JP4817773B2 - Imaging optical system and image reading apparatus using the same - Google Patents

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an imaging optical system and an image reading apparatus using the same, and in particular, various aberrations are corrected in a well-balanced manner, and a monochrome image or color can be obtained using a line sensor in an image scanner or a digital copying machine that can read an image with high resolution. This is suitable for reading an image.

  Conventionally, a line sensor in which a plurality of light receiving elements are arranged in the main scanning direction is used as an image reading apparatus for reading image information on a document surface, and the image information is imaged on a line sensor (CCD) surface, and the document and line Various image reading apparatuses have been proposed in which image information of the original or the like is read by using an output signal obtained from the line sensor by displacing the position relative to the sensor in the sub-scanning direction.

  FIG. 13 is a schematic diagram of a conventional carriage-integrated scanning image reading apparatus. In the figure, the light beam emitted from the illumination light source 1 directly illuminates the document 7 placed on the platen glass 2, and the reflected light beam from the document 7 is sequentially converted into the first, second, and third folding mirrors 3 a, 3 b, The optical path is bent inside the carriage 7 through 3c, and an image is formed on the surface of the line sensor 5 by the imaging lens (imaging optical system) 4.

  Then, the image information of the document 7 is read by moving the carriage 6 in the arrow A direction (sub-scanning direction) shown in FIG. The line sensor 5 in the figure has a configuration in which a plurality of light receiving elements are arranged in a one-dimensional direction (main scanning direction).

  FIG. 14 is an explanatory diagram of a basic configuration of the image reading optical system of FIG.

  In the figure, 144 is an imaging optical system, and 145R, 145G, and 145B are for R, G, and B that read image information relating to each color of R (red), G (green), and B (blue) constituting the line sensor 145, respectively. The line sensors 147R, 147G, and 147B are reading ranges on the document surface 147 corresponding to the line sensors 145R, 145G, and 145B.

  By scanning the document surface 147, the same portion can be read with different colors at a certain time interval. In the above-described configuration, when the imaging optical system 144 is composed of a normal refraction system, axial chromatic aberration and lateral chromatic aberration are generated. Therefore, the line image formed on the line sensors 145B and 145R is defocused with respect to the reference line sensor 145G. Or a positional shift occurs. Therefore, when the color images are superimposed and reproduced, the image becomes conspicuous in color blurring and deviation. In other words, when high aperture and high resolution performance is required, the request cannot be met.

  On the other hand, recently, it has been proposed that even in non-coaxial optical systems, an optical system in which aberrations are sufficiently corrected can be constructed by introducing the concept of a reference axis and making the constituent surface an asymmetric aspheric surface (patent) Literatures 1-3). The design method is shown in Patent Document 1, and the design example is shown in Patent Document 2 and Patent Document 3.

  Such a non-coaxial optical system is an off-axial optical system (a curved surface whose surface normal at the intersection with the reference axis of the component surface is not on the reference axis when considering a reference axis along a ray passing through the image center and the pupil center ( This is an optical system defined as an optical system including an off-axial curved surface, and at this time, the reference axis has a bent shape).

  In this off-axial optical system, the constituent surfaces are generally non-coaxial and vignetting does not occur even on the reflecting surface, so that it is easy to construct an optical system using the reflecting surface. In addition, the optical path can be routed relatively freely, and it has a feature that it is easy to make an integrated optical system by a method of integrally forming the constituent surfaces.

  A technique using such a technique in an imaging optical system for image reading is disclosed (Patent Documents 4 and 5). With the technique disclosed therein, an off-axial optical system including five and six reflecting surfaces in which an aberration is sufficiently corrected without chromatic aberration is achieved in the image reading apparatus.

Since Patent Document 4 also aims to achieve miniaturization of the optical system at the same time, each embodiment is an optical system suitable for a carriage integrated type. Further, in the embodiment disclosed as an imaging optical system for image reading in Patent Document 5, an off-axial optical system including three reflecting surfaces is shown, which is applied to a 2: 1 mirror scanning type scanner. The optical path length is sufficient.
Japanese Patent Laid-Open No. 9-5650 (no corresponding foreign countries) US Pat. No. 5,825,560 USP5847887 US AA20033032828 US AA20033076066

  In a reflection type off-axial optical system, it is difficult to keep good optical performance while all the reflecting surfaces are formed of spherical surfaces. Therefore, good optical performance is achieved by introducing a rotationally asymmetric aspherical surface (free curved surface) on at least one surface.

  By the way, it is generally known that an optical system composed of a reflecting surface has a great decrease in optical performance due to an eccentric error. When a reflective optical element having a rotationally asymmetric aspherical surface (free-form surface) is incorporated in an off-axial optical system, the arrangement of the members for holding it is very high as compared with a normal spherical reflecting surface. Is required.

