lens 7 forms an image of a certain portion of the incident beam in the sub-scanning direction 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 forms an image on the surface of the photoreceptor drum 15, which is parallel to the main scanning direction by an fθ lens 56 and rotates in the direction of arrow B. After being refracted as shown in FIG. Since the third cylinder lens 58 images a certain portion of the transmitted beam in the sub-scanning direction on the photoreceptor drum 15, the beam is imaged into a spot and the main scanning line 15a of the photoreceptor drum 15.
Main scan the top in the direction of arrow C. The sub-scanning direction of the beam focused on the sub-deflection element 4 by the first cylinder lens 3 is again focused on the reflective surface 55a of the polygon mirror 55 by the second cylinder lens 7, and then Since the image is formed three times on the main scanning IIA 15a of the photoreceptor drum 15 by the three-cylinder lens 58, the sub-deflection element 4 and the main polarizer 8 (
The reflective surface 55a) of the photoreceptor drum 15 (main scanning surface of the photoreceptor 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 4 is such that the beam passes through a position where it is not affected by the beam cross-section modulation means 6. It is fixed, and the LD unit outputs a laser beam that is turned on and off according to binary image data to form an image. FIG. 1 is a basic configuration diagram for explaining the principle of multi-gradation image formation according to the present invention, in which the main deflector 8, fθ
Main scanning-related parts such as the lens 56 and mirror-related parts that change the optical axis direction are omitted.

In addition, Figure 1 shows the deflection surface by the sub-deflection element 4 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 is also omitted since it is sufficient to focus the image on the image. A laser diode (not shown), which is a laser light source (hereinafter referred to as r
The LD unit 50, which outputs a parallel beam, is controlled by the LD driver 2, which is a laser output control means, and outputs a beam with an amount of light according to image data (same as gradation data). The parallel beam output from the LD unit 50 is imaged on the sub-deflection element 4 at the focal point of the first cylinder lens 3, and then imaged again on the spot imaging plane 9 by the second cylinder lens 7. That is, the sub-deflection element 4 and the spot imaging surface 9 are located at an optically common position due to the second cylinder lens 7, and therefore the spot imaging surface 9 is aligned with the surface of the photoreceptor drum 15 shown in FIG. Equivalent to. The amount of beam deflection of the sub-deflection element 4 is controlled in accordance with image data by a sub-deflection driver 5, which is a sub-deflection element control means. The sub-deflection element 4 is composed of, for example, an acousto-optic element, a photoelectric deflection element, etc., which will be described later. The beam deflected by the sub-deflection element 4 is then modified according to the amount of deflection by the beam cross-section modulating means 6, which also includes a light shielding plate, an optical filter, a reflector, etc., which will be described later. The light intensity distribution is changed (this is called "cross-sectional modulation").The cross-sectionally modulated beam is imaged as a spot on the spot imaging surface 9 by the second cylinder lens 7, but the beam cross-section is changed due to the cross-sectional modulation. The total light intensity of the spot decreases depending on the changed shape or light intensity distribution, that is, the state of the beam cross section, and the spread changes due to light diffraction, which also depends on the state of the beam cross section.Here, the illuminance distribution of the spot is Gaussian. Since it closely approximates the distribution curve, the "spreading" of the spot can be calculated by setting the illuminance at a certain percentage of the peak illuminance, for example 50%, 25%, or e-" = 1, with the spot's "spread" as the center.
