JP2019211711A - Optical system, image capturing device having the same, and image capturing system - Google Patents

Abstract

To provide an optical system which is compact and yet capable of applying different aberration correction for each wavelength, and to provide an image capturing device having the same, and an image capturing system.SOLUTION: An optical system 10 provided herein comprises a front group 11, a light shield member 4, and a rear group 12 arranged in order from the object side to the image side, the light shield member 4 having an aperture elongate in a first direction. The front group 11 is configured to form no object image on the aperture in a first cross-section parallel to the first direction, and to form an intermediate object image on the aperture in a second cross-section perpendicular to the first direction. The rear group 12 includes a diffractive surface 5 for splitting light passing through the aperture in the second cross-section into a plurality of light components of different wavelengths, and an anamorphic optical surface 6 configured to focus the plurality of light components onto different points in the second cross-section. In the second cross-section, a curvature radius of the anamorphic optical surface 6 on one side of an optical axis gets smaller with distance from the optical axis, and a curvature radius on the other side of the optical axis gets greater with distance from the optical axis.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an optical system used in an imaging apparatus that acquires image information by splitting a light beam from an object, and is suitable for evaluation and inspection in industrial fields such as agriculture, food, and medicine.

  2. Description of the Related Art Conventionally, an optical system that splits a light beam from a test object (object) into a plurality of light beams having different wavelengths and condenses each light beam at different positions is known. Patent Document 1 describes an optical system in which a light beam reflected by a cylindrical mirror is dispersed by a diffraction grating and each light beam is collected by a lens.

US Pat. No. 7,1998,877

  However, in the optical system described in Patent Document 1, since the incident angles of the respective light beams with respect to the lens are different from each other, coma aberration that varies depending on the wavelength occurs. In order to correct such coma aberration that differs for each wavelength, it is necessary to use a large number of lenses, and the configuration of the optical system becomes complicated.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an optical system that can correct different aberrations for each wavelength while having a simple configuration, and an imaging apparatus and imaging system including the optical system.

  In order to achieve the above object, an optical system according to one aspect of the present invention is an optical system including a front group, a light shielding member, and a rear group arranged in order from the object side to the image side. A long opening is provided in the first direction, and the front group does not form an object on the opening in the first cross section parallel to the first direction, and the first group extends in the first direction. In the second vertical section, an intermediate image of the object is formed on the opening, and the rear group splits the light beam that has passed through the opening in the second cross section into a plurality of light beams having different wavelengths. And an anamorphic optical surface for condensing the plurality of light beams at different positions in the second cross section, and one of the anamorphic optical surfaces with respect to the optical axis in the second cross section. The curvature on the side decreases as it goes off-axis, Curvature on the other side with respect to the axis is characterized by increases toward the off-axis.

  According to the present invention, it is possible to provide an optical system that can correct different aberrations for each wavelength while having a simple configuration, an imaging apparatus including the optical system, and an imaging system.

The principal part schematic in the XY cross section of the optical system which concerns on embodiment. The principal part schematic in the ZX cross section of the optical system which concerns on embodiment. FIG. 2 is a schematic diagram of a main part of a diffractive optical element according to an embodiment. The figure which shows the partial curvature in the zx cross section of the 4th reflective surface which concerns on embodiment. The figure which shows the condensing state of the light beam of a several different wavelength which concerns on embodiment. The figure which shows the condensing state of the light beam of a several different wavelength which concerns on a comparative example. FIG. 3 is a diagram illustrating an MTF of the optical system according to the first embodiment. FIG. 6 is a schematic diagram of a main part of an optical system according to a second embodiment. FIG. 6 is a diagram showing an MTF of an optical system according to Example 2. FIG. 6 is a schematic diagram of a main part of an optical system according to a third embodiment. FIG. 6 is a diagram showing an MTF of an optical system according to Example 3. FIG. 7 is a schematic diagram of a main part of an optical system according to a fourth embodiment. FIG. 10 is a diagram showing an MTF of an optical system according to Example 4. The principal part schematic of the imaging system as the usage example 1 of the optical system which concerns on embodiment. The principal part schematic of the imaging system as the usage example 2 of the optical system which concerns on embodiment.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. Each drawing may be drawn on a different scale for the sake of convenience. Moreover, in each drawing, the same reference number is attached | subjected about the same member and the overlapping description is abbreviate | omitted.

  In the following description, an XYZ coordinate system is defined as an absolute coordinate system, and an xyz coordinate system is defined as a local coordinate system for each optical surface. In the local coordinate system, the x axis is the normal axis (optical axis) at the apex (origin) of each optical surface, the y axis is parallel to the Y axis and orthogonal to the x axis at the origin, the z axis is the x axis, and It is an axis orthogonal to the y-axis. Further, the Y direction and the y direction are the first direction (reading direction), the Z direction and the z direction are the second direction (spectral direction), the XY cross section and the xy cross section are the first cross section (read cross section), the ZX cross section, and The zx cross section is also referred to as a second cross section (spectral cross section).

  1 and 2 are schematic views of a main part of an optical system 10 according to an embodiment of the present invention. FIG. 1 shows a first cross section, and FIG. 2 shows a second cross section. 1 and 2 show the shape of each member in a cross section including the optical axis. In FIG. 1, each member is shown in the same sheet for convenience. In FIGS. 1 and 2, the diffraction grating on the diffraction surface is omitted for convenience. In the present embodiment, the test object is disposed at a position near Z = 0 on the object plane parallel to the YZ plane, and the light receiving surface 7 of the image sensor is disposed on the image plane of the optical system 10. To do. In addition, it is assumed that the test object is illuminated with white light (light having a plurality of wavelength components) such as sunlight.

