JP2000233529A - Imaging optical system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an imaging lens system capable of precisely imaging by correcting variation in a light quantity distribution due to position shift of a light source. SOLUTION: A diffraction grating 110 and an imaging lens 112 are provided in a casing 68 of an imaging lens system 26 toward an exposing head 26 from a substrate 82. Each of imaging spots on a photosensitive material is imaged by a magnification greater than that determined by the imaging lens 112. Even when an optical beam is shifted in a sub-scanning direction, variation in a light quantity distribution can be reduced and the image can be precisely formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an imaging optical system that emits light from a plurality of light sources in accordance with image information and forms an image of light.

[0002]

2. Description of the Related Art At present, in an image exposure apparatus, a plurality of light sources are arranged vertically and horizontally in a matrix, and a plurality of light sources and a photosensitive material are relatively scanned in a main scanning direction and a sub-scanning direction for exposure. There are types. Then, light from the plurality of light sources is imaged on the photosensitive material by the imaging optical system. As the light source, for example, a semiconductor element such as an LED is employed, and the semiconductor elements are arranged in a matrix at a predetermined pitch.

[0003] However, the arrangement pitch accuracy of the light sources is relatively rough, and a positional shift (so-called pitch shift) may occur. This pitch shift may cause unevenness in the light amount distribution during scanning of the photosensitive material, thereby deteriorating the image quality.

[0004] In order to reduce the unevenness of the light amount distribution due to such a pitch shift, for example, it is conceivable to reduce the imaging magnification of a lens constituting an imaging optical system. That is, by reducing the imaging magnification, the absolute amount of the pitch shift on the photosensitive material is reduced. However, when the image forming magnification of the lens is reduced in this manner, the spot size on the photosensitive material is also reduced, so that even if the pitch shift amount is small, the uneven light amount distribution becomes large.

[0005]

SUMMARY OF THE INVENTION In view of the above, it is an object of the present invention to provide an imaging optical system capable of correcting unevenness of a light amount distribution due to a displacement of a light source and forming an image with high accuracy.

[0006]

According to the first aspect of the present invention, a plurality of light sources which emit light in accordance with image information, and a distance between the light sources is a predetermined value with respect to the distance between the light sources. Imaging means for imaging light from these light sources so as to have an image magnification, and a spot size for forming an image spot of each of the plurality of light sources at a spot magnification larger than the imaging magnification by the imaging means. And enlarging means.

Accordingly, the light from the plurality of light sources is imaged by the image forming means so that each interval has a predetermined imaging magnification with respect to the interval between the light sources.

On the other hand, the image forming spots of the plurality of light sources are formed by the spot size expanding means at a spot magnification larger than the image forming magnification. That is,
The spot size at the image forming position is larger than when the image is formed only by the image forming unit. For this reason, even if the positions of the plurality of light sources are shifted (so-called pitch shift), the unevenness of the light amount distribution of the plurality of light sources as a whole is corrected, and as a result, an image can be formed with high accuracy.

As the spot size expanding means, for example, a structure including a diffraction grating as described in claim 2 or a structure including a birefringent material as described in claim 3 can be used. it can. In any case, the spot size enlarging means can be provided with a simple configuration.

The spot size enlarging means may be an image forming means and a lens having a predetermined spherical aberration. Thereby, the spot size enlarging means is integrated with the image forming means, and it is not necessary to provide a separate member such as a diffraction grating or a birefringent material, so that the number of parts of the image forming optical system is reduced, and the weight and the cost are reduced. Become.

According to a fifth aspect of the present invention, in any one of the first to fourth aspects, light from a plurality of light sources is diffused between the plurality of light sources and the imaging means. A diffusion member to be disposed is provided.

Even when the light amount distribution varies due to the shape of the light emitting surface of the light source, the light amount distribution from the light source is made uniform (so-called beam shaping) by the diffusion member. Thereby, an image can be formed with higher accuracy.

According to a sixth aspect of the present invention, in the first aspect of the present invention, the plurality of light sources are constituted by at least two types of light sources having different wavelengths, and the diffusing member is provided. , Wherein the plurality of light sources have a diffusivity corresponding to different wavelengths.

When a diffusing member having the same degree of diffusion is used, the beam shape of the light diffused by the diffusing member differs for each color. On the other hand, by using a diffusion member having a diffusion degree corresponding to different wavelengths, the beam shape of the light diffused by the diffusion member becomes the same for any color.

According to a seventh aspect of the present invention, in the first aspect of the present invention, the plurality of light sources are arranged along a main scanning direction and a sub-scanning direction, and It is characterized in that the spot magnification of the enlarging means is set to be larger in the sub-scanning direction than in the main scanning direction.

Accordingly, the spot size increases in the sub-scanning direction, and unevenness in the light amount distribution can be reduced. Further, since the spot size is smaller in the main scanning direction than in the sub scanning direction, a clearer image can be obtained.

According to the present invention, a plurality of light sources that emit light in accordance with image information and an interval between lights from the plurality of light sources are set to have a predetermined imaging magnification with respect to an interval between the light sources. An image forming means for forming an image of light from these light sources, and a diffusing means provided at or near a pupil position of the image forming means and diffusing light from the plurality of light sources.

Therefore, the light from the plurality of light sources is imaged by the image forming means so that each interval has a predetermined imaging magnification with respect to the interval between the light sources.

Further, the image forming spots of the plurality of light sources are diffused by the diffusing means disposed at or near the pupil position of the image forming means, and the image is formed at a spot magnification larger than the image forming magnification of the image forming means. Is done. That is, the spot size at the image forming position is larger than when the image is formed only by the image forming unit. For this reason, even if the positions of the plurality of light sources are shifted (so-called pitch shift), the unevenness of the light amount distribution of the plurality of light sources as a whole is corrected, and as a result, an image can be formed with high accuracy.

Further, even if the light amount distribution is varied due to the light emitting surface shape of the light source, the light from the plurality of light sources is diffused by the diffusing means to make the light amount distribution uniform (so-called beam shaping). ) Is done. This allows
An image can be formed with higher accuracy.

In addition, since the means for making the spot magnification larger than the imaging magnification and the means for beam shaping also serve as the diffusing means, when these means are provided separately, The number of parts is reduced as compared with, and the weight and cost are reduced.

