Optical system and image reading device

Abstract

To provide an optical system for focusing light beams from an object to a light receiving part extending in a first direction, the optical system exhibiting small change in contrast performance due to focus deviation in a cross section perpendicular to the first direction without increasing the number of components.SOLUTION: An optical system for focusing light beams from an object to a light receiving part extending in a first direction includes a first lens, an aperture stop, and a second lens including a rotationally asymmetric anamorphic surface, where the second lens is disposed closer to an image side than to the aperture stop, and in a cross section including an optical axis and perpendicular to the first direction, a first portion of the second lens on one side with respect to the optical axis advances a phase of a wavefront of an incident light beam, while a second portion on the other side delays the phase of the wavefront of the incident light beam, and a distance from the optical axis to the first portion is equal to a distance from the optical axis to the second portion.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an optical system and an image reading device. [Background technology]

[0002] Conventionally, as an image reading device for reading image information on a document surface, a device that acquires image information of a document using a linear image sensor in which multiple pixels are arranged in a first direction and an imaging optical system that focuses the image information on the linear image sensor is known.

[0003] In this configuration, in order to widen the angle of view while effectively reducing field curvature aberration and precisely read the image information of the original, an imaging optical system is known that uses an anamorphic optical system in which the power (refractive power) differs between a cross section (first cross section) parallel to a first direction and the optical axis and a cross section (second cross section) perpendicular to the first direction.

[0004] However, by using an anamorphic optical system, the imaging performance in the first cross section and the second cross section is no longer the same, and therefore, when incorporating the optical element into the imaging optical system, an adjustment mechanism is required to correct the deterioration of the imaging performance in both cross sections.

[0005] In response to this, Patent Document 1 proposes a configuration in which an imaging optical system using an anamorphic optical system is provided with an optical phase-changing filter located near an aperture stop in the optical path, which has a phase-advancing effect that advances the phase of the wavefront based on the wavefront at the center of the incident light beam, and a phase-delaying effect that delays the phase of the wavefront. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-109331 Summary of the Invention [Problem to be solved by the invention]

[0007] However, in the imaging optical system of Patent Document 1, since the optical phase shift filter has a rotationally asymmetric surface shape, when incorporating it into the optical system, it is required to accurately align the main scanning direction or sub-scanning direction of the optical system with a predetermined surface of the optical phase shift filter, which results in an increase in the number of components, restrictions on placement, and ease of assembly.

[0008] The present invention aims to provide an optical system that focuses a light beam from an object onto a light receiving section extending in a first direction, and that has little change in contrast performance due to focus shift in a cross section perpendicular to the first direction, without increasing the number of components. [Means for solving the problem]

[0009] In order to achieve the above object, the optical system of the present invention is an optical system that focuses a light beam from an object onto a light receiving section extending in a first direction, and includes a first lens, an aperture stop, and a second lens including a rotationally asymmetric anamorphic surface, the second lens being arranged closer to the image side than the aperture stop, and in a cross section that includes the optical axis and is perpendicular to the first direction, a first portion of the second lens on one side with respect to the optical axis advances the phase of a wavefront of the incident light beam, and a second portion on the other side delays the phase of the wavefront of the incident light beam, and the distance from the optical axis to the first portion and the distance from the optical axis to the second portion are equal to each other. [Effects of the Invention]

[0010] The present invention provides an optical system that focuses a light beam from an object onto a light receiving portion extending in a first direction, and that has little change in contrast performance due to focus shift in a cross section perpendicular to the first direction, without increasing the number of components. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a schematic diagram of a main part of an image reading apparatus equipped with an imaging optical system according to an embodiment. [Figure 2]1A and 1B are a first cross-sectional view and a second cross-sectional view of an imaging optical system according to a first embodiment. [Figure 3] 3A to 3C are diagrams showing various aberrations of the imaging optical system according to the first embodiment. [Figure 4] 1A and 1B are diagrams showing the shape of an anamorphic surface of the imaging optical system according to the first embodiment, and diagrams showing the cross-sectional shape in a second cross section and the relationship between the distance from the optical axis in a second direction and the second cross-sectional shape. [Figure 5] FIG. 3 is a diagram showing the MTF defocus characteristic of the imaging optical system according to the first embodiment. [Figure 6] 1A and 1B are a first cross-sectional view and a second cross-sectional view of an imaging optical system according to a second embodiment. [Figure 7] 10A to 10C are diagrams showing various aberrations of the imaging optical system according to the second embodiment. [Figure 8] 10A and 10B are diagrams showing the shape of an anamorphic surface of an imaging optical system according to a second embodiment, the cross-sectional shape in a second cross section, and the relationship between the distance from the optical axis in a second direction and the second cross-sectional shape. [Figure 9] FIG. 10 is a diagram showing the MTF defocus characteristic of the imaging optical system according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] The imaging optical system according to this embodiment will be described in detail below with reference to the drawings. Note that the drawings may be drawn at a scale different from the actual scale in order to facilitate understanding of this embodiment.

