Lens Device

1. Field of the Invention

This invention relates to a laser scanning apparatus suitable for use on a laser printer or the like.

2. Description of the Prior Art

In the art of laser scanning apparatus, it has been the usual practice to deflect a laser beam by means of a rotary polygonal mirror, thereby scanning a photosensitive medium with the laser beam after correcting its focal point according to the deflection angle through a correction lens. For example, Japanese Patent Laid-Open No. 46713/1989 describes a laser scanning apparatus of this sort employing a correction lens which is formed by linearly cutting off upper and lower portions of a circular lens in such a manner as to provided parallel upper and lower faces on the opposite sides of the optical axis of the lens. The lower face of the correction lens, serving as a mounting surface, is held in intimate contact with a base plate which has a lens positioning means projectingly provided thereon, and a resilient lid member is fitted over the upper face of the correction lens and fittingly engaged with the positioning means. A projection formed on the resilient lid member is abutted against the upper marginal edge of the correction lens to push the same against the base plate and the positioning means, thereby holding the lens fixedly in position.

Generally, at the time of mounting a correction lens on a laser scanning apparatus, high accuracy is required with regard to the position of the optical axis of the lens. However, in the conventional laser scanning apparatus in which the position of the optical axis is determined on the basis of the mounting surface formed by a lens cutting operation, high accuracy is required not only with respect to the base plate and the positioning member but also on the mounting surface of the correction lens. It follows that the mounting surface needs high precision cutting and machining operations which lengthen and complicate the lens machining process and considerably increase production costs.

It is therefore an object of the present invention to provide a laser scanning apparatus which contributes to simplifying the machining process of the correction lens while achieving highly accurate positioning of the correction lens to guarantee picture images of high quality without impairing the compactness of the apparatus.

According to the present invention, there is provided a laser scanning apparatus in which laser light emitted from a light source is deflected in a predetermined direction by a rotary polygonal mirror to scan the surface of a photosensitive medium, the laser scanning apparatus comprising: a correction lens for correcting the focal point of the laser light deflected by the rotary polygonal mirror according to a deflection angle, the correction lens being formed by cutting a circular lens along a plane off the optical axis thereof and removing a segment of shorter arc therefrom; a lens holder including a frame arranged to abut against an arcuate peripheral portion of the correction lens to hold the latter in position both in the radial direction of the lens and in the direction of the optical axis; and pressing means for pressing the correction lens both in the radial direction and in the direction of the optical axis thereof to hold the lens in position relative to the lens holder.

The above and other objects, features and advantages of the invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings which show the invention by way of a preferred embodiment and in which:

FIG. 1 is a side view of a laser scanning apparatus embodying the present invention;

FIG. 2 is a plan view of the laser scanning apparatus;

FIG. 3 is a perspective view of an f.theta. lens according to the invention, showing the outer shape of the lens;

FIG. 4 is a view taken in the direction of arrow W in FIG. 2 with the f.theta. lens in its operative orientation;

FIG. 5 is a sectional view taken on line 5--5 of FIG. 4; and

FIG. 6 is a sectional view taken on line 6--6 of FIG. 2.

Hereafter, the invention is described more particularly by way of the preferred embodiment shown in the drawings.

Referring to FIGS. 1 and 2 which show the laser scanning apparatus of the invention in side and plan views respectively. The reference numeral 1 denotes a base plate for mounting thereon a laser beam producing unit 2, a polygon scanner unit 4 and a correction lens unit 7.

The laser beam producing unit 2 (hereinafter referred to simply as "laser unit" for brevity) has a semiconductor laser-emitting device 8 (hereinafter referred to simply as "LD" for brevity) retained in an LD holder 10 as a light source. The laser unit 2 is provided with a collimator lens 12 for collimating the laser light which is emitted from the LD 8 toward a polygon mirror 34, which will be described later. The collimator lens 12 is retained in position by a collimator lens holder 14. Further, the laser unit 2 is provided with a collimator lens focusing member 18 with a knob 16 for adjusting the distance between the LD 8 and the collimator lens 12. The collimator lens focusing member 18 is fixed in position by a fixing member 20. The laser unit 2 is further provided with a beam-forming lens 22, which forms the collimated laser light into a beam, and a beam-forming lens holder 24 which retains the beam-forming lens 22 in position. In turn, the beam-forming lens holder 24 is fixed in position by a spring member 26. These component parts are integrally supported on a frame 28 and mounted on the base plate 1 through the frame 28 which is fixed to the base plate 1.