According to a ninth aspect of the present invention, in the eighth aspect of the invention, the diffusing means has different degrees of diffusion in two directions orthogonal to the optical axes from the plurality of light sources. It is characterized by the following.

As described above, by using the diffusing means having different diffusivities in two directions orthogonal to the optical axis, a desired spot magnification and a desired beam shaping can be obtained according to the directions. A clear image can be obtained. For example, as described in claim 10, the plurality of light sources are arranged along the main scanning direction and the sub-scanning direction, and the degree of diffusion of the diffusing unit is relatively larger in the sub-scanning direction than in the main scanning direction. In this configuration, the spot magnification by the diffusing unit can be made larger in the sub-scanning direction than in the main scanning direction. Accordingly, the spot size becomes larger in the sub-scanning direction than in the main scanning direction, and unevenness in the light amount distribution can be reduced. Further, since the spot size is smaller in the main scanning direction than in the sub scanning direction, a clearer image can be obtained.

[0024] In the eleventh aspect of the present invention, claims 8 to
The invention according to claim 10, wherein the diffusion means is movable along an optical axis direction from the plurality of light sources to the diffusion means.

As described above, since the spot magnification is changed by moving the diffusing means along the optical axis direction, the diameter of the image forming beam of the light to be imaged can be set to a desired value. For this reason, it is possible to effectively correct unevenness in the light amount distribution caused by the pitch shift of the plurality of light sources, and to form an image with higher accuracy.

According to the twelfth aspect of the present invention, the eighth to eighth aspects are described.
In the invention according to any one of the eleventh to eleventh aspects, the diffusing means is constituted by a plurality of diffusing plates disposed at or near the pupil position so as to be able to advance and retreat.

Therefore, by moving one or a plurality of diffusion plates toward or near the pupil position, the number of diffusion plates at or near the pupil position can be changed, and the diffusion degree can be changed as a whole of the diffusion means. . Accordingly, the spot magnification also changes, so that the imaging beam diameter of the light to be imaged can be set to a desired diameter. For this reason, it is possible to effectively correct unevenness in the light amount distribution caused by the pitch shift of the plurality of light sources, and to form an image with higher accuracy.

[0028] In the invention according to claim 13, claims 8 to
The invention according to claim 12, wherein the diffusing unit is rotatable at or near the pupil position.

As described above, since the spot magnification is changed by rotating the diffusing means, the diameter of the image forming beam of the light to be formed can be set to a desired value. For this reason, it is possible to effectively correct unevenness in the light amount distribution caused by the pitch shift of the plurality of light sources, and to form an image with higher accuracy.

[0030] According to the fourteenth aspect of the present invention, claims 8 to
The invention according to claim 13, wherein the diffusing means is made of a material whose degree of diffusion can be changed.

As described above, when the diffusing means is made of a material whose diffusivity can be changed, the spot magnification changes by changing the diffusivity, so that the image forming beam diameter of the light to be imaged is set to a desired diameter. can do. For this reason, it is possible to effectively correct unevenness in the light amount distribution caused by the pitch shift of the plurality of light sources, and to form an image with higher accuracy. The material whose diffusivity can be changed in this way is not particularly limited. For example, as described in claim 15, a scattering liquid crystal whose scattering degree changes according to an applied voltage can be given. This allows
The diffusivity can be changed with a simple configuration that only changes the applied voltage.

[0032] According to the invention described in claim 16, claims 1 to 1 are provided.
16. The invention according to claim 15, wherein a slit plate having a transmission hole corresponding to light from the plurality of light sources is arranged between the plurality of light sources and the imaging unit. It is characterized by.

Generally, as compared with the mounting accuracy of mounting a light source to a member to be mounted such as a substrate, the transmission hole of the slit plate is
It can be formed with much higher positional accuracy. Therefore, even when the positions of the plurality of light sources are shifted from the desired positions, the shift is corrected by the transmission holes.
That is, since the accuracy of the arrangement position of the light sources is improved, an image can be formed with higher accuracy.

The interval between the transmission holes is not particularly limited. For example, when the light sources are arranged in the sub-scanning direction, the intervals can be equal.

In consideration of the aberration of the image forming means (for example, when a lens is used as the image forming means), a plurality of light sources are arranged at equal intervals on the photosensitive material. As described above, the interval between the transmission holes can be adjusted in advance. Further, in consideration of the shape of the photosensitive surface of the photosensitive material, the interval between the transmission holes may be adjusted in advance so that a plurality of light sources are equally spaced on the photosensitive material.

According to the seventeenth aspect, in the sixteenth aspect,
In the invention described in (1), a sub-transmission hole corresponding to a position outside the arrangement direction of the plurality of light sources is formed in the slit plate.

Therefore, a part of the light from the plurality of light sources reaches the photosensitive material from the sub-transmission holes through the image forming means and the spot size expanding means. Similarly to the central light source, the end light sources also have transmission holes on both sides in the direction in which the light sources are arranged, so that a decrease in the amount of light at the end portions in the arrangement direction of the plurality of light sources is prevented, and unevenness in the light amount distribution is reduced. it can.

According to the invention described in claim 18, in claim 17
In the invention described in (1), a pseudo light source is provided at a position corresponding to the sub-transmission hole.

When considering light from a specific light source among a plurality of light sources, generally, this light is affected by other light sources (for example, when an LED is used as a light source, a light from a specific LDE is used). Some of the light passes through or is reflected by other LEDs and enters the image forming means and the spot size enlarging means). Therefore, by providing the sub-transmission hole and the pseudo light source, the light source at the center in the arrangement direction of the light sources and the light source at the end also emit light from the plurality of sub-transmission holes in the same manner,
A uniform spot can be obtained on the photosensitive material.

[0040]

FIG. 2 shows an exposure section of an image forming apparatus to which an image forming optical system according to a first embodiment of the present invention is applied. FIG. 1 shows a schematic configuration of the image forming apparatus 10.

As shown in FIG. 1, a photosensitive material magazine 14 is disposed in a housing 12 of the image forming apparatus 10. The supply reel 16 is rotatably and detachably attached to the photosensitive material magazine 14. The photosensitive material 18 is set on the supply reel 16 in a wound state. The photosensitive material 18 is unwound when the supply reel 16 is rotated by a driving unit (not shown).