[0013] Conventionally, a known image reading device for reading image information on a document surface uses a linear image sensor with multiple pixels arranged in one dimension (first direction). In this device, an imaging optical system focuses image information on the linear image sensor, and the relative positions of the document and the linear image sensor are displaced in a direction perpendicular to the first direction (second direction), and the image information of the document is acquired using the output signal obtained from the linear image sensor.

[0014] In this configuration, an imaging optical system using an anamorphic optical system that is asymmetric between a first direction and a second direction is known to precisely read the image information of the document, which makes it possible to widen the angle of view of the imaging optical system while effectively reducing field curvature aberration.

[0015] However, when an anamorphic optical system is used, the imaging performance is no longer the same in the first direction and the second direction, so an adjustment mechanism is required to correct the deterioration of the imaging performance in both directions when the optical element is incorporated into the imaging optical system. Specifically, the amount of movement required to meet the standard value is calculated from the change in optical characteristics (sensitivity) when the optical element is moved by a unit amount in the first direction or the second direction, and the deterioration of the imaging performance is corrected. In this case, an imaging optical system based on a linear image sensor generally has the characteristic of being high in sensitivity in the first direction with a wide angle of view and low in sensitivity in the second direction.

[0016] Therefore, if the same degree of degradation in imaging performance, such as defocusing, occurs in both directions, the amount of movement required to satisfy the standard value will be greater in the second direction than in the first direction. For this reason, if defocusing occurs in both the first and second directions due to, for example, a local manufacturing error in the peripheral portion of the optical element in the first direction, it may be possible that only the defocusing in the second direction cannot be corrected, resulting in a failure to satisfy contrast performance, i.e., MTF (Modulation Transfer Function). To address this type of defocusing, an optical system is required that has MTF defocus characteristics that minimize changes in contrast performance even when defocusing occurs.

[0017] In an imaging optical system using an anamorphic optical system, a configuration is known in which an optical phase-changing filter is provided near an aperture stop provided in the optical path, and the optical phase-changing filter has a phase-advancing effect that advances the phase of the wavefront based on the wavefront at the center of the incident light beam, and a phase-delaying effect that delays the phase of the wavefront.

[0018] The optical phase shift filter has a surface shape component that is rotationally symmetric with respect to a predetermined plane including the surface normal at the center of the light beam, and has a rotationally asymmetric surface shape component that has a phase-leading effect on one side and a phase-delaying effect on the other side in a first direction or a second direction including the surface normal at the center of the light beam. This allows an imaging optical system with little change in contrast performance due to focus shift to be obtained by causing partial focus shift in the incident light beam.

[0019] However, in an imaging optical system including an optical phase shifting filter, the optical phase shifting filter needs to be placed near the aperture stop to provide a desired optical phase shifting effect to incident light beams over the entire field angle in the first and second directions. Furthermore, since the optical phase shifting filter has a rotationally asymmetric surface shape, when incorporating it into the optical system, it is required to accurately align the first or second direction of the optical system with a predetermined surface of the optical phase shifting filter. Therefore, an increase in the number of components, restrictions on placement, and ease of assembly become issues. An object of one embodiment of the present invention is to provide an imaging optical system in which the change in contrast performance due to focus deviation in the second direction is small, without increasing the number of components compared to conventional imaging optical systems.

[0020] The imaging optical system 15 according to this embodiment can be particularly suitably used as an imaging optical system mounted in an image reading device such as an image scanner, a copier, or a facsimile, which focuses (concentrates) a light beam from an original onto a linear image sensor (on a light receiving section).

[0021] [First embodiment] FIG. 1 is a schematic diagram showing the main parts of an image reading device 10 equipped with an imaging optical system 15 according to this embodiment. The image reading device 10 includes a document table glass 12, a carriage 17, an illumination unit 13, first, second, third, and fourth reflecting mirrors 14a, 14b, 14c, and 14d, an imaging optical system 15, an imaging element 16, and a motor 18. The illumination unit 13 is composed of a light source such as an LED, fluorescent lamp, or halogen lamp, a light guide, a reflector, and the like.

[0022] The imaging element 16 is a linear image sensor (light receiving unit) such as a CCD (Charge Coupled Device) sensor or a CMOS (Complementary Metal Oxide Semiconductor) sensor in which multiple pixels (light receiving elements) are arranged (extended) in one dimension (first direction).

[0023] The illumination unit 13, the first, second, third and fourth reflecting mirrors 14a, 14b, 14c and 14d, the imaging optical system 15 and the imaging element 16 are arranged within a carriage 17 and can move integrally with the carriage 17. In the image reading device 10 , a light beam emitted from an illumination unit 13 illuminates an original 11 placed on an original table glass 12 .

