JPH08142399A - Jitter component removing apparatus - Google Patents

Abstract

PURPOSE: To prevent the image quality deterioration such as banding due to jitter by placing an optical fiber for removing spatial frequency components generated by the interference of a jitter component and the carrier frequency component of an image signal in a scanning optical system. CONSTITUTION: A semiconductor laser light source 1 is so placed as to coincide with the focal point of a first optical system 3 at an object side, and the image forming surface of a laser beam LB emitted from the source 1 coincides with the focal point of a second optical system 5 at the image side. An optical shut- off member 7 as an optical spatial frequency filter is arranged at the position of the system 5 corresponding to the focal point of the object side at the image side focal point of the system 3. The member 7 is so formed as to remove the interference frequency component occurring from the interference between the jitter component generated due to the rotation unevenness of a rotary polygon mirror 4 and the carrier frequency component of the modulation signal of the image signal. Thus, the image of high quality having no banding due to jitter can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser printer for forming an image by modulating a laser beam according to an image signal and scanning and exposing the modulated laser beam onto a photosensitive member by a rotating polygon mirror. The present invention relates to a jitter component removing device used in an image forming apparatus such as a digital copying machine and for removing a jitter component generated due to uneven rotation of a rotary polygon mirror.

[0002]

2. Description of the Related Art Conventionally, in an image forming apparatus such as a laser printer or a digital copying machine which forms an image by scanning and exposing a laser beam of this kind on a photosensitive drum by a rotary polygon mirror, a binary image is mainly used. A character image is formed based on the image signal. In the image forming apparatus such as the laser printer or the digital copying machine, the image quality of the character image is deteriorated due to the rotation fluctuation of the rotating polygon mirror, the photosensitive drum, or the like.

Therefore, in order to improve the deterioration of the image quality of a character image in such a digital image, the relationship between the image quality and the image quality is studied by paying attention to the raggedness and blur of the edge portion of the character. "An Analy
sis of Edge Raggedness and Blur "Journal of
Applied Photographic Engineering, Vol.7, No.6
(1981): p148-151.

In a laser printer for printing a character image, as a research for preventing the deterioration of the image quality due to the rotation fluctuation of the photosensitive drum, the second non-impact printing technology symposium proceedings 1-1 (198).
As disclosed in 5), there is a method in which the conditions of the beam diameter and the laser intensity that do not cause a problem in image quality are obtained even when the rotation of the photosensitive drum changes.

Through these researches and technical developments, in image forming apparatuses such as laser printers and digital copying machines, it is possible to prevent deterioration of the image quality of character images due to fluctuations in rotation of the photosensitive drum.

However, in recent years, in an image forming apparatus such as a laser printer or a digital copying machine which forms an image by scanning and exposing the above-mentioned laser beam onto a photosensitive drum by a rotary polygon mirror, diversification of document processing in an office. Accordingly, it has been required to form not only a character image but also a halftone image such as a photographic image. In order to reproduce a halftone image in the image forming apparatus such as the laser printer or the digital copying machine, for example, as disclosed in JP-A-53-19201, a reference signal and an image are generated by a binarizing circuit. A method of performing comparison with a signal and turning on / off the output of a laser beam according to the comparison result is adopted as a basic technique.

The image signal conversion device according to Japanese Patent Application Laid-Open No. 53-19201 is a device for converting an electrical analog input signal representing an image into an output signal in the form of a corresponding dot pattern. A device for generating a time-varying function which is a function of the first and second signals having different frequencies; and (b) said as a series of continuous scans during the generation of each dot forming said dot pattern. A device for receiving an analog signal, and (c) coupled to the generator and the receiver to compare the continuous scan with the function and generate a difference signal when the function differs from the continuous scan. And a device for generating an output signal in the form of the dot pattern in response to the difference signal.

