JPH0653420B2 - Laser optical scanning device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a laser beam scanning device, and more particularly to a laser beam scanning device capable of preventing color misregistration of a multicolor recording laser printer. Regarding

(Prior Art) Conventionally, image information is recorded by modulating a laser beam according to an image signal, irradiating the information recording medium with the modulated laser beam through an optical system, and scanning exposing the information recording medium. Laser printers are known. FIG. 8 is a block diagram showing an example of a conventional device of this type.

In the figure, 1 is a laser that emits a light beam, 2 is an optical modulator that modulates the light beam B emitted from the laser 1 in accordance with an image signal, and 20 is a predetermined light receiving the output light of the optical modulator 2. It is a beam expander that converts to the beam size of. The beam expander 20 is composed of, for example, optical lenses 3 and 4.

Reference numeral 5 is a polygon mirror that distributes the transmitted light of the beam expander 20 in the main scanning direction, 6 is a motor that rotates the polygon mirror 5 at a constant speed, and 7 is a constant speed scan that receives reflected light from the polygon mirror 5 f θ It is a lens. An information recording medium 8 receives the scanning light from the fθ lens 7. Reference numeral 9 denotes a position detection sensor that receives a light beam to detect an initial recording position, 10 a conveying roller that conveys the information recording medium 8 in the sub-scanning direction (winding direction), and 11 and 12 conveys the information recording medium 8. It is a pressure bonding roller that is pressure bonded to. The outline of the operation of the apparatus configured as described above is as follows.

The light beam B emitted from the laser 1 is modulated by the light modulator 2 according to the image signal. The light beam is converted into a predetermined beam diameter by the beam expander 20. The converted light beam is distributed by the Poygon mirror 5 in the main scanning direction of the information recording medium 8 and is irradiated toward the information recording medium 8 via the f θ lens 7. At this time, when the light beam hits the position detection sensor 9, a counter (not shown) starts counting the clock signal, and when the count value reaches a predetermined value, the light beam modulated by the image signal becomes the information recording medium. Irradiation of 8 is started. L indicates the scanning line at this time.
Then, when the count value of the counter reaches a predetermined value, the light beam is turned off and the exposure operation is ended. When the scanning of one line is completed, the transport roller 10 rotates to transport the information recording medium 8 for one line in the sub-scanning direction (winding direction). Then, a new scan is performed. The information recording medium 8 that has been subjected to scanning exposure is developed and fixed, and predetermined image information is recorded. If a laser that can be directly modulated (for example, a semiconductor laser) is used as the laser 1, the optical modulator 2 can be omitted.

In recent years, laser light of a plurality of wavelengths, for example, red (R), green (G), and blue (B) laser light is irradiated onto an information recording medium,
Laser printers capable of multicolor recording by scanning exposure have been developed. In this type of laser printer, the deviation of the beam spot of each wavelength on the imaging surface due to the chromatic aberration of the imaging lens (f θ lens) becomes a problem. 9th
As shown in the figure, consider a case where laser light B having wavelengths λ ₁ and λ ₂ is incident on the polygon mirror 5 and the reflected light is applied to the information recording medium 8 via the f θ lens 7.

Assuming that the surface of the information recording medium 8 is the imaging position, f
The light passing through the θ lens 7 is slightly deviated into the light having the wavelength λ _{1 and} the light having the wavelength λ ₂ . Such a phenomenon (called chromatic aberration) occurs because the refractive index of the lens differs for each wavelength. The solid line in the figure is the light of λ ₁ , and the broken line is the light of λ ₂ . When λ ₁ and λ ₂ are deviated, color misregistration occurs on the information recording medium 8, which causes deterioration of image quality and reduction of resolution due to thick line width. FIG. 10 is a diagram showing a state of color shift. For example, A
Assuming that the part is an irradiation region by the light of the wavelength λ _{1 and} the part B is an irradiation region by the light of the wavelength λ ₂ , the A ₁ , B ₁ parts form the color misregistration region, and the line width is apparently thickened as it is. .

