JPS5868063A - Image forming apparatus - Google Patents

Abstract

PURPOSE:To enable uneven distribution of image illumination on the face of a photoreceptor to be fully corrected, by providing a correction member moving to a position having constant relative relationship to the position of a lens at the time of changing magnification, and making the position of this member controllable in the direction crossing the optical axis. CONSTITUTION:Since a plate 8 for correcting the law of the 4th power of cosine is held with a support for holding a lens 7 at a distance (a) apart from the lens 7 in the optical direction, the distance (a) between the lens 7 and the plate 8 is kept constant when the lens 7 is moved by changing magnification, and image illumination is made uniform in a longitudinal slit direction with the tip of the plate 8 protruding in the light path. Besides, when the difference in illumination distribution between the center and the both end parts in the luminous flux occurs and deviates from the designed value, the distribution of illumination can be made uniform by loosening a screw 18 and adjusting the plate 8 in the vertical direction with respect to the support 15.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image forming apparatus such as a variable magnification copying machine that projects an original image onto a photosensitive surface at different magnifications, and in particular to correcting uneven distribution of image light amount on the photosensitive surface. Concerning improvements to equipment.

Image variable magnification. In the A-area forming device, when the magnification is changed and the optical path length ratio before and after the lens is changed, the distribution of the image light amount on the photosensitive surface changes before and after the magnification change, resulting in fluctuations in image quality and other problems. , there is a disadvantage that it also causes a change in the characteristics of the photoreceptor. In order to solve this problem, it is known that a different slit plate is placed in and out of the optical path each time the magnification is changed.
This not only complicates and enlarges the mechanical structure, but also requires subtle changes in the shape of each slit plate, complicating the device configuration and significantly increasing the cost.

There is also a known device in which slits are placed on the original surface side and on the photosensitive surface side, and each slit is used depending on the magnification, but this also requires the shape of each slit to be slightly changed, so it is similar to the above. There are drawbacks.

1. There are also known methods in which the light emission distribution of the original lamp is changed according to the magnification, or in which the distribution is corrected by moving the lens relative to a fixed light intensity distribution correction plate, but none of them. It is difficult to make sufficient corrections.

The present invention provides a device that can solve all of the above-mentioned disadvantages.

The present invention will be described in detail below with reference to the drawings. In the figure, a drum having an electrophotographic photosensitive layer on its circumferential surface rotates in the direction of the arrow. As the drum D rotates, it is uniformly charged by the charger 10, and is then subjected to slit exposure of an optical image of the original O at an exposure position E by an optical system, which will be described later. An image is formed. This latent image is developed by the developer 11, and the toner image is transferred to the copy paper 1 which is conveyed in the direction of the arrow under the action of the transfer charger 120.
Transferred to 3. After the transfer, the toner image is fixed on the paper 13, while the toner remaining on the drum D is removed by the cleaning device 1.
4 from the drum D, and the drum D is reused.

The document O is placed on a document table 1 that moves in the direction of the arrow in synchronization 17 with the rotation of the drum D. The document O is scanned by the movement of the table 1 in the direction of the arrow. At this time, the document O is scanned by a long fluorescent lamp, a fluorescent lamp, etc. It is illuminated by a light source 2 having a light emission distribution in the direction. During this scanning, the light beam from the document 0 illuminated by the light source is transmitted through the slit 3 (11711Q in the longitudinal direction of the slit) placed at the foot position near the document table l
(a direction perpendicular to the document scanning direction) and is directed toward the fixed mirror 4. The light beam reflected by mirror 4 is then sequentially reflected by mirrors 5 and 6 and enters lens 7. The light flux exiting the lens 7 is then reflected by the fixed mirror 9 and enters the drum D at the exposure position E. That is, the original image is exposed onto the drum D, and the exposure area at this position E is defined by the area where the image of the slit 3 is projected onto the position E by the lens 7. (However, the above image of slit 3 is not a focused image because slit 3 is conjugate with the drum with respect to lens 7 and is slightly away from the document surface.) When the document scanning is completed, table 1 moves in the opposite direction to the arrow. Then, it performs a backward movement I7 and returns to the forward movement starting position.

Now, when the lens 7 and the mirrors 5 and 6 are at the positions indicated by solid lines, a same-size image of the document is formed on the drum D, and the lens 7'
By moving the mirrors 5 and 6 to the position 5j6', a reduced image of the original is formed on the drum D. The copying magnification is changed by changing the optical path length ratio between the front and rear lenses.