On the other hand, the output of the laser beam is turned on and off based on the image signal obtained by the image signal converting device constructed as described above, and this laser beam is scanned and exposed on the photosensitive drum by using the rotating polygon mirror. Thus, in an image forming apparatus that forms a halftone image, a direction (sub-scanning direction) orthogonal to the rotation of a rotary polygon mirror called a wobble that is synchronized with the rotation period of the rotary polygon mirror, that is, along the rotation direction of the photosensitive drum. Fluctuations are likely to appear. The fluctuation in the sub-scanning direction of the rotary polygon mirror appears visually as band-shaped density unevenness along the sub-scanning direction called banding (band-shaped) noise, and image signals with particularly small density changes are continuously input. Therefore, it is necessary to take measures to significantly deteriorate the quality of the image.

[0009] Therefore, as a study that can contribute to the prevention of banding noise caused by fluctuations in the sub-scanning direction of a rotary polygon mirror called such a wobble, for example, the 3rd Non-Impact Printing Technology Symposium Proceedings (1986) ) As disclosed in p133, an attempt has been already made to clarify the evaluation scale of the countermeasure by clarifying the relationship between the displacement of the rotary polygon mirror in the sub-scanning direction and the banding and defining the range of fluctuation. ing.

Further, as a countermeasure against banding noise caused by such positional deviation of the rotary polygon mirror in the sub-scanning direction,
As disclosed in Japanese Patent Application Laid-Open No. 63-177190, the rotary polygon mirror and the image surface, which is the surface of the photosensitive drum, are arranged so as to have an optically conjugate relationship only in the sub-scanning direction. Attempts have been made to eliminate the influence of the tilting of the reflecting surface of the rotary polygon mirror, which is effective in reducing banding noise due to fluctuations in the sub-scanning direction of the rotary polygon mirror.

[0011]

However, the above-mentioned prior art has the following problems. That is, in the image forming apparatus that forms a halftone image by scanning the laser beam with the rotary polygon mirror, another problem inherent in image quality is that the rotary polygon mirror rotates along the rotation direction. The unevenness or the processing accuracy of the mirror surface of the rotary polygon mirror, the occurrence of exposure position shift along the rotation direction (main scanning direction) of the rotary polygon mirror called jitter, which is caused by the vibration generated in the drive system of the rotary polygon mirror, etc. . The image quality deterioration due to this jitter is not only the image quality deterioration such as line fluctuations for characters and thin line images, but also banding noise as shown in FIG. 7 as image unevenness in the spatial frequency in the sub-scanning direction corresponding to the frequency of the jitter. There was a problem that it appeared. Although banding noise due to this jitter causes deterioration of the sensory value of the image quality of the halftone image more than that for characters and thin line images, it has not been theoretically supported or considered in the past.
At present, neither effective measures nor evaluation methods have been proposed.

Further, both the banding due to the wobble and the banding due to the jitter described above occur in the sub-scanning direction of the rotary polygon mirror, and the directions in which the image quality deterioration occurs are the same, so visually. It is difficult to separate because it is indistinguishable, and in particular, it cannot be evaluated by a measuring method considering visual characteristics.
Further, in many cases, the banding spatial frequency due to the jitter is included in the spatial frequency region of the image signal, and therefore it is difficult to remove only the banding due to the jitter separated from the image signal.

Therefore, conventionally, an effort to suppress the jitter to about 15 μm to 20 μm has been made based on an argument without theoretical support such as “If the jitter is suppressed to about 15 μm to 20 μm, the banding is not conspicuous”, relying solely on experience. Has been done. Therefore, a drastic increase in cost due to unnecessary high precision rotation of the rotary polygon mirror, and a result of deteriorating the spatial frequency response of the optical system, the developing system, etc. as a countermeasure against banding, resulting in graininess and gradation Has a problem in that important image quality items such as spatial frequency response as an image forming system are newly deteriorated.