3. If defined as an area of 5% or more, the spread will increase due to cross-sectional modulation, but the absolute value of the illuminance will decrease. The beam focused on the spot by the sub-deflection element 4 located at the optical intersection position of the photoreceptor surface, that is, the spot imaging surface 9, begins to spread again, and at the beam cross-section modulating means 6 located away from the optical intersection position, Although the cross section has a certain extent, the light intensity distribution in the cross section before undergoing cross-sectional modulation also closely approximates a Gaussian distribution. Therefore, the transmission (reflection) of the optical filter (reflector) is reflected in the light intensity distribution of the beam cross section shifted according to the amount of deflection.
(
The light intensity distribution (including shape) can be obtained. Next, by Fourier transforming the light intensity distribution of the beam cross section obtained in this way, the illuminance distribution of the spot on the spot imaging plane 9 is calculated, and its peak illuminance and spread are determined, so the amount of deflection can be calculated. Accordingly, you can see how much the output of the five LD units should be increased in order to keep the peak illumination constant. The amount of deflection of the sub-deflection element 4 relative to the input value varies depending on the structure of the sub-deflection element, but is known in advance. Finally, the amount of deflection to obtain the optimum spot for the image data can be determined and controlled by the sub-deflection driver 5. therefore. The output of the LD unit 50 can also be controlled to a substantially constant peak light amount by the LD driver 2 according to the image data. FIG. 5 shows an acousto-optic element 4a as a sub-deflection element 4,
FIG. 3 is a schematic structural diagram showing a first embodiment in which a light shielding plate 6a is used as the beam cross-section modulating means 6 to deflect the beam in the sub-scanning direction. In FIG. 5, since the main scanning direction is perpendicular to the plane of the paper, the main deflector 8 is shown as a straight line representing the reflecting surface, and the area to the right shows the portion after reflection. As is already known, when ultrasonic waves are input into a transparent medium from one side and interfere with the 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. An analytical grid with a pitch of 1/2 is created. The acousto-optic element 4a uses this diffraction grating to deflect a beam, and when light is incident on the diffraction grating, the 0th order light travels straight, and the 1st and 2nd orders are placed on both sides of the 0th order light, respectively.
Diffracted light of -1st order, -2nd order, etc. is generated at different deflection angles, and the higher the order, the larger the deflection angle, but the light intensity decreases rapidly.
Second light is used. The deflection angle is inversely proportional to the wavelength of the ultrasound, that is, proportional to the frequency, so the deflection angle can be controlled by changing the frequency. Therefore, in order to prevent the mixing of zero-order and other higher-order light, the reference optical axis (without cross-sectional modulation) is bent from the beginning, that is, a grating with a certain frequency fO is created even during normal binary image formation. When forming multi-tone images, it is necessary to control the amount of deflection by shifting the ultrasound frequency from fO. The sub-deflection driver 5a performs the above-described control on the acousto-optic element 4a according to image data.