  The optical system 10 according to the present embodiment includes a front group 11, a light shielding member (slit member) 4, and a rear group 12 arranged in order from the object side to the image side. The optical system 10 forms an image of the test object on the light receiving surface (image surface) 7 by collecting a light beam from a test object (not shown) located on the −X side. The front group 11 includes a diaphragm 1, a first reflection surface 2, and a second reflection surface 3. The rear group 12 has a third reflecting surface (diffraction surface) 5 and a fourth reflecting surface 6. Note that a cover glass G is disposed immediately before the light receiving surface 7, but this is treated as not contributing to image formation.

  The diaphragm 1 is a member for regulating the width of the light beam from the test object in the second direction, and is arranged so that the opening surface thereof is perpendicular to the X direction. However, the diaphragm 1 may be provided outside the optical system 10. As shown in FIGS. 1 and 2, it is desirable that the light beam entrance (aperture 1) and the light exit (light receiving surface 7) in the optical system 10 be disposed on opposite sides of each optical surface. Thereby, when the optical system 10 is applied to an imaging device, it is possible to easily avoid that the light beam from the test object is blocked by the imaging element, wiring, or the like.

  The light shielding member 4 is provided with a long opening (slit) in the first direction. The light shielding member 4 serves as a stop for restricting the width of the light beam in the first direction while restricting the angle of view in the second cross section of the optical system 10 to shield unnecessary light. Note that the width of the opening of the light shielding member 4 is determined according to the required light quantity, resolution, and the like. The width of the opening of the light shielding member 4 in the second direction is preferably shorter than the width (several mm) in the first direction and is several μm to several 100 μm. If the width of the opening of the light shielding member 4 in the second direction is too large, the resolution on the light receiving surface 7 is lowered, and if it is too small, the effective light beam contributing to image formation is easily shielded. More preferably, it is 10 μm or more and 0.2 mm or less.

  The region other than the aperture in the aperture stop 1 and the light shielding member 4 is a light shielding surface that does not transmit light in at least the use wavelength band (design wavelength band) of the optical system 10. As the diaphragm 1 and the light shielding member 4, it is possible to employ one having a hole in a sheet metal or one having a chromium vapor deposited on the surface of a glass plate. By adopting such a light shielding member 4, the optical system 10 can form an image of a linear reading region (test region) that is long in the first direction.

  The 1st reflective surface 2, the 2nd reflective surface 3, and the 4th reflective surface 6 are reflective surfaces obtained by giving a reflective coating to the base surface which has a free-form surface shape. The base surface of each reflecting surface is formed by processing (cutting, polishing, molding with a mold, etc.) a block material made of glass, resin, metal or the like. The reflective coating desirably has a spectral reflection characteristic that can realize sufficient energy efficiency (light utilization efficiency) in the used wavelength band. When the base surface has a sufficient reflectance in the used wavelength band, the reflective coating may be omitted.

  In the present embodiment, each of the first reflecting surface 2, the second reflecting surface 3, and the fourth reflecting surface 6 is an anamorphic optical surface (anamorphic reflection) in which the first cross section and the second cross section have different curvatures (power). Surface). Thereby, different optical actions can be generated in the first cross section and the second cross section. In particular, in the second cross section, the curvature of one side of the fourth reflecting surface 6 with respect to the optical axis decreases as it goes off-axis, and the curvature of the other side with respect to the optical axis decreases toward the off-axis. It has a shape that increases. As a result, coma that varies depending on the wavelength can be corrected well (details will be described later).

  Each reflective surface of the front group 11 may not be an anamorphic optical surface. For example, each reflective surface may be a spherical surface, and an anamorphic refractive surface may be provided instead. However, in order to reduce the number of optical surfaces in the front group 11, it is desirable that at least one of the first reflecting surface 2 and the second reflecting surface 3 is an anamorphic optical surface. The rear group 12 may have at least one diffractive surface and at least one anamorphic optical surface arranged on the image side thereof.

  However, in order to suppress the occurrence of aberration by sharing power between the optical surfaces, it is more preferable that all the optical surfaces of the front group 11 and the rear group 12 are anamorphic optical surfaces. When the diffractive surface 5 is provided in the front group 11, only a part of the light beams having a certain wavelength can pass through the opening of the light shielding member 4. Therefore, the diffractive surface 5 needs to be provided in the rear group 12.

  In the present embodiment, since all the optical surfaces are reflecting surfaces, the optical path is bent to realize downsizing of the optical system 10 while suppressing the occurrence of chromatic aberration. At this time, in order to reduce the size of the optical system 10, as shown in FIG. 2, the reflecting surfaces are arranged so that the optical paths intersect in each of the front group 11 and the rear group 12 (so that it becomes a character of 4). It is desirable to arrange. If necessary, at least one optical surface may be a refracting surface (transmission surface).

  However, particularly in the rear group 12, since holding members and wirings (not shown) are arranged around the light shielding member 4 and the light receiving surface 7, it is difficult to secure a sufficient space for arranging the refractive optical element. . Even if a sufficient space can be secured, it is necessary to arrange a plurality of refractive optical elements in order to correct chromatic aberration satisfactorily, so that the entire system becomes large. Therefore, it is desirable that at least all optical surfaces included in the rear group 12 are reflective surfaces. Furthermore, it is more preferable that all the optical surfaces included in the front group 11 are reflecting surfaces.

  As described above, the configuration of the optical system 10 is not limited to this embodiment. For example, the front group 11 is configured by one or three or more optical surfaces, and the rear group 12 is configured by three or more optical surfaces. May be. That is, the front group 11 only needs to have at least one reflecting surface or refracting surface, and the rear group 12 only needs to have at least two reflecting surfaces or refracting surfaces (one of which is a diffractive surface). At this time, it is good also as a structure which combined the reflective surface and the refractive surface as needed. However, in order to realize downsizing of the entire system and reduction in the number of parts, it is desirable that each of the front group 11 and the rear group 12 is constituted by two reflecting surfaces as in this embodiment.