To further explain, for example, when feedback control such as PLL control is adopted in the driving portion of the rotary polygon mirror in order to suppress the occurrence of jitter, the feedback cycle will be taken into consideration when the rotational speed of the rotary polygon mirror is taken into consideration. Although it is necessary to make it extremely short, the effect of control can hardly be expected in consideration of the rotational characteristics of the rotary polygon mirror having a rotational speed of several thousand to several tens of thousands rpm. Even if feedforward control is introduced in the drive control of the rotary polygon mirror, it is necessary to apply a large amount of energy to the rotary polygon mirror in a short time in order to suppress the occurrence of jitter, which only increases the cost. However, various secondary problems such as interference with the image signal and increase in power consumption due to abrupt changes in the control signal are newly generated, and it cannot be an effective technique for suppressing the jitter. It was

On the other hand, when electrical control is applied to the image signal in order to suppress the jitter, interference of the control signal with the image signal causes a new image quality deterioration and the effect of suppressing the jitter is not sufficient. Alternatively, various problems such as causing new deterioration of image quality due to overcorrection newly occur.

Further, as a technique for optically providing a filter characteristic for blocking a banding spatial frequency band, a technique according to US Pat. No. 4,884,083 has already been proposed. But in the case of this technology,
Although the banding can be suppressed by the wobble, there is a problem in that the cutoff region interferes with the spatial frequency region of the image signal, which causes blurring of the image and deterioration of gradation.

Therefore, the present invention has been made to solve the above-mentioned problems of the prior art, and the object thereof is to avoid an increase in cost and complication of the structure.
An object of the present invention is to provide a jitter component removing apparatus which can prevent image quality deterioration such as banding due to jitter with a low cost and a simple configuration.

[0018]

SUMMARY OF THE INVENTION The present invention is based on the fact that banding noise caused by jitter is caused by interference between a jitter component in the spatial frequency domain and a carrier frequency component of the image signal. By arranging an optical filter for removing the spatial frequency component generated by the interference of the carrier frequency component in the scanning optical system, it is possible to prevent the image quality deterioration such as banding due to the jitter.

That is, in the jitter component removing apparatus according to the first aspect of the present invention, in the laser beam scanning optical system for scanning and exposing the laser beam modulated by the image signal on the photosensitive member by the rotating polygon mirror, the rotation is performed. An optical spatial frequency filter for removing an interference frequency component appearing from interference between a jitter component generated by uneven rotation of a polygon mirror and a carrier frequency component of the modulation signal is formed by an optical image forming laser beam on a photoconductor. It is configured so as to be arranged near the focus on the object side of the system.

[0020]

In many cases, the spatial frequency domain in which the image information exists is set to be lower than or equal to the carrier frequency of the modulated signal of the image, that is, the screen frequency component from the viewpoint of moire prevention and sampling theory.

Here, when the jitter occurs, a frequency component spread in the sub-scanning direction of the screen frequency component appears. Since this frequency component is outside the spatial frequency region where the image information exists, by optically shielding it with an optical spatial frequency filter, the influence of jitter can be prevented without deteriorating the spatial frequency response. It can be removed or greatly reduced.

As described above, according to the present invention, it is possible to remove banding due to jitter, which cannot be removed by image processing. Further, since the spatial frequency domain to be cut off exists outside the spatial frequency domain of the image signal, there is no factor of image quality deterioration. Further, since the blocking is performed optically, there is no need for electrical / mechanical feedback, and there are effects such as low cost and high reliability.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to illustrated embodiments.

FIG. 1 shows an embodiment of a laser printer to which the jitter component removing apparatus according to the present invention is applied.

In FIG. 1, reference numeral 1 denotes a semiconductor laser light source, and this semiconductor laser light source 1 emits a laser beam LB modulated according to an image signal output from an image signal generator 2. The image signal generator 2 is adapted to generate an image signal having a pulse width according to the image density, and to form a halftone image by a screen structure such as a parallel line screen or halftone dots. There is. The laser beam LB emitted from the semiconductor laser light source 1 is converted into parallel light by a first optical system 3 including a collimator lens and guided to a rotary polygon mirror 4 and reflected by a mirror surface 4 a of the rotary polygon mirror 4. To be done. The rotary polygon mirror 4 is driven to rotate at a high speed in the arrow direction by a drive motor (not shown), and the laser beam LB reflected by the mirror surface 4a of the rotary polygon mirror 4 has a predetermined scanning speed. The scanning is deflected along the main scanning direction. The laser beam LB deflected and scanned by the rotary polygon mirror 4 is focused on the photosensitive drum 6 as a spot image along the main scanning direction via the second optical system 5 including an f-θ lens. It The photoconductor drum 6 rotates at a predetermined rotation speed in the sub-scanning direction, and an electrostatic latent image corresponding to image information is formed on the photoconductor drum 6. In this laser printer, an image is formed by developing the electrostatic latent image formed on the photosensitive drum 6.

