JP2840839B2 - fθ lens - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an fθ lens used in an optical scanning device that scans a surface to be scanned by deflecting a light beam by an optical deflector. The present invention relates to an easy and high-performance fθ lens.

(Prior Art) Conventionally, an fθ lens has been used as a scanning lens in an optical scanning device that scans a surface to be scanned by deflecting a light beam by an optical deflector. That is, the light beam is reflected and deflected at an equal angular velocity by an optical deflector such as a rotary polygon mirror or a galvanometer mirror. In general, the sheet may be a flat sheet conveyed in any direction, and in any case, the deflecting direction is flat. Therefore, the deflection speed (deflection angular velocity) of the deflector and the scanning surface In order to make the scanning speed of the light beam proportional, an fθ lens having a characteristic of making the amount of movement of the light beam on the surface to be scanned proportional to the deflection angle θ is provided between the optical deflector and the surface to be scanned. There is a need.

The fθ lens is usually a combination of a plurality of lenses, and various fθ lenses have been proposed for the purpose of minimizing the number of lenses and improving the performance as much as ever. For example, Japanese Patent Application Laid-Open No.
An fθ lens composed of two lenses is disclosed.

(Problems to be Solved by the Invention) By the way, in order to obtain a practically preferable fθ lens,
It is necessary to consider the ease of manufacture and assembly.
Heretofore, assembling the lens in a structure to be incorporated into the lens barrel has made it possible to relatively easily assemble the lens with high accuracy. However, in recent years, it is customary to cut the lens into a strip shape in order to reduce the size of the apparatus, thereby reducing the size of the lens body. In that case, the fθ lens is configured such that each lens constituting the fθ lens is fixed to the mount, so that if both surfaces of the lens have a curvature, it becomes difficult to process the lens. The mount also needs to be processed in accordance with the curvature of the lens, and there is a problem that the processing of both the lens and the mount takes time and costs increase. Further, in the case of mounting by pressing a surface having a curvature against the plane of the mount or the like, it is difficult to increase the accuracy of the mounting position, and there is a problem that an assembling error increases and the performance as an fθ lens deteriorates. If the fθ lens is composed of two lenses as described above, a lens having a curvature on both sides is required, and the above-mentioned inconvenience is expected in processing and mounting the lens.

The present invention has been made in view of the above-described problems, and has as its object to provide an fθ lens that can be easily processed and attached, and has good performance.

(Means for Solving the Problems) As a result of intensive studies based on the above objects, the present invention has
If three lenses are used, it is possible to provide a curvature on only one surface and flatten the other surface for all the lenses, and in this case, the refractive power of the lens becomes negative in order from the optical deflector side. I came to find that it would be better if the order was negative, then positive. There are eight ways of arranging the lens system depending on whether the flat surface of each lens is directed to the optical deflector side or the surface to be scanned, and the fθ lens is used in each of these eight cases. When the overall lens characteristics were examined, the results shown in Table 1 were obtained.

Each lens type is evaluated by appropriately combining several types of lenses that are considered to have the best performance among the lens types, and the value of the lens with the best result is shown. The image surface curvature indicates the maximum amount of curvature from the reference position in both the main scanning main direction and the sub-scanning direction. (H is the height at the shake angle θ on the image plane) indicates the maximum value of the values defined. In addition, the value of * of the curvature of the image surface indicates that the value is too large. In each case, an optical deflector is disposed on the left side of the lens system, and a surface to be scanned is disposed on the right side.

As shown in Table 1, the lens types are (2), (4),
In the cases of (5), (6), (7), and (8), the curvature of the image plane becomes large, so that it is not preferable to use the fθ lens. Therefore, it is necessary to arrange the three lenses having one flat surface as lens type (1) or (3), and the fθ lens of the present invention is a negative lens arranged in order from the optical deflector side. A first lens having a refractive power and a flat surface on the scanned surface side, a second lens having a negative refractive power and a flat surface on the scanned surface side or the deflector side, and a positive refraction; It is characterized by comprising a third lens having a force and a flat surface on the optical deflector side.

(Function) According to the fθ lens of the present invention, the lens has good lens characteristics, and all the surfaces of each lens are flat, so that the lens can be easily processed and the manufacturing cost can be reduced. Processing of the lens mount and highly accurate mounting on the mount are also facilitated.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a perspective view of an optical scanning device provided with an fθ lens according to one embodiment of the present invention, and FIG. 2 is a side view thereof.