The polygon scanner unit 4 includes a polygon mirror 34 which is rotated by a polygon motor 30 to serve as a rotary polygonal mirror. These parts are integrally supported on a polygon mirror drive base plate 36. The polygon motor drive base plate 36 is fixed on the base plate 1 to support the polygon scanner unit 4 in a predetermined position on the base plate 1.

Referring to FIGS. 3-6, details of the correction lens unit 7 are discussed below.

FIG. 3 shows in a perspective view the shape of the f.theta. lens 40 which is employed as a correction lens in the present embodiment. The periphery 48 of the original lens, including a segment indicated by the two-dot chain line in FIG. 3, is a high precision true circle having an optical axis C at the center thereof, and a plane of high precision flatness defined by the back edges 48a of the peripheral portion 48, the plane lying perpendicular to the optical axis C. This f.theta. lens 40 has a greater arc than a semi-circle and is formed by cutting the original lens along a plane off the optical axis C and removing a segment 46 with a shorter arc. The cutting operation does not require especially high accuracy in forming the flat surface on a cut area 44 and can therefore be performed by a simple cutting operation. The flat cut surface 44 is preferred to be formed at a position as close to the optical axis C as possible to minimize the size of the f.theta. lens 40.

As shown in FIGS. 4 to 6, a lens holder 42 is provided with an adapter plate 47 for fixation to the base plate 1. A pair of opposing columns 49a integrally extending from the adaptor plate 47 are provided with flat surfaces 50b in face to face relation with each other. A beam 49b which is bridged between the columns 49a is provided with a U-shaped groove 49c linearly along the length thereof, the groove 49c having its bottom surface 50a disposed in face to face relation with the adapter plate 47. The bottom surface 50a of the groove 49c and the opposingly disposed flat surfaces 50b of the columns 49a constitute a radial reference surface 50 which is to be held in contact with the arcuate peripheral portion 48 of the f.theta. lens 40 at three spaced positions to determine the position of the f.theta. lens in the radial direction thereof.

Each one of the columns 49a is provided with a ledge 51c which extends longitudinally therealong in face to face relation with a similar ledge 51c on the other column 49a. A surface 51b of each of these two ledges 51c is located in the same plane as side surface 51a of the groove 49c to provide an axial reference surface 51 which determines the position of the f.theta. lens 40 in the direction of the optical axis C.

In this manner, in the present embodiment, the frame-like lens holder 42 is constituted by the adapter plate 47, the pair of columns 49a and beam 49b. The radial and axial reference surfaces 50 and 51 are not restricted to the U-shape as employed in this embodiment, and may be formed in an arcuate or V-shape or in any other shape as long as they can determine the position of the f.theta. lens 40 in the radial and axial directions by circumscribing contact with the peripheral portion 48 of the f.theta. lens 40.

The adapter plate 47 is provided with a couple of positioning projections 52 in predetermined spaced positions on its bottom surface 47a. These positioning projections 52 are fittingly engageable with positioning holes 53, which are formed in the base plate 1, thereby fixedly mounting the lens holder 42 in a predetermined position on the base plate 1.

The correction lens unit 7 includes an axial pressing member 54 which is abutted against the convex surface 40a of the f.theta. lens 40 to press the peripheral portion 48 of the f.theta. lens 40 against the axial reference surface 51 upon mounting the f.theta. lens 40 on the lens holder 42. In this particular embodiment, the axial pressing member 54 is in the form of a resiliently deformable leaf spring of rectangular shape which is folded at a longitudinally median point to provide a ridge 56 to be abutted against the convex surface 40a of the f.theta. lens 40. Needless to say, the axial pressing member 54 may be in a form other than a leaf spring as long as it is capable of pressing the peripheral portion 48 against the axial reference surface 51.