The leading end of the photosensitive material 18 is
Is held by a pull-out roller 20 provided in the vicinity of the take-out port. The pull-out roller 20 pulls out the photosensitive material 18 under predetermined conditions and sends it out to the guide plate 22, or forms a buffer (a slack portion of the photosensitive material 18, indicated by a two-dot chain line) in the photosensitive material 18.

The photosensitive material 18 that has passed through the guide plate 22 is
It is wound around an exposure drum 24 and an image is exposed by an exposure head 26. The exposure drum 24 has a cylindrical shape, and the exposure surface (outer peripheral surface) has a constant curvature about the rotation axis C1. By exposing the photosensitive material 18 around the exposure drum 24, wrinkles and the like do not occur in the width direction of the photosensitive material 18, and a certain flatness can be secured.

The photosensitive material 18 to which the image has been exposed is sandwiched between the support 28 and the pressure plate 30, and the coating tank 32
Sent below. The coating tank 32 is provided with a coating member 34 formed of a highly water-absorbing material such as a sponge, and the coating member 34 applies water to the photosensitive material 18.

The photosensitive material 18 to which water has been applied is wound around the heat drum 36 having a built-in heating means such as a halogen lamp by tension rollers 38 and 40 at a constant pressure. The wound photosensitive material 18 is superimposed on an image receiving paper 48 to be described later while being heated.
The image is transferred to 8.

The photosensitive material 18 having the image transferred to the image receiving paper 48
Is wound up on a waste reel 42. As described above, by transferring the photosensitive material 18 from the supply reel 16 to the waste reel 42 without being cut off in the middle, the photosensitive material 18 itself functions as a belt that applies a constant pressure to the image receiving paper 48.

On the other hand, in the housing 12, an image receiving paper magazine 44 is disposed above the photosensitive material magazine 14. A supply reel 46 is rotatably and detachably attached to the image receiving paper magazine 44. The image receiving paper 48 is set on the supply reel 46 in a wound state. The image receiving paper 48 is pulled out by the nip roller 50, cut by the cutter 52 to a predetermined length, guided by the transport roller 54 and the guide plate 56, and wound around the heat drum 36 while the photosensitive material 18 is overlapped.

The image receiving paper 48 onto which the image has been transferred from the photosensitive material 18 is peeled off from the heat drum 36 by a peeling claw (not shown), guided by the transport roller 58 and the guide plate 60, and reaches the tray 62.

As shown in FIG. 2, two shafts 64 are arranged above the exposure drum 24 in parallel with the rotation axis C1. The shaft 64 is inserted into a support hole passing through the support block 66, and the support block 66 can slide along the shaft 64.

The casing 68 of the exposure head 26 is fixed to the support block 66. A connecting plate (not shown) is attached to the casing 68, and an endless timing belt 72 is fixed to the connecting plate. The timing belt 72 is wound around a sprocket 74 provided near both ends of the shaft 64.
Rotational force is transmitted to the sprocket 74 from a stepping motor 76 via a transmission, and the
6 rotates the exposure head 26 to the shaft 64.
Reciprocate along.

The driving of the stepping motor 76 is controlled by the controller 78, and is synchronized with the step driving of the photosensitive material 18. That is, in the state where the photosensitive material 18 is stopped by moving stepwise, the stepping motor 76 is stopped.
Rotates forward, the exposure head 26 moves forward along the shaft 64, and moves in the width direction (main scanning direction) of the photosensitive material 18.

After confirming the predetermined pulse, the photosensitive material 18 is further moved stepwise and stopped.
By reversing the stepping motor 76, the exposure head 26 moves back along the shaft 64, and the photosensitive material 18
In the width direction (main scanning direction).

Substrates 82 (82R, 82G, 82B) corresponding to the three colors RGB are fixed to the upper plate 80 of the casing 68. As shown in FIG. 3 and FIG.
A flexible cable 84 of a controller 78 in which image signals are stored is connected to R, 82G, and 82B, and an LED chip 96 that emits light in response to a signal from the controller 78 is attached. In the LED chip 96, a plurality of LEDs 98 corresponding to the substrates 82R, 82G, and 82B (in the present embodiment, 31 red LEDs on the substrate 82R, 31 green LEDs on the substrate 82G, and 31 blue LEDs on the substrate 82B, respectively) ) Are arranged linearly in the sub-scanning direction, and the plurality of LEDs 98 are turned on in response to the image signal (in FIG. 3, for convenience of illustration, the substrate 82R is shown upside down. , LE
The light of D98 is irradiated upward in the figure).

The substrate 82R shown in FIG. 3 is a substrate to which the R (red) LED 98 is attached, and is formed in a substantially T shape. On the other hand, as shown in FIG. 4, the boards 82G and 82B to which the G (green) and B (blue) LEDs 98 are attached are formed in a substantially rectangular shape. By arranging the board 82R between the boards 82G and 82B, each flexible cable 84
Can be radially extended in three directions.
For this reason, even the flexible cables 84 having a line width can be connected to the respective boards 82R, 82G, and 82B in the same plane without interfering with each other. Hereinafter, the substrate 82R will be described as an example, but the basic configuration (the substrate 82R) will be described.
Are the same for any of the substrates 82.

A protection frame 86 is fixed to the substrate 82R. An accommodation hole 88 corresponding to the LED chip 96 is formed in the protection frame 86. In addition, the thickness of the protection frame 86 is higher than a fixing portion (not shown) for connecting the LED chip 96 and the controller 78 (when fixed by bonding or the like, the fixing portion is a swelled portion due to bonding). . Therefore, the fixed portion is protected by the protection frame 86.

Further, a slit plate 90 is fixed on the emission side of the protection frame 86. In the slit plate 90,
A plurality of transmission holes 92 corresponding to the plurality of LEDs 98 are formed. In general, the mounting position of the LED 98 with respect to the substrate 82R has variations and deviations.
Accuracy (position and shape accuracy) of transmission hole 92 with respect to
Can be much higher than the mounting accuracy of the LED 98. For this reason, the light of the LED 98 is transmitted through the transmission hole 92 of the slit plate 90, so that the variation and deviation of the position of the LED 98 is corrected, and the light (spot light) is emitted from an accurate position in a fixed spot shape. . An area corresponding to the transmission hole 92 constitutes a substantial light emitting area of the LED 98.