Although not shown in the above embodiment,
In order to correct the surface tilt of the rotary polygon mirror 4, a cylindrical lens or the like having an optical power only in the sub-scanning direction having a mirror surface of the rotary polygon mirror as a focal point is provided between an optical shielding member described later and the rotary polygon mirror 4. An optical member is inserted, and further, the second optical system 5 is anamorphicized so that the mirror surface 4a of the rotary polygon mirror 4 and the image surface, which is the surface of the photosensitive drum 6, are arranged to be conjugate with each other in the sub-scanning direction. It may be configured.

By the way, in this embodiment, the optical spatial frequency for removing the interference frequency component appearing from the interference between the jitter component generated by the uneven rotation of the rotary polygon mirror and the carrier frequency component of the modulation signal of the image signal. The filter is arranged near the focus on the object side of the optical system that forms the laser beam on the photoconductor.

That is, in this embodiment, as shown in FIG. 1, the optical blocking member 7 as an optical spatial frequency filter is the image-side focal point of the first optical system 3 and the second optical system. 5 is located at a position corresponding to the object-side focal point. It should be noted that the optical blocking member 7 does not necessarily have to be exactly aligned with the image-side focus of the first optical system 3 and the object-side focus of the second optical system 5, and the optical blocking member 7 of the first optical system 3 is not necessarily required. If it is arranged in the vicinity of the image-side focus and the object-side focus of the second optical system 5, the same operation / effect can be obtained. As will be described later, the optical blocking member 7 is configured to remove an interference frequency component that appears due to the interference between the jitter component generated by the uneven rotation of the rotary polygon mirror 4 and the carrier frequency component of the modulation signal of the image signal. It is configured.

FIG. 2 schematically shows an optical system portion of a laser printer to which the jitter component removing apparatus according to this embodiment is applied.

As shown in FIG. 2, the semiconductor laser light source 1 is arranged so as to substantially coincide with the object-side focal point of the first optical system 3. On the other hand, the image plane of the laser beam LB emitted from the semiconductor laser light source 1 is substantially coincident with the image side focus of the second optical system 5. Now, as described above, the optical blocking member 7 is arranged at the image-side focal point of the first optical system 3, and the optical blocking member 7 is the second focal point.
If it is placed at the object-side focal point of the optical system 5, the Fourier imaging theory (Optical Technology Handbook, Supplement p145-157
(1975, Junpei Tsujiuchi), a spectrum obtained by Fourier-transforming the light amplitude distribution at the emission point of the semiconductor laser light source 1 appears on the optical blocking member 7, and the image plane which is the surface of the photoconductor drum 6 appears. It is known that the light amplitude distribution in (3) has the property that it generally matches the inverse Fourier transform spectrum of the light amplitude distribution passing through the optical blocking member 7, which is called a pupil function. At this time, even if the optical blocking member 7 does not exactly match the image-side focus of the first optical system 3 and the object-side focus of the second optical system 5, the effect on the effect of the present invention Since it is small, the description will proceed assuming that they match.

Here, the pupil function means a complex amplitude distribution generated on the exit pupil of the image forming system by a point light source placed on the object plane, the amplitude transmittance of the lens is t (ξ, ζ), and the wavefront aberration is W
Assuming that (ξ, ζ), the pupil function f (ξ, ζ) is f (ξ, ζ) = t (ξ, ζ) exp {ikW (ξ, ζ)} (ξ ² + ζ ² ≦ ρ ² ) = 0 (Ξ ² + ζ ² > ρ ² ) It is expressed by (1). Here, ρ is the radius of the exit pupil, and k is the phase constant.