  In particular, when the number of surfaces is three or more, it is necessary to obtain the relative positional accuracy of each surface. Conversely, in order to achieve high accuracy, the structure of the holding member becomes complicated, the size is increased, and the assemblability is deteriorated. This makes it difficult to manufacture.

  In addition, when the off-axial reflecting surface is a free-form curved reflecting surface, the manufacturing process becomes complicated if it is manufactured using ordinary glass.

  Therefore, it is conceivable to manufacture with polycarbonate, acrylic, polyolefin-based plastics, etc., but when the number of surfaces is large, there arises a problem that the manufacturing cost such as mold cost corresponding to the number of surfaces increases.

  The present invention provides an imaging optical system capable of reading an image with a very simple configuration while maintaining good image performance with little occurrence of asymmetric aberration even when configured with an off-axial reflecting surface, and an image reading apparatus using the same The purpose is to provide.

  In particular, an object of the present invention is to provide an imaging optical system suitable for easily realizing an image reading system such as a digital copying machine or an image scanner.

前記第１のオフアキシャル反射面の非球面係数Ｃ０２ｒ２と前記第２のオフアキシャル反射面の非球面係数Ｃ０２ｒ４は、
−１．０＜Ｃ０２ｒ２／Ｃ０２ｒ４＜−０．０１
を満たしていることを特徴とする。 Therefore, in the present invention, image information on the document surface is imaged on a line sensor extending in the main scanning direction, and the relative position between the document surface and the line sensor is changed in the sub-scanning direction, and the line sensor An imaging optical system for image reading for reading image information,
The imaging optical system comprises only two surfaces, a first off-axial reflective surface and a second off-axial reflective surface, and
The shapes of the first off-axial reflective surface and the second off-axial reflective surface in the sub-scan section are asymmetric with respect to a reference axis, and
In the sub-scan section, an off-axial reflection surface that deflects the reference axis light beam in the first direction with a straight line perpendicular to the sub-scan section as the rotation axis,
Within the sub-scan section, an off-axial reflection surface that deflects a reference axis ray in a second direction opposite to the first direction with a straight line perpendicular to the sub-scan section as a rotation axis is a plus deflection surface,
When defining
The first off-axial reflecting surface and the second off-axial reflecting surface are formed by the imaging optics in the order of the plus deflection surface, the minus deflection surface or the minus deflection surface, and the plus deflection surface from the document surface side. Arranged in the optical path of the system, and
In the sub-scan section, when the angle formed by the reference axis ray incident on the first off-axial reflection surface and the reference axis ray emitted from the second off-axis reflection surface is θ,
−30 ° <θ <+ 30 °
Satisfying the following conditions, and
A stop is disposed in the optical path between the first off-axial reflective surface and the second off-axial reflective surface; and
The reference axis passes through the central point of the effective diameter of the light beam of the stop and reaches the center of the imaging plane on the line sensor, and is reflected by the i-th off-axial reflecting surface (i = 1, 2). And the local coordinates (x, x) of the i-th off-axial reflective surface (i = 1, 2) with the origin at the point where the reference axis and the i-th off-axial reflective surface (i = 1, 2) intersect. y, z), the shape of the i-th off-axial reflective surface (i = 1, 2) can be defined as follows:

The aspheric coefficient C02r2 of the first off-axial reflective surface and the aspheric coefficient C02r4 of the second off-axial reflective surface are:
−1.0 <C02r2 / C02r4 <−0.01
It is characterized by satisfying.

  According to the present invention, an image reading apparatus using a line sensor such as an image scanner or a digital copying machine has a very simple structure with two chromatic aberrations and two off-axial reflecting surfaces, and a high performance image reading result. An image optical system is obtained.

  Before describing each embodiment, a description will be given of how to represent the configuration specifications of the image forming optical system (optical system) used in each embodiment and common items of the entire embodiment.

  FIG. 15 is an explanatory diagram of a coordinate system defining configuration data of the imaging optical system of the present invention. In the embodiment, the i-th surface is defined as the i-th surface along one light beam (indicated by a one-dot chain line in FIG. 15 and referred to as a reference axis light beam) La1 traveling from the object side to the image plane.

  In FIG. 15, the first surface R1 is a stop, the second surface R2 is a refracting surface coaxial with the first surface, the third surface R3 is a reflecting surface tilted with respect to the second surface R2, the fourth surface R4, and the fifth surface. The surface R5 is a reflective surface shifted and tilted with respect to each front surface, and the sixth surface R6 is a refractive surface shifted and tilted with respect to the fifth surface R5. Each surface from the second surface R2 to the sixth surface R6 is formed on one optical element formed of a transparent medium such as glass or plastic.

