Lens and an optical apparatus with the lens

A lens comprises a lens body having a lens surface with a convex-concave portion and a filler whose optical characteristics are the same as or much similar to that of said lens body, wherein said convex-concave portion of said lens is filled with said filler.

BACKGROUND OF THE INVENTION
 1. Field of the invention
 The present invention relates to optical elements of optical products such
 as cameras, image formation apparatuses in which electrophotographic
 techniques are used, and relates to a method of producing the optical
 elements.
 2. Description of the Related Art
 The grinding method is the most well known method for producing a lens. As
 an example, Japanese Unexamined Patent Publication No. Sho. 62-203744
 discloses the following grinding method.
 A grinding bowl having a reciprocal shape of a lens to be ground is
 prepared in advance. The grinding bowl is pushed against a glass material.
 Abrasive grains are provided between the grinding bowl and the glass
 material, so that the grinding bowl can grind the glass material to form a
 lens. Since super fine abrasive grains are used, there is little grind
 stress applied to the glass material, and little distortion arising in the
 inside of the lens. However, this lens producing method can not be
 conducted unless a shape of the lens to be produced is simple, and thus
 not all lens shapes can be produced. That is, the shape of the lens is
 limited, having only a few degrees of freedom.
 As higher grades of lens performance become required, techniques for making
 the lens surface aspherical and more precise become more important.
 Because it is difficult to produce an aspherical lens using the above
 grinding method, the aspherical surface is generally formed using a
 machine controlled by numerical control (hereinafter referred to as "NC").
 However, it is difficult to obtain a mirror surface that is suitable for
 use as a lens after processed using the NC, and a shape error remains in
 the lens in the order of sub micrometers, and some flaws remain on the
 processed surface by a working tool.
 Ductility mode grinding methods may be used to produce a surface that is
 suitable for use as a lens, even when grinding fragile materials such as a
 glass. For example, Japanese Patent Application No. 2-53557 discloses a
 method of grinding an object to have an aspherical surface. The object to
 be ground is disposed on a turntable turned by a motor, and a grindstone
 for grinding the object is disposed in an air spindle turning at a speed
 of about 10000 rpm. A pulse is detected by a rotary encoder that is
 directly connected to a rotation axis of the turntable, grinding data is
 provided to a piezo-actuator on the basis of the detected pulse, and a
 movable table is continuously moved forward and backward. The air spindle
 is fed one step for each rotation of the turntable and its position is
 changed so that the position where the grindstone contacts the object
 changes. Using this method, it is possible to grind freely the object to
 asymmetrical and aspherical shapes. Moreover, because it is possible to
 control a cutting depth of the grindstone against the object in the order
 of sub-micrometers, fragile materials can be ground to obtain a mirror
 surface that may be used as a lens.
 However, this method is time-consuming, as a significant amount of time is
 required to grind the aspherical parts of the lens surface. Moreover, a
 ground surface of the lens after the grinding has some convex or concave
 flaws in the order of sub micrometers. In the case of requiring to produce
 a lens with high resolution, there arises a problem where the optical
 characteristics of the lens are adversely affected by these slight flaws.
 Thus, a final step is required when processing the optical element such as
 a lens by the NC.
 A polishing method is often used in the final step. For example, the lens
 surface is further polished with diamond grindstones and a soft pad such
 as felt, thereby forming a better lens surface.
 Three kinds of factors indicating the error of the lens surface are well
 known.
 The first is an undulation (in the order of from several hundreds
 micrometers to several millimeters).
 The second is a surface roughness (in the order of from sub micrometers to
 several micrometers).
 The third is a flaw made by the grinding (in the order of from several
 dozen micrometers to several hundreds micrometers).
 In the above, the undulation of the lens strongly depends on a machine
 precision. Thus, it is difficult to correct the undulation of the lens
 surface in the final step, and it is required that the undulation is made
 quite little in the step of the NC.
 On the other hand, the surface roughness may be fully improved in the final
 step (polishing).
 The flaw is in a place between the undulation and the surface roughness.
 When polishing the flaws to improve the lens surface, the whole lens shape
 is degraded and the undulation is increased. Furthermore, because these
 flaws result from contacting tools which hit the lens surface, the flaws
 can not be reduced as the undulation which can be reduced by the NC.
 As mentioned above, the NC may process the object to the complicated lens
 shape. However, since the NC is conducted by machines, it is impossible to
 prevent the lens surface from making the flaws. In the worse case,
 deterioration of the lens characteristics may occur.