In order to eliminate such a problem, the following method is adopted.

f θ Lens (imaging lens) is completely achromatic.

The chromatic aberration is corrected by delaying the pixel clock. This method is divided into one clock unit correction and one clock correction. That is, the correction in clock units is performed by delaying the data, and the correction in the clock is performed by D / A.
This is done by delaying the converter clock.

FIG. 11 is a circuit diagram for chromatic aberration correction. The image data passes through the clock unit correction circuit 21 and the D / A converter 22.
Is input to the D / A converter 22 as a conversion clock via the intra-clock correction circuit 23. The operations of the clock unit correction circuit 21 and the in-clock correction circuit 23 are controlled by the control circuit 24. That is, the correction in clock units is performed by controlling the clock given to the clock unit correction circuit 21 by the control circuit 24 to delay the data. On the other hand, the clock correction is performed by the control circuit 24 controlling the in-clock correction circuit 23 and delaying the conversion clock provided from the in-clock correction circuit 23 to the D / A converter 22.

In the case of (Problems to be solved by the invention), a perfect achromatic lens cannot be realized for light having wavelengths of three or more colors. Even in the case of two colors, it is necessary to use a special material in order to manufacture a lens with little chromatic aberration, and a highly accurate assembly is required. Therefore, the yield of the fθ lens also deteriorates, resulting in an increase in cost.

In the case of the method shown in, the pixel clock is constant, so
It is necessary to slightly move the beam deviated from the original position due to chromatic aberration to bring it to the original position. This will be described with reference to FIG. Assuming that the unit delay time between the basic pixel clock and the delayed pixel clock is Δt, it is necessary to sequentially increase the delay amounts to 2Δt, 3Δt, ... To correct the color shift. Therefore, the correction circuit 13 (see FIG. 11)
The delay element used in (1) requires an element capable of delaying up to the period T in units of Δt, for example, a delay line and a selector for selecting a clock of each stage delayed by the delay line.

FIG. 13 is a diagram showing an example of a delay circuit, in which the pixel clock input to the delay line 25 is delayed from Δt to T in steps of Δt. The optimum delay pixel clock is selected by the selector 26 at each of these stages, and is output as the delay pixel clock.

However, such a delay line is expensive (for example, 4000 yen in the case of 8 taps) and the accuracy is not good (for example, Δt ± 5%). Further, in the case of this method, the pixel clock can be delayed so that the image forming position can be moved backward, but it cannot be moved forward.

The present invention has been made in view of such points,
The object is to realize a laser beam scanning device which can move an image forming position back and forth arbitrarily without using an expensive delay line.

(Means for Solving Problems) According to the present invention for solving the above problems, a plurality of laser beams having different wavelengths are modulated in accordance with image information, and an information recording medium is provided via a deflecting unit and an imaging optical system. In a laser beam scanning device for guiding and scanning a laser beam, the storage unit for storing frequency division information of the laser beam and a frequency divider for changing a frequency division ratio according to the output of the memory unit are provided. It is characterized in that the image information is modulated based on the above.

(Operation) The frequency division information of the laser light is stored in advance, and the frequency division ratio of the pixel clock is changed (the pixel clock is increased or decreased) based on the frequency division information. As a result, the pixel clock can be advanced or delayed, and any chromatic aberration pattern can be corrected without causing color shift.

FIG. 14 is an explanatory diagram of a color misregistration correction method according to the present invention. Assuming that the scanning direction is the direction of the arrow in the figure, (a)
Shows the case where the beam position B ₂ displaced from the original beam position B ₁ by chromatic aberration occurs rearward, and (b) shows the case where the beam position B ₂ displaced from the original beam position B ₁ due to chromatic aberration occurs forward. Shown respectively.

To correct the beam position shown in (a), the pixel clock is delayed. In other words, it is sufficient to increase the frequency division ratio N, and
To correct the beam position shown in, advance the pixel clock. That is, the frequency division ratio N may be reduced. In order to achieve such an object, first, a basic pixel clock is created, and this basic pixel clock is increased or decreased.