  3A and 3B are schematic views of a main part of a diffractive optical element (reflective optical element) 50 including the diffractive surface 5 according to the present embodiment, FIG. 3A is a perspective view, and FIG. 3B is a front view (+ x). Figure viewed from the direction). The diffraction surface 5 includes a base surface 51 and a diffraction grating 52 provided on the base surface 51. In FIG. 3, only a part of the diffraction grating 52 is shown enlarged for convenience.

  The base surface 51 in the diffractive surface 5 has a free-form surface shape like other reflective surfaces. The diffraction grating 52 is composed of a plurality of gratings (convex portions) arranged at a pitch on the order of submicron to micron, and the height of each grating is also on the order of submicron to micron. As the diffraction grating 52, one having a step shape, a rectangular uneven shape, a blazed shape, a SIN wave shape, or the like in the zx section can be adopted. The shape of the diffraction grating 52 is selected in consideration of the required diffraction efficiency and ease of manufacture.

  In the present embodiment, a blazed shape is employed, in which it is relatively easy to improve both diffraction efficiency and ease of manufacture. In the blazed diffraction grating, the portion farthest in the x direction with respect to the base surface 51 is the lattice vertex, the portion that reflects (diffracts) incident light is the blazed surface (lattice surface), and contributes to diffraction adjacent to the blazed surface. The part which is not called is called a lattice wall surface. The diffractive surface 5 according to this embodiment is arranged so that the blaze surface faces the light receiving surface 7 side (image side) and the grating wall surface faces the object side. As a result, a short wavelength light beam enters the + Z side of the light receiving surface 7 in FIG. 2, and a long wavelength light beam enters the -Z side.

  The base surface 51 is formed by the same method as the other reflective surfaces described above. The diffraction grating 52 can be formed by processing the base surface 51 by cutting or polishing. However, the diffraction grating 52 may be formed at the same time when the base surface 51 is formed. For example, the diffractive optical element 50 in which a fine concavo-convex structure is provided on the surface of a mirror piece constituting the mold and the diffraction grating 52 is provided by molding using the mold may be manufactured. In order to improve the diffraction efficiency, a reflective coating may be applied to the surface of the diffraction grating 52.

  Further, as shown in FIG. 3B, it is desirable to configure the diffraction grating 52 so that a plurality of ridge lines corresponding to the vertices of each grating in the zx section are parallel to each other. Furthermore, it is more preferable that the intervals between the ridge lines of the diffraction grating 52 be constant. Thereby, the process of the base surface 51 by cutting or grinding | polishing can be made easy. However, the intervals between the ridge lines may be non-constant if necessary. For example, in order to realize more advanced aberration correction in the optical system 10, the interval between the ridge lines may be changed in the z direction.

  The base surface 51 of the diffractive surface 5 is preferably an anamorphic surface having different curvatures in the xy section and the zx section. This makes it possible to share power with other anamorphic optical surfaces, thus facilitating correction of aberrations. In the present embodiment, the base surface 51 of the diffractive surface 5 is an anamorphic surface. However, the base surface 51 may be configured as a flat surface or a spherical surface in consideration of the ease of manufacturing the diffraction grating 52.

  The operation of the optical system 10 will be described with reference to FIGS. 1 and 2.

  The light beam emitted from the test object passes through the aperture of the diaphragm 1, is reflected by the first reflecting surface 2 and the second reflecting surface 3, and reaches the light shielding member 4. At this time, the front group 11 does not image the test object on the opening of the light shielding member 4 in the first cross section (XY cross section), and on the opening of the light shielding member 4 in the second cross section (ZX cross section). An intermediate image of the test object is formed in That is, the front group 11 is configured such that the focal position does not coincide with the object plane in the first cross section. Thus, a line-shaped intermediate image (line image) that is long in the first direction is formed on the opening of the light shielding member 4. Here, “on the opening” is not limited to the exact position of the opening, but also includes the vicinity (substantially above the opening) of the opening slightly separated from the position of the opening in the optical axis direction.

  The light beam that has passed through the opening of the light shielding member 4 is split into a plurality of light beams having different wavelengths from each other by the diffraction surface 5 in the second cross section. At this time, as shown in FIG. 3, the diffraction grating 52 on the diffraction surface 5 is composed of a plurality of gratings (ridge lines) arranged in the z direction, so that the light beam incident on the diffraction surface 5 has a spectral action only in the z direction. In the y direction, no spectral action is applied.

  The plurality of light beams from the diffractive surface 5 are reflected by the fourth reflecting surface 6 and enter the light receiving surface 7 disposed on the image surface. At this time, a plurality of light beams having different wavelengths are condensed at different positions on the light receiving surface 7 in the second cross section. That is, according to the optical system 10 according to the present embodiment, since a plurality of images for each wavelength can be formed on the light receiving surface 7, the light receiving surface 7 can acquire a plurality of image information for each wavelength.

  As described above, the optical system 10 according to the present embodiment generates different optical effects in the first cross section including the reading direction and the second cross section including the spectral direction. Specifically, in the first cross section, the test object is imaged on the light receiving surface 7 without being imaged once on the opening of the light shielding member 4, but in the second cross section, the test object is imaged on the light shielding member 4. The image is once formed on the aperture and then re-imaged on the light receiving surface 7. That is, the test object is imaged once in the first cross section, while the test object is imaged twice in the second cross section.

  According to this configuration, in the first cross section, the convergence state of the light flux when passing through the opening of the light shielding member 4 is not limited, so that the degree of freedom in designing the optical system 10 can be improved. Accordingly, the front group 11 and the rear group 12 can appropriately share the power to form an image of the test object on the light receiving surface 7, and various aberrations can be easily corrected. Wide area) and high definition of captured images.

