JPS60178769A - Conversion method of picture element density for optical scan system - Google Patents

Abstract

PURPOSE:To perform the conversion of picture element density with low cost by setting the diameter of a scan spot at the constant value in the main scan direction in response to the main scan direction of the minimum picture element and increasing gradually said diameter in the subscan direction in response to reduction of the picture element density. CONSTITUTION:The diameter of a scan spot is decided constant in the main scan direction in response to the size of a picture element with the maximum picture element density. While the spot diameter is gradually increased in the subscan direction as the picture element density is reduced. That is, an optical system consists of a spherical lens 1, an optical modulator 2, a cylindrical lens 3, spherical lens 4, an ftheta lens 5 and a cylindrical lens 6. In this case, the position of a deflecting start point P is conjugate to the position of a main scan line in the subscan direction. Therefore the scan spot in the subscan direction is equal to a reduced image of the section of aluminous flux at the point P. Thus the spot diameter can be changed just by changing the diameter of the luminous flux section in the subscan direction at the point P. For this purpose, the lens 3 is replaced with another one having a proper focal distance in response to the picture element density.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a pixel density layer conversion method in an optical scanning method.

(Prior art) Using deflection means such as a rotating polygon mirror or a diffraction grating,
The laser beam is deflected, the laser beam is focused as a scanning spot on the surface to be scanned, the surface to be scanned is main-scanned by this scanning bond, and the surface to be scanned is moved in the sub-scanning direction perpendicular to the main-scanning direction. An optical scanning method that performs sub-scanning has been proposed.

The locus of movement of the scanning spot on the surface to be scanned is called a main scanning line, and the main scanning direction is the direction of this main scanning line.

In addition, when optical scanning is performed for reading an original, the scanned surface refers to the original image from which information is to be read, and when optical scanning is performed for writing optical information, it refers to the original conductive sensitive material. The surface of a photosensitive recording medium such as

The moving direction of the surface to be scanned is a sub-scanning direction, and this sub-scanning direction is orthogonal to the main scanning direction.

When reading a document by optical scanning, the information to be read is divided into pixels of minute area and converted into a clear signal for each pixel, and when writing the original information by optical scanning, 1ihII
It's getting hotter and hotter. Therefore, the area of this one pixel is S
When , the number of pixels per unit area, that is, 1/S
The sum is the pixel density. The smaller the pixel area S,
The pixel density increases.

By the way, it is reasonable to change the pixel density depending on the image information to be read or written. for example,
In cases where the power of solving the above image is not a problem,
Reading scans and writing scans may be performed with a relatively small pixel density, but when reading or digging a single fine image, scanning must be performed with a large pixel density.

Conventionally, as a method for converting pixel density in optical scanning, a method of using multiple beam expanders and switching conversion appropriately according to the pixel density has been intended, but this method requires multiple expensive beam expanders. However, there was a problem in that the cost of the optical travel device was high.

(Purpose) In view of the above problems, the present invention provides a pixel density conversion method in an optical scanning method that can effectively convert the pixel density of an optical scanning method without using a plurality of expensive beam expanders. The purpose is to provide.

(composition) The present invention will be explained below.

The special features of the present invention are as follows.

The shape of the scanning spot is generally set as an ellipse with the main axis aligned with the main scanning direction and the sub-scanning direction. In other words, this is determined according to the size of the minimum pixel in the main scanning direction. It is assumed to be constant.

The diameter of the scanning spot in the sub-scanning direction is changed so that it becomes larger as the pixel density becomes smaller.

Here, general conditions for performing pixel conversion will be described, taking as an example the case where original information is written and scanned.

In this case, it must be taken into account that the laser source is a Gaussian beam and the light intensity distribution at the scanning spot follows a Gaussian curve.

Now, when writing original information by optical scanning, first looking at the main scanning direction, if the recording medium is exposed and non-exposed for each pixel, sufficient exposure is obtained for the exposed and non-exposed areas. It is necessary that there be a difference.

In addition, regarding the sub-scanning direction, when the photosensitive recording medium, which is the surface to be scanned, is exposed to light over the entire surface, there is little exposure unevenness in the sub-scanning direction. When exposure and non-exposure are repeated, the exposed area/
It is necessary that there be a sufficient exposure difference in the non-exposed area, in other words, it is necessary that every other main scanning line be clearly visualized.

From this, it follows that there is an appropriate value range for the size of the scanning spot depending on the pixel density.

Now, assuming that the X coordinate of the center of the scanning spot is C and the Y coordinate is D, the light intensity distribution near the scanning spot E(x.

y) is approximated by the following equation.

