GB2330918A

GB2330918A - Creating images with a laser beam without ghosting

Info

Publication number: GB2330918A
Application number: GB9821566A
Authority: GB
Inventors: Yoshihiro Hama; Toshimitsu Saito
Original assignee: Asahi Kogaku Kogyo Co Ltd
Current assignee: Pentax Corp
Priority date: 1997-10-03
Filing date: 1998-10-02
Publication date: 1999-05-05
Anticipated expiration: 2018-10-02
Also published as: GB2330918B; JPH11109275A; GB9821566D0; HK1020782A1; KR19990036778A

Abstract

An apparatus for creating an image with a laser beam comprises a laser light source 1, a polygonal mirror 4 which reflects the emitted laser light onto a photoreceptor's surface, an optional f# lens 5 and an optional photoreceptor drum 6. A light attenuating element 9 is provided in the optical path of laser light between the polygonal mirror 4 and the photoreceptor drum 6. The quantity of the laser light that is reflected from the photoreceptor's surface to the polygonal mirror 4, from which it is reflected back to be projected again onto the photoreceptor drum 6, is sufficiently reduced by the light attenuating element 9 so that the generation of a ghost image due to the second reflection of the laser light is suppressed. Alternatively, the f# lens 5 may be used as the light attenuating element (Figs 4(a), 4(b)).

Description

APPARATUS FOR CREATING PATTERNS WITH A LASER BEAM The present invention relates to an apparatus for forming a latent image with laser light that is projected onto a photoreceptor's surface. More particularly, the invention relates to an apparatus that is capable of creating patterns of improved quality by reducing the generation of ghost images on the photoreceptor's surface.

A typical example of apparatus to which the present invention may be applied is a laser scan recording apparatus that uses a polygonal mirror and which is intended for use in laser printers and the like. As shown diagrammatically in Fig. 5, a laser light source 1, such as a semiconductor laser device, emits laser light. The emitted laser light passes through a collimator lens 2 to form a beam of parallel light having an elliptical cross section. The parallel beam of light then passes through a beam shaping prism 3 to form a circular beam, which is projected onto a specular surface of a rotating polygonal mirror 4. As the polygonal mirror 4 rotates, it changes the angle of reflection of the laser light Further, the reflected light passes through an fO lens assembly 5 (which in this case, is composed of two lenses 5A and 5B), and is projected onto the surface of the photoreceptor drum 6 (i.e., onto the photoreceptor's surface). This light from the fO lens assembly is also scanned along the rotating axis of the photoreceptor drum 6 thereby performing horizontal scanning". Since the photoreceptor drum 6 rotates about its own axis, vertical scanning is also performed. These two kinds of scanning cooperate to create a specified pattern on the photoreceptor's surface. The fO lens assembly 5 is provided to ensure that the scan speed, which varies with the change in the angle of reflection of the laser light from the polygonal mirror 4, is made uniform on the photoreceptor's surface.

A problem with the laser scan recording apparatus employing a polygonal mirror is that the latent image formed on the photoreceptor's surface has a ghost. Referring to Fig.

5, consider the behavior of laser light travelling from the polygonal mirror to the photoreceptor drum 6. The laser light reflected from the polygonal mirror 4 passes through the fO lens assembly 5 to be projected onto the photoreceptor drum 6. Thereafter, as shown by a dashed line, the laser light is reflected back toward and passes through the fO lens assembly 5 to return to the polygonal mirror 4. The laser light is again reflected from the polygonal mirror 4 and travels along the path indicated by another dashed line, passes through the fO lens assembly 5, and projects onto the photoreceptor drum 6 to form a ghost pattern on the photoreceptor's surface.

The polygonal mirror 4 rotates at high speed, so when the laser light has been reflected back to the mirror, a reflecting surface of the mirror has rotated to a different angular position than when the first reflection occurred.

Hence, the second reflection of the laser light from the polygonal mirror 4 is directionally offset from the optical axis for the first reflection. This means that due to the variation in the angular position of the rotating mirror 4, pattern drawing occurs in a different position on the photoreceptor drum 6 than that from the first reflected light.