  Specifically, by configuring the front group 11 so that the focal position in the first cross section does not coincide with the object plane, the light beam passing through the opening of the light shielding member 4 can be made non-parallel light. Thereby, it becomes easy to realize a wide angle of view in the first cross section. If the light beam passing through the opening of the light shielding member 4 is parallel light, it is necessary to arrange a large number of optical elements in the rear group 12 in order to increase the angle of view of the optical system 10. The system becomes larger. In the present embodiment, a wide angle of view is realized by using a divergent light as a light beam passing through the opening of the light shielding member 4. However, a light beam passing through the opening of the light shielding member 4 as necessary. May be convergent light.

  Further, in the first cross section, when the test object is once imaged on the opening of the light shielding member 4, each of the front group 11 and the rear group 12 must independently correct the aberration. Therefore, it is necessary to increase the power of each optical surface, for example, the degree of freedom in designing each optical surface is lowered, and it is difficult to increase the angle of view of the optical system 10. On the other hand, in the second cross section, since it is not necessary to widen the angle of view, it is possible to increase the NA by once imaging the test object on the opening of the light shielding member 4.

  In the configuration described above, each of the front group 11 and the rear group 12 has different powers in the first cross section and the second cross section. In order to realize this configuration, it is necessary to provide an anamorphic optical surface for each of the front group 11 and the rear group 12. At this time, the anamorphic optical surface included in the front group 11 should be positively given power not only to the second cross section but also to the first cross section (make the absolute value of curvature larger than 0). desirable.

  In the second cross section, the front group 11 and the rear group 12 have positive power in order to form an image of the test object once on the opening of the light shielding member 4 and then re-image the light receiving surface 7. It is necessary to make it. On the other hand, in the first cross section, since it is not necessary to once form an image of the test object on the opening of the light shielding member 4, in order to realize a further wide angle of view, the front group 11 is given negative power. It is desirable to give the rear group 12 positive power. Thereby, since the optical system 10 becomes a retro focus type in the first cross section, the focal length of the entire system is shortened, and a wide angle of view can be realized. However, when the test object is sufficiently separated from the optical system 10, the optical system 10 is made a telephoto optical system by giving the front group 11 positive power and giving the rear group 12 negative power. Also good.

  The manner in which the light beam is split by the diffractive surface 5 will be described with reference to FIG. Here, a case is considered in which a white light beam emitted from one point of the test object is split into light beams having respective wavelengths of λ1 [nm], λ2 [nm], and λ3 [nm] (λ2 <λ1 <λ3). . However, in FIG. 2, only the chief ray and the marginal ray are shown among the light beams.

  The chief ray L1P and the marginal rays L1U and L1L in the white light beam emitted from the test object are line-shaped intermediately on the opening of the light shielding member 4 via the stop 1, the first reflecting surface 2, and the second reflecting surface 3. Form an image. The principal ray L2P and the marginal rays L2U and L2L that have passed through the opening of the light shielding member 4 are caused by the diffraction surface 5 to be rays L3P, L3U, and L3L having the wavelength λ1, rays L4P, L4U, and L4L having the wavelength λ2, and rays having the wavelength λ3. Spectroscopy into L5P, L5U, and L5L. Then, each of the light beams having the wavelength λ1, the wavelength λ2, and the wavelength λ3 is condensed at the first position 73, the second position 74, and the third position 75 on the light receiving surface 7.

  Next, the shape and action of the fourth reflecting surface 6 will be described in detail.

FIG. 4 shows the partial curvature (aspheric amount) of the fourth reflecting surface 6 in the second cross section. In FIG. 4, the horizontal axis indicates the position [mm] in the z direction with respect to the origin in the local coordinate system of the fourth reflecting surface 6, and the vertical axis indicates the partial curvature 1 / R _z [1 / mm] of the fourth reflecting surface 6. Is shown. In addition, _Rz is the partial curvature radius of the 4th reflective surface 6 in a 2nd cross section. In FIG. 4, solid lines, broken lines, and alternate long and short dash lines are y = 0.0 [mm], y = −6.0 [mm], and y = −10.0 [mm] in the local coordinate system. The graph in position is shown.

As can be seen from FIG. 4, the partial curvature 1 / R _z is asymmetric on the −z side and the + z side with respect to the optical axis (x axis). Specifically, the partial curvature 1 / R _z is larger on the −z side than on the + z side. Further, the partial curvature 1 / R _z has different changes (asymmetric changes) on the −z side and the + z side with respect to the optical axis. In the present embodiment, the partial curvature 1 / _Rz increases as it goes off-axis on the −z side, and decreases as it goes off-axis on the + z side. Further, the partial curvature 1 / _Rz increases as the distance from the position of y = 0 [mm] increases.

  FIG. 5 shows a condensing state on the light receiving surface 7 in the second cross section (y = 0 [mm]) including the optical axis of light beams having different wavelengths according to the present embodiment. 5A, FIG. 5B, and FIG. 5C, the principal rays and the marginal rays of the light beams having the wavelengths of λ1 = 700 nm, λ2 = 400 nm, and λ3 = 1000 nm reach the light receiving surface 7, respectively. The optical path is shown. In the present embodiment, it is designed so that a chief ray having a wavelength λ1 = 700 nm is incident on the optical axis (x axis) of the fourth reflecting surface 6. In FIG. 5, the reference numerals of the light beams having the respective wavelengths are the same as those shown in FIG. 2, and the diffraction grating on the diffraction surface 5 is omitted as in FIG.