[0024] The light reflected from the document 11 is reflected by the first, second, third, and fourth reflecting mirrors 14a, 14b, 14c, and 14d so as to bend the optical path, and is guided onto the light receiving surface of the image sensor 16 by the imaging optical system 15. Then, the carriage 17 is moved in the direction of arrow H by a motor 18 to obtain two-dimensional image information on the document 11. The obtained image information is sent to an external device such as a personal computer via an interface (not shown).

[0025] The imaging optical system 15 according to the first embodiment will be described in detail below. Fig. 2 shows cross-sectional views of the imaging optical system 15 according to this embodiment in a first direction (Y direction) (a) and a second direction (Z direction) (b). In this specification, a plane parallel to the first direction and the optical axis direction will be referred to as a first cross section, and a cross section parallel to the second direction and the optical axis direction will be referred to as a second cross section.

[0026] The imaging optical system 15 according to this embodiment includes a front group 31 and a rear group 32, and guides light reflected from the original 11 to an imaging plane 33. In this specification, of the distances on the optical axis between adjacent optical elements among the multiple optical elements included in the imaging optical system 15, the front group 31 is defined as the portion closer to the original (object side) than the largest distance, and the rear group 32 is defined as the portion closer to the image sensor (image side). In other words, the front group and the rear group are separated from each other by the largest distance between optical elements adjacent to each other in the optical axis direction within the imaging optical system.

[0027] The front group 31 includes one or more lenses (first lenses), and in this embodiment, it includes a first lens L1, an aperture stop AP, a second lens L2, and a third lens L3, with at least one surface being configured as a lens surface that is rotationally symmetrical about the optical axis. In the front group 31 according to this embodiment, the first lens L1 has positive refractive power, the second lens L2 has negative refractive power, and the third lens L3 has positive refractive power, and all are configured as lens surfaces that are rotationally symmetrical about the optical axis. Note that the lens configuration is not limited to the above, and other lenses may be included as necessary.

[0028] Here, in this embodiment, the optical axis is the central axis of the imaging optical system 15, is defined by the reference axis of the rotationally symmetric optical surface of the imaging optical system 15, is perpendicular to the first direction and the second direction, and is an axis passing through the surface vertex of the lens included in the imaging optical system 15.

[0029] Furthermore, the aperture diaphragm AP according to this embodiment is disposed between the first lens L1 and the second lens L2, and is configured to have a shape that is rotationally symmetric with respect to the optical axis. However, the aperture diaphragm AP is not limited to this, and may have a shape that is not point-symmetric with respect to the optical axis (in a cross section perpendicular to the optical axis), such as an elliptical or rectangular shape, or may have a configuration in which the arrangement is reversed within the front group 31. However, it is preferable that the aperture diaphragm AP is disposed within the front group 31, and that at least one lens is disposed on the image side of the aperture diaphragm AP. Furthermore, the aperture diaphragm AP may be disposed closest to the object within the front group 31.

[0030] The rear group 32 is composed of an anamorphic lens (second lens) L4, which has at least one anamorphic surface formed of an aspherical shape that is rotationally asymmetric with respect to the optical axis. This makes it possible to effectively correct field curvature in both the first and second directions, even with a small number of lenses. Furthermore, the anamorphic lens L4 according to this embodiment has positive refractive power at least within the range of the on-axis light beam diameter centered on the optical axis.

[0031] 3 shows various aberration diagrams of the imaging optical system 15 according to this embodiment. The various aberration diagrams include spherical aberration diagrams, astigmatism diagrams, distortion diagrams, and chromatic aberration of magnification diagrams. In each diagram, e, g, C, and F represent the e-line, g-line, C-line, and F-line, respectively, and M and S represent the meridional image plane and the sagittal image plane. Furthermore, Fno is the F-number, and ω is the half angle of view.

[0032] Fig. 4(a) shows a cross-sectional view of the anamorphic surface of the imaging optical system 15 according to the first embodiment. As shown in Fig. 4(a), the cross-sectional shapes in the first cross section and the second cross section are both non-arcuate, and the amount of non-arcuateness of the shape in the second cross section changes continuously with increasing distance from the optical axis in the first direction.

[0033] 4(b) shows the cross-sectional shape of the second cross section including the optical axis of the anamorphic surface of the imaging optical system 15. In this embodiment, the cross-sectional shape of the second cross section including the optical axis of the anamorphic surface is asymmetric with respect to the optical axis. That is, the cross-sectional shape of one side and the cross-sectional shape of the other side at the same distance from the optical axis are different in the second cross section.

[0034] This provides a lens effect that brings about a converging or diverging effect on the light beam incident on the anamorphic lens L4. Also, the phase advancing effect (first phase advancing effect) that is given to the light beam incident on one side in the second direction with respect to the optical axis (to converge) can be made greater than the phase advancing effect (second phase advancing effect) that is given to the light beam incident on the other side.