πWxWy Wx Here, P is the maximum value of the light intensity distribution, that is, the light intensity at the center of the scanning spot, and Wx and Wy are the scanning spot radii in the sub-scanning direction and the main scanning direction (− of the above maximum value P), respectively. distance given).

Therefore, the pixel density q 300 DPI (dots /
1nch), and the spot diameter in the sub-scanning direction is set to 10[j~1
The exposure distribution in the sub-scanning direction when two full-surface exposures are performed for each spot diameter by varying the diameter in 20-μm steps within a range of 80 μm is calculated and illustrated according to the above formula for E(x, y). , it will look like the one shown in Figure 1.

・ In Figure 1, the vertical axis shows the exposure amount, and the horizontal axis shows the distance in the sub-travel strain direction. The scanning spot diameters in the sub-scanning direction are curves 1-A, 17B, 1-C, and 1-D.

1-E, 180 μm, 160 μm, 1
40 μm.

120 μm, 100 μm.

Similarly, at 300 DPI, the scanning spot diameter in the sub-scanning direction is changed every 9 μm in the range of 100 to 180 tx, and the diameter of the scanning spot in the sub-scanning direction is changed every 9 μm, and the spot diameter is
The exposure amount distribution in the sub-scanning direction when 50 scans are performed is as shown in Figure 2. The scanning spot diameter in the sub-scanning direction is curve 2-A.

180/En, 160 μm, 140 μm, 12[
]μm.

It is 100 μm.

Also, at 300 DPI, the scanning spot diameter in the main scanning direction is changed every 20 μm in the range of 60 μm to 140 μm, and exposure and non-exposure are repeated in the main scanning direction for every other pixel. The quantity distribution will be as shown in Figure 6. The horizontal axis is the distance in the main scanning direction. The scanning spot diameter in the main scanning direction is curves 3-A, , 3-B.
, 1 according to each of 3-C, 3-D, and 3-E
40 tm, 120 ttm, 100 μm, 80
μm, 601 m.

As is clear from FIGS. 1 and 2, in order to reduce exposure unevenness in the sub-scanning direction during full-surface exposure, it is better to have a larger diameter of the scanning spot in the sub-scanning direction. but,
In order to obtain a sufficient exposure difference between exposed and unexposed areas when every other main scanning line is exposed, the diameter of the scanning spot in the sub-scanning direction is preferably small. Therefore, these two conditions in the sub-scanning direction are harmonized in a certain area of the scanning spot diameter in the sub-scanning direction. The above region in the scanning spot diameter in the sub-scanning direction is referred to as an appropriate value range.

On the other hand, in the main scanning direction, in order to obtain a sufficient exposure difference between the exposed and non-exposed areas when exposing every other pixel, the diameter of the scanning spot in the main scanning direction needs to be smaller than a certain value.

Therefore, let us consider the case of switching the pixel density.

For example, change the pixel density from 400 DPI to 300 DPI
Let us consider the case where the diameter of the scanning spot in the main scanning direction is a, and the diameter in the sub-scanning direction is b, and the condition for giving good scanning at 400 DPI is a≦a1+bi≦b≦
b2 and that at 300 DPI is a≦a
2,1) If 5≦O≦04, obviously a
l<adzOl<I)s. So, first,
Considering the main scanning direction, when a≦a1, al<a2, so the main scanning can be performed satisfactorily whether the pixel density is 300 DPI or 400 DPI.

Therefore, when converting pixel density, in the main scanning direction, the diameter of the scanning spot in the main scanning direction is determined according to the size of one pixel at the maximum pixel density, and this is kept constant. Regarding the scanning direction, as the pixel density decreases, the scanning spot diameter in the sub-scanning direction may be changed to gradually increase (1 degree, for example, pixel illuminance of 480 DPI, 400 DPI).
DPI, 300DPI, 240DPI, the scanning spot diameter a in the main scanning direction is, when exposing every other pixel at a pixel density of 400DPI,
The diameter aO is determined to be such that a sufficient exposure difference can be obtained between the exposure area and the non-exposed area, and this is set as a constant, and the diameter aQ is kept constant even when converting the pixel density.

On the other hand, in the sub-scanning direction, the pixel density is 480DPI.
, 400 DPI +' 300 DPI , 240D
From each suitable value range according to PI, spot diameter')'0,
')'1, "2 + b'5 (b#o<b'1<
b/, , <b'3), and a spot diameter to be called another name is selected in accordance with the switching of the pixel density.