  5A, FIG. 5B, and FIG. 5C, respectively, + z side with respect to the optical axis of the fourth reflecting surface 6, that is, the same as the vertex 54 of the base surface of the diffractive surface 5. The curvature on the side (near side) becomes smaller toward the off-axis. On the other hand, the curvature on the −z side with respect to the optical axis of the fourth reflecting surface 6, that is, on the side opposite to the apex 54 of the base surface of the diffractive surface 5 (the far side) increases toward the off-axis. Thereby, each of the marginal ray L3L incident on the + z side of the fourth reflection surface 6 and the marginal ray L3U incident on the −z side of the fourth reflection surface 6 is reflected so as to approach the principal ray L3P. As can be seen from FIG. 5, the coma aberration on the light receiving surface 7 is well corrected for the light flux of any wavelength.

  Here, in order to explain the effect of the fourth reflecting surface 6 according to the present embodiment, a comparative example in which only the fourth reflecting surface 6 in the optical system 10 is changed will be considered.

The fourth reflecting surface 6 according to the comparative example is obtained by setting a part of the aspheric coefficient in the expression representing the shape of the second cross section of the fourth reflecting surface 6 according to the present embodiment to zero. Specifically, the values of aspherical coefficients M ₀₁ , M ₂₁ , M ₄₁ , M ₀₃ , M ₂₃ , M ₄₃ , M ₀₅ , M ₂₅ , and M ₄₅ in formula (Equation 2) described later are set to zero. . However, the values of M ₀₁ , M ₀₅ , M ₂₅ , and M ₄₅ are zero in this embodiment. These aspheric coefficients relate to the symmetry of the fourth reflecting surface 6 with respect to the optical axis. That is, the partial curvature 1 / _Rz of the fourth reflecting surface 6 according to the comparative example is a target with respect to the optical axis, unlike the present embodiment.

  FIG. 6 shows the condensing state on the light receiving surface 7 in the second cross section including the optical axis of the light beams having different wavelengths according to the comparative example, as in FIG. 6 (a), 6 (b), and 6 (c), the principal rays and the marginal rays of the light beams having the wavelengths of λ1 = 700 nm, λ2 = 400 nm, and λ3 = 1000 nm reach the light receiving surface 7, respectively. The optical path is shown. As shown in FIG. 6A, FIG. 6B, and FIG. 6C, in the comparative example, the condensed positions of the principal ray and the marginal ray at each wavelength on the light receiving surface 7 do not match. The coma is not corrected well.

  Thus, in the present embodiment, in the second cross section, the anamorphic optical surface disposed on the image side of the diffractive surface 5 becomes smaller as the curvature on one side with respect to the optical axis goes off-axis, The shape is such that the curvature on the other side with respect to the shaft increases as it goes out of the shaft. Thereby, since the marginal ray of the light flux of each wavelength can be condensed so as to be close to the principal ray, it is possible to satisfactorily correct the coma aberration that differs for each wavelength.

  Specifically, in the present embodiment, the curvature of the fourth reflecting surface 6 on the same side as the vertex of the base surface of the diffractive surface 5 with respect to the optical axis becomes smaller as it goes off-axis, and is diffracted with respect to the optical axis. The curvature of the surface 5 opposite to the vertex of the base surface is configured to increase as it goes off-axis. The method of changing the curvature of both sides with respect to the optical axis of the fourth reflecting surface 6 is opposite to that of the present embodiment in accordance with the direction of deflection of the light beam (direction of each optical surface) by the diffractive surface 5 and the fourth reflecting surface 6. You may comprise so that it may become the relationship of these. However, it is advantageous for downsizing the entire system that the optical path from the light shielding member 4 to the diffractive surface 5 and the optical path from the fourth reflecting surface 6 to the light receiving surface 7 intersect in the second cross section. It is desirable to configure as in the present embodiment.

  As described above, according to the optical system 10 according to the present embodiment, it is possible to correct different aberrations for each wavelength while having a simple configuration.

[Example 1]
The optical system 10 according to Example 1 of the present invention will be described below. The optical system 10 according to the present example has the same configuration as the optical system 10 according to the above-described embodiment.

  In this embodiment, the distance (object distance) from the test object to the diaphragm 1 is 300 mm, the width of the reading area in the first direction is 300 mm, and the angle of view in the first cross section is ± 24.17 °. In the present embodiment, the wavelength band used is 400 nm to 1000 nm, and the width of the image forming area (incident area) of the light beam on the light receiving surface 7 in the second direction is 2.7 mm.

  The composite focal lengths in the first cross section of the front group 11 and the rear group 12 according to this example are -16.27 mm and 28.30 mm, respectively, and the composite focal lengths in the second cross section of the front group 11 and the rear group 12 are respectively. The focal lengths are 19.99 mm and 25.76 mm, respectively. As described above, the optical system 10 according to the present embodiment improves the imaging performance by performing the intermediate in the second cross section, while widening the angle of view (reading) by adopting the retrofocus type in the first cross section. Realization of wide area).

  Here, the expression of the surface shape of each optical surface of the optical system 10 according to the present embodiment will be described. In addition, the expression of the surface shape of each optical surface is not limited to that described later, and each optical surface may be designed using another expression as necessary.

  The shape (bus shape) of the first reflecting surface 2, the second reflecting surface 3, the third reflecting surface (diffraction surface) 5, and the fourth reflecting surface 6 in the first cross section of the first reflecting surface 2, the second reflecting surface 3, and the fourth reflecting surface 6 according to the present embodiment. Is expressed by the following expression in each local coordinate system.

Here, R _y is a radius of curvature (bus curvature radius) in the xy section, and K _y , B ₂ , B ₄ , and B ₆ are aspheric coefficients in the xy section. The numerical values of the aspheric coefficients B ₂ , B ₄ , and B ₆ may be different from each other on both sides of the x axis (−y side and + y side) as necessary. As a result, the bus shape can be asymmetric in the y direction with respect to the x axis. In the present embodiment, secondary to sixth-order aspheric coefficients are used, but higher-order aspheric coefficients may be used as necessary.