  Accordingly, in the configuration of FIG. 15, the medium from the object surface (not shown) to the second surface R2 is air, the common medium from the second surface R2 to the sixth surface R6, and the seventh surface (not shown) from the sixth surface R6. The medium up to (for example, the image plane) R7 is composed of air.

  Since the imaging optical system of the present invention is an off-axial optical system, the surfaces constituting the imaging optical system do not have a common optical axis.

  Therefore, in the embodiment, first, an absolute coordinate system having the origin at the center of the effective ray diameter of the first surface is set. In the embodiment, the center point of the effective ray diameter of the first surface is used as the origin, and the path of the light beam (reference axis light beam) passing through the origin and the center of the final imaging surface is the reference axis of the imaging optical system. It is defined as Furthermore, the reference axis in the embodiment has a direction (orientation). The direction is the direction in which the reference axis ray travels during imaging.

  In the embodiments of the present invention, the reference axis used as the reference of the imaging optical system is set as described above. However, the method of determining the axis used as the reference of the imaging optical system is based on the optical design, the collection of aberrations, or the result. An axis that is convenient in expressing the shape of each surface constituting the image optical system may be employed. However, in general, the reference axis that serves as a reference for the optical system is the center of the image plane and the path of the light beam that passes through either the stop, the entrance pupil or the exit pupil, or the center of the first surface or the center of the final surface of the optical system. Set to.

  In other words, in the embodiment of the present invention, the reference axis passes through the center point of the effective diameter of the light beam on the first surface, that is, the diaphragm surface, and the light beam that reaches the center of the final imaging surface (reference axis light beam) The path that is refracted and reflected by the surface is set as the reference axis. The order of each surface is set in the order in which the reference axis rays are refracted and reflected.

  Therefore, the reference axis finally reaches the center of the image plane while changing its direction in accordance with the law of refraction or reflection along the set order of each surface.

All the tilt surfaces constituting the imaging optical system of each embodiment of the present invention are basically tilted within the same plane. Therefore, each axis of the absolute coordinate system is determined as follows.
Z axis: Reference axis passing through the origin toward the second surface R2 Y axis: Straight line passing through the origin and tilting 90 ° counterclockwise with respect to the Z axis in the tilt plane (in the paper of FIG. 15) X axis: Origin Straight lines perpendicular to the Z and Y axes (straight lines perpendicular to the paper surface of FIG. 15)
In addition, in order to represent the surface shape of the i-th surface constituting the imaging optical system, the local coordinates with the origin at the point where the reference axis and the i-th surface intersect are represented by the shape of the surface in the absolute coordinate system. It is easier to understand when the system is set and the surface shape of the surface is expressed in the local coordinate system for recognizing the shape. Therefore, in the embodiment for displaying the configuration data regarding the imaging optical system of the present invention, the i-th surface The surface shape of is expressed in the local coordinate system.

Further, the tilt angle of the i-th surface in the YZ plane is represented by an angle θi (unit: °) with the counterclockwise direction being positive with respect to the Z axis of the absolute coordinate system. Therefore, in the embodiment of the present invention, the origin of the local coordinates of each surface is on the YZ plane in FIG. There is no surface eccentricity in the XZ and XY planes. Furthermore, the y and z axes of the local coordinates (x, y, z) of the i-th surface are inclined at an angle θi in the YZ plane with respect to the absolute coordinate system (X, Y, Z). Set as follows.
z-axis: a straight line that passes through the origin of the local coordinate and forms an angle θi in the counterclockwise direction in the YZ plane with respect to the Z direction of the absolute coordinate system. y-axis: passes through the origin of the local coordinate and runs in the YZ plane with respect to the z direction. A straight line that forms 90 ° in the clockwise direction: a straight line that passes through the origin of the local coordinates and is perpendicular to the YZ plane. Di is a scalar quantity that represents the distance between the origins of the local coordinates of the i-th and (i + 1) -th planes. , Ndi and νdi are the refractive index and Abbe number of the medium between the i-th surface and the (i + 1) -th surface. Moreover, in the Example of this invention, sectional drawing and numerical data of an optical system are shown.

  A spherical surface is a shape represented by the following formula:

  Further, the imaging optical system in the embodiment of the present invention has a rotationally asymmetric aspheric surface, and the shape thereof is expressed by the following equation.

  Since the curved surface formula is only an even-order term with respect to x, the curved surface defined by the curved surface formula is a plane-symmetric shape with the yz plane as the symmetry plane.