The first embodiment shown in FIG.
A driver 2, an acousto-optic element 4a, a sub-deflection driver 5a, and a data conversion circuit that inputs image data and converts it into conversion data in a form that is easy to control by the LD driver 2 and sub-deflection driver 5a. Boa (this effect will be described later) and a light shielding plate 6 which is the cross-sectional modulation means 6.
a, first and second cylinder lenses 3a and 7a each having power in the sub-scanning direction, a main deflector 8, an fθ lens 58a, and a photosensitive drum 15.

The LD unit 50 includes a laser diode 1 that is a laser light source, a collimator lens 51 that converts a diverging beam outputted from the laser diode 1 into a parallel light beam, and a cross-sectional shape including the thickness of the laser beam that has become the parallel light. The aperture 52 outputs a parallel light beam regulated by the aperture 52. The aperture 52 typically has a circular or square window, but may have a modified window to suit cross-sectional modulation. The l-th cylinder lens 3a images the sub-scanning direction component of the parallel light beam onto the acousto-optic element 4a. Acousto-optic element 4a
When the beam is bent by , it enters the second cylinder lens 7a while expanding as shown by the broken line when the cross section is not modulated.
The second cylinder lens 7a forms an image on the main deflector 8 again.

The light shielding plate 6a includes the acousto-optic element 4a and the second cylinder lens 7.
In the vicinity of the beam shown by this broken line,
It is installed in a position that does not cover the beam. The beam deflected in the main scanning direction by the main polarizer 8 is imaged on the photoreceptor drum 15 in both the main scanning direction and the sub-scanning direction by the fθ lens 58a, and is scanned as a spot in the main scanning direction. ．． When forming a multi-tone image, as the frequency is shifted higher than fO according to the image data, the amount of beam deflection increases, so the beam is deflected upward as shown by the solid line. part is blocked by the light shielding plate 6a, and the remaining part whose cross section is modulated is imaged on the main polarizer 8 by the second cylinder lens 7a, and the fθ lens 56
The image is re-formed on the photoreceptor drum 15 by a. Figure 6 is a diagram showing an example of the change in illuminance distribution in each section due to cross-sectional modulation, with the vertical axis representing the illuminance and the horizontal axis representing the coordinates in the sub-deflection direction with the origin at the center of the beam or spot. It shows. FIG. 6(a) is a diagram showing the illuminance distribution of the laser beam before cross-sectional modulation, with the peak illuminance at the beam center normalized to 1.

As already explained, since the illuminance distribution of the laser beam can be regarded as a Gaussian distribution, the normalized illuminance y is expressed by the following equation, where X is the distance from the beam center.

y=axp (-2x"/w") Here, when x=w, y=e", or 0.l35, and W is a constant that is generally defined as the effective radius of the laser beam. Therefore, W If is large, the beam effective diameter or spot diameter is also large.

In Fig. 6(a), A is a state where the amount of deflection is large and the right side of the beam is blocked from the center of the beam, and E is a state where the amount of deflection is small and there is no cross-sectional modulation, and the intermediate states are W/4, W/2, W
The states in which the right side is shaded are B, C, and D, respectively. FIG. 6(b) is a diagram showing the illuminance distribution immediately after cross-sectional modulation, and the vertical axis represents the relative illuminance when the output of the laser light source is modulated in light quantity so that the peak illuminance of the spot is uniform.

For example, A, in which 1/2 of the beam cross section is blocked, has four times the output compared to E, in which the cross section is not modulated. Figure 6 (c
) is a diagram showing the illuminance distribution of a spot due to a cross-sectionally modulated beam, and the illuminance distributions A to E are the results obtained by Fourier transforming the illuminance distribution shown in (b) of the same figure. ．． It is clear from these results that spots with equal peak illuminance can be obtained by light intensity modulation, and spots with different spreads can be obtained by cross-sectional modulation. The illuminance distribution of these spots is also a Gaussian distribution, and since the peak illuminances are aligned, that is, normalized, the ratio of the spot diameters of A to D to E does not change no matter what level you slice. Even if the overall density of the image changes due to changes in the sensitivity (density/illuminance) of the photoreceptor due to conditions or other factors, the gradation itself is not affected. This is the effect of keeping the peak illuminance constant, but in practice, even if the peak illuminance varies slightly depending on the image data, no actual harm will be observed, so it is sufficient to keep the peak illuminance approximately constant. FIG. 7 is an explanatory diagram showing an example of spot change due to cross-sectional modulation, from E with no modulation to A with deep modulation, as the modulation deepens, the spot diameter increases and the output of the laser diode increases. It shows a trend. In general, in order to maintain linearity between the gradation of the image obtained in this way and the gradation indicated by the image data, the relationship between the image data and the amount of deflection, the relationship between the amount of deflection and output modulation, or the relationship between the image data The relationship between and output modulation is not necessarily linear.

Therefore, the LD driver 2 and the sub-deflection driver 5
a is a laser diode 1 and an acousto-optic element 5, respectively.
In order to easily control a, it may be better to input converted data obtained by converting the image data instead of inputting the image data as is.