(1) How to make a basic pixel clock In order to make an image without a sitter, place a photodetector near the scan start position outside the recording area as shown in 9 in FIG. 8 and synchronize with the signal from this photodetector. Then, the image data is read. That is, it is necessary to generate a pixel clock synchronized with the output of the photodetector. In general, as a method of generating this pixel clock, the output signal of the photodetector divides the original oscillation N times the pixel clock by N. Therefore, in principle, there is always 1 / N jitter of 1 dot, but if N is made sufficiently large, it will not be noticeable. FIG. 15 shows a block diagram of the pixel clock generation circuit, and FIG. 16 shows a timing chart of each part.

The oscillator 30 outputs a clock having a frequency Nf ₀ shown in FIG. 16B and enters the N frequency divider 31. Here, the position detector signal as shown in FIG.
When it enters 1 as the reset signal R, the output of the N frequency divider 31 is in the reset state during this period. When the reset signal disappears, the N frequency divider 31 starts the N frequency division and outputs the basic pixel clock f ₀ of the synchronization T ₀ as shown in FIG. 16c.

(2) Method of Increasing / Decreasing Basic Pixel Clock As is apparent from FIG. 15, it is understood that the frequency f ₀ of the basic pixel clock can be increased / decreased by changing the frequency division ratio N. For example, three types of frequency division ratios N ₁ , N ₂ ,
Consider N ₃ (N ₁ <N ₂ <N ₃ ). Then, when it is desired to delay the pixel clock, the division ratio is increased to N ₃ (clock cycle T ₀ becomes longer), and when the pixel clock is advanced, the division ratio is decreased to N ₁ (clock cycle T ₀ becomes shorter).
Thus, the frequency division ratio is changed for each pixel. By such frequency division ratio control, an arbitrary clock cycle pattern can be created.

The case of FIG. 15 will be described as an example. Assuming that the pixel clock frequency when dividing by N ₂ is f ₀ , the pixel clock frequency when dividing by N ₁ is (N ₂ / N ₁ ) × f ₀ , which is high. The pixel clock frequency when divided by N ₃ is (N ₂
/ N ₃ ) × f _{0 and the} frequency becomes low.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a configuration diagram of a main part showing an embodiment of the present invention. The embodiment shown in the figure is used for a blue (B), green (G) and red (R) color laser printer, and the synchronous clock generating circuit shown in FIG. 15 is provided for each of B, G and R. is there. In the figure, 40 is a blue synchronous clock generating circuit, 50 is a green synchronous clock generating circuit, and 60 is a red synchronous clock generating circuit. The synchronous clock generation circuit 40 for blue is 60M
A clock pattern for correcting the color misregistration (the above N ₁ , N ₂ , N ₃ minutes) by receiving the output of the N frequency divider 41, the output of the N frequency divider 41, and the output of the counter 42 as an address. PROM 43 that stores the
It is composed of The output of the PROM 43 is the N frequency divider 4
Input to 1 to determine the division ratio. For example, N divider 4
If 0, 1, 2 are input to the load input of 1, respectively,
The division ratios are set to 16, 15, and 14, respectively. or,
The blue light detection required signal is the N frequency divider 41 and the counter 42.
Is given as a reset pulse. The above configuration is exactly the same for the green synchronous clock generating circuit 50 and the red synchronous clock generating circuit 60.

Here, it is assumed that the division ratios of N divided periods in the synchronous clock generation circuit are, for example, N ₃ = 16, N ₂ = 15, and N ₁ = 14. Here, if the frequency division ratio of N ₁ and N ₃ is set to a value that is far from N ₂ , the corrected portion becomes conspicuous, which is not preferable. The operation of the circuit thus configured will be described below with reference to the timing chart shown in FIG.