In addition, since all embodiments of the imaging optical system are not coaxial optical systems, it is difficult to directly calculate the focal length based on paraxial theory. Therefore, the converted focal length f _eq according to the following definition is used.

  By definition, when there are an odd number of reflecting surfaces, the sign of the focal length is expressed opposite to the normal sign.

Where h _{1 is} the incident height ak ′ of the light beam incident on the first surface parallel to the reference axis and infinitely incident on the reference axis, ak ′: the angle that the light beam makes with the reference axis when emitted from the final surface

  FIG. 1 is a schematic view of the main part in the sub-scan section of Example 1 when the imaging optical system of the present invention is applied to an image reading apparatus. FIG. 2 is a schematic view in the sub-scan section when the imaging optical system 4a of FIG. 1 is extracted.

  In the figure, reference numeral 1 denotes a light source (light source means), which includes a fluorescent lamp, a xenon lamp, and the like. Reference numeral 2 denotes a platen glass, and 3a, 3b, and 3c are first, second, and third reflecting mirrors in this order. An image forming optical system 4a for image reading has two off-axial optical elements R2 and R4 each made of a reflecting surface. Reference numeral 5 denotes a line sensor (light receiving means) composed of a CCD or the like, which is arranged at a position corresponding to the image plane. Reference numeral 6 denotes a carriage (housing) that houses the members 1, 3a, 3b, 3c, 4a, 5 and the like.

  Here, the direction in which the pixels of the line sensor 5 are arranged (perpendicular to the paper surface) is the main scanning direction (in the main scanning section), and the direction orthogonal to the direction (in the paper surface direction) is the sub-scanning direction (in the sub-scanning section).

  In this embodiment, as shown in FIG. 15 described above, the off-axial reflecting surface that deflects the reference axis light beam in the clockwise direction is a minus deflection surface, and the off-axial reflecting surface that deflects the reference axis light beam in the counterclockwise direction is added. When defined as a deflecting surface, the imaging optical system 4a has off-axial optical elements R2 and R4 composed of two reflecting surfaces. The two off-axial optical elements R2 and R4 are configured to be a plus deflection surface, a minus deflection surface, a minus deflection surface, and a plus deflection surface in order from the document surface 7 side. As a result, good optical performance is obtained.

  The off-axial optical element R2 is a plus deflection surface that deflects the reference axis beam in the counterclockwise direction, and the off-axial optical element R4 is a minus deflection surface that deflects the reference axis beam in the clockwise direction.

  The imaging optical system 4a for image reading of this embodiment does not have a refractive surface having power, and is composed of two off-axial optical elements R2 and R4. As a result, if a configuration having a refractive action on the entrance surface and exit surface is used like a prism, the problem due to decentration is reduced, but chromatic aberration due to the characteristics of the glass material of the prism occurs, resulting in a color shift problem in the read image. Is preventing.

  The original 7 (object) placed on the original platen glass 2 is illuminated with the light flux from the light source 1, and the light from the original 7 is detected by the imaging optical system 4a via the reflecting mirrors 3a, 3b, 3c. 5 is imaged. At this time, the document 7 is read two-dimensionally by changing the relative position between the document 7 and the carriage 6 in the sub-scanning direction (arrow A direction).

  In order to make the document reading apparatus compact, the optical path is folded by the first, second, and third reflecting mirrors 3a, 3b, and 3c. The imaging optical system 4a also contributes to folding the optical path. In the imaging optical system 4a, the optical path is folded in an approximate Z shape to easily cancel the decentration aberrations generated on the off-axial reflecting surfaces of each other. Yes.

  That is, since the two off-axial optical elements R2 and R4 are arranged in order from the document surface 7 side, the deflection surfaces having different signs are arranged in order so as to be a plus deflection surface, a minus deflection surface or a minus deflection surface, and a plus deflection surface. An effect of canceling decentration aberrations generated on the mutual off-axial reflecting surfaces is obtained.

  If the deflection surfaces having the same sign are arranged in order so as to be a plus deflection surface, a plus deflection surface or a minus deflection surface, and a minus deflection surface in order from the document surface 7 side, decentration aberration generated on each off-axial reflecting surface. Problems that are amplified.

  In order to obtain better imaging performance, the off-axial reflecting surface is composed of a free-form surface that is symmetric with respect to the reference axis in the main scanning direction and asymmetric in the sub-scanning direction, so that the eccentric aberration caused by bending the optical path in the sub-scanning direction Corrected well.

  Further, by providing a stop SP between the off-axial reflecting surfaces R2 and R4, it contributes to downsizing of the off-axial reflecting surface. In order to further reduce the size of the reflecting surface, it is sufficient to configure an intermediate imaging surface. There is no intermediate imaging.