The light beam 2 emitted from the light source 1 is expanded to a desired beam diameter by a beam expander 3 and then incident as a parallel light beam on a rotating other-surface mirror 4 rotating in the direction of arrow A. The light beam 2 reflected and deflected by the rotary polygon mirror 4 passes through an fθ lens 8 arranged on the optical path, and is then conveyed (sub-scanned) at a constant speed in the direction of arrow C (sub-scanning). Main scan in the direction. The fθ lens 8 is a lens that converges the light beam 2 deflected at an equal angular velocity by the rotary polygon mirror 4 on a flat running surface 9 and scans the light beam 2 at a constant speed, and has a negative refractive power and has a negative refractive power. A first lens 5 having a flat surface 9, a second lens 6 having a negative refractive power and a flat surface to be scanned 9, and a third lens 6 having a positive refractive power and having a flat surface on the side of the rotary polygon mirror 4. Lens 7.

Using the fθ lens having the above-described refractive power and three lenses each having a flat surface oriented in the above-described direction, the light beam is scanned on the scanned surface 9 at a constant speed, High-accuracy two-dimensional scanning can be realized. Examples of each lens of the fθ lens will be described below. Note that r ₁
~r ₆ is shown in FIG. 2, the radius of curvature of the lens surface of each lens of the fθ lens, d _1, d _3, d ₅ is the first respectively the axis of the second, third lens 5, 6 and 7 The upper wall thickness, d ₂ and d ₄ are the air intervals on the axis between the lenses, d ₀ is the interval from the deflection point to the lens surface of r ₁ ,
d ₆ is the distance from the lens surface of r ₆ to the scanning point (in mm), and n ₁ , n ₂ , and n ₃ are the refractive indices of the first, second, and third lenses, respectively. The wavelength of the light beam is 632.8 nm, the beam deflection angle by the rotary polygon mirror is ± 15 °, and the focal length of the entire fθ lens is 100 mm.

Each lens of the fθ lens shown in FIG. 2 the r ₁ ~
r ₆ , d _{0 to} d ₆ are shown for the sake of convenience, and the specific lens shapes of the fθ lenses of the first to ninth embodiments described below are shown in FIG. ) To 14th
It is shown in FIG. FIGS. 3 (b) to 14 (b) show aberration diagrams of the fθ lenses of the first to twelfth examples, respectively.