[0035] As a result, a light beam incident on an area away from the optical axis in the second direction will be out of focus with respect to the image forming surface 33. As a result, it is possible to obtain MTF defocus characteristics in the second direction that reduce changes in contrast performance even with focus deviations caused by manufacturing errors or positional errors of the lenses included in the imaging optical system 15.

[0036] 4(c) is a diagram showing the relationship between the distance from the optical axis in the second direction and the difference in cross-sectional shape between one side and the other side at the same distance from the optical axis. In this embodiment, the anamorphic surface of the imaging optical system 15 is configured so that the difference in cross-sectional shape between one side and the other side at the same distance from the optical axis in the second direction gradually increases with increasing distance from the optical axis in the second direction. In a cross section that includes the optical axis and is perpendicular to a first direction, a first portion on one side of the optical axis of the anamorphic lens is configured to advance the phase of the wavefront of the incident light beam, and a second portion on the other side is configured to delay the phase of the wavefront of the incident light beam, where the distance from the optical axis to the first portion is equal to the distance from the optical axis to the second portion.

[0037] In other words, the anamorphic lens L4 is configured such that, in the cross-sectional shape in the second cross section, the phase-advancing effect (first phase-advancing effect) of one side portion at the same distance from the optical axis is greater than the phase-advancing effect (second phase-advancing effect) of the other side portion, and the difference in magnitude between the first and second phase-advancing effects increases with increasing distance from the optical axis. This makes it possible to obtain MTF defocus characteristics in the second direction that minimize the decrease in the MTF value at the image plane 33 and reduce the change in contrast performance due to focus shift.

[0038] In the imaging optical system 15 according to this embodiment, the anamorphic surface that provides the phase advance described above is the optical surface of the anamorphic lens L4 on the original 11 (object) side, but it is not limited to this and may be the optical surface on the image sensor 16 side. In this embodiment, the rear group 32 is composed of only one lens, the anamorphic lens L4, but it is not limited to this and may include other lenses as needed. In this embodiment, the optical phase change between one side and the other side at the same distance is provided by the cross-sectional shape of the anamorphic surface of the rear group 32, but it is not limited to this and the optical element may have a refractive index distribution characteristic in the second direction.

[0039] Next, the characteristics of the imaging optical system 15 according to this embodiment are shown in Tables 1, 2 and 3 below. In Tables 1, 2 and 3, R denotes the radius of curvature of the optical surface (the sign is positive when the surface is convex toward the object side), D denotes the distance between the optical surfaces, Nd denotes the refractive index of the optical element, and νd denotes the Abbe number of the optical element. AP1 denotes the diameter of the aperture stop AP in the first direction, and AP2 denotes the diameter of the aperture stop AP in the second direction. Furthermore, Fno1 denotes the F-number in the first direction, and Fno2 denotes the F-number in the second direction. Here, the F-number is the reduction-side F-number of the imaging optical system 15. Furthermore, f denotes the focal length of the imaging optical system 15, β denotes the imaging magnification, Y denotes the most off-axis image height, and ω denotes the half angle of view. In Table 3, "Ex" denotes "×10 -x " means.

[0040] [Table 1]

[0041] [Table 2]

[0042] [Table 3]

[0043] Here, when the intersection point with the optical axis of the anamorphic lens is taken as the origin, the optical axis direction is taken as the X axis, the first direction is taken as the Y axis, and the second direction is taken as the Z axis, the line cut by a first cross section that includes the optical axis and is parallel to the X axis and Y axis is taken as the generatrix. In other words, the line of intersection with the first cross section of the anamorphic surface is taken as the generatrix. Also, the line cut by a second cross section that is parallel to the optical axis and perpendicular to the first cross section is taken as the sagittal. In this case, the generatrix shape X is expressed by the following equation (1).

number

[0044] Moreover, a sagittal shape S, which is a shape in a second cross section perpendicular to the first direction, is expressed by the following formula (2).

number

[0045] The radius of curvature r' is expressed by the following formula (3).

number

[0046] (Explanation of the invention principle) Next, a characteristic configuration of the imaging optical system 15 according to this embodiment will be described in detail. In this embodiment, the anamorphic surface of the anamorphic lens L4 has different cross-sectional shapes on one side and the other side at the same distance in the second direction from the optical axis, characterized in that the phase-advancing effect on a light beam incident on the one side is greater than the phase-advancing effect on a light beam incident on the other side at the same distance in the second direction from the optical axis.