  In this embodiment, since the length of the document 7 in the sub-scanning direction is short, the size of the reflecting surface does not become so large even if the intermediate image plane is not formed, and the surface interval can be reduced.

  In this embodiment, by using such an imaging optical system 4a, an image reading apparatus of a carriage-integrated optical system can be configured with a small number of parts including three plane folding mirrors and two off-axial optical elements. Manufacture is very easy and it is possible to cope with downsizing, and thus high-speed reading is possible.

  Numerical data will be shown below for Numerical Example 1 of the imaging optical system 4a for image reading corresponding to Example 1 of the present invention described above. Numerical data is also shown for Numerical Examples 2 and 3 of the imaging optical system 4a for image reading corresponding to the similar embodiment.

  4 and 6 are sub-scanning sectional views of the imaging optical system for image reading corresponding to Numerical Examples 2 and 3. FIG. 3, 5, and 7 show aberration diagrams at five points (image heights) in the line direction of the line sensors of Numerical Examples 1, 2, and 3. FIG. X in the figure represents the height (image height) on the document surface.

In the present embodiment, as shown in FIG. 2, the angle formed by the reference axis ray incident on the first off-axial optical element R2 of the imaging optical system 4a and the reference axis ray emitted from the second off-axial optical element R4 is θi. -30 ° <θi <30 ° (1)
Satisfy the following conditions.

  The technical significance of this conditional expression (1) will be described.

  Conditional expression (1) defines the angle formed between the light beam incident on the image forming optical system and the light beam emitted from the image forming optical system. More specifically, the conditional expression (1) is relative to the arrangement angle of the two off-axial reflecting surfaces. It defines the relationship. In the off-axial optical system, decentration aberration is generated and needs to be corrected. However, since the off-axial reflecting surface is composed of only two surfaces, it is difficult to obtain good optical performance only by correcting the surface shape. . Therefore, by folding the reference axis ray into the shape of Z, it becomes easy to cancel the decentration aberration generated on the off-axial reflecting surfaces of each other. However, the decentration aberration can be canceled by making it within the range of the conditional expression (1). It becomes easy. If the off-axial reflecting surfaces are relatively arranged outside the upper and lower limits of the conditional expression, the amount of decentering aberration generated on one surface increases, and there is a problem that it is difficult to cancel with only two off-axial reflecting surfaces.

More preferably, the conditional expression (1) is set as follows.
−15 ° <θi <15 °
In this embodiment, as shown in FIG. 2, the aspherical surface of the aspherical coefficient of the first off-axial optical element R2 (reflective surface) and the second off-axial optical element R4 (reflective surface) of the imaging optical system 4a is used. When the coefficients C02 are C02r2 and C02r4, respectively -1.0 <C02r2 / C02r4 <-0.01 (2)
Satisfy the following conditions.

  The technical significance of this conditional expression (2) will be described.

  Conditional expression (2) defines the ratio of the aspheric coefficients of the off-axial optical element R2 and the off-axial optical element R4. More specifically, the ratio of the refractive powers of the two off-axial optical elements in the sub-scanning direction. Is specified.

  In general, an off-axial optical system generates decentration aberrations, which need to be corrected. However, in order to obtain good optical performance while achieving compactness, the arrangement angle of the off-axial optical element is limited. .

  In order to obtain good optical performance in this limited angular range, the relationship between the refractive powers in the sub-scanning direction, which is the same as the sub-scanning direction in which the light beam is bent, is important.

  If the difference between the refractive powers in the sub-scanning direction between the two surfaces exceeds the lower limit of the conditional expression (2), there arises a problem that the optical path length becomes long in order to maintain a predetermined magnification. Conversely, if the difference in refractive power between the two surfaces in the sub-scanning direction exceeds the upper limit of conditional expression (2), the refractive power in the sub-scanning direction is reversed between the two surfaces, and the optical performance is improved. In order to bend, it is necessary to make a figure 4 shape, and this also causes a problem that the imaging optical system is enlarged.