 Conventionally, a coating method is used to improve the lens. However, its
 purpose is to prevent a ray of light reflecting on the lens and the
 coating is conducted along the shape of the lens surface. Therefore, the
 convex or concave of the lens surface may not be modified.
 SUMMARY OF THE INVENTION
 The object of the present invention is to provide a shape formation method
 by which an optical element such as a lens may be processed in high
 precision.
 According to the present invention, there is provided a lens comprising:
 a lens body having a lens surface with a convex-concave portion; and
 a filler whose optical characteristics are the same as or much similar to
 that of the lens body,
 wherein the convex-concave portion of the lens is filled with the filler.
 The filler may contain a SiO.sub.2.
 The SiO.sub.2 maybe filled with a method comprising the steps of:
 coating a polysilazane on the lens surface; and burning the polysilazane to
 be transferred to the SiO.sub.2.
 The filler may contain a polymer. The filler may contain an acrylic
 polymer. The filler may contain an ultraviolet cure resin. The
 convex-concave portion is no more than 500 .mu.m in width, 1 .mu.m in
 depth and 500 .mu.m in pitch.
 To achieve the above object, there is provided a lens process method
 according to the present invention as follows. Namely, the lens is filled
 in the flaw portions of the lens with a material whose optical
 characteristics are similar to the lens, so that the lens surface may be
 smoothed and the flaws may be reduced. For example, inorganic materials
 such as SiO.sub.2 or organic materials such as a polymer are suitable to
 the filled material.
 There are several methods to fill the material in the flaw. SiO.sub.2 may
 be filled in the flaw by a vacuum evaporation, a plasma CVD or the like.
 Polysilazane may be coated on the surface and burnt thereon to be
 transformed into silica (SiO.sub.2). (Polysilazane is a polymer including
 Si, N and H (sometimes including organic group).) And, a resin material is
 coated on the lens to form a coated film.

DETAILED DESCRIPTION OF THE PRESENT INVENTION
 The present invention will now be described with reference to the accompany
 drawings.
 (Embodiment 1)
 FIG. 1 shows an axisymmetric aspherical f.theta. glass lens used in the
 Embodiment 1. BK7 is used as a glass material of the lens. An A-surface of
 the glass material has a horizontal scanning direction radius of curvature
 of R1, and perpendicular scanning directions radii of curvature of r1, r2
 and r3 are different from one another depending on positions thereof. The
 A-surface is referred to as a modified toric shape. A B-surface has a flat
 shape. Both of the A-surface and B-surface have a mirror surface. The
 A-surface is processed by the NC to have an aspherical shape. In the
 Embodiment 1, a ductility mode grinding is conducted, resulting in that
 the ground surface of the glass almost becomes the mirror surface by only
 the grinding. However, flaws 1 are made by grindstones in the horizontal
 scanning direction of the lens 2, which results from a process principle
 of the NC. In this case, the flaws are about 100 .mu.m in width, about
 0.087 .mu.m in maximum depth, about 100 .mu.m in pitch. The surface
 roughness is measured to be 0.019 .mu.m in average, and the undulation of
 the glass lens surface is measured to be 0.28 .mu.mP-P in the region of 8
 mm in the perpendicular scanning direction. FIG. 2 shows a measured result
 of the lens surface shape. The undulation of the lens surface is measured
 with a contact-type shape measuring instrument. The shape measurement of
 the flaws and the surface roughness measurement are conducted with
 non-contact-type shape measurement instrument. Only the measurement result
 of the lens surface shape is shown in FIG. 2 in this embodiment, as same
 as the other embodiments.
 As shown in FIG. 3, the convex and concave portions of the lens surface are
 filled with a transparent material 3 as a filter. SiO.sub.2 is used as the
 transparent material in this embodiment. Here, the method is used in which
 polysilazane is coated on the lens surface and burnt to be transformed
 into SiO.sub.2. Polysilazane is a polymer including Si, N and H (sometimes
 including organic groups). The polysilazane is transformed into silica
 when the polysilazane is burnt. Here, Silazane is a compound having a
 Si--N bonding and the Polysilazan it has SiH.sub.2 NH as a composition.