The correction lens unit 7 is further provided with a radial pressing member 58 which is abutted against the cut surface 44 of the f.theta. lens 40 when the latter is mounted on the lens holder 42, thereby pressing the f.theta. lens 40 against the radial reference surface 50. In this particular embodiment, the radial pressing member 58 is a pair of rectangular leaf springs. The leaf springs are abutted against opposite end portions of the cut surface 44 of the f.theta. lens 40 to press the peripheral portion 48 of the f.theta. lens 40 against the radial reference surface 50. Needless to say, the axial and radial pressing members 54 and 58 which are formed separately in this embodiment may be integrated into a unitary structure if desired.

The correction lens unit 7 is assembled in the following order. First, the f.theta. lens 40 is inserted into the lens holder 42 from the side away from the cut surface 44 until the peripheral portion 48 fits in the groove 49c. By so doing, the peripheral portion 48 of the f.theta. lens 40 is abutted against the radial and axial reference surfaces 50 and 51 of the lens holder 42. At the time of inserting the f.theta. lens 40 into the lens holder 42, the assembling job can be facilitated by dropping the f.theta. lens 40 into the lens holder 42 which is held in such a posture that the bottom surface 50a of the groove 49c on the beam 49b is faced upward. At this time, the f.theta. lens 40 is unlikely to drop in the direction of the convex surface 40a because the peripheral portion 48 fits in the groove 49c. Then, the axial pressing member 54 is mounted on the columns 49a, whereupon the ridge portion 56 is abutted against the convex surface 40a of the f.theta. lens 40, exerting a pressure on the f.theta. lens 40 to hold the peripheral portion 48 against the axial reference surface 51. Then, the radial pressing member 58 is mounted on the columns 49a, the f.theta. lens 40 is thereby pressed in the radial direction to hold the peripheral portion 48 against the radial reference surface 50. In this manner, the f.theta. lens 40 is fixed in position in the radial and axial directions relative to the lens holder 42 by the actions of the pressing members 54 and 58.

After positioning and fixing the f.theta. lens 40 relative to the lens holder 42 in the above-described manner, the projections 52 on the lens holder 42 are engaged with the positioning holes 53 on the base plate 1 to mount the f.theta. lens 40 on the base plate 1 with the cut surface 44 in face to face relation with the base plate 1.

During operation of the laser scanning apparatus of this embodiment, the laser light emitted from LD 8 is collimated through the collimator lens 12. The resultant collimated laser light is formed into a beam through the beam-forming lens 22. The laser beam is deflected in a predetermined direction by the rotating polygon mirror 34 to scan the surface of a photosensitive medium through the accurately positioned f.theta. lens 40 which corrects the focal point according to the deflection angle of the beam.

As is clear from the foregoing description, in the laser scanning apparatus according to the present invention, the location of the optical axis of the f.theta. lens 40 is determined by the accurately shaped peripheral portion 48 of the original lens instead of the cut surface 44. In contrast to the prior art which requires a lengthy and complicated machining process involving high precision operations for forming a cut surface to be relied upon for determination of the optical axis position, what is required in the present invention involves only a simple cutting operation free of any high precision requirement, simplifying and reducing the cost of the machining process for the f.theta. lens itself. In addition, since the peripheral portion 48 has an arcuate shape, the direction of the optical axis C is not changed even if the contacting positions of the radial and axial reference surfaces 50 and 51 and the f.theta. lens 40 are shifted relative to the peripheral portion 48 due to rotation of the f.theta. lens 40 about the optical axis C. Accordingly, the f.theta. lens 40 and the lens holder 42 can be assembled with high efficiency because there is no need to pay special attention to positional deviations due to rotation of f.theta. lens 40 about its optical axis C during assembly.

Furthermore, the f.theta. lens 40 is fixed in the lens holder 42 with the cut surface 44 faced toward the base plate 1, so that it is unnecessary to provide a wide space between the base plate 1 and the position of the optical axis C. It follows that the polygon mirror 34 may be located in a position close to the base plate 1, without spacing from the latter, to maintain the compactness of the laser scanning apparatus as a whole.

It is to be understood that the present invention is not restricted to the particular forms shown in the foregoing embodiment, and can be put into practice in various modified forms without departing from the scope of the invention as encompassed by the appended claims.