As the result, the ghost image overlaps the true image to create a different pattern than what should be produced.

Hence, the quality of the resulting image created by the printer is deteriorated.

The foregoing description assumes that two patterns formed by double reflection overlap to produce a ghost image.

If triple or more reflections occur, three or more patterns will overlap to produce a ghost image that results in the creation of a quite unnatural pattern.

An object of the present invention is to produce an apparatus that eliminates multiple projections of laser light onto the photoreceptor's surface so as to ensure the production of high-quality image patterns.

According to an aspect of the present invention, there is provided apparatus for creating patterns with a laser beam comprising a laser light source, a light reflecting element, and a light attenuating element.

The laser light source emits laser light. The light reflecting element reflects the laser light and projects it onto a photoreceptor's surface. The light attenuating element is located on the optical path of the laser light between the light reflecting element and the photoreceptor's surface.

Preferably, the light attenuating element has a light transmittance p greater than 0.2 but smaller than 0.7.

The light attenuating element may be composed of a transparent plate having a specified light transmittance. If the apparatus is of such a type that an fO lens that allows the laser light to be scanned over the photoreceptor's surface at a uniform speed is provided between the polygonal mirror and the photoreceptor's surface, the fO lens composes the light attenuating element. If desired, a surface of the fO lens may be provided with a coat that permits reduced light transmission.

The present disclosure relates to the subject matter contained in Japanese patent application No. Hei. 9-270880 (filed on October 3, 1997) which is expressly incorporated herein by reference in its entirety.

Examples of the present invention will now be described with reference to the accompanying drawings, in which Fig. 1 is a perspective view showing the general layout of a laser scan recording apparatus of the present invention; Fig. 2 is an elevation of part of the apparatus shown in Fig. 1; Fig. 3 shows schematically how laser light is attenuated in the apparatus; Figs. 4(a) and 4(b) show, in cross section, two examples of a modification in which an fO lens is used as a light attenuating element; and Fig. 5 is a diagrammatic plan view showing a general layout of a related laser scan recording apparatus.

An apparatus for creating patterns with a laser beam will now be described with reference to the accompanying drawings.

Fig. 1 is a diagrammatic perspective view showing the general layout of the laser scan recording apparatus.

Fig. 2 is an elevation of part of the apparatus shown in Fig.

1. A semiconductor laser 1 working as a laser light source emits laser light. Provided on the optical path of the laser light are a collimator lens 2 that converts the emitted laser light into a parallel beam and a cylindrical lens 3 that shapes the laser beam. The shaped laser beam is projected onto a polygonal mirror 4. The polygonal mirror 4 is a prism having an equilateral polygonal cross section, and its side faces, which are six in number in the apparatus under consideration, serve as reflecting surfaces. The polygonal mirror 4 is driven to rotate clockwise about the central axis 4a. The shaped laser beam incident on a reflecting side surface of the mirror 4 is reflected by that side surface.

As the mirror 4 rotates, the reflected laser beam is deflected and passes through an fO lens assembly 5 made up of more than one lens element such that the laser beam is projected onto the surface of a photoreceptor drum 6 (i.e., onto the photoreceptor's surface). As a result, the laser beam is scanned over the photoreceptor's surface in the axial direction of the photoreceptor drum 6 to effect a "horizontal scan.

The fO lens assembly 5 corrects the angular velocity during deflection of the reflected laser beam to a uniform scan speed on the surface of the photoreceptor drum 6. In the case shown, the fO lens assembly 5 is made up of two lens element a first lens 5A positioned closer to the polygonal mirror 4 and a second lens 5B positioned closer to the photoreceptor drum 6. The photoreceptor drum 6 is rotated about its axis to effect a "vertical" scan. Part of the laser beam passing through the fE lens assembly 5 is reflected by a reflecting mirror 7 positioned outside the area of the photoreceptor drum 6 where pattern drawing is to occur, and the reflected beam is received by a beam detector 8 composed of a light-receiving device. The beam detector 8 is connected to a control circuit (not shown) so as to produce a horizontal sync signal for the scanning of the laser light.