The above description has been made regarding the case of writing optical information, but the same applies to the case of reading an original by optical scanning.

Below, a description will be given of how the scanning spot diameter is switched only in the sub-scanning direction.

FIG. 4 shows an example of an optical scanning device for writing optical information with an optical arrangement developed in the optical axis direction.

Figure 4 (1) shows the state seen from the main scanning direction, and the same figure (I
) shows the state viewed from the sub-scanning direction.

In Figure 4, symbol LS is a gas laser as a laser light source, symbol 1 is a spherical lens, symbol 1 is a spherical lens, symbol 2 is an optical modulator, symbol 3 is a cylindrical lens, symbol 4 is a spherical lens, symbol 5 is an fθ lens, symbol Reference numeral 6 indicates a cylindrical lens, and reference numeral 7 indicates a photoconductive photoreceptor.

The photoreceptor 7 is drum-shaped, and its surface is a surface to be scanned. When the photoreceptor 7 rotates in the direction of the arrow, the surface to be scanned moves in the sub-scanning direction.

Further, the position indicated by the symbol P is the position of the starting point of deflection of laser light by a deflection means such as a rotating polygon mirror or a diffraction grating disk.

Laser light from the gas laser LS is first focused by a spherical lens 1 and then enters an optical modulator 2.
A beam waist is formed in the modulator 2. The laser light then becomes a divergent light beam and enters the Norindril lens 5VC.

As shown in Figure 4 (1), the laser beam is parallelized by the cylindrical lens 5 in the sub-scanning direction and enters the spherical lens 4, and by the power of the spherical lens 4 and the fθ lens 5, A beam is formed between fθ lens 5 and cylindrical lens 6, and then focused onto the surface to be scanned by lens 1,7 and cylindrical lens 6-.

On the other hand, in the main scanning direction, as shown in Figure 4 (11), the diverging laser beam after the optical modulator 2 is converted into a parallel beam by the spherical lens 4, and is reflected by the power of the fθ lens 5. It is focused on the running surface. The laser light is thus focused as a scanning spot on the surface to be scanned.

When the laser beam LIJ'' is periodically deflected by the deflection means, the scanning spot repeats main scanning of the surface to be scanned. By rotating the photoreceptor 7 in the direction of the arrow, sub-scanning of the surface to be scanned is realized. Therefore, depending on the image information, the light modulator 2VC
Original information can be written on the photoreceptor 7 by performing optical modulation and converting the laser beam into an optical signal.

In order to prevent the fluctuation of the main scanning line in the sub-scanning direction due to the light flux blurring of the deflected laser light in the sub-scanning direction based on the light deflecting means, for example, the light flux blurring caused by the so-called surface sagging of a rotating polygon mirror, the laser beam is The starting point P of deflection and the position of the main scanning line are the fθ lens 5 and the cylindrical lens 6.
are connected to a conjugate relationship by

Note that, in principle, the arrangement of the fθ lens 5 and the cylindrical lens 6 may be reversed on the optical axis. In addition, in order to reduce the (image surface curvature) on the scanned surface,
A toroidal lens may be used instead of the drical lens 6.

Now, in order to switch the pixel density by switching only the diameter of the scanning spot in the sub-scanning and forward directions, as in the present invention, the following procedure may be used.

As mentioned earlier, the position of the deflection starting point P and the position of the main scanning line are connected in a conjugate relationship in the sub-scanning direction, so in the sub-scanning direction, the scanning spot is It is a reduced image of the beam cross section of the laser beam at the starting point P of deflection. Therefore, in order to change the spot diameter of the scanning spot in the sub-scanning direction, it is only necessary to change the diameter in the sub-scanning direction of the beam cross section of the laser beam at the starting point P of deflection. To do this, the cylindrical lens 6 may be replaced with one having a different focal length. At that time, if you choose the focal length (of the cylindrical lens being replaced) appropriately, you can create a new cylindrical lens.
It can be placed at the same position as the cylindrical lens 6 and still achieve desired pixel conversion.

Let me give you a concrete example. In the example shown in Figure 4, the focal length of the spherical lens 1 is 90 mm, the focal length of the cylindrical lens 6 is 85 mm, and the focal length of the spherical lens 4 is 4.
40ii,, fθ Focal length of lens 5: 296 mm
, 7 The focal length of the rhinodrical lens 6 is 68 cages,
The distance between the spherical lens 4 and the lindrical lens 6 is 35.
8 mm, and the valley lens is placed at a predetermined position (- then, a diameter of 100 μm in the main scanning direction, 1iflh) is placed on the scanned surface.
A scanning spot with a diameter of 120 ca in the t-scanning direction can be obtained. This scanning spot has a pixel density of 300 D
Suitable for PI.