A light intensity distribution (Point Spread Fun) on the photoconductor drum 6 which is an image forming surface of the laser beam LB emitted from the semiconductor laser light source 1.
ction: PSF) is equal to the power of the light amplitude distribution on the above-mentioned image plane, that is, the square of the absolute value of the light amplitude distribution. Here, PSF is an intensity distribution of an image of an optical system with respect to a point object, is a function of an image plane position and a wavelength, and in terms of wave optics, a pupil function is subjected to Fraunhofer diffraction integration to obtain a point image. It is obtained by calculating the complex amplitude distribution and then taking the square of the absolute value of the distribution.

By the way, since the image signal input from the image signal generator 2 to the semiconductor laser light source 1 is a time-series signal which changes with time, it differs on the image plane composed of the surface of the photosensitive drum 6. The laser beams LB emitted to the positions are emitted from the semiconductor laser light source 1 at different times and do not interfere with each other. Therefore,
The optical system that scans and exposes the laser beam LB emitted from the semiconductor laser light source 1 according to the image signal on the photosensitive drum 6 is considered to be an imaging system including an incoherent (non-coherent) optical system. Therefore, the actual energy distribution of the laser beam LB imaged on the image plane composed of the surface of the photoconductor drum 6 can be obtained by considering the image formation state of the incoherent (incoherent) optical system. Yes, the integration of the PSF at each instant, that is, the composite product of the PSF and the input image signal (Convol
It is known that the

That is, this composite product (Convolut
ion) h (x) is the PSF of the laser beam LB g
(X), where the input image signal is f (x), it is expressed by the following integral formula. h (x) = ∫g (x ′) f (x−x ′) dx ′ = ∫g (x−x ′) f (x ′) dx ′ (3) The integral extends from −∞ to + ∞. Shall be taken.

This integral calculation can also be expressed as follows using the symbol *. h (x) = g (x) * f (x) (4) Here, h (x), g (x), and f (x) are Fourier transformable functions, and these functions h (x), g (x), f
Fourier transform of (x) is H (μ), G (μ),
If it is F (μ), it can be expressed as H (μ) = G (μ) F (μ) (5). In the integration calculation, physically, a certain signal sequence f (x) is linearly filtered (impulse response:
It corresponds to the output signal string (h (x)) when passing through g (x). That is, in an optical imaging system, its characteristics can be described as linear filtering. A laser beam LB emitted from the semiconductor laser light source 1 according to an image signal is applied to the photosensitive drum 6
As described above, the optical system that scans and exposes the light is an incoherent (incoherent) optical system, and the isoplanatism (imaging near the axis of the axisymmetric optical system is on-axis). Equality), that is, a shift-invariant imaging system, and its point spread function is h (x, y)
Then, the intensity distribution i (x, y) of the image of the object whose intensity distribution is o (x, y) by the imaging system is the object function o (x,
y) and the point spread function h (x, y) composite product (Convo
i) (x, y) = ∫∫o (x ′, y ′) h (x−x ′, y−y ′) × dx′dy ′ (6). Note that the integration is taken over -∞ to + ∞.

As described above, when the imaging by the optical system is dealt with, the relation between the spatial Fourier spectra of each object is, as described above, except that the relationship between the object and the amplitude distribution or the intensity distribution of the image is discussed. Fourier imaging theory discussed as a relationship between
(1975) Junpei Tsujiuchi) can be applied. In this Fourier imaging theory, the image formation from the object plane to the image plane is to be investigated by what kind of image Fourier spectrum of the object the Fourier spectrum of the object is transmitted to.