When a photodetection signal from a photodetector (not shown) is input as shown in FIG. 2A, the N frequency divider 41b and the output of the counter 42 are output as shown in FIGS. ) Cleared as shown. When the light detection signal falls, the N frequency divider 4
1 starts frequency division of the 60 MHz clock a shown in FIG. It is assumed that the PROM 43 output initial value is 0. When the output of the N frequency divider 41 rises as shown in (C), the count 42 counts this rise at this rise and rises as shown in (E). When the output of the counter 42 rises, the input address of the PROM 43 changes and P
The ROM 43 gives "1" to the N frequency divider 41.

When the N divider 41 reaches the count value of 15 by this,
The carry signal C as shown in (d) is output, the signal is taken into the own input and the load input "1" is loaded. As a result, the N frequency divider 41 is set to the frequency division ratio (15 frequency division) according to "1". Next, when the output of the N frequency divider 41 rises as shown in (c), the counter 42
The output falls as shown in (e). This allows PROM
43 outputs "0". Next, carry C and "0" is N
The frequency divider 41 is loaded, and the N frequency divider 41 is set to a frequency division ratio (16 frequency divisions) according to “0”. Further N divider 41
When the output rises, the counter 42 output falls. As a result, the PROM 43 outputs "2". Then, in the next carry C, “2” is loaded into the N frequency divider 41, and the N frequency divider 41 is set to the frequency division ratio (14 frequency division) according to “2”.

The operation of the blue synchronous clock generating circuit 41 has been described above, but the operation of the remaining green synchronous clock generating circuit 50 and red synchronous clock generating circuit 60 is exactly the same. As described above, according to the present invention, the synchronization clock can be arbitrarily increased or decreased according to the value of the PROM, and thus the color shift due to the chromatic aberration of the lens can be corrected.

In this embodiment, the light detection signals of the respective colors are independent, but in FIG. 8, the color shift (light detection signal of the respective colors by the above-described correction is performed during the interval from the position detection sensor 9 to the effective image area (left margin). Can be corrected, it is possible to obtain a light detection signal with light of a specific one color.
This will be described in more detail.

For example, in FIG. 1, the light detection signal from red light is also used as the blue and green detection signals. It is assumed that the beam spots of each color at the position of the position detection sensor 9 are lined up as shown in FIG. As shown in the figure, when the light detection signal is taken in red, the other two colors are matched with the number of pixels of the left margin and the above-mentioned correction to make the three colors coincide at the recording start position.

For example, if the left margin value of red is 100 clock counts, the other two colors are set to 102 clock counts for green and 98 clock counts for blue in the example of FIG. Furthermore, since a shift of Δt ₁ and Δt ₂ remains, this is canceled in the manner of color shift correction.

Furthermore, if an arbitrary one color is used as a reference, it is sufficient to match the reference one color of the other two colors. Therefore, the PROM clock pattern is not required for all three colors, and only two colors are required.

FIG. 4 is a main part structural view showing another embodiment of the present invention, and shows a case where red is taken as a reference. In the embodiment shown in the figure, the synchronous clock generating circuit for red is only an N frequency divider.
It is used as a frequency divider by 5. Regarding the other blue synchronous clock generating circuit and green synchronous clock generating circuit, the N frequency divider is configured to be variable by the PROM output.

Next, a modified example will be described. If the same pixel is corrected (clock increase / decrease) in each line, unevenness may appear in the sub-scanning direction. FIG. 5 is a diagram showing the occurrence of unevenness in the sub-scanning direction. The spots shown by the diagonal lines in the figure indicate unevenness. To prevent this, pixels to be corrected are randomly arranged in each line. This is the first
The counter unit shown in the figure is divided into upper and lower parts as shown in FIG. 6, and only the upper part is reset by the position detection sensor output.

With this, the lower bits take random values for each line, so if n bits are allocated to the lower bits, 2 ⁿ
The pixels to be corrected among the pixels randomly vary in each line. Alternatively, the counter may be loaded with a random value by the photodetector output.

Furthermore, when the gradation characteristics differ between the pixel to be corrected and the pixel not to be corrected, it is possible to prepare another γ correction table for the pixels to be divided by N ₁ , N ₂ , and N ₃ , respectively.