[0047] In this case, the sagittal shape S, which is the cross-sectional shape in the second cross section (second cross section) (Y=0) including the optical axis of the anamorphic surface in this embodiment, is expressed by the following formula (4) because, based on the aspherical coefficients in Table 3, the first-order and higher terms of Y for the third term of formula (2) become 0.

number

[0048] In this embodiment, the aspherical coefficient M 0,2n+1 (n is an integer of 1 or more) is added to the third term of the formula (2). 0,3 has a value other than 0, and M for other odd k 0,k is set to 0. However, the present invention is not limited to this, and two or more aspherical coefficients M 0,2n+1 (n is an integer of 1 or more) may have an aspherical shape having a value other than 0. As a result, the anamorphic surface in this embodiment has a sagittal shape that is asymmetric with respect to the optical axis in the second cross section (Y=0) that includes the optical axis.

[0049] In this embodiment, the aspherical coefficient M 0,2n+1(n is an integer of 1 or more), resulting in a sagittal shape that is asymmetric with respect to the optical axis. Furthermore, in this embodiment, the amount of non-circularity in the second direction continuously changes as the distance from the optical axis increases along the first direction, but the difference in cross-sectional shape between one side and the other side at the same distance in the second direction is made equal at any position on the generatrix. Therefore, it is possible to achieve the same optical phase shift effect at all image heights, thereby obtaining the same MTF defocus characteristics in the second direction.

[0050] In this embodiment, M 0,1 and M 0,2n However, M 0,2n+1 If all values ​​of (n is an integer of 1 or more) are set to 0, the degradation of MTF at the imaging plane 33 and the defocus position becomes significant, and the desired MTF defocus characteristics in the second direction cannot be obtained. Of the light beams incident on the anamorphic surface, the intersection point between the marginal ray of the light beam incident at the position farthest on one side from the optical axis in the second direction (second direction) and the anamorphic surface is defined as the first position, the distance from the optical axis in the second direction to the first position is defined as d, and a position away from the optical axis by d on the other side is defined as the second position. Furthermore, when the intersection point between the anamorphic surface and the optical axis is defined as a reference position, the distance between the reference position and the first position in the direction of the optical axis is defined as Sa, and the distance between the reference position and the second position in the direction of the optical axis is defined as Sb, the following conditional expression (5) is satisfied:

[0051]

number

[0052] Furthermore, it is more preferable that the following conditional expression (5a) be satisfied.

number

[0053] In this embodiment, the cross-sectional shape Sb on one side refers to the cross-sectional shape at a position of +1.50 mm relative to the horizontal axis (second direction) of Figure 4(b), and the cross-sectional shape Sa on the other side refers to the cross-sectional shape at a position of -1.50 mm relative to the horizontal axis (second direction) of Figure 4(b). Here, the above ±1.50 mm represents the position of the most off-axis light beam that is incident on the anamorphic surface at the position farthest from the optical axis in the second direction including the optical axis.

[0054] In this embodiment, d=1.50 mm, Sa=−0.0506, and Sb=−0.0494.

number

[0055] Furthermore, the anamorphic lens in this embodiment is made (formed) of resin. This makes it possible to easily and inexpensively mold a lens having the above-described rotationally asymmetric aspherical shape. Furthermore, unlike Patent Document 1, a filter having an optical phase shifting effect is not configured as a separate component. The anamorphic lens in this embodiment can obtain the same MTF defocus characteristics in the second direction as a conventional lens, simply by adding an optical phase shifting effect to the anamorphic surface.

[0056] Furthermore, unlike Patent Document 1, there is no need to provide a filter with an optical phase-shifting effect near the aperture stop AP of the front group 31, and this effect can be achieved with an anamorphic lens positioned sufficiently far away from the aperture stop AP in the optical axis direction. Therefore, the configuration according to this embodiment can be realized by simply changing the shapes of optical elements, and another feature is that it allows for a high degree of freedom in terms of placement.

[0057] This is a characteristic effect that results from the imaging optical system 15 according to this embodiment being configured on the premise of a linear image sensor and having almost no angle of view in the second direction. In particular, because the front group 31 according to this embodiment is composed entirely of optical elements with rotationally symmetric shapes, it is possible to perform rotational adjustment using a conventionally known rotational adjustment mechanism, making it possible to correct imaging performance due to decentering errors of optical elements and the like, and it is possible to follow conventional assembly and adjustment processes.

[0058] Here, the MTF defocus characteristics of the imaging optical system according to the first embodiment are shown in FIG. FIG. 5 shows the MTF defocus characteristics at 6.0 Line Pair / mm on the original 11 (object) side, with the solid line representing the MTF in the second direction (S_MTF) and the dashed line representing the MTF in the first direction (M_MTF). The center of the horizontal axis represents the image plane 33. FIG. 5(a) shows the MTF defocus characteristics at a field angle of 0°, FIG. 5(b) shows the MTF defocus characteristics at a field angle of 13.9°, and FIG. 5(c) shows the MTF defocus characteristics at a field angle of 27.8°. Here, the field angle of 27.8° in FIG. 5(c) corresponds to half the field angle.