  In addition, the shape (sub-wire shape) in the second cross section at an arbitrary position in the y direction of each base surface of each optical surface according to the present embodiment is expressed by the following expression.

However, K _z and M _jk are aspheric coefficients in the zx section. Further, r ′ is a radius of curvature (sub-wire curvature radius) in the zx section at a position separated by y from the optical axis in the y direction, and is represented by the following equation.

Here, r is the radius of curvature of the strand on the optical axis, and E ₂ and E ₄ are the strand changing coefficients. When r = 0 in Expression (Expression 3), the first term on the right side of Expression (Expression 2) is treated as zero. Note that the numerical values of the child line change coefficients E ₂ and E ₄ may be different from each other on the −y side and the + y side, if necessary. As a result, the aspherical amount of the child wire shape can be made asymmetric in the y direction. Moreover, although the formula (Equation 3) includes only even terms, odd terms may be added as necessary. Further, a higher order strand change coefficient may be used as necessary.

Note that the first-order term of z in the equation (Equation 2) is a term that contributes to the tilt amount (child tilt amount) of the optical surface in the zx section. Therefore, by setting M _jk to different values on the −y side and the + y side, it is possible to asymmetrically change the sub-line tilt amount in the y direction. However, the amount of sub-line tilt may be changed asymmetrically by using odd terms. In addition, the quadratic term of z in the equation (Equation 2) is a term that contributes to the radius of curvature of the optical surface. Therefore, in order to simplify the design of each optical surface, the subsurface curvature radius may be given to the optical surface using only the quadratic term of z in Expression (Expression 2) instead of Expression (Expression 3).

The shape of the diffraction grating on the diffraction surface 5 is not particularly limited as long as it is expressed by a phase function based on a known diffractive optical theory. In this embodiment, when the fundamental wavelength (design wavelength) is λ [mm] and the phase coefficient in the zx section is C1, the shape of the diffraction grating on the diffraction surface 5 is defined by the following phase function φ. However, in this embodiment, it is assumed that the diffraction order of the diffraction grating is 1.
φ = (2π / λ) × (C1 × z)

  Note that the fundamental wavelength here is a wavelength for determining the height of the diffraction grating, the spectral characteristics of the illumination light with respect to the object to be examined, the spectral reflectance of the reflective surface other than the diffraction surface 5, and the imaging including the light receiving surface 7. It is determined based on the spectral light receiving sensitivity of the element, the required diffraction efficiency, and the like. In other words, the fundamental wavelength corresponds to a wavelength that is desired to be emphasized during detection by the light receiving surface 7. In this embodiment, by setting the fundamental wavelength λ to 542 nm, the visible range in the used wavelength band can be intensively observed. However, for example, by setting the fundamental wavelength to about 850 nm, the near infrared region can be intensively observed, or by setting the fundamental wavelength to about 700 nm, the near infrared region can be observed in a balanced manner from the visible region. Also good.

Table 1 shows the position of the vertex of each optical surface of the optical system 10 according to the present embodiment, the direction of the normal line at the vertex, and the radius of curvature in each cross section. In Table 1, the position of the vertex of each optical surface is indicated by the distance X, Y, Z [mm] from the origin in the absolute coordinate system, and the direction of the normal line (x axis) is the X axis in the ZX section including the optical axis. Is represented by an angle θ [deg]. Moreover, d [mm] shows the space | interval (surface space | interval) between each optical surface, d '[mm] shows the space | interval between the reflective points of the principal ray in each optical surface, and _Ry , _Rz [mm] Each indicates the radius of curvature in the XY section and the ZX section at the reflection point of the principal ray. In addition, when the value of the radius of curvature of each reflecting surface is positive, it indicates a concave surface, and when it is negative, it indicates a convex surface.

Table 2 shows the surface shapes of the optical surfaces of the optical system 10 according to this example. In Table 2, each unit of R _y , r, and λ is [mm]. As shown in Table 2, the fourth reflecting surface 6 according to the present example has an aspherical shape that is symmetric with respect to the zx cross section in the y direction, and is non-symmetrical with respect to the xy cross section in the z direction. It has a spherical shape.

  Table 3 shows the aperture [1] in the y direction and the z direction of the aperture of the diaphragm 1, the aperture of the light shielding member 4, and the light receiving surface 7. In the present embodiment, all of the aperture of the diaphragm 1, the aperture of the light shielding member 4, and the light receiving surface 7 are rectangular.

  FIG. 7 shows an MTF (Modulated Transfer Function) of the optical system 10 according to the present embodiment. In FIG. 7, the MTF for each wavelength of 700 nm (frq1), 400 nm (frq2), and 1000 nm (frq3) is obtained when the object height [mm] in the reading area is Y = 0, 30, 60, 90, 120, 150. Shows about each of them. As shown in FIG. 7, the spatial frequency [lines / mm] for each wavelength of the imaging element including the light receiving surface 7 is 27.8, 41.7, and 55.6. As can be seen from FIG. 7, the aberration is satisfactorily corrected over the entire reading region, and a sufficient depth of focus is ensured.

[Example 2]
Hereinafter, the optical system 10 according to Embodiment 2 of the present invention will be described. In the optical system 10 according to the present embodiment, the description of the configuration equivalent to that of the optical system 10 according to Embodiment 1 described above is omitted.

  FIGS. 8A and 8B are schematic views of a main part of the optical system 10 according to the embodiment of the present invention. FIG. 8A shows a first cross section, and FIG. 8B shows a second cross section. The optical system 10 according to the present embodiment has a shorter optical path length from the stop 1 to the light receiving surface 7 than the optical system 10 according to the first embodiment, thereby realizing further downsizing of the entire system.