 TONEN POLYSILAZANE, produced by Tonen Corporation, is dissolved with
 xylene and the glass lens is put into the solution. Thereafter, the lens
 is removed from the solution, so that the convex and concave portions of
 the lens are filled with the polysilazan. After naturally drying the
 coated lens, the coated surface is burnt in an electric heating furnace at
 500.degree. C. for one hour. Thereafter, the polysilazane has transformed
 to a pure silica glass of substantially 100 percent when analyzing the
 filled portions. In this manner, the filled material whose optical
 characteristics are substantially as well as those of the lens material
 may be easily produced with the polysilazane. The silica glass film layer
 is 0.3 .mu.m in thickness, 0.003 .mu.m in average surface roughness and
 0.33 .mu.mP-P in undulation in the center region of 8 mm.
 FIG. 4 shows a result of the shape measurement, as apparent from which a
 flaw depth is improved from 0.087 .mu.m to 0.022 .mu.m as compared with
 FIG. 2, and it can be understood that the lens surface is smoothed by
 filling SiO.sub.2.
 (Embodiment 2)
 In this Embodiment, the polysilazane is coated on the surface by a
 spincoater. Here, a nonaxisymmetric aspherical F.theta. glass lens that is
 the same as that of Embodiment 1 is used as a sample. FIG. 5 shows a
 result of shape measurement of the lens surface before coating the
 polysilazane. In this case, the lens flaws are about 100 .mu.m in width,
 about 0.084 .mu.m in maximum depth and about 100 .mu.m in pitch. The
 surface roughness is 0.016 .mu.m in average, the undulation of the glass
 lens surface is 0.32 .mu.mP-P in the region of 8 mm in the perpendicular
 scanning direction.
 Lens 2 is mounted in the spincoater (not shown). As shown in FIG. 6,
 Polysilazanes 4 are dropped at 5 points on the lens of which each of
 Polysilazanes is dropped apart from 20 mm respectively. And, the
 spincoater is rotated at a rotation speed of 2500 rpm to coat the
 polysilazane thereon. After naturally drying the lens surface, the coated
 surface is burnt in an electric heating furnace at 500.degree. C. for one
 hour, so that the pure silica glass film is coated. FIG. 7 shows a result
 of the shape measurement, which is upgraded on optical characteristics.
 The silica glass film layer is 0.2 .mu.m in average thickness, 0.001 .mu.m
 in average surface roughness, 0.22 .mu.mP-P in undulation in the center
 region of the sample of 8 mm and 0.013 .mu.m in maximum depth. From the
 measurement results, it may be understood that the lens surface is
 smoothed by filling SiO.sub.2.
 The finished lens is mounted on an optical system in a laser printer. When
 measuring light gathering characteristics thereof, a uniform beam whose
 radius is 60 .mu.m is obtained in the whole scanning width. And, when
 printed out using the laser printer mounting the finished lens, the laser
 printer can conduct a high quality printing compared with a laser printer
 using a lens which is made by only the NC.
 (Embodiment 3)
 Using an ultraviolet cure resin including acrylic acid group, it is tried
 to reduce the flaw on the lens surface. Optical characteristics such as
 refraction index of the ultraviolet cure resin including acrylic acid
 group are much similar to those of the material of which the lens is made.
 The ultraviolet cure resin is diluted with an isopropyl alcohol, so that
 the ultraviolet cure resin having a viscosity of about 15 cp is produced.
 Using the spincoater in the manner as mentioned in Embodiment 2, the
 convex and concave portions of the lens are filled with the ultraviolet
 cure resin. Thereafter, a ray of ultraviolet is irradiated on the lens
 surface on which the ultraviolet cure resin is coated to cure the
 ultraviolet resin. The same lens as used in embodiment 1 and 2 is used in
 this embodiment.
 FIG. 8 shows a shape measurement result of the lens surface which has not
 yet been coated with the resin. In this case, the flaw is measured to be
 about from 100 .mu.m to 200 .mu.m in width, 0.106 .mu.m in the maximum
 depth and about from 150 .mu.m to 250 .mu.m in pitch. The lens surface is
 measured to be 0.026 .mu.m in average surface roughness and 0.41 .mu.m in
 undulation in the range of 8 mm in the perpendicular scanning direction.
 FIG. 9 shows a shape measurement result of the lens surface after the
 surface was coated with the resin. The resin layer is 0.25 .mu.m in
 average surface roughness. The lens surface is improved to 0.005 .mu.m in
 average surface roughness and 0.30 .mu.m in undulation in the range of 8
 mm in the perpendicular scanning direction. These results explain that the
 surface has been smoothed.
 Using methods according to the present invention as mentioned above, the
 flaws which are made during the machine process may be reduced efficiently
 and easily.
 As a result, a lens which has a complicated shape and is required high
 precision may be easily produced in a short period of time.