[0059] As shown in FIG. 5, the MTF in the second direction (S_MTF) is smaller than the MTF in the first direction (M_MTF) in terms of the MTF at the image plane 33, i.e., the MTF peak value. However, the change in MTF with respect to the defocus direction, i.e., the change in contrast performance, is smaller. In other words, the defocus range in which a specific MTF, for example, an MTF of 0.4 (40%) or greater, is achieved is expanded. Specifically, as can be seen from FIG. 5(a), which shows the MTF defocus characteristics at a 0° angle of view, the defocus range that satisfies the standard value of 0.4 (40%) is expanded to approximately 0.26 mm for S_MTF compared to 0.23 mm for M_MTF.

[0060] Similarly, in Figure 5(b), which shows the MTF defocus characteristics at a field angle of 13.9°, the M_MTF defocus is 0.23 mm, while the S_MTF defocus is increased to approximately 0.25 mm. Also, in Figure 5(c), which shows the MTF defocus characteristics at a field angle of 27.8°, the M_MTF defocus is 0.22 mm, while the S_MTF defocus is increased to approximately 0.24 mm.

[0061] This means that there is little change in contrast performance even when focal shift occurs due to manufacturing errors or positional errors in each lens, meaning that the defocus range in which the MTF standard value is satisfied can be expanded, which also means that the conventional adjustment process can be omitted. Furthermore, as described above, an optical phase change effect is produced at all image heights, and the desired MTF defocus characteristics in the second direction can be obtained regardless of the angle of view.

[0062] As described above, it is known that the sensitivity in the second direction is lower than the sensitivity in the first direction in the imaging optical system 15 according to this embodiment. In particular, it may be difficult to correct the field curvature aberration in the second direction due to local manufacturing errors at the end of the anamorphic lens L4, so it is very effective to expand the defocus range that satisfies the standard value of MTF. As described above, the imaging optical system 15 according to this embodiment makes it possible to provide an imaging optical system in which the change in contrast performance due to focus shift in the second direction is small, without increasing the number of components of the imaging optical system or the adjustment steps.

[0063] [Second embodiment] In the imaging optical system 15 according to the second embodiment, the definitions of the coordinate system of each optical surface and the definitions of the surface shape of each optical surface are the same as those in the imaging optical system 15 according to the first embodiment. Furthermore, a description of the configuration having the same features as those in the imaging optical system 15 according to the first embodiment will be omitted, and the following will describe in detail the characteristic parts of the imaging optical system 15 according to the second embodiment.

[0064] FIG. 6 shows cross-sectional views of the imaging optical system 15 according to this embodiment in the first direction (Y direction) (a) and the second direction (Z direction) (b). Moreover, since the imaging optical system 15 according to this embodiment has the same configuration as the imaging optical system 15 according to the first embodiment, the same members are given the same reference numerals and the description thereof will be omitted.

[0065] The specifications of the imaging optical system 15 according to this embodiment are shown in Tables 4, 5, and 6 below. In Tables 4, 5, and 6, R denotes the radius of curvature of the optical surface (the sign is positive when the surface is convex toward the object side), D denotes the distance between the optical surfaces, Nd denotes the refractive index of the optical element, and νd denotes the Abbe number of the optical element. AP1 denotes the diameter of the aperture stop AP in the first direction, and AP2 denotes the diameter of the aperture stop AP in the second direction. Furthermore, Fno1 denotes the F-number in the first direction, and Fno2 denotes the F-number in the second direction. Here, the F-number is the reduction-side F-number of the imaging optical system 15. Furthermore, f denotes the focal length of the imaging optical system 15, β denotes the imaging magnification, and Y denotes the most off-axis image height. In Table 6, "Ex" denotes "×10 -x " means.

[0066] [Table 4]

[0067] [Table 5]

[0068] [Table 6]

[0069] As shown in Tables 4, 5, and 6, the aperture stop AP according to this embodiment is disposed between the first lens L1 and the second lens L2 and has a shape that is rotationally asymmetric with respect to the optical axis. Specifically, the aperture has an elliptical shape with a diameter of 2.8 mm in a first direction including the optical axis and a diameter of 3.7 mm in a second direction including the optical axis.

[0070] In the imaging optical system 15 according to this embodiment, the focal lengths in the first direction and the second direction are the same, and therefore the F-numbers are different between the first direction and the second direction. As shown in Tables 4, 5, and 6, the F-number in the first direction is 8.5, and the F-number in the second direction is 6.5.

[0071] 7 shows various aberration diagrams of the imaging optical system 15 according to this embodiment. The various aberration diagrams show spherical aberration, astigmatism, distortion, and chromatic aberration of magnification, respectively. In each diagram, e, g, C, and F represent the e-line, g-line, C-line, and F-line, respectively, and M and S represent the meridional image plane and the sagittal image plane. Furthermore, Fno is the F-number, and ω is the half angle of view.