The number of the transmission holes 92 may be the same as the number of the LEDs 98, and furthermore, a sub-transmission having the same shape as the transmission holes 92 is provided at both ends in the arrangement direction (sub-scanning direction) of the transmission holes 92. Preferably, holes 94 are formed. That is, LE
The light from D98 not only transmits through the corresponding transmission holes 92, but also partially transmits through the transmission holes 92 around the corresponding holes.
The light that reaches the photosensitive material 18 is a collection of all the light that has passed through the surrounding transmission holes 92. When the sub-transmission holes 94 are not formed, light cannot be transmitted further outside in the arrangement direction than the transmission holes 92 at both ends, so that the amount of light near both ends in the arrangement direction is lower than other portions. However, since the sub-transmission holes 94 are formed in this manner, light is transmitted through the sub-transmission holes 94 outside the transmission holes 92 in the arrangement direction, so that a decrease in the amount of light near both ends in the arrangement direction is prevented, and the uniformity is obtained as a whole. A large amount of light can be obtained.

When such a sub-transmission hole 94 is formed, the LED chip 96 is correspondingly provided with the LE.
Dummy light source 10 having substantially the same configuration as D98
It is preferable to provide 0. That is, the light transmitted through the transmission hole 92 is not only the light transmitted directly from the LED 98, but also a part thereof is reflected by another LED 98,
After transmitting another LED 98, the transmission hole 92
Through. Accordingly, the light transmitted through the sub-transmission hole 94 is also reflected or transmitted by the pseudo light source 100 having substantially the same configuration as the LED 98, and the light source (LED 98) is provided at the position of the pseudo light source 100. Similar light can be used. Note that the pseudo light source 100 does not necessarily have to have substantially the same configuration as the LED 98 as long as it can transmit or reflect light in the same manner as the LED 98 as the original light source. For example, a translucent mirror or the like having a predetermined shape may be used.

The distance between the transmission holes 92 and the distance between the transmission holes 92 and the sub-transmission holes 94 can be, for example, equal, but this distortion is taken into account in consideration of the distortion of the imaging lens 112 described later. Is preferably set in advance so that the image spots are arranged at equal intervals on the photosensitive material 18 after correction.

Further, since the exposure drum 24 is curved at a constant curvature in the sub-scanning direction, the light hits the exposure drum 24 from an oblique direction as it approaches the both ends in the arrangement direction of the LEDs 98, and the light spot of the image forming spot is formed. The interval widens. Therefore, it is more preferable to adjust in advance so that the imaging spots are arranged at equal intervals on the photosensitive material 18 in consideration of the curvature of the exposure drum 24.

The diffusion plate 102 is fixed on the light irradiation side of the slit plate 90. The light transmitted through the transmission hole 92 is diffused by the diffusion plate 102, and the light is made uniform in the light emitting region corresponding to the transmission hole 92. For example, a portion where the wire is fixed to the LED chip 96 creates a shadow with respect to the light from the LED 98, and the light amount distribution of the irradiating light may be uneven, but even in such a case, the diffusion plate 102 This unevenness is eliminated, and uniform light can be obtained in the light emitting region.

An ND filter 106 is fixed to the irradiation side of the diffusion plate 102 via a filter frame 104. The ND filter 106 adjusts the light emitted from the LED 98 so that the light amount of the light from the LED 98 becomes appropriate for the photosensitive material 18.

The above-described protective frame 86, slit plate 90, diffusion plate 102, and filter frame 104 are fixed to the substrate 82R by screws. Of course, the structure for fixing to the substrate 82R is not limited to this.

As shown in FIG. 5, in the casing 68, in order from the substrate 82R toward the exposure drum 24, the light amount sensor 108, the diffraction grating 110, and the imaging lens 11 are arranged.
2 are provided.

The light quantity sensor 108 measures the quantity of light emitted from the LED 98. Then, based on the measurement data, the controller 78 (see FIG. 2)
The LEDs 98 are controlled so as to always obtain a desired amount of light by adjusting the variation of the amount of light in the above or adjusting the increase or decrease of the amount of light with time.

As will be described in detail later, the light emitted from the LED 98 spreads in the sub-scanning direction by the diffraction grating 110, and unevenness in the light amount distribution due to a minute positional deviation (pitch deviation) of the transmission hole 92 is eliminated. Is done.

The imaging lens 112 includes one or more lenses and a diaphragm, and is moved in the optical axis direction by a variable power motor 114 fixed to the casing 68. The imaging lens 112 collects light from the LED 98 and forms an image on the photosensitive material 18 wound around the exposure drum 24. This imaging magnification (1 / k)
Can be changed by driving the imaging lens 112 in the optical axis direction by the variable power motor 114.

By providing the diffraction grating 110 in addition to the imaging lens 112, each of the imaging spots on the photosensitive material 18 has an imaging magnification (1 / k) determined by the imaging lens 112. An image will be formed at a larger magnification. For example, when the distance (light source length) from one end to the other end of the plurality of LEDs 98 is L, the photosensitive material 18
The spot length L ′ of the imaging spot formed above is

[0069]

(Equation 1)

Is obtained.

On the other hand, the light emitted from each LED 98 and transmitted through the transmission hole 92 has the size (length of a specific portion) of the transmitted light as S, and forms an image on the photosensitive material 18. When the spot size of the spot (the length of the corresponding specific portion) is S ′,

[0072]

(Equation 2)

The relationship is as follows.

The image forming optical system according to the present embodiment includes the above-described image forming lens 112 and diffraction grating 110.

Next, the operation of the image forming optical system according to the present embodiment will be described.

When an image signal corresponding to the image information is input to the controller 78 and the LED 98 is turned on, this LE
The light of D98 is transmitted through the transmission hole 92 of the slit plate 90.
The positions are corrected on the photosensitive material 18 so as to be at equal intervals in the sub-scanning direction, and the light is made uniform by the diffusion plate 102 in the light emitting region. And ND filter 1
06, the light amount is set to an appropriate amount for the photosensitive material 18 and is incident on the diffraction grating 110.