More preferably, the conditional expression (2) is set as follows.
−0.8 <C02r2 / C02r4 <−0.2

Numerical example 1
Document reading width 220 mm Imaging magnification -0.189
Document side NA 0.016 feq 29.680

Aspherical surface R2
C02 = -4.5460E-03 C03 = 1.8570E-05 C04 = -1.9333E-05
C05 = 1.2781E-06 C06 = 1.5282E-06 C07 = -2.6256E-07
C08 = 4.0099E-08 C20 = -4.7534E-03 C21 = 6.3040E-05
C22 = -4.0753E-06 C23 = -5.3171E-08 C24 = 5.5317E-07
C25 = 2.3713E-08 C26 = -4.2652E-08 C40 = 2.5124E-06
C41 = -6.5956E-08 C42 = -2.5347E-09 C43 = -8.2587E-10
C44 = -8.2235E-10 C60 = -3.3116E-09 C61 = 2.3522E-11
C62 = 6.4321E-11 C80 = 3.3081E-12
R4 surface
C02 = 6.8081E-03 C03 = 6.1342E-05 C04 = -8.7196E-05
C05 = -4.3217E-05 C06 = 1.5179E-05 C07 = 1.0360E-05
C08 = 6.6517E-07 C20 = 6.9865E-03 C21 = 8.4961E-05
C22 = 8.3621E-06 C23 = 5.2533E-07 C24 = 3.4662E-06
C25 = -1.4766E-07 C26 = -5.7376E-07 C40 = -6.0197E-06
C41 = -1.6224E-07 C42 = 2.0695E-08 C43 = 7.9461E-10
C44 = -6.8943E-09 C60 = 1.5482E-08 C61 = 3.6681E-10
C62 = -1.4023E-10 C80 = -3.0158E-11

Numerical example 2
Document reading width 220 mm Imaging magnification -0.165
Document side NA 0.014 feq 28.927

Aspherical surface R2
C02 = -4.8409E-03 C03 = 1.2949E-05 C04 = 1.4067E-06
C05 = 3.5016E-07 C06 = -1.3856E-07 C20 = -4.9915E-03
C21 = 3.6762E-05 C22 = -3.3290E-06 C23 = -1.9140E-08
C24 = 1.9491E-08 C40 = 7.0626E-07 C41 = -9.2190E-09
C42 = 2.3060E-09 C60 = -2.2285E-10
R4 surface
C02 = 1.0388E-02 C03 = 1.8869E-05 C04 = -5.8565E-05
C05 = -5.3466E-06 C06 = 1.1605E-05 C20 = 1.0488E-02
C21 = 6.3485E-05 C22 = 1.5856E-05 C23 = 1.9689E-07
C24 = 6.9101E-07 C40 = -2.0168E-06 C41 = 7.2103E-09
C42 = 1.4145E-08 C60 = 2.2083E-09

Numerical Example 3
Document reading width 220 mm Imaging magnification -0.255
Document side NA 0.023 feq 49.819

Aspherical surface R2
C02 = -3.2910E-03 C03 = 2.8869E-06 C04 = 4.4281E-07
C05 = 6.6841E-08 C06 = -3.4875E-08 C07 = -1.0234E-09
C08 = 7.2439E-10 C20 = -3.3779E-03 C21 = 1.3092E-05
C22 = -2.7252E-07 C23 = 1.4027E-09 C24 = 1.9624E-10
C25 = -3.9379E-11 C26 = -5.2793E-12 C40 = 2.5615E-07
C41 = -2.5287E-09 C42 = -8.2932E-11 C43 = -9.6523E-13
C44 = 2.2800E-13 C60 = -6.3385E-11 C61 = 6.1354E-13
C62 = 8.2234E-14 C80 = 1.2858E-14
R4 surface
C02 = 5.3904E-03 C03 = -7.1792E-06 C04 = -1.8231E-06
C05 = 1.9647E-07 C06 = 4.9083E-07 C07 = 5.2920E-09
C08 = -2.9427E-08 C20 = 5.5433E-03 C21 = 3.5404E-05
C22 = 5.2311E-06 C23 = 2.3961E-08 C24 = 1.0051E-08
C25 = -1.5006E-09 C26 = -3.9259E-10 C40 = -5.9770E-07
C41 = 3.3155E-10 C42 = 3.5756E-10 C43 = 9.4700E-12
C44 = 8.5528E-12 C60 = 5.2122E-10 C61 = 1.5001E-12
C62 = -2.3248E-12 C80 = -2.2990E-13

  FIG. 8 is a schematic view of the essential portions of Embodiment 2 when the imaging optical system of the present invention is applied to an image reading apparatus. FIG. 9 is a schematic diagram in the sub-scanning direction when the imaging optical system 4b of FIG. 8 is extracted.

  In the figure, reference numeral 1 denotes a light source (light source means), which includes a fluorescent lamp, a xenon lamp, and the like. Reference numeral 2 denotes a platen glass, and 3a, 3b, and 3c are first, second, and third reflecting mirrors in this order. Reference numeral 4b denotes an image forming optical system for image reading, which has two off-axial optical elements R2 and R4. Reference numeral 5 denotes a line sensor (light receiving means) composed of a CCD or the like.