An incoherent optical system that satisfies isoplanatism (image formation near the axis of an axially symmetric optical system is equal to that on the axis), that is, shift-in. It is a variant imaging system, where the intensity distribution of the geometrical image of the object is o (x, y), the intensity distribution of the actual image is i (x, y), and the point image intensity distribution is h (x, y). Then, between these, the composite product (Convol
relationship) is established. When the Fourier transforms of these functions are each normalized by the spectrum of zero frequency (N _x = 0, N _y = 0), O (N _x , N _y ) = [∫∫o (x ′, y ′) × exp _{{-2πi (N x x '+} N y y')} dx'dy ' ] / ∫∫o (x', y ' ) dx'dy' (7) I (N x, N y) = [∫∫i (X ', y') x exp {-2πi (N _x x '+ N _y y')} dx'dy '] / ∫∫i (x', y ') dx'dy' (8) H (N _x , N _y) = [∫∫h (x ', y') × exp {-2πi (N x x '+ N y y')} dx'dy ' ] / ∫∫h (x', y ' ) dx' It can be expressed as dy ′ (9).

In the case of an incoherent optical system, the integral value of these denominators used to normalize the Fourier spectrum represents the total amount of light in each image, and the image has no structure (N _x = N _y = 0) corresponds to the Fourier spectrum of the background, that is, the DC component of the image. Therefore, in the Fourier spectrum of the image structure in the numerator of each equation, the ratio of the DC component to the background may be taken into consideration. Now, the composite product (Convol
If both sides of the imaging equation of the equation (6), which are in the relationship of (1) are substituted by the above equations (7) to (9), I (N _x , N _y ) = H (N _x , N _y ) O (N _x , N _y ) (10) is obtained. This is an expression representing the transfer of the Fourier spectrum of the object of the incoherent optical system to the image, and H
(N _x , N _y ) is called the spatial frequency transfer function (OTF) of the incoherent optical system.

Therefore, in the optical system that scans and exposes the laser beam LB emitted from the semiconductor laser light source 1 according to the image signal onto the photosensitive drum 6, when the semiconductor laser light source 1 is an object, this semiconductor laser light source 1
Are arranged so as to substantially match the object-side focus of the first optical system 3, and the optical blocking member 7 is arranged at the image-side focus of the first optical system 3, as shown in FIG. Has been done. Therefore, a spectrum obtained by Fourier transforming the light amplitude distribution at the emission point of the semiconductor laser light source 1 appears on the optical blocking member 7. The semiconductor laser light source 1 is modulated by a screen having a parallel line structure having a resolution of, for example, 200 dpi to form a halftone image. Therefore, in an ideal state in which there is no jitter due to the rotation fluctuation of the rotary polygon mirror 4, as shown in FIG. 3, the optical amplitude distribution at the emission point of the semiconductor laser light source 1 is Fourier-transformed on the optical blocking member 7. The converted spectra 10 and 11 appear.

However, in the presence of the jitter due to the rotational fluctuation of the rotary polygon mirror 4, etc., the optical amplitude distribution at the emission point of the semiconductor laser light source 1 is set on the optical blocking member 7 as shown in FIG. Fourier transformed spectra 10, 1
In addition to 1, the jitter component generated by the uneven rotation of the rotary polygon mirror and the carrier frequency component of the modulation signal of the image signal (200
From the interference with the (dpi resolution), the spatial frequency component 12 generated by the jitter appears.

In view of this, in this embodiment, based on the feedforward concept, the spatial frequency component appearing in the presence of jitter due to the rotational fluctuation of the rotary polygon mirror 4 is blocked by an optical spatial frequency filter. Therefore, as shown in FIG. 5, a spectrum obtained by Fourier-transforming the regular light amplitude distribution of the semiconductor laser light source 1 is transmitted through the optical blocking member 7, and the spatial frequency component generated by the jitter is removed. The pattern 7a is formed. As the optical blocking member 7, for example, a transparent glass plate in which a portion for removing a spatial frequency component generated by jitter is colored black in a cross shape is used.

FIG. 6 and FIG. 7 concretely show the above relationship.