A light attenuating plate 9 for reducing the intensity of light is provided between the polygonal mirror 4 and the first lens 5A of the fO lens assembly 5. The light attenuating plate 9 is a flat plate formed of resin or glass having a specified light transmittance. The plate 9 is positioned on the optical path of the laser beam reflected from the polygonal mirror 4 toward the fO lens assembly 5 in such a way that the reflected laser beam passes through the light attenuating plate 9.

The apparatus having the above-described layout operates in the following manner. The laser light emitted from the semiconductor laser 1 is passed through the collimator lens 2 and the cylindrical lens 3 to be converted to a parallel circular beam. This parallel circular beam becomes incident on the polygonal mirror 4 and reflected therefrom as the beam is deflected by the rotating mirror 4.

The reflected laser beam passes through the light attenuating plate 9 so that its quantity is slightly reduced. Thereafter, the laser beam passes through the fO lens assembly 5 so that its speed becomes uniform in the direction of deflection; the laser beam is then projected onto the photoreceptor drum 6 to effect a horizontal scan over the photoreceptor's surface.

This horizontal scan, combined with the vertical scan due to the rotation of the photoreceptor drum 6, allows a latent image to be formed in a desired pattern on the surface of the photoreceptor drum 6. The laser light reflected from the photoreceptor drum 6 travels on a return path until it reaches the polygonal mirror 4, from which it is again reflected and projected onto the photoreceptor drum 6.

As shown schematically in Fig. 3, when the laser light that has been first reflected from the photoreceptor's surface (which is hereinafter referred to as the first laser light and indicated as Ll) travels on the return path, it passes through the fO lens assembly 5 and the light attenuating plate 9. The laser light is then reflected again from the polygonal mirror 4 to be directed toward the photoreceptor drum 6. This second laser light L2 passes through the light attenuating plate 9 again before it is passes through the fe lens assembly 5 to be projected onto the photoreceptor drum 6. Thus, the first laser light Ll passes through the light attenuating plate 9 only once whereas the second laser light L2 makes three passes therethrough, so that the second laser light L2 is attenuated by the plate 9 by a greater amount than the first laser light Ll. As a result, the ratio of the quantity of the first laser light L1 to that of the second laser light L2 on the photoreceptor's surface is sufficiently increased so as to suppress the generation of a ghost image due to the second laser light L2.

Assume that the light attenuating plate 9 has a light transmittance p of 0.7. If the quantity of the laser light which is first reflected. from the polygonal mirror 4 is represented as 1o and the quantity of the first laser light L1 falling on the photoreceptor's surface is represented as II, the following relation exists: Ii = 0.7Io If the quantity of the second laser light L2 that falls on the photoreceptor's surface after making three passes through the light attenuating plate 9 is represented as I2, the following relation exists: I2 = (0.7)3I0 (Note that, to simplify the explanation, it is assumed here that the light is not attenuated by the reflections on the photoreceptor's surface and the polygonal mirror reflection surface. That is, although other such incidental attenuation would actually occur, the simplified equations omit their effect.) Therefore, the ratio R2 between the quantity of the first laser light Ll and that of the second laser light L2 is expressed by: R12 = (0.7)3I0/0.7I0 = (o. 7) 2 O.S Thus, I2, which is the quantity of the second laser light, is about one half of I, which is the quantity of the first laser light. This difference contributes to a satisfactory suppression of the generation of a ghost image.

In general, the quantity of the second laser light is equal to p2 times the quantity of the first laser light, p representing the light transmittance of the light attenuating plate 9.

Considering all relevant factors including the quantity of the first laser light, the sensitivity characteristics of the photoreceptor drum 6 in receiving light and the density of the visible image formed by the latent image on the photoreceptor's surface, one may adjust the light transmittance of the light attenuating plate 9 to a value so as to provide a ratio Rlz that produces the least noticeable ghost image. According to the experiments conducted by the present inventors, it has been found that the light transmittance p is preferably greater than 0.2 but smaller than 0.7. If p is equal to 0.7 or more, the effectiveness of the light attenuating plate in suppressing ghosts is impractically small. If p is equal to 0.2 or less, the light attenuating plate is very effective in suppressing ghosts but, on the other hand, the luminous efficiency (utilization) of the semiconductor laser 1 is impractically low.