[0072] 8(a) is a cross-sectional view of the anamorphic surface of the imaging optical system 15 according to the second embodiment. FIG. 8(b) shows the cross-sectional shape of the anamorphic surface of the imaging optical system 15 according to the second embodiment in a second cross section including the optical axis. FIG. 8(c) is a diagram showing the relationship between the distance from the optical axis in the second direction of the anamorphic surface of the imaging optical system 15 according to the second embodiment and the difference in cross-sectional shape between one side and the other side at the same distance from the optical axis.

[0073] In this embodiment, as in the first embodiment, there is a difference in cross-sectional shape within the second cross section between one side and the other side at the same distance from the optical axis in the second direction, and the anamorphic surface is configured so that the difference in cross-sectional shape gradually increases as the distance from the optical axis along the second direction increases.

[0074] Furthermore, as with the first embodiment, the formula (2) representing the cross-sectional shape of the anamorphic surface in the second cross section including the optical axis also has an aspherical coefficient M 0,2n+1 (n is an integer of 1 or more).

[0075] In addition, in the imaging optical system 15 according to this embodiment, M 0,3has a value other than 0, and M for other odd k 0,k is set to 0. However, the imaging optical system 15 of the present invention is not limited to this and may have two or more aspherical coefficients M 0,2n+1 (n is an integer of 1 or more) may have an aspherical shape having a value other than 0.

[0076] Furthermore, in equation (5), which is derived by assuming that the distance between the optical axis and a position through which a marginal ray of a light beam incident on the anamorphic surface at the farthest position on one side from the optical axis in the second direction passes is d, the cross-sectional shape on one side at a position distanced from the optical axis is Sa, and the cross-sectional shape on the other side is Sb, in this embodiment, d=1.50 mm, Sa=-0.0505, and Sb=-0.0495.

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[0077] FIG. 9 shows the MTF defocus characteristics of the imaging optical system according to the second embodiment. FIG. 9 shows the MTF defocus characteristics at an axial image height of 6.0 Line Pair / mm, with the solid line indicating the MTF in the second direction (S_MTF) and the dashed line indicating the MTF in the first direction (M_MTF). The center of the horizontal axis represents the image plane 33. FIG. 9(a) shows the MTF defocus characteristics at a field angle of 0°, FIG. 9(b) shows the MTF defocus characteristics at a field angle of 13.8°, and FIG. 9(c) shows the MTF defocus characteristics at a field angle of 27.7°. Here, the field angle in FIG. 9(c) corresponds to half the field angle.

[0078] 9, the MTF in the first direction (M_MTF) and the MTF in the second direction (S_MTF) are roughly the same at the image plane 33 and the defocus position nearby. This is because, as described above, increasing the F-number in the first direction (darkening it) reduces the peak value of the MTF in the first direction and expands the defocus range when viewed at a specific MTF.

[0079] Also, the F-number in the second direction is smaller (brighter) than the F-number in the first direction. However, as described above, the anamorphic surface of the anamorphic lens L4 in the rear group 32 has an aspherical shape that imparts an optical phase shifting effect, and therefore the defocus range when viewed at a specific MTF is similarly expanded.

[0080] When viewed at a specific MTF, for example 0.4 (40%), the defocus range at a 0° angle of view is approximately 0.29 mm for M_MTF and 0.25 mm for S_MTF. Similarly, at a 13.8° angle of view, the defocus range is 0.28 mm for M_MTF and approximately 0.24 mm for S_MTF, and at a 27.7° angle of view, the defocus range is 0.27 mm for M_MTF and approximately 0.24 mm for S_MTF.

[0081] Therefore, compared to conventional imaging optical systems in which the F-numbers in both the first and second directions are 6.5, there is little change in contrast performance even with focus shifts caused by manufacturing errors or positional errors in each lens, meaning that the defocus range in which the MTF standard value is satisfied can be expanded.

[0082] As described above, the imaging optical system 15 according to this embodiment makes it possible to provide an imaging optical system in which the change in contrast performance due to focus shift in the second direction is small, without increasing the number of components of the imaging optical system.

[0083] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments and various modifications and changes are possible within the scope of the gist of the present invention.