FIG. 6A shows an image signal for modulating the laser beam LB emitted from the semiconductor laser light source, and this image signal is repeatedly turned on and off at a constant cycle. FIG. 6B shows the light intensity distribution of the laser beam in a stationary state on the image plane.
FIG. 7A shows the energy distribution of the laser beam LB which is modulated by the image signal shown in FIG. 6A and is irradiated onto the image plane. It is represented by a composite product (Convolution).

FIG. 7B shows the Fourier transform image of FIG. 7A, that is, the spatial frequency distribution. Although a so-called parallel line screen in which the same pattern is repeated in the sub-scanning direction is used as the screen here, the same discussion can be made in any screen form.

In FIG. 7B, 13 is the image information O.
(N_x, N_y) DC component (N_x= N _y0) mean strength
Which appears at coordinates (0,0). 1
4 is the basic spectrum of the screen (x₁, 0) (-
x₁, 0) and x₁Is a line screen solution
If the image resolution is 200 dpi, it corresponds to 8 dot / mm.
Value. 15 is a pixel pitch (0,
y₂) (0, -y₂) Is represented. 16 is a screen
Harmonic component of (2 × x₁, 0), (-2 × x₁, 0),
(3 × x₁, 0), (-3 × x₁, 0) ………, dotted line 1
The area surrounded by 7 is the spatial frequency area where the image information exists.
Is shown.

FIGS. 8 and 9 show the state with jitter as in FIGS. 6 and 7. FIG. 9B and FIG.
When the spatial frequency distributions of (b) are compared, it is shown in FIG. 9 (b) that the spectrum 18 which does not exist in FIG. 7 (b) appears due to the jitter as described above. That is, in FIG. 9 (b), a basic spectrum (x _1, 0) of the screen - the (x _1, 0) and the position of the same x-coordinate, y
It can be seen that a plurality of spatial frequency components appear along the direction. A plurality of spatial frequency components generated along the y direction at the same x-coordinate position as the basic spectrum of this screen are generated by the jitter.

From this, the spectrum is generated by the interference between the jitter and the screen, and can be considered as "the transverse wave called jitter is modulated by the screen". In this case, the spectrum 13 can be regarded as the sideband of the spectrum 2 of the screen. Also, the spectrum appears outside the spatial frequency domain of the image signal,
It does not have a region in common with the spatial frequency region of the image signal.

Therefore, the spectrum 13 is located at the position 7 in FIG.
The banding due to the jitter can be eliminated by disposing the optical blocking member 7 that blocks the optical frequency band and the spectrum of the screen and the spectrum of the screen.

FIGS. 10 and 11 show a state in which the optical blocking member is inserted into the system having jitter shown in FIGS. 8 and 9. FIG. 10A shows the input signal distribution on the image plane where the jitter is superimposed, which is the same as FIG. 8A. FIG. 10B is a light intensity distribution of the laser beam in a stationary state on the image plane when the optical blocking member is inserted, and FIG. 11A is an actual energy distribution on the image plane. b) represents the spatial frequency distribution of energy on the image plane. It can be seen that the influence of jitter is eliminated as compared with FIG.

Next, parameters that define the shape of the optical blocking member 7 will be described. Generally, it is assumed that various parameters are as follows. Focal length of the second optical system ...... f (mm) Wavelength of the laser beam of the semiconductor laser light source 1 ... λ (nm) Screen pitch on the image plane ...... d (μm) Pixel pitch on the image plane ...... p (μm) Number of mirror surfaces 4a of the rotary polygon mirror 4 ... N

In many cases, the second optical system 5 includes
Since it is necessary to linearly convert the angular velocity of the rotary polygon mirror 4 into the scanning velocity on the image plane, a negative distortion aberration called an f · θ characteristic is provided. That is, when the angle formed by the light beam incident on the second optical system 5 and the optical axis is θ (rad), the image height h on the image plane is represented by h = f · θ, but on the image plane When the beam diameter is in the range of several tens of μm, the beam narrowing angle is very small (several tens in terms of F / No) tan θ = θ = sin θ.

