JP3648350B2 - Optical element - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical element used in an image forming apparatus that forms an electrostatic latent image on a photoreceptor with a laser beam, and more particularly to a technique for preventing distortion due to thermal expansion or the like.
[0002]
[Prior art]
A beam scanning optical system using a laser beam is used as an optical system such as a laser printer, a digital copying machine, and a plain paper fax machine. As a feature of the recent beam scanning optical system, a long optical element is often used. ing. The long optical elements are frequently used because of the cost reduction due to the resinization of lenses, and the use of resin materials with low refractive power and the formation of strips is relatively easy. Therefore, the glass lens tends to be longer in the main scanning direction than the conventionally used glass lens.
[0003]
FIG. 6 is a diagram showing an example of a beam scanning optical system conventionally used. In the figure, a laser beam 9 generated by the laser unit 1 enters the cylindrical lens 2 and forms an image on the rotary polygon mirror 3. The rotating polygon mirror 3 rotates at a constant angular velocity in a fixed direction, the laser beam 9 is deflected in the main scanning direction (the arrow direction shown in the photosensitive member 6), and the imaging lens 4 and the imaging lens 5 An fθ characteristic is provided so as to perform constant speed scanning on the scanning line, and an image is formed so as to have a predetermined beam spot diameter. The mirror 7 reflects a part of the scanning line and guides it to the synchronization detection sensor 8. The synchronization detection sensor 8 detects that the scanning beam 9 has reached a predetermined position, and generates a synchronization signal of the image writing position. .
[0004]
Some of these optical elements are usually fixed to an integrally formed holding member having an optical element positioning portion and mounting surface called an optical housing in order to stably maintain a desired positional accuracy. Conventionally, such a holding member is often made of a high-strength synthetic resin, for example, a PC resin containing glass fiber, a PPE resin, an unsaturated polyester resin, or an aluminum / zinc die-cast.
[0005]
The imaging lens 4 is made of a transparent synthetic resin and is made of, for example, PC or PMMA. Since the refractive index of these resins is smaller than that of glass, the aperture is made larger than that of a glass lens having the same performance (that is, This can be done by increasing the length). On the other hand, the imaging lens 5 is a long lens having an anamorphic aspherical surface having a refractive index also in the sub-scanning direction, and is difficult to manufacture with glass for processing, and is transparent like the imaging lens 4 described above. For example, it is made of PC, PMMA or the like.
[0006]
Such a long optical element is subject to thermal expansion due to a temperature rise during use and deformation of parts due to moisture absorption of the synthetic resin, so that it is important to increase rigidity. Therefore, the imaging lens 5 of FIG. 6 includes a box-shaped reinforcing rib 5a that extends in front of the lens (incident side) as a reinforcing structure.
[0007]
FIG. 7 is a diagram showing a state in which the same reinforcing rib 10b as that in FIG. 6 is formed on the reflective optical element 10. FIG. When the box-shaped reinforcing rib 10b similar to the transmissive optical element is formed on the reflective optical element in this way, the root of the reinforcing rib 10b is formed when forming a vapor deposition pattern on the reflective surface 10a for the reinforcing rib 10b. And the reflective surface that can be used effectively becomes narrow. Further, the periphery of the laser beam is provided by the reinforcing rib 10b, the incident angle is limited, and only a certain range in the reflecting surface 10a can be used effectively. A two-dot chain line in FIG. 7 illustrates the effective region 10c. The effective area 10c is wide at both ends of the optical element 10 but narrow at the center. That is, since the reinforcing rib 10b is formed in front of the reflecting surface 10a, the effective area 10c is obstructed by the reinforcing rib 10b and is remarkably narrowed as it approaches the optical axis.
[0008]
Therefore, in such an optical element 10, as shown in FIG. 8, the reinforcing rib 10 d is formed on the side opposite to the reflecting surface 10 a, that is, on the rear side of the optical element 10. The rigidity of the part is weak.
[0009]
It is needless to say that the shape shown in FIG. 7 is the most effective for the above bending in view of the cross-sectional shape, but the effective area 10c of the reflecting surface 10a is narrowed, so that it is difficult to align the laser beam. . If the component height H (FIG. 7) is increased so as to widen the effective area 10c, not only will the apparatus be increased in size and cost, but also the optical element 10 itself will be difficult to mold (due to variations in mold temperature). The degree of sink, residual strain, etc.) is further amplified.
[0010]
[Problems to be solved by the invention]
The present invention was conceived from the above facts. In an optical element having a long synthetic resin concave reflecting surface, buckling deformation due to expansion / contraction or vibration due to a change in temperature and humidity and the reflection surface An object of the present invention is to propose an optical element having a rib structure for giving a sufficient vapor deposition surface while giving sufficient rigidity to a change in curvature.
[0011]
[Means for Solving the Problems]
In order to achieve the above object , in claim 1, in a long optical element made of a synthetic resin and having a concave reflecting surface in the main scanning direction for use in a light beam scanning device, an optical axis is provided in front of the reflecting surface. And a distance between the reinforcing rib and the longitudinal axis of the optical element is greater on the center side than on both ends of the optical element .