  In this embodiment, the distance from the test object to the diaphragm 1 is 300 mm, the width of the reading region in the first direction is 300 mm, and the angle of view in the first cross section is ± 24.46 °. In this embodiment, the used wavelength band is 400 nm to 1000 nm, and the width of the imaging region in the second direction on the light receiving surface 7 is 2.7 mm. The composite focal lengths in the first cross section of the front group 11 and the rear group 12 according to the present embodiment are respectively -14.21 mm and 16.69 mm, and the composite focal lengths in the second cross section of the front group 11 and the rear group 12 are respectively. The focal lengths are 19.33 mm and 11.01 mm, respectively.

As in Example 1, Table 4 shows the position of the vertex of each optical surface of the optical system 10 according to this example, the direction of the normal line at the vertex, and the radius of curvature at each cross section, and Table 5 shows each optical surface. Table 6 shows the diameter of the aperture of the diaphragm 1, the aperture of the light shielding member 4, and the diameter of the light receiving surface 7. However, as for the shape of the third reflecting surface 5 in the second cross section, the local line that differs from position to position so that the normal line at each position on the generatrix represented by the equation (Equation 1) matches the x axis. After defining the coordinate system, it is expressed by the above equation (Equation 2). The reason why the values of the curvature radii _Ry do not match between Table 4 and Table 5 is that the values of the curvature radii in Table 4 consider the tilt angle in the second cross section.

  FIG. 9 shows the MTF of the optical system 10 according to the present embodiment as in FIG. As can be seen from FIG. 9, the aberration is satisfactorily corrected over the entire reading region, and a sufficient depth of focus is ensured.

[Example 3]
The optical system 10 according to Example 3 of the present invention will be described below. In the optical system 10 according to the present embodiment, the description of the configuration equivalent to that of the optical system 10 according to Embodiment 1 described above is omitted.

  FIGS. 10A and 10B are schematic views of a main part of the optical system 10 according to the embodiment of the present invention. FIG. 10A shows a first cross section, and FIG. 10B shows a second cross section. The optical system 10 according to the present embodiment has a configuration in which the emission side (image side) Fno is smaller (brighter) than the optical system 10 according to the first embodiment. Specifically, the Fno on the emission side of the optical system 10 according to the first embodiment is 4.7 and 4.0 in the first and second cross sections, respectively, whereas the optical system according to the present embodiment. Fno on the exit side of 10 is 4.1 and 3.5 in the first and second cross sections, respectively.

  In this embodiment, the distance from the test object to the diaphragm 1 is 300 mm, the width of the reading region in the first direction is 300 mm, and the angle of view in the first cross section is ± 24.44 °. In this embodiment, the wavelength band used is 400 nm to 1000 nm, and the width of the imaging region in the second direction on the light receiving surface 7 is 2.64 mm. The combined focal lengths in the first cross section of the front group 11 and the rear group 12 according to the present embodiment are -14.46 mm and 26.85 mm, respectively, and the combined focal lengths in the second cross section of the front group 11 and the rear group 12 are respectively. The focal lengths are 19.34 mm and 24.98 mm, respectively.

  As in Example 1, Table 7 shows the position of the vertex of each optical surface of the optical system 10 according to this example, the direction of the normal at the vertex, and the radius of curvature at each cross section. Table 8 shows each optical surface. Table 9 shows the aperture of the diaphragm 1, the aperture of the light shielding member 4, and the diameter of the light receiving surface 7.

  FIG. 11 shows the MTF of the optical system 10 according to the present embodiment, as in FIG. As can be seen from FIG. 11, the aberration is satisfactorily corrected over the entire reading region, and a sufficient depth of focus is ensured.

[Example 4]
The optical system 10 according to Example 4 of the present invention will be described below. In the optical system 10 according to the present embodiment, the description of the configuration equivalent to that of the optical system 10 according to Embodiment 1 described above is omitted.

  FIGS. 12A and 12B are schematic views of a main part of the optical system 10 according to the embodiment of the present invention. FIG. 12A shows a first cross section, and FIG. 12B shows a second cross section. The optical system 10 according to the present embodiment has a shorter optical path length from the stop 1 to the light receiving surface 7 than the optical system 10 according to the first embodiment, thereby realizing further downsizing of the entire system.

  In this embodiment, the distance from the test object to the diaphragm 1 is 300 mm, the width of the reading region in the first direction is 300 mm, and the angle of view in the first section is ± 24.49 °. In this embodiment, the used wavelength band is 400 nm to 1000 nm, and the width of the imaging region in the second direction on the light receiving surface 7 is 2.37 mm. The composite focal lengths in the first cross section of the front group 11 and the rear group 12 according to the present embodiment are -13.23 mm and 16.78 mm, respectively, and the composite focal lengths in the second cross section of the front group 11 and the rear group 12 are respectively. The focal lengths are 17.53 mm and 11.25 mm, respectively.

As in Example 1, Table 10 shows the position of the vertex of each optical surface of the optical system 10 according to this example, the direction of the normal line at the vertex, and the radius of curvature at each cross section. Table 11 shows each optical surface. Table 12 shows the aperture of the diaphragm 1, the aperture of the light shielding member 4, and the diameter of the light receiving surface 7. The reason why the values of the curvature radii _Ry do not match between Table 10 and Table 11 is that the values of the curvature radii in Table 10 take into account the tilt angle in the second cross section.

In the present embodiment, the sub-wire shapes of the first reflecting surface 2, the second reflecting surface 3, the third reflecting surface 5, and the fourth reflecting surface 6 are the following formulas instead of the above formula (Equation 3). It is expressed using Further, the shape of the sub-line of the third reflecting surface 5 is expressed by the above formula (Equation 2) after defining a different local coordinate system for each position on the bus line as in the second embodiment.
r ′ = r (1 + E ₂ y ² + E ₄ y ⁴ )

  FIG. 13 shows the MTF of the optical system 10 according to the present embodiment as in FIG. As can be seen from FIG. 13, the aberration is satisfactorily corrected over the entire reading region, and a sufficient depth of focus is ensured.