FIG. 6A qualitatively shows a light quantity distribution of light emitted from each of the plurality of LEDs 98 when the diffraction grating 110 is not provided. FIG. 7A qualitatively shows a light amount distribution of the light transmitted through the diffraction grating 110 (FIG. 6B).

As can be seen from the graph shown in FIG. 6A, when the diffraction grating 110 is not provided, the light intensity is reduced by the transmission holes 92 of the slit plate 90 and the diffusion plate 102.
, And the spread of each light in the sub-scanning direction (the spread of the light amount distribution) is reduced. Then, the adjacent light beams overlap each other, so that the total light amount is reduced as shown in FIG.
As shown by the thick line in (A), it is almost constant in the sub-scanning direction.

Here, for example, of the plurality of transmission holes 92 of the slit plate 90, the position of one specific transmission hole 92 is
Considering a case where the transmission holes 92 are slightly shifted in the sub-scanning direction, in this case, as shown in FIG.
Is moved in the sub-scanning direction, the distance from the light transmitted through the transmission hole 92 in the moving direction (left direction in this graph) is shortened, and the total light amount has a local maximum. A point appears. Further, since the distance between the transmitting hole 92 and the moving direction opposite to the moving direction is widened, the light amount as a whole is
Local minima also appear. Then, the maximum point and the minimum point cause unevenness in the light amount distribution, and the image quality deteriorates.

On the other hand, in the image forming optical system according to the present embodiment, light passes through the diffraction
As shown in (B), in addition to the original zero-order beam,
Higher-order beams, such as a primary beam, are generated in order. In other words, as shown by the above equation (2), the spot size S ′ of the imaging spot is enlarged at a magnification substantially larger than the imaging magnification (1 / k) determined by the imaging lens 112. As a result, there is a spread in the sub-scanning direction. Then, as shown in FIG. 7A, when considering the light amount as a whole, the light amount at a specific position in the sub-scanning direction is the light collected from many LEDs 98. Therefore,
Even if the position of one specific transmission hole 92 of the slit plate 90 is slightly shifted in the sub-scanning direction, FIG.
As shown by the bold line in FIG. 3A, the light quantity distribution as a whole is hardly affected by this deviation, and the variation of the light quantity at the local maximum point and the minimum is small. That is, the unevenness of the light amount distribution caused by the maximum point and the minimum point is reduced, and the image is formed with high accuracy, so that the deterioration of the image quality is prevented.

The direction in which the light is spread by the diffraction grating 110 may include a component in the main scanning direction as well as the above-described sub-scanning direction. However, deviation of the imaging spot in the main scanning direction is eliminated by the exposure drum 24 performing main scanning. In addition, in consideration of the accuracy of an image to be formed, the smaller the size of the formed image spot, the better the accuracy in many cases. For this reason, it is preferable to use the diffraction grating 110 in which the light spreads only in the sub-scanning direction and does not spread in the main scanning direction.

As described above, in the image forming optical system according to the present embodiment, the spot size of the image forming spot in the sub-scanning direction is substantially larger than the image forming magnification of the image forming lens 112. Since the imaging spot spreads in the sub-scanning direction, unevenness of the light amount distribution due to uneven displacement of the position of the LED 98 can be eliminated, and an image can be formed with high accuracy.

As described above, the spot size enlarging means for making the spot size of the image spot in the sub-scanning direction substantially larger than the image forming magnification of the image forming lens 112 includes the above-described diffraction grating 110. It is not limited to.
For example, a material (a so-called birefringent material) in which a phenomenon of birefringence appears may be used instead of the diffraction grating 110. That is, since an extraordinary ray appears in addition to the ordinary ray due to birefringence, the image size increases in the direction in which the extraordinary ray spreads. Therefore, it is preferable to arrange the birefringent material in consideration of the direction of the birefringent material so that the direction in which the extraordinary ray appears is the sub-scanning direction. Specific examples of birefringent materials include fluorite, quartz,
Calcite and the like.

Further, instead of the diffraction grating 110, the diffusion plate 1
A new diffusion plate different from 02 may be provided. Also in this case, a diffusion plate that diffuses light only in the sub-scanning direction is preferable. In general, the diffusivity of the diffuser depends on the wavelength of light, and the longer the wavelength, the smaller the diffusivity. Therefore, by providing a diffusion plate for each color and setting the diffusion degree of the diffusion plate to be larger for light of a longer wavelength, the size of the image beam on the photosensitive material 18 (particularly, , The size in the sub-scanning direction) can be made constant.

Further, the imaging lens 112 can be used without using the diffraction grating 110, the birefringent material and the diffusion plate.
The spot size of the imaging spot in the sub-scanning direction may be made larger than the imaging magnification of the imaging lens 112 using the spherical aberration of the imaging lens. That is, since a lens generally has a spherical aberration by nature, the spherical aberration is positively used. For example, spherical aberration can be increased by using a condenser lens or the like. As described above, by utilizing the spherical aberration of the imaging lens 112 itself, it is not necessary to provide a separate member such as the diffraction grating 110, so that the number of components constituting the imaging optical system is reduced, and the weight and cost are reduced. Become.

Examples of the diffusion plate include frosted glass, opal glass, non-glare plate, resin diffusion plate, beam shaping diffuser, liquid crystal panel, and the like.

Further, a plurality of types of the spot size expanding means described above may be used in combination.

FIG. 8 shows an exposure head 120 having an imaging optical system according to the second embodiment of the present invention. In the second embodiment, only the configuration of the exposure head 120 is different from that of the first embodiment, and the other configuration is the same. Therefore, only the exposure head 120 will be described. Also, the same components as those of the first embodiment,
The same reference numerals are given to members and the like, and the description is omitted.

In the exposure head 120 of the second embodiment, a diffusing plate 122 for diffusing light is provided in place of the diffraction grating 110 of the first embodiment (see FIG. 5).
The diffusion plate 122 is held by the holding plate 124 and is arranged at or near the pupil position of the imaging lens 112.

Therefore, also in the second embodiment, the diffusion plate 122 diffuses light similarly to the diffusion plate 102 of the first embodiment. Thus, the light transmitted through the transmission hole 92 (see FIG. 3) is made uniform within the light emitting region corresponding to the transmission hole 92. That is, even if the light amount distribution of the irradiation light becomes uneven, the unevenness is eliminated by the diffusion plate 122 and uniform light can be obtained in the light emitting region.