If a cylindrical lens with a focal length of 100 mm is placed at the position of the cylindrical lens 6 in place of the cylindrical lens 6, only the spot diameter in the sub-scanning direction of the scanning spot will be
This results in a scanning spot of approximately 150 lxn, which is suitable for a pixel density of 240 DPI.

In the example described above, FIG. 5 shows how the spot diameter of the scanning spot in the sub-scanning direction changes when the focal length S of the solid/drical lens B changes. Incidentally, since the cylindrical/trical lens 5 has no power in the main scanning direction, it goes without saying that the diameter of the scanning spot in the main scanning direction does not change even if its focal length changes.

Figure 6 shows another example of the optical scanning device for writing optical information.
It is shown in the same manner as in the figure. The reference numeral 7 indicates a photoconductive photoreceptor as in FIG. 4, and the reference numeral P indicates the starting point of deflection by the laser beam deflection means, as in FIG.

In Figure 6, the symbol LSI is a semiconductor laser as a laser light source, the symbol 11 is a collimator lens, the symbol 12 is an /su/trical lens, the symbol 1 is an fθ lens, and the symbol 14 is a serial/trical lens. shown below.

The laser beam L' emitted from the semiconductor laser LSI is collimated by the collimator lens 11, and
It enters the serial/trical lens 12, and in the sub-scanning direction, it becomes convergent as shown in Figure 6 (1), and fθ
Form the beam waist in front of the /z 16, then f
θ After passing through the lens 15, the serial/trical lens 14 and f
After forming a beam waist again with the θ lens 16, the beam is focused onto the surface to be scanned by the lintrical lens 14. In addition, regarding the main scanning direction, the fθ lens 16
Due to the action of rays, it is focused on a surface. In this way, a scanning spot is obtained on the surface to be scanned.

In this example, the semiconductor laser LSI serving as the light source is
Since the emission intensity can be modulated by the drive current, a dedicated optical modulator is not required.

To convert the pixel drop, the cylindrical/trical lens 12 may be replaced with another 7-linodrical lens having a different focal length.

In the example shown in Figure 4, an aperture member for shaping the shape of the light beam may be provided immediately after the spherical lens 4, and in the example shown in Figure 6, immediately after the silica/trical lens 12. In this case, the pixel density conversion of the present invention can also be performed by switching the aperture member to one with a different aperture shape. That is, it is only necessary to change the opening width in the sub-scanning direction in the aperture shape.

When converting pixel density, it is possible to replace the silicate/trical lens or aperture member one by one, but it is also possible to form these into a turret type and change the silicate/trical lens etc. according to the desired pixel density. , it is also possible to perform VC so as to arrange the S4 scale on the optical path.

Note that the f.theta. lens in the optical system shown in Figures 4 and 6 is for equalization in the main scan, and in some cases, a normal spherical lens is used.

(Effects) As described above, according to the present invention, a novel pixel density conversion method in an optical scanning method can be provided.

In this invention, when converting the pixel density, only the spot diameter of the scanning spot is changed, so there is no need to change the expensive beam expander. , pixel conversion can be performed.

[Brief explanation of drawings]

]・Figures 1 to 3 are diagrams for explaining how the set meal spot diameter affects optical scanning in the optical scanning method, and Figures 4 to 6 are diagrams for explaining the present invention. This is a diagram for LS...Gas laser, LS1 peak semiconductor laser, L
. L'... Laser light, 1... Spherical lens, 2... Optical modulator, 6... Serial/trical lens, 4... Spherical lens, 5... fθ lens, 6...・7S/Trical lens, 7... Photoconductive photoreceptor, P... Starting point of deflection of laser light by deflection means 7F) 4E]

Claims (1)

[Claims] Laser light is periodically deflected and focused as a scanning spot on a surface to be scanned, the scanning spot is used to main scan the surface to be scanned, and the surface to be scanned is perpendicular to the main scanning direction. In an optical scanning method in which sub-scanning is performed by moving in the sub-scanning direction, a method of converting the pixel density of optical scanning into multiple stages, the diameter of the scanning spot in the main scanning direction, the pixel that provides the maximum pixel density, It is determined according to the size in the main scanning direction and is constant. A pixel density conversion method in an optical scanning method, characterized in that the diameter of the scanning spot in the sub-scanning direction is converted so as to increase sequentially as the pixel density decreases.