In this case, the sine wave image having a period of D μm appears as a spectrum at a position of f × λ / D × 10 ⁻³ (mm) from the optical axis on the optical Fourier transform plane (reference, “Visio”).
n and Visual Perception ”,
John Wiley and Sons Inc.
N. Y. (See (1965)), the spectral distribution as shown in FIG. 12, that is, the spectral position (x ₁ , y ₁ ) of the screen pitch is √ (x ₁ ² + y ₁ ² ) = F × λ / d × 10 ⁻³ (mm). In addition, the spectral position of the pixel pitch (x ₂ , y
₂ ) becomes x ₂ = 0 y ₂ = ± f × λ / p × 10 ⁻³ (mm).

At this time, the position (x, y) of the spectrum due to the jitter is x ₃ = x ₁ (mm) y ₃ = ± k / n × y ₁ (mm), where k is an integer of 0 <k <n Appears in the position. The spectral region of the image signal is {(x ₄ , y ₄ ): √ (x ₃ ² + y ₃ ² ) ≦ 1/2 × √ (x ₂ ² + y ₂ ² )}, The shaded area 7a in FIG. 9, that is, "area including {(x ₃ , y ₃ )} and not including {(x ₄ , y ₄ )}" is shielded. As a result, the jitter component can be removed.

More specifically, the optical blocking member 7 described above.
The parameters that define the shape will be described.

Now, various parameters of the laser scanning optical system are the focal length f of the second optical system 5 = 320 (mm), the wavelength of the laser beam of the semiconductor laser light source 1 λ = 780 (nm), and the screen pitch d on the image plane. = 84 (μm) Pixel pitch on image plane p = 42 (μm) Number of mirror surfaces 4a of rotary polygon mirror 4 n = 8 In a system of (x ₁ , y ₁ ), (x ₂ ,
y ₂ ), (x ₃ , y ₃ ), and (x ₄ , y ₄ ) are represented as shown in FIG. 13, respectively.

Therefore, by creating an optical shield member that shields the area given by these coordinates,
It is possible to prevent deterioration of image quality due to jitter.

[0057]

EFFECTS OF THE INVENTION The present invention has the above-described structure and operation, and can eliminate the jitter component caused by the uneven rotation, vibration, processing accuracy, etc. of the rotary polygon mirror without affecting the image quality. Therefore, a high-quality image without banding due to jitter can be obtained.

[Brief description of drawings]

FIG. 1 is a perspective configuration diagram showing an embodiment of a laser printer to which a jitter component removing device according to the present invention is applied.

FIG. 2 is a schematic diagram showing the optical system of FIG.

FIG. 3 is an explanatory diagram showing a spatial frequency distribution.

FIG. 4 is an explanatory diagram showing a spatial frequency distribution.

FIG. 5 is a front view showing an optical blocking member.

6 (a) and 6 (b) are explanatory views respectively showing a state where there is no jitter.

7 (a) and 7 (b) are explanatory views respectively showing a state where there is no jitter.

FIG. 8A and FIG. 8B are explanatory diagrams respectively showing states with jitter.

9 (a) and 9 (b) are explanatory views respectively showing states with jitter.

10 (a) and 10 (b) are explanatory views showing states in which the influence of jitter is prevented.

11A and 11B are explanatory diagrams showing states in which the influence of jitter is prevented.

FIG. 12 is an explanatory diagram showing design values of the optical blocking member.

FIG. 13 is a table showing design values of the optical blocking member.

[Explanation of symbols]

1 semiconductor laser light source, 3 first optical system, 4 rotating polygon mirror, 5 second optical system, 6 photosensitive drum, 7 optical blocking member.

Claims (1)

[Claims] 1. A laser beam scanning optical system for scanning and exposing a laser beam modulated by an image signal onto a photoconductor by a rotary polygon mirror, and carrying a jitter component and the modulation signal generated by uneven rotation of the rotary polygon mirror. Jitter characterized by arranging an optical spatial frequency filter for removing an interference frequency component appearing due to interference with a frequency component in the vicinity of a focus on the object side of an optical system for forming a laser beam on a photoconductor. Component removal device.