Incidentally, the ratio of the document scanning speed (the moving speed of the table 1 in the direction of the arrow) and the speed of the drum D is also changed in accordance with the selected magnification.

By the way, as is well known, the lens transmittance decreases as the angle of incidence (θ) of a light beam on a lens increases. This is the i1'4 uJS4 power law, but in any case, in the above-mentioned slit exposure type copying apparatus, the lens transmittance decreases as the light beam reaches the end in the longitudinal direction of the slit (drum generatrix direction). Since the lens angle of view changes when the magnification is changed, the image light amount distribution at the exposure position often changes every time the magnification is changed.The present invention solves this problem with simple means.

8 is a correction plate for the ωS fourth power law according to the present invention.

This correction plate 8 is arranged at a lens transmittance position where the cross-sectional area of the projected light beam changes little even when the magnification is changed. In the illustrated example, the correction plate 8 is supported by a support 15 that supports the lens 7 at a distance a from the lens 7 in the optical axis direction. Therefore, the correction plate 8 moves integrally with the lens 7 when changing the magnification. Therefore, during reduction copying, the correction plate 8 is located at a position 8' at a distance a from the lens (7) in the optical axis direction.In other words, even if the magnification is changed, the distance a between the lens 7 and the correction plate 80 remains constant. drooping

The tip of the correction plate protruding into the light beam, that is, the tip that corrects the light intensity distribution, corrects the light intensity distribution in the longitudinal direction of the slit in accordance with the CO34 law, and corrects the image in the longitudinal direction of the slit at the exposure position. The shape is configured so that the amount of light is uniform. Now, the illuminance distribution on the document surface is smooth]・
Assuming that it is uniform in the longitudinal direction, by making the tip 81 of the slit 8 into, for example, an arc shape as shown in FIG. A uniform image light amount distribution can be obtained in the longitudinal direction of the slit.

FIG. 2 is a view of the correction plate 8 and the lens 7 viewed from behind the lens on the optical axis. The upper part of the lens 7 is hidden by a correction plate having an edge 81. In FIG. 2, the correction plate 8 is centered at a point spaced a distance H from the optical axis.
An experimental example of exposure unevenness correction using a correction plate made of a part of such a disk is shown below.

A lens 7 with a focal length of 160 mm and an F value of 6 is used, and a correction plate with an H of 210 x m and an R of 2081 nm is installed at a position 100 mm away from the rear pupil position of the lens, and the magnification is the same (cropping) and reduced. When the exposure unevenness distribution in the longitudinal direction of the slit was measured at (X O, 64), it was suppressed to within 4% at most even when the magnification was changed. In addition to a circle, the correction plate 8 may be a quadratic curve or a polygonal approximation.

By the way, when copying at any magnification, align the side edge of the original O (in the direction perpendicular to the original scanning direction) with the common reference position of the side edge of the document table 1, and when copying at any magnification, In a copying apparatus that projects an image onto a side end portion of a photoreceptor, the lens is moved in a direction oblique to the optical axis when changing magnification. FIG. 3 shows an optical path development diagram of such an apparatus.

In this figure 3, O is the optical position of the original when copying at the same size, O'
is the optical position of the original at the time of reduction copying, and the above-mentioned side edge of the original coincides with the reference line R, and by moving the lens 7 in a direction oblique to the optical axis X, it is aligned with the line R. The original image side edge is imaged at the S position of the drum side edge both at the same magnification and when reduced. In this case as well, the principal ray that enters the lens 7 at an angle of θ at the same magnification and the principal ray that enters the lens 7 at an angle of 0 during reduction are at the same position on the correction plate 8, that is, the light Ilη
The projection light flux incident at a position a tan θ away from 11 is regulated at the same position of the correction plate, and the co
It is confirmed that the s fourth law distribution is corrected. By the way, when the magnification is changed, the maximum angle of view at each magnification differs, but as long as the correction plate 8 is extended in the longitudinal direction of the slit to the extent that it can regulate the incident light flux at the maximum angle of view, it can be used at any angle. The exposure distribution of the incident projection light beam is corrected at all magnifications, giving a uniform exposure distribution on the image plane. (This also applies to devices that move the lens only in the optical axis direction when changing magnification.