[0012]
According to a second aspect of the present invention, in the optical element according to the first aspect, the reinforcing ribs are formed stepwise along the longitudinal direction of the optical element.
[0013]
According to a third aspect of the present invention, in the optical element according to the first or second aspect, the effective area of the concave reflecting surface can ensure at least substantially the same width in the longitudinal direction of the optical element .
[0014]
According to a fourth aspect of the present invention, in the optical element according to any one of the first to third aspects, the reinforcing rib extends behind the reflecting surface in parallel with the optical axis.
According to a fifth aspect of the present invention, in the optical element according to any one of the first to fourth aspects, the reflective surface has a constant width in the longitudinal direction, and the connection surface recedes from the reflective surface between the reflective surface and the reinforcing rib. It is connected .
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 shows an optical element 11 having a concave reflecting surface in the first embodiment of the present invention. The optical element 11 is provided with reinforcing ribs 11b on the back surface side of the reflecting surface 11a (this back surface side is referred to as “rear side” of the optical element). However, unlike the conventional one, this reinforcing rib 11b increases the rigidity of the optical element 11, particularly at the center, so that the height h in the optical axis direction is substantially the same from any position on the reflecting surface. Is formed. With such a configuration, the rigidity can be increased without affecting the shape accuracy of the reflecting surface 11a, and deterioration of image quality such as vertical lines due to thermal expansion or vibration, or banding can be prevented.
[0016]
FIG. 2 shows a second embodiment of the present invention. In some cases, the rear reinforcing rib 11b in FIG. 1 may not be sufficiently rigid with respect to the bending force received from the mounting portion as a whole as a bow. Therefore, in the optical element 12 of the second embodiment, the reinforcing rib 12b is also extended to the reflecting surface 12a side (this is referred to as “front”). An effective area 12c shown by a two-dot chain line in FIG. 2B is an area where vapor deposition is performed normally without being obstructed by the ribs, and light rays can also be incident and reflected. That is, although the vicinity of the base of the reinforcing rib 12b is an unusable region, the reinforcing rib 12b is formed on both the front and rear sides of the optical element 12, so that the height h 'on the front side of the reinforcing rib 12b can be reduced, and effective It is possible to increase the area of the region 12c.
[0017]
FIG. 3 is a diagram of a third embodiment of the present invention. In this embodiment, reinforcing ribs 13b are formed before and after the reflecting surface 13a of the optical element 13. The height h in the optical axis direction on the rear side of the reinforcing rib 13b is substantially the same over the entire reflecting surface. In contrast, the front edge 13d of the reinforcing rib 13b is a straight line perpendicular to the optical axis c, and the height h 'increases toward the center of the optical element 13. Further, in this embodiment, the reinforcing rib 13b has a curved shape in which the distance L from the longitudinal axis a of the optical element 13 is wider at the center L0 than at both ends L1. Therefore, the rigidity of the optical element 13 is improved, and the area of the effective region 13c indicated by the two-dot chain line can be increased.
[0018]
As a result, the height H of the cross section at the central portion of the optical element 13 is increased, so that the bending rigidity of the optical element 13 increases toward the central portion. Therefore, compared to the conventional example, the displacement can be suppressed to be smaller with respect to vibration and deformation in the x direction and the z direction, and banding due to vertical lines on the image can be reduced.
[0019]
FIG. 4 shows a fourth embodiment of the present invention. While the reinforcing rib 13b of FIG. 3 is a curved surface, the reinforcing rib 14b of the optical element 14 changes in a step shape. For this reason, the arrangement position of the reinforcing ribs 14b changes stepwise regardless of the height change of the reinforcing ribs 14b, so that the effective area 14c of the reflecting surface 14a changes discontinuously as shown in the figure. Actually, since the laser beam is scanned smoothly, the arrangement of the reinforcing ribs 14b may be determined so as to ensure a substantially rectangular effective range 14d having a minimum width d. The effect of the present embodiment is that when the mold is manufactured, the alignment of the blocks constituting the mold becomes a flat surface, so that the processing becomes easy. Further, the increase in size of the component due to the improvement in rigidity can be partially stopped, so that the molding efficiency can be suppressed and the increase in the size of the unit in which the component is incorporated can be suppressed.
[0020]
FIG. 5 shows a fifth embodiment of the present invention. The optical elements 13 and 14 of FIGS. 3 and 4 have reinforcing ribs 13b and 14b formed on the extension of the reflecting surfaces 13a and 14a. In this embodiment, the reflecting surface 15a is substantially constant in the longitudinal direction of the optical element. The width of the reinforcing rib 15b and the reflecting surface 15a are connected by a discontinuous connecting surface 15d that recedes from the reflecting surface. The advantage of this embodiment is that the mold manufacturing cost can be reduced by minimizing the mold member corresponding to the reflective surface requiring high accuracy and making it a rectangular cross-section that is easy to process.