It is also possible to correct the deviation from the fθ characteristic of the fθ lens at the same time as the color shift.

As described above, according to this method, a pattern having an arbitrary frequency division ratio written in advance in the PROM can be realized, the accuracy of clock increase / decrease and the accuracy of original oscillation can be achieved, and the cost is low.

Finally, a concrete example will be described. The frequency division ratio is assumed to be the same as that of the previous embodiment, namely, three types of frequency divisions of 14, 15, and 16, and the frequency division of 15 is basically used. At this time, for example, as shown in FIG. 7, with red as a reference, the image height (scanning signal) from the center of the f θ lens is set to Y
Then, at the point of Y = 30 mm, if the beam is imaged at −50 μm for blue (before red, in FIG. 14 (a)) and at +70 μm for green (before red, in FIG. 14 (b)) I assume. Pixel pitch 50 μm in main scanning direction (20 dots /
mm), it is (16/15) × 50 = 5 when divided by 16.
A pixel of 3.3 μm is also divided by 14 (14/15) ×
Pixels of 50 = 46.7 μm are realized, and ± 3.3 μm pixel pitch can be shifted from standard 50 μm.

The number of pixels at an image height of Y = 30 mm is 30 × 20 = 600 pixels based on the center of the lens. Therefore, for blue, 50/600 pixels from -50 μm
3.3 = 15 pixels divided by 16 If it is divided evenly, it is divided by 16 once for 40 pixels (2 mm). Similarly, for green, 70 / 3.3 = 21 pixels are divided by 14. If divided evenly, 1 for every 28 pixels (1.4 mm)
Divide by 4 By performing these processes, Y = 30mm
At this point, the three beam spots of B, G, and R can be matched.

(Effect of the Invention) As described in detail above, according to the present invention, the pixel clock can be increased or decreased by appropriately changing the division ratio according to the value of the PROM when dividing the clock. Thus, the deviation of the beam spot of each color can be adjusted. Therefore, it is possible to realize a laser light scanning device that can arbitrarily move the imaging position back and forth without using an expensive delay line. According to the present invention, color misregistration of a color laser printer can be eliminated.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a main part showing an embodiment of the present invention, FIG. 2 is a timing chart showing the operation of each part, FIG. 3 is a view showing a positional relationship of beams of respective colors, and FIG. 4 is a view of the present invention. FIG. 5 is a diagram showing the construction of a main part of another embodiment, FIG. 5 is a diagram showing the occurrence of unevenness in the sub-scanning direction, FIG. 6 is a diagram showing the configuration of a counter, and FIG. 7 is an explanatory diagram of a specific example of color misregistration. FIG. 8 is a diagram showing a conventional configuration example of a laser printer, FIG. 9 is a schematic diagram of color misregistration, FIG. 10 is a diagram showing a color misregistration state, and FIG. 11 is a diagram showing an example of a chromatic aberration correction circuit. FIG. 13 is an explanatory diagram of a pixel clock delay state, FIG. 13 is a pixel clock delay circuit diagram by a delay line, FIG. 14 is an explanatory diagram of the principle of color misregistration correction, and FIG. 15 is an explanatory diagram of how to make a pixel clock. FIG. 16 is a timing chart of FIG. 40 ... Synchronous clock generation circuit for blue 41 ... N frequency divider, 42 ... Counter 43 ... PROM 50 ... Synchronous clock generation circuit for green 60 ... Synchronous clock generation circuit for red

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location H04N 1/04 D 7251-5C 104 A 7251-5C

Claims (1)

[Claims] 1. A laser beam scanning device that modulates a plurality of laser beams having different wavelengths according to image information and guides the laser beam to an information recording medium through a deflecting unit and an image forming optical system to perform scanning. It is characterized by comprising storage means for storing information and a frequency divider for changing the frequency division ratio according to the output of the storage means, and configured to modulate the image information based on the output of the frequency divider. Laser scanning device.