The data conversion circuit 60a is a circuit provided for this purpose, and is configured with a diode matrix, for example, to convert image data into arbitrary conversion data and output the converted data to the LD driver 2 and sub-deflection driver 5a, respectively. In addition, in order to adjust the relationship between the amount of deflection and the amount of output modulation, and to adjust the shape of the modulated cross section, the edge of the light shielding plate 6a is not necessarily a straight line as shown in FIG. 8(a). , it is also possible to have a curve as shown in the same figure (b).
In the case of the light shielding plate 6a having this curve, the light shielding plate 6
For example, a second cylinder lens 7 configured in a circular shape instead of a
You may also use the mirror frame in a. FIG. 9 shows a photoelectric deflection element 4b as a sub-deflection element 4,
This is a schematic diagram showing a second embodiment in which optical filters (hereinafter referred to as "filters") 6b are used as the beam cross-section modulating means 6 to deflect the beam in the main scanning direction, and the sub-scanning direction is on the paper surface. It will be orthogonal to As is already known, for example, PL placed in an electric field
There are transparent media such as ZT ceramics whose refractive index changes depending on the strength of the electric field. This transparent medium is used as a prism to form a rectangular parallelepiped in combination with an ordinary optical glass prism with a similar refractive index, and by applying a voltage at right angles to the direction in which the light travels, it is possible to deflect the light according to the voltage. It can be done. In this case, there is no need to apply a voltage when there is no polarization, and the optical axis is in a straight line. However, since the polarization angle in the direction of the electric field and the polarization component orthogonal to it are different, a polarizer (not shown) In combination, only the polarized light components with a larger deflection angle (absolute value) are used.

The second embodiment shown in FIG. 9 has an LD unit 5Q, an LD
Driver 2, photoelectric deflection element 4b, and sub-deflection driver 5
b, a data conversion circuit 60b, a filter 6b which is the cross-sectional modulation means 6, first and second cylinder lenses 3b and 7b each having power in the main scanning direction, a main deflector 8, an fθ lens 58b and It consists of a photoreceptor drum 15. In this case, since the deflection direction by the photoelectric deflection element 4b is the main scanning direction, the second cylinder lens 7b does not focus the beam emitted from the photoelectric deflection element 4b as a diverging light onto the main deflector 8, but instead parallelizes the beam. Another difference from the first embodiment is that the beams are changed to nearly parallel beams. Further, the intensity of the beam in the sub-scanning direction is imaged onto the main deflector 8 by a cylinder lens or the like (not shown). Therefore, the f.theta. lens 58b is also different from the f.theta. lens 5B& of the first embodiment.

Furthermore, depending on the difference in the characteristics of the sub-deflection element 4, the sub-deflection driver 5b, and therefore the data conversion circuit 60b,
Sub-deflection driver 5a and data conversion circuit 60a, respectively
Although the purpose is the same, it goes without saying that each has different characteristics.

Components other than the above and the filter 6b to be described later, those with the same reference numerals are the same parts, and the ones with different suffixes are equivalent parts, and their explanation will be omitted.

The filter 6b has a transmittance that changes within the effective diameter of the beam incident thereon, i.e., except for the non-modulated transparent part and the part that hardly transmits the beam, the transmittance changes relatively significantly at both ends of the effective diameter of the beam. This is a filter with distribution characteristics such that the transmittance changes. The reflector is equivalent to a filter if the transmittance is replaced by the reflectance, except that the optical axis direction changes due to reflection, so a description thereof will be omitted.

FIG. 10 is a diagram showing an example of the transmittance distribution characteristics of the filter 6b, with the horizontal axis representing the coordinate in the main scanning direction and the vertical axis representing the transmittance.

The filter 6b shown in the figure has a transparent portion on the left side from the origin of the horizontal axis, and has transmittance characteristics that match one side of a Gaussian distribution curve on the right side.

Figure 11 is a diagram showing changes in illuminance distribution in each cross section due to cross-sectional modulation, and (a) and (b) in the same figure correspond to Figures 6 (b) and (C), respectively, and The illuminance distribution is the same as that in Figure 6(a), so it is omitted. However, the points corresponding to A to E shown in FIG. 6(a) are points A to F indicating the center position of the incident beam in FIG. 10, and F is the position when no modulation is performed. In Fig. 11(a), the beam centers other than F appear to be shifted from the origin 0, but this is because the peak illuminance has shifted from the beam center due to cross-sectional modulation by the filter, and the same as in Fig. 6(b). The beam center is at the origin. Also, the illuminance distribution is modulated in light amount so that the peak illuminance at the spot is constant. The illuminance distribution of the spot shown in FIG. 11(b) is also
Similar to l(c), the spot size increases as the cross-sectional modulation becomes deeper. Further, unlike the first embodiment, the intensity of the beam deflection direction is not imaged on the main deflector 8, but the photoelectric deflection element 4b and the photosensitive drum 15 are connected to the second cylinder lens 7b and fθ
Since the spot is in an optically coextensive position with the lens 56b, the spot position does not change even if the beam is deflected by the photoelectric deflection element 4b.