) By the way, in Fig. 3, when changing the magnification, the lens 7 is in the optical axis direction,
Then, it moves in the direction perpendicular to this and reaches the position 7', and the angular range of incidence on the lens 7' becomes asymmetrical with respect to the optical axis X. However, this is simply due to the magnification It's just that it differs depending on IJ. That is, in the case of an animal, the range in which the correction plate 8' contributes to regulating the projection light flux is only a range that is 1 to 1 asymmetric with respect to the optical axis. Therefore, the present invention is particularly effective for such an apparatus in which the lens is moved in a direction oblique to the optical axis when changing the magnification. (Of course, the present invention can also be applied to a device that moves the lens only in the optical axis direction.) In any case, the correction plate 8 is provided in the optical path near the lens 7 and moved integrally with the lens 7 when changing the magnification. (-te,
With one correction plate, it is possible to easily correct the uneven distribution of light liquid according to the cos fourth power law for all magnifications. However, when the correction plate 8 matches the pupil position of the lens 7, it acts as a complete aperture diaphragm and does not work to correct the cos fourth power law, so the correction plate 8 does not match the pupil position of the lens 7. It is necessary to provide the lens at a position near the lens.

By the way, the shape of the above correction plate 8 is set to correspond to the illuminance distribution in the longitudinal direction of the slit on the document surface. However, when a halogen lamp, a fluorescent lamp, or the like is used as an actual light source, a completely desired illuminance distribution on the document surface cannot be obtained.

This corrects for variations in tilt, lip, force, etc. due to variations in filament position, temperature, and the reflective shade that makes up the illumination system.

FIG. 4 shows the relationship between the movement of the correction plate 8 across the optical path and the exposure distribution on the photosensitive surface according to the present invention.

As shown in Fig. 4(a), when the correction plate 8, which is provided with a slight deviation from the center of the lens, is moved up and down in Fig. 2, the upward movement is shown by the curve ■. As shown, the exposure distribution on the photoreceptor surface shows a decrease in the light amount at the periphery and an increase in the light amount at the center, and conversely, when moving downward, the light amount distribution shows an increase at the periphery and a decrease at the center as shown by the curve (→).

Also, when moving the correction plate 8 to the left and right in Fig. 2, curves appear respectively. ), the distribution is upward to the right and upward to the left as shown in 0,
Only the change in slope does not affect the correction of the ωs fourth power law.

Therefore, we utilized these characteristics to distribute lamp light.

To correct the position of the correction plate 8 in response to deviations from the design value of the luminous flux distribution to the correction plate 8 due to a reflective shade, mirror, etc., and to obtain a more perfect uniform exposure distribution in the longitudinal direction of the slit on the image plane. Can be done.

FIG. 5 shows the adjustment means. FIG. 5 is a view of the lens and correction plate viewed from above the optical axis, and the lens 7 is fixed to the lens support 15. 16 is a plate for mounting the correction plate.

The plate 16 has a vertically long long hole 17 with a difference (-
The plate 16 is fixed to the support 15 by screwing screws 18 into the support 15.

The warped correction plate 8 has a long hole 19 that is long in the left-right direction, and the correction plate 8 is fixed to the plate 16 by inserting a screw 20 into this long hole and screwing the screw 20 to the plate 16. Ru.

If a distribution error occurs in the projected light flux in which the difference between the center and both ends deviates from the design value, loosen the screw 18 and adjust the plate 8 in the vertical direction (see Fig. 4 (a)). Correct this. On the other hand, if a distribution error occurs in the projected light flux in which the difference between both ends deviates from the design value, loosen the screw 20 and adjust the plate 8 in the left-right direction (see Fig. 4(b)).
), correct this.

When these adjustments have been completed and the light intensity distribution in the longitudinal direction of the image slit has become uniform, screw screw 18.20
is tightened again to fix the relative relationship between the lens 7 and the correction plate 8. This allows the light amount distribution to be kept uniform at any magnification.

In the illustrated example, the correction plate 8 is located near the image plane side of the lens 7, but it may be located near the object side of the lens 7. It may be distributed to both sides to block light.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of an embodiment of the present invention, Fig. 2 is an explanatory diagram of one of the main parts of the embodiment, Fig. 3 is a developed optical path diagram of the embodiment of the invention, and Fig. 4 is the same. FIG. 5 is an explanatory diagram of a change in light amount distribution according to the embodiment. FIG. 5 is an explanatory diagram of one of the main parts of the embodiment. 2 is a light source, 3 is a slit, 7 is a lens, and 8 is a correction plate.

Claims (1)

[Claims] In a variable magnification image forming apparatus that selectively projects an original image onto a photosensitive surface at different magnifications, a correction member for correcting the cos fourth power law provided near a lens,
An image forming apparatus comprising: a correction member that moves to a position that maintains a constant relative relationship with a lens when changing magnification; and means for adjusting the position of the correction member in a direction across an optical path.