First Example d ₀ = 4.4118 r ₁ = −22.0588 d ₁ = 0.5882 n ₁ = 1.5151 r ₂ = ∞ d ₂ = 2.1622 r ₃ = −218.8953 d ₃ = 0.6195 n ₂ = 1.5667 r ₄ = ∞ d ₄ = 2.1434 _{r 5 = ∞ d 5 = 0.8912} n 3 = 1.7786 r 6 = -24.5680 d 6 = 113.8701 second embodiment _{d 0 = 4.4118 r 1 = -29.4118} d 1 = 1.5986 n 1 = 1.5151 r 2 = ∞ d 2 = 7.4718 r ₃ = -1850.6882 d ₃ = 0.4960 n ₂ = 1.5667 r ₄ = ∞ d ₄ = 1.9412 r ₅ = ∞ d ₅ = 2.2059 n ₃ = 1.7786 r ₆ = -33.8616 d ₆ = 121.1452 Third embodiment d ₀ = 4.4118 r ₁ = -14.7059 d ₁ = 0.5882 n ₁ = 1.5151 r ₂ = ∞ d ₂ = 0.2941 r ₃ = −124.2074 d ₃ = 1.0504 n ₂ = 1.5667 r ₄ = ∞ d ₄ = 1.9412 r ₅ = ∞ d ₅ = 0.5882 n ₃ = 1.7786 r ₆ = −17.8716 d ₆ = 114.1307 Fourth embodiment d ₀ = 4.4118 r ₁ = -44.1177 d ₁ = 0.5882 n ₁ = 1.5151 r ₂ = ∞ d ₂ = 0.2941 r ₃ = −52.6042 d ₃ = 1.0504 n ₂ = 1.5667 r ₄ = ∞ d ₄ = 1.9412 r ₅ = ∞ d ₅ = 0.5882 n ₃ = 1.7786 r ₆ = −25.7099 d ₆ = 107.3483 Fifth Embodiment d ₀ = 4.4118 r ₁ = -51.4706 d ₁ = 0.5882 n ₁ = 1.5151 r ₂ = d ₂ = 0.2941 r ₃ = -35.8532 d ₃ = 0.5882 n ₂ = 1.5667 r ₄ = ∞ d ₄ = 1.3468 r ₅ = ∞ d ₅ = 0.5882 n ₃ = 1.7786 r ₆ = −22.9771 d ₆ = 105.9313 Sixth embodiment d ₀ = 4.4118 r ₁ = −58.8235 d ₁ = 0.5882 n ₁ = 1.5151 r ₂ = ∞ d ₂ = 0.2941 r ₃ = − _{31.9188 d 3 = 0.9034 n 2 =} 1.5151 r 4 = ∞ d 4 = 1.9412 r 5 = ∞ d 5 = 0.5882 n 3 = 1.7786 r 6 = -23.9803 d 6 = 107.7183 seventh embodiment d ₀ = 4.4118 r ₁ = - _{14.7059 d 1 = 0.5882 n 1 =} 1.5151 r 2 = ∞ d 2 = 0.2941 r 3 = -122.6857 d 3 = 1.0000 n 2 = 1.5667 r 4 = ∞ d 4 = 1.9412 r 5 = ∞ d 5 = 0.5882 n 3 = 1.7786 r ₆ = −17.8334 d ₆ = 114.0192 Eighth embodiment d ₀ = 4.4118 r ₁ = −29.4122 d ₁ = 0.5882 n ₁ = 1.5151 r ₂ = ∞ d ₂ = 14.9938 r ₃ = −48.7320 d ₃ = 0.5882 n ₂ = 1.5151 r ₄ = ∞ d ₄ = 1.9412 r ₅ = ∞ d ₅ = 1.2232 n ₃ = 1.7786 r ₆ = -25.9230 d ₆ = 136.3936 Ninth embodiment d ₀ = 4.4118 r _{1 = -23.5294 d 1 = 0.5882 n 1} = 1.5151 r 2 = ∞ d 2 = 3.1897 r 3 = -296.1479 d 3 = 0.5882 n 2 = 1.5151 r 4 = ∞ d 4 = 1.9412 r 5 = ∞ d 5 = 1.4596 n 3 = 1.7786 r ₆ = -26.5912 d ₆ = 115.2886 10th embodiment d ₀ = 4.4118 r ₁ = -21.47059 d ₁ = 1.64860 n ₁ = 1.5151 r ₂ = ∞ d ₂ = 0.897059 r ₃ = ∞ d ₃ = 0.588235 n ₂ = 1.566704 r ₄ = 261.89510 d ₄ = 1.941176 r ₅ = ∞d ₅ = 1.176471 n ₃ = 1.7786 r ₆ = −24.14667 d ₆ = 112.474114 11th embodiment d ₀ = 4.4118 r ₁ = −23.44727 d ₁ = 0.588235 n ₁ = 1.5151 r ₂ = ∞ d ₂ = 3.169007 r ₃ = ∞ d ₃ = 0.586182 n ₂ = 1.5151 r ₄ = 321.07367 d ₄ = 1.934400 r ₅ = ∞ d ₅ = 1.154459 n ₃ = 1.7786 r ₆ = −26.51284 d ₆ = 114.774824 12th Example d ₀ = 4.4118 r ₁ = −29.39958 d ₁ = 1.597941 n ₁ = 1.5151 r ₂ = ∞ d ₂ = 7.468756 r ₃ = ∞ d ₃ = 0.495822 n ₂ = 1.566704 r ₄ = 1849.92280 d ₄ = 1.940373 _{r 5 = ∞ d 5 = 2.204969} n 3 = 1.7786 r 6 = -33.84757 d 6 = 121.148132 ( the invention According to the present invention as fruits) above description, one side is flat 3
A high-performance f-theta lens can be obtained by combining two lenses, so that it is possible to reduce costs by using easily processed lenses, simplify the processing of the lens mount, and attach it to the lens mount. It can be performed easily and with high precision.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an outline of an optical scanning device provided with an fθ lens according to one embodiment of the present invention, FIG. 2 is a side view of the above-mentioned device, FIGS. 3 (a), (b) and FIG. (A), (b), FIGS. 5 (a), (b), FIGS. 6 (a), (b), FIGS. 7 (a), (b), FIGS. 8 (a), (b) ), FIGS. 9 (a) and (b), FIGS. 10 (a) and (b), FIGS. 11 (a) and (b), FIGS. 12 (a), (b) and 13 ( 14 (a), 14 (b), and 14 (a), 14 (b) are a schematic diagram and an aberration diagram, respectively, showing the configuration of an fθ lens according to an embodiment of the present invention. 2 ... light beam, 4 ... rotary polygon mirror 5 ... first lens, 6 ... second lens 7 ... third lens, 8 ... fθ lens 9 ... scanning surface

 ────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Masaru Noguchi 798 Miyadai, Kaisei-cho, Ashigara-gun, Kanagawa Prefecture Fuji Photo Film Co., Ltd. (56) References JP-A-58-21711 (JP, A) JP-A-58- 153908 (JP, A)

Claims (1)

(57) [Claims] An optical deflector for deflecting a light beam at a constant angular velocity and a scanning surface having a flat surface in a deflecting direction are provided, and the light beam is focused on the scanning surface and scanned at a constant speed. Fθ lens, a first lens having a negative refractive power and a flat surface on the surface to be scanned, which is arranged in order from the optical deflector, and a surface having a negative refractive power and the surface to be scanned Or a second lens having a flat surface on the optical deflector side, and a third lens having a positive refractive power and a flat surface on the optical deflector side.