  The imaging optical system 4b for image reading of this embodiment is composed of an off-axial optical element 4b composed of two reflecting surfaces R2 and R4, and the off-axial reflecting surface for deflecting the reference axis light beam in the clockwise direction is minus. When the deflection surface and the off-axial reflection surface for deflecting the reference axis light beam in the counterclockwise direction are defined as the positive deflection surface, the positive deflection surface, the negative deflection surface or the negative deflection surface, and the positive deflection surface are formed from the document surface 7 side. It is configured as follows.

  In FIG. 8, the light beam irradiated onto the document 7 is reflected by the document 7 and travels toward the first mirror 3a placed on the first mirror base M1. The reflected light includes image information of the document 7, and a second mirror 3b and a third mirror 3c are arranged to guide the light flux to the light receiving means 5 as a reading means. The second mirror 3b and the third mirror 3c also have an integral structure (C-shaped mirror) and are placed on the second mirror base M.

  When scanning (reading) a document, the first mirror stage M1 scans the document 2 with a motor (not shown), and the second mirror stage M2 follows at a half speed. Thus, the optical distance between the document 7 being scanned and the imaging optical system 4b (reading system) is maintained. In this way, the imaging optical system 4b forms an image on the surface of the line sensor 5 in which the light beam from the document 7 is arranged in a conjugate relationship.

  The light receiving means 5 is composed of a one-dimensional line sensor (CCD) or the like in which a plurality of elements are arranged in the main scanning direction as in the first embodiment, and its position is defined along the main scanning direction on the document surface 7. Thus, the main scanning direction of the document 7 placed on the document stacking table 2 is read once, and the first and second mirror tables M1 and M2 are sequentially scanned to read the sub-scanning direction of the document 7. It is out.

  The second embodiment is different from the first embodiment in that the configuration of the image reading apparatus is changed from the one-carriage scanning type to the 2: 1 mirror scanning type and the image reading imaging optical system 4b is 2: 1. Corresponding to the mirror scanning method, the optical path length from the original surface to the off-axial reflecting surface on the original side is increased.

  By using such an imaging optical system 4b for image reading, a 2: 1 mirror scanning type image reading apparatus is configured with a small number of parts including three plane folding mirrors and two off-axial optical elements R2 and R4. Can be very easy to manufacture.

  Furthermore, since it is composed of only the reflective surface, no chromatic aberration occurs, and there is no color shift in color reading, so the processing with the accompanying software can be simplified and the image reading apparatus can be manufactured more easily. Can do.

  Numerical data will be shown below for a numerical example 4 of the imaging optical system for image reading corresponding to the example 2 of the present invention described above. Numerical data is also shown for Numerical Example 5 of the imaging optical system for image reading corresponding to the same Example.

  FIG. 11 shows a sub-scanning sectional view of an image-reading imaging optical system corresponding to Numerical Example 5. 10 and 12 show aberration diagrams at five points (image heights) in the line direction of the line sensors of Numerical Examples 4 and 5. FIG. X in the figure represents the height on the document surface.

Numerical Example 4
Document reading width 304.8 mm Imaging magnification -0.111
Document side NA 0.009 feq 45.093

Aspherical surface R2
C02 = -3.0020E-03 C03 = 4.7838E-06 C04 = 2.3504E-07
C05 = 9.9324E-08 C06 = -2.8659E-09 C20 = -3.1491E-03
C21 = 2.5084E-05 C22 = -5.4761E-07 C23 = -1.0084E-08
C24 = -6.8492E-10 C40 = 7.8808E-07 C41 = -1.6587E-08
C42 = 1.2190E-09 C60 = -5.1022E-10
R4 surface
C02 = -4.4171E-03 C03 = -1.4059E-05 C04 = 4.6758E-07
C05 = 8.9070E-07 C06 = 4.8105E-08 C20 = 4.6431E-03
C21 = 6.1598E-05 C22 = 5.6242E-06 C23 = 1.6523E-09
C24 = 3.4457E-09 C40 = -1.5530E-06 C41 = -3.2913E-08
C42 = -8.0017E-09 C60 = 9.5119E-10

Numerical Example 5
Document reading width 304.8 mm Imaging magnification -0.220
Document side NA 0.017 feq 80.432

Aspherical surface R2
C02 = -1.6082E-03 C03 = 3.9135E-06 C04 = 7.1800E-08
C05 = 2.6687E-08 C06 = -1.1885E-09 C20 = -1.6933E-08
C21 = 1.4997E-05 C22 = -3.1463E-07 C23 = -6.9537E-09
C24 = -9.9100E-11 C40 = 5.1882E-07 C41 = -1.2070E-08
C42 = 1.0832E-09 C60 = -3.2503E-10
R4 surface
C02 = 1.9433E-03 C03 = 2.6548E-06 C04 = 6.9000E-08
C05 = 6.2702E-08 C06 = 4.8170E-09 C20 = 2.0246E-03
C21 = 2.1504E-05 C22 = 9.6888E-07 C23 = -5.0452E-09
C24 = -2.2503E-10 C40 = -6.6966E-07 C41 = -1.5781E-08
C42 = -2.2432E-09 C60 = 3.9269E-10
In each embodiment, the imaging optical systems 4a and 4b are configured as described above, so that the structure is very simple with no chromatic aberration and two off-axial reflecting surfaces, and a high-performance imaging optical system for image reading and An image reading apparatus using the same can be achieved.