As explained above, according to the present invention, a sub-deflection element that deflects at high speed in units of dots is arranged at or near a position optically coextensive with the photoreceptor surface, and image data (gradation data) is
The spread of the spot (W in Gaussian distribution) is changed by deflecting the beam in accordance with the beam and modulating the cross section of the deflected beam using a beam cross-section modulating means such as a light shielding plate, filter, or reflector.

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, since the laser output control means controls the decrease in light intensity due to this cross-sectional modulation so that the peak illuminance of the spot is approximately constant, the spot is always normalized to the peak illuminance. 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 laser printers, and can be applied to laser scanning image forming apparatuses built into office automation equipment such as digital copying machines and high-speed facsimiles. Needless to say, it can be applied to . 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 type control device according to the present invention does not cause an increase in the clock frequency.
It is possible to form high-quality images, that is, high-resolution, high-gradation images.

[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 diagram of the internal structure of a laser printer showing an embodiment of the present invention, and FIG. Figure 4 is a block diagram showing an example of the control section; Figure 4 is a perspective view of the main parts of the writing unit; Figure 5 is a schematic diagram showing the first embodiment of the cross-sectional modulation section; Figure 6 is a line diagram showing an example of the illuminance distribution in each section of the first embodiment, Figure 7 is an explanatory diagram showing an example of the change in the spot, and Figure 8 is the shape of the edge of the light shielding plate. FIG. 9 is a schematic diagram showing a second embodiment of the cross-sectional modulation portion; FIG. 10 is a line showing an example of transmittance distribution characteristics of an optical filter according to the second embodiment. FIG. 11 is a diagram showing an example of the illuminance distribution in each section of the same. 1... Laser diode (LD; laser light source) 2...
・LD driver (laser output control means) 5, 3at
3b... Lth cylinder lens 4... Sub-deflection element 4a... Acousto-optic element 4b... Photoelectric deflection element s, 5at 5b... Sub-deflection driver (sub-deflection element control means) 6... Beam cross-section modulating means 6a...light shielding plate 6b...optical filter 7.7at 7b...second cylinder lens 8...main deflector 9...spot imaging surface (photoreceptor surface) 15... Photosensitive drum 26...Writing unit 30...Controller 38...Host machine 50...LD unit 55...Polygon mirror 5B. 5 6 a, 5 6 b-fθ lens Boa, 60b...Data conversion circuit Fig. 3 3B Fig. 4 Fig. 6 Fig. 7 Maximum Figure 8 (Q) (b) Dense 9 @◆-- 匪瑞0 匪目O

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 as a sub-deflector, the sub-deflector is disposed at or near an optically concentric position of the photoreceptor surface scanned by the spot and deflects the laser beam dot by dot at high speed. a sub-deflection element control means for controlling the amount of deflection of the laser beam by the sub-deflection element in accordance with image data; a beam cross-section modulating means for cross-sectionally modulating the cross-sectional shape of the laser beam or its light intensity distribution in accordance with the amount of deflection; and illuminance of a spot formed on the photoreceptor by the laser beam cross-sectionally modulated by the beam cross-section modulating means. 1. A laser scanning surface image forming apparatus, comprising: a laser output control means for controlling an output light amount of the laser light source so that the peak illuminance of the distribution is substantially constant. 2. The laser scanning image forming apparatus according to claim 1, wherein the beam cross-section modulating means is constituted by a light-shielding plate that blocks part of the cross-section of the laser beam according to the amount of deflection of the laser beam by the sub-deflection element. A laser scanning image forming device. 3. The laser scanning image forming apparatus according to claim 1, wherein the beam cross-section modulating means is arranged mainly in a sub-deflection direction such that its transmittance or reflectance changes within an effective diameter of the laser beam at a position where the beam cross-section modulating means is disposed. 1. A laser scanning image forming apparatus comprising an optical filter or a reflector having distribution characteristics.