Further, the diffusion plate 122 is
Since light is diffused at or near the pupil position 2,
Light is imaged at a spot magnification larger than the imaging magnification of the imaging lens 112, and the light substantially spreads at the imaging position. Thus, similarly to the diffraction grating 110 according to the first embodiment, the unevenness in the light amount distribution due to a minute positional shift (pitch shift) of the transmission hole 92 is eliminated.

As described above, in the second embodiment, by providing the diffusion plate 122 at or near the pupil position of the imaging lens 112, it is possible to eliminate unevenness in the light amount distribution of the irradiation light (so-called beam shaping) and reduce the spot. The enlargement of the size is performed by one member. For this reason, as compared with the case where members having these functions are separately provided, the number of components can be reduced, the exposure head 120 can be lightened, and the exposure head 120 can be manufactured at low cost. For example, in the example shown in FIG. 8, it is not necessary to provide the diffusion plate 102 of the first embodiment.

FIG. 9 shows an exposure head 130 according to the third embodiment of the present invention. In the third embodiment, only the configuration of the exposure head 130 is different from that of the second embodiment, and the other configuration is the same. Therefore, only the exposure head 130 will be described. Also,
The same components and members as those in the first embodiment and the second embodiment are denoted by the same reference numerals, and description thereof is omitted.

In the exposure head 130 of the third embodiment, the same diffusion plate 122 as that of the second embodiment
24, and a holding plate 124 is held by a diffusion plate holder 132. The diffusion plate holder 132 projects through a movement hole 134 formed in the casing 68 and into a movement box 136 provided in the casing 68. An insertion hole 138 is formed in a protruding portion of the diffusion plate holder 132, and a guide shaft 140 is inserted into the insertion hole 138 to allow the diffusion plate holder 13 to be inserted.
2 is movable in the optical axis direction. A female screw 142 is formed on the projecting portion of the diffusion plate holder 132, and a male screw 144 screwed to the female screw 142 is
The motor 146 rotates. Therefore, when the motor 146 rotates, the diffusion plate holder 132
Move in the optical axis direction along the guide shaft 140, and the diffusion plate 122 also moves in the optical axis direction. Then, the diffusion plate 12
2 can change the spot magnification. At this time, since the diffusion plate holder 132 is guided by the guide shaft 140, the diffusion plate 122 always maintains a state perpendicular to the optical axis X.

The moving box 136 has the moving hole 13
4 is completely covered so that no extra light enters the exposure head 130.

In the exposure head 130 of the third embodiment having such a configuration, the diffusion plate 122 is provided at or near the pupil position of the imaging lens 112, similarly to the exposure head 120 of the second embodiment. Since it is provided, it is possible to eliminate unevenness (beam shaping) in the light amount distribution of irradiation light and to increase the spot size.

In addition, in the third embodiment, the motor 1
By rotating the diffusing plate 122 in the optical axis direction by rotating 46, the spot magnification by the diffusing plate 122 can be changed, so that the spot size of the spot imaged by the imaging lens 112 (imaging beam diameter) ) Can have a desired diameter. For this reason, unevenness of the light amount distribution due to a minute positional deviation (pitch deviation) of the transmission hole 92 and the like can be effectively corrected, and an image can be formed with higher accuracy.

In the third embodiment, the adjustment of the position of the diffusion plate 122 by rotating the motor 146 may be manually performed by an operator. However, the motor 146 is controlled by a controller such as the controller 78. It is also possible to connect and automate this position adjustment. After the adjustment, the diffusion plate can be fixed with an adhesive or the like, and the adjustment means can be removed. By removing the adjustment means, the weight of the entire exposure head can be reduced, and stable main scanning can be performed.

FIG. 10 shows an exposure head 150 according to a fourth embodiment of the present invention. The fourth embodiment is different from the second embodiment in that the exposure head 15
Only the configuration of the exposure head 150 is different, and the other configuration is the same. Therefore, only the exposure head 150 will be described. The same components, members, and the like as those in the first to third embodiments are denoted by the same reference numerals, and description thereof is omitted.

In the exposure head 150 according to the fourth embodiment, a plurality of diffusion plates 122 are prepared in a state where they are held by respective holding plates 124, and are stored in a diffusion plate storage box 152 provided in the casing 68. I have. And
One or a plurality of desired diffusion plates 122 are selected from the diffusion plate storage box 152 and disposed at the pupil position of the imaging lens 112 or at a position near the pupil position of the imaging lens 112 through the arrangement hole 153 of the casing 68, or the diffusion plate It can be returned into the storage box 152. As described above, the configuration for moving the selected diffusion plate 122 to the pupil position of the imaging lens 112 or a position in the vicinity thereof is not particularly limited. For example, a holding member (for example, a holding arm) holding the diffusion plate 122 And the like, and the desired diffusion plate 122 is disposed at or near the pupil position, or the diffusion plate at or near the pupil position is returned into the diffusion plate storage box 152 by gripping with this gripping member. It is possible to adopt a configuration in which

The diffusing degrees of the plurality of diffusing plates 122 may be the same or may be different from each other.

Note that the diffusion plate housing box 152 of the fourth embodiment is also the moving box 136 of the third embodiment.
Similarly to the above, the arrangement hole 153 is completely covered so that no extra light enters the exposure head 130.

In the exposure head 150 according to the fourth embodiment having such a configuration, the exposure head 120 according to the second embodiment and the exposure head 130 according to the third embodiment are used.
Similarly to the above, since the diffusion plate 122 is provided at or near the pupil position of the imaging lens 112, it is possible to eliminate unevenness in the light amount distribution of the irradiation light (beam shaping) and increase the spot size. I have.

In addition, in the fourth embodiment, by selecting an arbitrary diffusion plate 122 and disposing it at or near the pupil position of the imaging lens 112, the diffusion plate substantially disposed at this position is selected. It is possible to change the degree of diffusion of the whole 122. Since the spot magnification can be changed by changing the diffusion degree to a desired diffusion degree, the spot size (imaging beam diameter) of the spot formed by the imaging lens 112 is desired. Of diameter. For this reason, unevenness of the light amount distribution due to a minute positional deviation (pitch deviation) of the transmission hole 92 and the like can be effectively corrected, and an image can be formed with higher accuracy. Note that, as long as a desired spot magnification can be obtained, the number of diffusion plates 122 arranged at or near the pupil position is not particularly limited, and may be one or more.