[0084] The disclosure of this embodiment includes the following configuration. (Configuration 1) An optical system that focuses a light beam from an object onto a light receiving portion extending in a first direction, a first lens, an aperture stop, and a second lens including a rotationally asymmetric anamorphic surface; the second lens is disposed closer to the image side than the aperture stop, In a cross section including the optical axis and perpendicular to the first direction, a first portion of the second lens on one side with respect to the optical axis advances the phase of a wavefront of the incident light beam, and a second portion on the other side delays the phase of the wavefront of the incident light beam; An optical system, characterized in that the distance from the optical axis to the first portion and the distance from the optical axis to the second portion are equal to each other. (Configuration 2) The optical system described in configuration 1, characterized in that, within the cross section, a first phase-advancing effect that advances the phase of the wavefront of the incident light beam in the first portion on one side at the same distance from the optical axis is greater than a second phase-advancing effect that advances the phase of the wavefront of the incident light beam in the second portion on the other side, and the difference between the first phase-advancing effect and the second phase-advancing effect increases with increasing distance from the optical axis. (Configuration 3) The direction of the optical axis is the X axis, the first direction is the Y axis, the second direction is the Z axis, the radius of curvature of the anamorphic surface on the optical axis in a second cross section that includes the optical axis and is perpendicular to the first direction is r0, the radius of curvature of the anamorphic surface on a generatrix that is an intersection line with the first cross section that includes the optical axis and is perpendicular to the second direction is r', and the aspheric coefficient is K z , D 2i , M jk , E2, E4, E6, E8, E 10 and the cross-sectional shape S of the anamorphic surface in a second cross section perpendicular to the first direction is

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[0085] 15 Imaging optical system (optical system) L1 First lens AP aperture stop L2 Second lens L3 Third lens L4 Anamorphic Lens (Second Lens)

Claims

1. an optical system that focuses a light beam from an object onto a light receiving portion extending in a first direction, a first lens, an aperture stop, and a second lens including a rotationally asymmetric anamorphic surface; the second lens is disposed closer to the image side than the aperture stop, In a cross section including the optical axis and perpendicular to the first direction, a first portion of the second lens on one side with respect to the optical axis advances the phase of a wavefront of the incident light beam, and a second portion on the other side delays the phase of the wavefront of the incident light beam; An optical system, characterized in that the distance from the optical axis to the first portion and the distance from the optical axis to the second portion are equal to each other.

2. The optical system of claim 1, wherein, in the cross section, a first phase-advancing effect that advances the phase of the wavefront of the incident light beam in the first portion on one side at the same distance from the optical axis is greater than a second phase-advancing effect that advances the phase of the wavefront of the incident light beam in the second portion on the other side, and the difference between the first phase-advancing effect and the second phase-advancing effect increases with increasing distance from the optical axis.

3. The direction of the optical axis is the X axis, the first direction is the Y axis, the second direction is the Z axis, and the radius of curvature of the anamorphic surface on the optical axis in a second cross section that includes the optical axis and is perpendicular to the first direction is r ０ , the radius of curvature of the anamorphic surface in the second cross section on a generatrix that is an intersection line with the first cross section that includes the optical axis and is perpendicular to the second direction is r', and the aspheric coefficient is K ｚ , D ２ｉ , M ｊｋ , E ２ , E ４ , E ６ , E ８ , E １０ and the cross-sectional shape S of the anamorphic surface in a second cross section perpendicular to the first direction is [Equation 1] [Equation 2] When expressed as M ０，２ｎ＋１ 2. The optical system according to claim 1, wherein at least one of (n is an integer of 1 or more) is a value other than 0.

4. When an intersection between the anamorphic surface and a marginal ray of a light beam incident on the anamorphic surface at a position farthest from the optical axis in the second direction among the light beams incident on the anamorphic surface is defined as a first position, a distance from the optical axis in the second direction to the first position is defined as d, a position away from the optical axis by d on the other side is defined as a second position, the intersection between the anamorphic surface and the optical axis is defined as a reference position, a distance between the reference position and the first position in the direction of the optical axis is defined as Sa, and a distance between the reference position and the second position in the direction of the optical axis is defined as Sb, [Equation 3] 2. The optical system according to claim 1, wherein the following condition is satisfied:

5. 2. The optical system according to claim 1, wherein the shape of the anamorphic surface in a second cross section perpendicular to the first direction is a non-arcuate shape.

6. 2. The optical system according to claim 1, wherein the shape of the anamorphic surface in a first cross section perpendicular to the second direction is a non-arcuate shape.

7. 2. The optical system of claim 1, wherein the F-number in the first direction is greater than the F-number in the second direction.

8. 2. The optical system of claim 1, wherein the aperture stop has a larger aperture in the first direction than in the second direction.

9. the optical system comprises a front group and a rear group arranged in this order from the object side to the image side, the front group and the rear group are separated from each other by the largest distance between optical elements adjacent to each other in the direction of the optical axis within the optical system, 2. The optical system according to claim 1, wherein the aperture stop is included in the front group.

10. 10. The optical system according to claim 9, wherein the front group has a lens arranged closer to the image side than the aperture stop.

11. 10. The optical system according to claim 9, wherein the rear group is composed of only the second lens.

12. 2. The optical system according to claim 1, wherein the second lens is made of resin.

13. 13. An image reading device comprising: the optical system according to claim 1; and a light receiving section that receives a light beam from an object that is converged by the optical system.

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