The light attenuating plate 9 may be provided in any position on the optical path between the polygonal mirror 4 and the photoreceptor drum 6. It should however be noted that the closer the plate 9 is located to the polygonal mirror, the smaller the required size of the plate. In the apparatus described above, the light attenuating plate 9 functioning as the light attenuating element is independent of the fO lens assembly. However, the invention is not limited to such a construction. If the f6 lens assembly can be adapted to have a specified light transmittance, it may be used as the light attenuating element and in this case there is no need to provide a separate independent light attenuating plate. If the fO lens assembly itself is to be used as the light attenuating element, the individual components of the assembly may be provided with a light attenuating film or surface coat in order to ensure a specified setting of light transmittance.

Figs. 4(a) and 4(b) illustrate two ways of providing such a light attenuating film or coat. In Fig. 4a, a film 10 having a specified light transmittance is attached to a surface of an fO lens element 5. In this respect, a film may be regarded as a thin layer that is preformed and adhered to the fO lens element in one piece. In Fig. 4b, a coat 11 that permits reduced light transmission to achieve similar light attenuation is provided on a surface of the fa lens element 5. In this respect, a coat may be regarded as a composition that is directly applied to the fO lens element 5 surface.

It should also be noted that the arrangement described above is applicable to any type of apparatus that has a light reflecting element provided on the optical path of laser light and in which the laser light reflected from the element is allowed to reach a photoreceptor's surface to create a specified pattern.

As described above, the apparatus comprises a laser light source, a light reflecting element by which the emitted laser light is reflected to be projected onto a photoreceptor's surface and a light attenuating element provided between the light reflecting element and the photoreceptor's surface. Because of this arrangement, the quantity of the laser light that is reflected from the photoreceptor's surface to the light reflecting element, from which it is reflected back to be projected again onto the photoreceptor's surface is sufficiently reduced by the light attenuating element. Therefore the generation of a ghost image due to the second reflection of the laser light is effectively suppressed to ensure the creation of high-quality image patterns. If the light attenuating element is composed of a light attenuating plate, it needs only to be inserted into the optical system of an existing apparatus for creating patterns with a laser beam, and the intended arrangement can be realized with great ease. If the light attenuating element is composed of an fO lens, the intended arrangement can be realized without any additional components.

Claims

CLAIMS : -

1. An apparatus for creating an image with a laser beam which comprises: a laser light source emitting laser light; a light reflecting element which reflects said laser light onto a photoreceptor's surface; and a light attenuating element positioned in an optical path of said laser light between said light reflecting element and said photoreceptor's surface.

2. Apparatus according to claim 1, wherein said light attenuating element has a light transmittance p greater than 0.2 but smaller than 0.7

3. Apparatus according to claim 1 or 2, wherein said light reflecting element comprises a polygonal mirror that is rotatable such that the laser light is scanned over said photoreceptor's surface.

4. Apparatus according to any preceding claim, wherein said light attenuating element comprises a light transmitting plate having a specified light transmittance.

5. Apparatus according to claim 3, wherein an fO lens that allows the laser light to be scanned over the photoreceptor's surface at a uniform speed is provided between said polygonal mirror and said photoreceptor's surface, said fE lens forming said light attenuating element.

6. Apparatus according to claim 5, wherein a surface of said fe is provided with a coat that permits reduced light transmission.

7. Apparatus according to claim 5, wherein a surface of said fE lens is covered with a film that permits reduced light transmission.

8. Apparatus according to any one of claims 1 to 3, wherein the light attenuating element comprises a coating provided on an optical element provided between the light reflecting element and the photoreceptor surface.

9. Apparatus according to any one of claims 1 to 3, wherein the light attenuating element comprises a film provided on an optical element provided between the light reflecting element and the photoreceptor surface.

10. Apparatus according to any preceding claim, wherein the apparatus creates patterns on the photoreceptor' s surface.

11. Apparatus substantially as hereinbefore described with reference to the accompanying drawings.