In the fourth embodiment, the desired diffusion plate 122 is disposed at the pupil position or a position near the pupil position by operating the above-described gripping member.
The work of returning to the inside may be performed manually by an operator, but it is also possible to connect the motor 146 to a control device such as the controller 78 and automate the position adjustment of the diffusion plate 122. Further, after making the diffusion plate housing box detachable and adjusting to a desired spot size, the diffusion plate housing box can be removed to reduce the weight of the exposure head.

FIG. 11 shows an exposure head 160 according to a fifth embodiment of the present invention. In the fifth embodiment, the exposure head 16 is different from the second embodiment.
Only the configuration of the exposure head 160 is different, and the other configuration is the same. Therefore, only the exposure head 160 will be described. Further, the same components, members, and the like as those in the first to fourth embodiments are denoted by the same reference numerals, and description thereof is omitted.

In the fifth embodiment, a diffusion plate 162 having a different degree of diffusion depending on the direction is used. That is, as shown in FIG.
The degree of diffusion in the direction of the arrow Y along the surface of the diffusion plate 162 is the highest (that is, light is most easily diffused in the direction of the arrow Y), and in the direction perpendicular to the direction of the arrow Y (the direction of arrow Z). Diffusivity is lowest. Then, the degree of diffusion continuously decreases as the light rotates around the optical axis X from the arrow Y direction to the arrow Z direction.

The holding plate 124 holding the diffusion plate 162 is formed with a flat cylindrical holding cylinder 164. The holding cylinder 164 is in contact with the outer peripheral surface of the imaging lens 112, and holds the diffusion plate 162 rotatably around the optical axis X. External teeth 166 are formed on the outer periphery of the holding plate 124, and partially protrude from a meshing hole 174 formed in the casing 68.

The gear housing box 1 is provided in the casing 68.
An external gear 170 rotated by a motor 172 is provided in the gear housing box 168. The external gear 170 and the external teeth 166 of the holding plate 124 are always engaged with each other. When the external gear 170 rotates, the holding plate 124 rotates, and the diffusion plate 162 also rotates about the optical axis X. The rotation of the diffusion plate 162 causes L
The desired spot magnification can be obtained by changing the degree of diffusion in the arrangement direction (sub-scanning direction) and the orthogonal direction (main scanning direction) of the ED 98 (see FIGS. 3 and 4).

Note that the gear housing box 168 of the fifth embodiment completely covers the engagement hole 174 similarly to the movement box 136 of the third embodiment, so that extra light Is not allowed to enter.

In the exposure head 160 of the fifth embodiment having such a configuration, the exposure head 120 of the second embodiment and the exposure head 13 of the third embodiment are also used.
0, and like the exposure head 150 of the fourth embodiment,
A diffusing plate 162 is provided at or near the pupil position of the imaging lens 112.
Is provided, it is possible to eliminate unevenness in the light amount distribution of the irradiation light (beam shaping) and enlarge the spot size.

In addition, in the fifth embodiment, the holding plate 1
24 is rotated about the optical axis X, the diffusion plate 16 is rotated.
2, the spot magnification of the spot formed by the imaging lens 112 can be made a desired diameter. For this reason, unevenness of the light amount distribution due to a minute positional deviation (pitch deviation) of the transmission hole 92 and the like can be effectively corrected, and an image can be formed with higher accuracy.

In the fifth embodiment, the rotation angle of the diffusion plate 162 by rotating the motor 172 may be adjusted manually by an operator.
2 to a control device such as a controller 78, and
It is also possible to automate the angle adjustment of 62.

It is not always necessary to rotate the diffusion plate 162 about the optical axis X. For example, the diffusing plate 162 may be rotated around an axis parallel to the optical axis X or around an axis inclined with respect to the optical axis X. Or you may. Further, after the adjustment, the diffusion plate can be fixed and the adjusting means can be removed to reduce the weight of the exposure head.

As described above, in the exposure heads of the second to fifth embodiments, since the diffusion plates 122 and 152 are provided at or near the pupil position of the imaging lens 112, the amount of irradiation light It is possible to eliminate uneven distribution (beam shaping) and increase the spot size. In addition, the number of parts can be reduced, the exposure head can be made lighter, and it can be manufactured at low cost.

In addition, in the exposure heads of the third to fifth embodiments, the spot size (imaging beam diameter) is changed by changing the spot magnification of the spot by the diffusion plates 122 and 152.
Has a desired diameter, thereby effectively correcting the unevenness of the light amount distribution and forming an image with higher accuracy.

Note that the specific configuration is not limited to the above as long as the spot magnification can be changed as described above. For example, by forming the diffusion plates 122 and 152 from a material whose scattering degree can be changed, the diffusion plates 12 and 152 can be formed.
A configuration in which the scattering degree itself of 2, 152 may be changed. For example, as the diffusion plates 122 and 152, a diffusion plate whose scattering degree changes according to the magnitude of the applied voltage may be used. With this configuration, the diffusivity of the diffusion plate can be changed only by changing the applied voltage, so that the configuration for changing the spot size is simplified.

In the diffusion plate 122 used in the exposure heads of the second to fourth embodiments, the directionality of the degree of diffusion in the direction along the surface of the diffusion plate 122 (that is, in the direction perpendicular to the optical axis X). Although there is no particular limitation, the diffusion plate 1 having different diffusivities in two different directions along the surface can be used. The use of such a diffusion plate is preferable because a desired spot magnification different depending on the direction and a desired beam shaping can be performed. For example, similar to the diffusion plate 162 of the fifth embodiment, a diffusion plate having different degrees of diffusion in two orthogonal directions along the surface can be used. In this case,
When the diffusion plate is arranged such that the spot magnification in the sub-scanning direction is larger than the spot magnification in the main scanning direction, the spot size in the sub-scanning direction is larger than that in the main scanning direction,
Irregularities in the light quantity distribution can be reduced. Further, since the spot size is smaller in the main scanning direction than in the sub scanning direction, a clearer image can be obtained.