[0021]
【The invention's effect】
As described above, according to the present invention, in a long optical element made of a synthetic resin and having a concave reflecting surface used for a light beam scanning device, it extends in parallel with the optical axis behind the concave reflecting surface. Since the reinforcing ribs are formed so that the height in the optical axis direction is the same, the bending rigidity of the reflective optical element can be increased, resulting in vertical lines due to thermal expansion and vibration, and deterioration of image quality such as banding. Can be prevented.
[0022]
If the reinforcing rib is extended to the front of the reflecting surface and the height of the reinforcing rib in the optical axis direction in front of the reflecting surface is substantially the same, the bending rigidity can be further increased. Further, since the reinforcing ribs are provided at the front and rear, the height of the rib in front of the reflecting surface can be reduced, and a large effective area can be secured.
[0023]
Further, in a long optical element made of a synthetic resin and having a concave reflecting surface used for a light beam scanning device, a reinforcing rib extending substantially parallel to the optical axis is formed in front of the reflecting surface, and the reinforcing rib and the optical element Since the distance from the longitudinal axis of the optical element is larger on the center side than the both ends of the optical element, the bending rigidity of the optical element can be increased, and the effective area of the reflecting surface can be increased even if the height of the rib is increased. Largely secured.
[0024]
If the reinforcing ribs are formed stepwise along the longitudinal direction of the optical element, the mating surfaces of the molds can be made flat.
If the reinforcing rib is extended to the rear of the reflecting surface, the height on the front side can be reduced and the effective area can be increased.
If the reflecting surface has a constant width in the longitudinal direction and the reflecting surface and the reinforcing rib are connected by a connecting surface that is recessed from the reflecting surface, the range of the reflecting surface that requires high accuracy is limited to a substantially rectangular shape. As a result, the corresponding mold member is kept to the minimum necessary and the processing becomes easy, so that there is an effect of suppressing the mold manufacturing cost.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram showing the configuration of a first embodiment of a long and reflective optical element according to the present invention, where (a) is a top view, (b) is a front view, and (c) is an I of (a). It is -I sectional drawing.
FIGS. 2A and 2B are diagrams showing a configuration of a second embodiment of the long and reflective optical element of the present invention, where FIG. 2A is a top view, FIG. 2B is a front view, and FIG. 2C is II of FIG. It is -II sectional drawing.
FIGS. 3A and 3B are diagrams showing the configuration of a third embodiment of the long and reflective optical element of the present invention, where FIG. 3A is a top view, FIG. 3B is a front view, and FIG. 3C is III of FIG. It is -III sectional drawing.
FIGS. 4A and 4B are diagrams showing the configuration of a fourth embodiment of the long and reflective optical element of the present invention, where FIG. 4A is a top view, FIG. 4B is a front view, and FIG. 4C is an IV of FIG. It is -IV sectional drawing.
FIGS. 5A and 5B are diagrams showing the configuration of a fifth embodiment of the long and reflective optical element of the present invention, where FIG. 5A is a top view, FIG. 5B is a front view, and FIG. It is -V sectional drawing.
FIG. 6 is a perspective view showing a configuration of a conventional scanning optical system.
FIGS. 7A and 7B are diagrams showing a state in which the same reinforcing rib as that of the transmission type is formed on the long and reflective optical element, where FIG. 7A is a top view, FIG. 7B is a front view, and FIG. It is VII-VII sectional drawing.
FIG. 8 is a perspective view showing a configuration of a conventional scanning optical system using a long and reflective optical element.
[Explanation of symbols]
11, 12, 13, 14, 15 Optical elements 11a, 12a, 13a, 14a, 15a Reflective surfaces 11b, 12b, 13b, 14b, 15b Reinforcement ribs 12c, 13c, 14c, 15c Effective area 15d Connection surface h Light of reinforcement ribs Rear height h ′ in the axial direction Front height a in the optical axis direction of the reinforcing rib a Longitudinal axis c of the optical element Optical axis

Claims (5)

In a long optical element made of a synthetic resin and used in a light beam scanning apparatus and having a concave reflecting surface in the main scanning direction , a reinforcing rib extending substantially parallel to the optical axis is formed in front of the reflecting surface, and the reinforcing rib An optical element characterized in that the distance from the longitudinal axis of the optical element is larger on the center side than on both ends of the optical element. 2. The optical element according to claim 1, wherein the reinforcing rib is formed in a step shape along the longitudinal direction of the optical element. 3. The optical element according to claim 1, wherein the effective area of the concave reflecting surface can ensure at least substantially the same width in the longitudinal direction of the optical element. 4. The optical element according to claim 1, wherein the reinforcing rib is extended behind the reflecting surface in parallel with the optical axis . 5. The optical element according to claim 1, wherein the reflective surface has a constant width in the longitudinal direction, and the reflective surface and the reinforcing rib are connected by a connection surface that is recessed from the reflective surface. An optical element.

Images