1 is a schematic diagram of the main part of a first embodiment of an image reading apparatus according to the present invention. Sectional drawing of Numerical Example 1 of the imaging optical system for image reading of this invention Aberration diagram of Numerical Example 1 of the imaging optical system for image reading of the present invention Sectional drawing of Numerical Example 2 of the imaging optical system for image reading of this invention Aberration diagram of Numerical Example 2 of the imaging optical system for image reading of the present invention Sectional drawing of numerical Example 3 of the imaging optical system for image reading of this invention Aberration diagram of Numerical Example 3 of the imaging optical system for image reading of the present invention Schematic diagram of main parts of Embodiment 2 of the image reading apparatus of the present invention. Sectional drawing of Numerical Example 4 of the imaging optical system for image reading of this invention Aberration diagram of Numerical Example 4 of the imaging optical system for image reading of the present invention Sectional drawing of Numerical Example 5 of the imaging optical system for image reading of this invention Aberration diagram of Numerical Example 5 of the imaging optical system for image reading of the present invention Schematic diagram of main parts of a conventional image reading apparatus Basic configuration diagram of a conventional color image reading apparatus Schematic explaining the definition of off-axial optical system

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Illumination light source 2 Original plate glass 3a, 3b, 3c Reflection mirror 4a, 4b Imaging optical system 5 Reading means (line sensor)
6 Carriage (housing)
7 Original SP Aperture

Claims (4)

The image information on the document surface is imaged on a line sensor extending in the main scanning direction, the relative position between the document surface and the line sensor is changed in the sub-scanning direction, and the image information is read by the line sensor. An imaging optical system for image reading,
The imaging optical system comprises only two surfaces, a first off-axial reflective surface and a second off-axial reflective surface, and
The shapes of the first off-axial reflective surface and the second off-axial reflective surface in the sub-scan section are asymmetric with respect to a reference axis, and
In the sub-scan section, an off-axial reflection surface that deflects the reference axis light beam in the first direction with a straight line perpendicular to the sub-scan section as the rotation axis,
Within the sub-scan section, an off-axial reflection surface that deflects a reference axis ray in a second direction opposite to the first direction with a straight line perpendicular to the sub-scan section as a rotation axis is a plus deflection surface,
When defining
The first off-axial reflecting surface and the second off-axial reflecting surface are formed by the imaging optics in the order of the plus deflection surface, the minus deflection surface or the minus deflection surface, and the plus deflection surface from the document surface side. Arranged in the optical path of the system, and
In the sub-scan section, when the angle formed by the reference axis ray incident on the first off-axial reflection surface and the reference axis ray emitted from the second off-axis reflection surface is θ,
−30 ° <θ <+ 30 °
Satisfying the following conditions, and
A stop is disposed in an optical path between the first off-axial reflective surface and the second off-axial reflective surface; and
The reference axis passes through the central point of the effective diameter of the light beam of the stop and reaches the center of the imaging plane on the line sensor, and is reflected by the i-th off-axial reflecting surface (i = 1, 2). And the local coordinates (x, x) of the i-th off-axial reflective surface (i = 1, 2) with the origin at the point where the reference axis and the i-th off-axial reflective surface (i = 1, 2) intersect. y, z), the shape of the i-th off-axial reflective surface (i = 1, 2) can be defined as follows:

The aspheric coefficient C02r2 of the first off-axial reflective surface and the aspheric coefficient C02r4 of the second off-axial reflective surface are:
−1.0 <C02r2 / C02r4 <−0.01
An imaging optical system characterized by satisfying −0.8 <C02r2 / C02r4 <−0.2
The imaging optical system according to claim 1, wherein the following condition is satisfied. In the sub-scan section, when the angle formed by the reference axis ray incident on the first off-axial reflection surface and the reference axis ray emitted from the second off-axis reflection surface is θ,
−15 ° <θ <+ 15 °
The imaging optical system according to claim 1 or 2, wherein the following condition is satisfied. An image reading apparatus comprising: the imaging optical system according to claim 1; and the line sensor.

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