Similarly, also in the fifth embodiment, the angle of the diffusion plate 162 is adjusted so that the spot magnification in the sub-scanning direction is larger than the spot magnification in the main scanning direction. By adjusting the angle, it is possible to reduce unevenness in the light amount distribution and obtain a clearer image.

In the second to fifth embodiments,
The position where the diffusion plates 122 and 162 are arranged may be any position as long as it is at or near the pupil position of the imaging means (imaging lens 112).
The specific position is appropriately determined according to the required spot magnification or the like. That is, as described in the third embodiment, by moving the positions of the diffusion plates 122 and 162 along the optical axis direction, the spot magnification changes, so that a range in which a desired spot magnification can be obtained. If the position is within the range, the position is included in “the pupil position of the imaging unit or its vicinity” in the present invention.

[0121]

According to the present invention having the above-described structure, it is possible to correct unevenness of the light amount distribution due to the displacement of the light source and to form an image with high accuracy.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing an image forming apparatus employing an exposure head having an imaging optical system according to a first embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of an exposure head having an imaging optical system according to the first embodiment of the present invention.

FIG. 3 is an exploded perspective view showing a substrate of the imaging optical system according to the first embodiment of the present invention and the vicinity thereof.

FIG. 4 is a plan view showing a substrate of the imaging optical system according to the first embodiment of the present invention.

FIG. 5 is a sectional view of an exposure head having an imaging optical system according to the first embodiment of the present invention.

6A and 6B are graphs qualitatively showing a light amount distribution in an imaging optical system in which a diffraction grating is not provided. FIG. 6A shows a case where light is equally spaced in the sub-scanning direction, and FIG. This is a case where the position is shifted in the sub-scanning direction.

FIGS. 7A and 7B are graphs qualitatively showing a light amount distribution in the imaging optical system according to the first embodiment of the present invention, wherein FIG. 7A shows a plurality of lights and FIG. Take out and show.

FIG. 8 is a sectional view of an exposure head having an imaging optical system according to a second embodiment of the present invention.

FIG. 9 is a sectional view of an exposure head having an imaging optical system according to a third embodiment of the present invention.

FIG. 10 is a sectional view of an exposure head having an imaging optical system according to a fourth embodiment of the present invention.

FIG. 11 is a sectional view of an exposure head having an imaging optical system according to a fifth embodiment of the present invention.

FIG. 12 is a perspective view showing a diffusion plate used in an imaging optical system according to a fifth embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 90 slit plate 92 transmission hole 94 sub-transmission hole 98 LED (light source) 100 pseudo light source 102 diffusion plate (diffusion member) 110 diffraction grating (spot size expansion unit) 112 imaging lens (imaging unit) 122 diffusion plate (diffusion unit) 152 diffusion plate (diffusion means)

Claims (18)

[Claims] A plurality of light sources that emit light in accordance with image information; and a light source configured to form light from the plurality of light sources such that a distance between the light sources from the plurality of light sources has a predetermined imaging magnification with respect to the distance between the light sources. Image forming means for forming an image, and spot size enlarging means for forming an image forming spot of each of the plurality of light sources at a spot magnification larger than the image forming magnification of the image forming means. Optical system. 2. The imaging optical system according to claim 1, wherein said spot size expanding means includes a diffraction grating. 3. An imaging optical system according to claim 1, wherein said spot size enlarging means includes a birefringent material. 4. The imaging optical system according to claim 1, wherein said spot size enlarging means is a lens constituting said imaging means and having a predetermined spherical aberration. 5. The light-emitting device according to claim 1, wherein a diffusion member for diffusing light from the plurality of light sources is disposed between the plurality of light sources and the imaging unit. 2. The imaging optical system according to item 1. 6. The plurality of light sources are constituted by at least two kinds of light sources having different wavelengths, and the diffusing member has a diffusivity corresponding to different wavelengths of the plurality of light sources. An imaging optical system according to claim 1. 7. The light source according to claim 1, wherein the plurality of light sources are arranged along a main scanning direction and a sub-scanning direction, and a spot magnification of the spot size expanding unit is set to be larger in the sub-scanning direction than in the main scanning direction. The imaging optical system according to claim 1, wherein: 8. A plurality of light sources that emit light in accordance with image information, and light from these light sources is formed such that an interval between the light from the plurality of light sources has a predetermined imaging magnification with respect to the interval between the light sources. An imaging optical system comprising: an imaging unit configured to form an image; and a diffusion unit provided at or near a pupil position of the imaging unit and configured to diffuse light from the plurality of light sources. 9. The imaging optical system according to claim 8, wherein said diffusing means has different degrees of diffusion in two directions orthogonal to the optical axes from said plurality of light sources. 10. The light source device according to claim 1, wherein the plurality of light sources are arranged along a main scanning direction and a sub-scanning direction, and a diffusion degree of the diffusing unit is relatively larger in the sub-scanning direction than in the main scanning direction. Claim 9
2. The imaging optical system according to item 1. 11. The connection according to claim 8, wherein said diffusion means is movable along the optical axis direction from said plurality of light sources to said diffusion means. Image optics. 12. The apparatus according to claim 8, wherein the diffusing means comprises a plurality of diffusing plates disposed at or near the pupil position so as to be able to advance and retreat. Imaging optics. 13. The imaging optical system according to claim 8, wherein said diffusion means is rotatable at or near said pupil position. 14. The apparatus according to claim 8, wherein said diffusion means is made of a material whose degree of diffusion can be changed.
An imaging optical system according to claim 13. 15. The imaging optical system according to claim 14, wherein said diffusing means is made of a scattering liquid crystal whose degree of scattering changes according to an applied voltage. 16. A slit plate in which a transmission hole corresponding to light from the plurality of light sources is formed between the plurality of light sources and the image forming means. An imaging optical system according to claim 15. 17. The imaging optical system according to claim 16, wherein a sub-transmission hole corresponding to a position outside the arrangement direction of the plurality of light sources is formed in the slit plate. 18. The imaging optical system according to claim 17, wherein a pseudo light source is provided at a position corresponding to the sub-transmission hole.