[Imaging system]
Hereinafter, an imaging apparatus (spectral reading apparatus) and imaging system (spectral reading system) as examples of use of the optical system 10 according to the above-described embodiment will be described.

  14 and 15 are schematic views of main parts of the imaging systems 100 and 200 according to the embodiment of the present invention. The imaging systems 100 and 200 are configured to change the relative positions of the imaging system 101 and 201 having an optical system 10 and an imaging device that receives an image formed by the optical system 10, and the imaging apparatus and the test objects 103 and 203. Sections 102 and 202. Each imaging system desirably has an image processing unit that generates an image based on image information obtained from the imaging element. The image processing unit is a processor such as a CPU, for example, and may be provided inside or outside each imaging apparatus.

  According to the imaging devices 101 and 201, a plurality of pieces of image information (one-dimensional images) corresponding to a plurality of wavelengths are obtained by imaging the linear reading regions 104 and 204 that are long in the first direction (Y direction) once. Can be obtained. At this time, it is desirable to configure each imaging device as a multispectral camera that can acquire image information corresponding to four or more types of wavelengths more than a general camera. Furthermore, it is more preferable to configure each imaging device as a hyperspectral camera that can acquire image information corresponding to 100 or more wavelengths.

  A CCD (Charge Coupled Device) sensor, a CMOS (Complementary Metal Oxide Semiconductor) sensor, or the like can be used as an imaging element in each imaging apparatus. The imaging device may be configured to photoelectrically convert not only visible light but also infrared light (near infrared light or far infrared light). Specifically, an image sensor using a material such as InGaAs or InAsSb may be employed according to the wavelength band used. In addition, the number of pixels of the image sensor is desirably determined based on the resolution required in the reading direction and the spectral direction.

  As illustrated in FIG. 14, the transport unit 102 in the imaging system 100 is a unit that moves the test object 103 in the second direction (Z direction). A belt conveyor or the like can be employed as the transport unit 102. On the other hand, as illustrated in FIG. 15, the conveyance unit 202 in the imaging system 200 is a unit that moves the imaging device 201 in the second direction. As the transport unit 202, a multicopter, an airplane, an artificial satellite, or the like can be employed. By using the transport unit 202, it is possible to perform imaging at a plurality of positions in the second direction even for a large test object that cannot be transported by a belt conveyor or the like that is difficult to move. .

  According to the imaging systems 100 and 200, a plurality of positions in the second direction can be obtained by causing each imaging device to sequentially image the reading area while changing the relative position of each imaging device and each test object in each transport unit. A plurality of pieces of image information corresponding to can be acquired. A two-dimensional image corresponding to a specific wavelength can be generated by performing rearrangement or arithmetic processing of the plurality of captured images by the image processing unit. In addition, since each image information represents the light and shade information in the first direction, the image processing unit may generate a spectrum distribution based on the light and shade information for each wavelength at a specific position in the second direction.

  In addition, you may comprise each conveyance part so that both each imaging device and each test object may be moved. Moreover, you may enable it to adjust the relative position in the optical axis direction (X direction) of each imaging device and each test object by each conveyance part. Alternatively, an optical member (focusing member) that can be driven inside or outside the optical system 10 is arranged, and the position of the optical member may be adjusted so that the object can be focused.

  The preferred embodiments and examples of the present invention have been described above, but the present invention is not limited to these embodiments and examples, and various combinations, modifications, and changes can be made within the scope of the gist.

4 light shielding member 5 third reflecting surface (diffraction surface)
6 Fourth reflective surface (anamorphic optical surface)
7 Light receiving surface (image surface)
10 optical system 11 front group 12 rear group

Claims (20)

An optical system composed of a front group, a light shielding member, and a rear group arranged in order from the object side to the image side,
The light shielding member is provided with a long opening in the first direction,
The front group does not image an object on the opening in a first cross section parallel to the first direction, and the object on the opening in a second cross section perpendicular to the first direction. An intermediate image of
The rear group condenses the light beams that have passed through the opening in the second cross section into a plurality of light beams having different wavelengths and condenses the light beams in different positions in the second cross section. An anamorphic optical surface,
In the second cross section, the curvature of one side of the anamorphic optical surface with respect to the optical axis decreases as it goes off-axis, and the curvature of the other side with respect to the optical axis increases as it goes off-axis. An optical system characterized by that.   In the second cross section, the curvature of the anamorphic optical surface on the same side as the vertex of the base surface of the diffractive surface with respect to the optical axis becomes smaller toward the off-axis, and is opposite to the vertex with respect to the optical axis. The optical system according to claim 1, wherein the curvature on the side increases as it goes off-axis.   3. The optical system according to claim 1, wherein in the second cross section, the curvature of the anamorphic optical surface changes so that a marginal ray of a light beam from the diffraction surface approaches a principal ray.   The optical system according to claim 1, wherein the anamorphic optical surface is a reflecting surface.   The optical system according to claim 1, wherein a base surface of the diffractive surface is an anamorphic surface.   6. The optical system according to claim 1, wherein a plurality of ridge lines in the diffraction grating of the diffraction surface are parallel to each other.   The optical system according to claim 6, wherein an interval between the plurality of ridge lines is constant.   The optical system according to claim 1, wherein the diffractive surface is a reflective surface.   The optical system according to claim 1, wherein all optical surfaces included in the front group and the rear group are reflection surfaces.   The optical system according to any one of claims 1 to 9, wherein the front group includes an anamorphic reflecting surface.   11. The optical system according to claim 1, wherein, in the first cross section, the front group has a negative power, and the rear group has a positive power.   The optical system according to claim 1, wherein the light shielding member regulates a width of the light beam from the object in the first direction.   The optical system according to any one of claims 1 to 12, wherein the front group includes a diaphragm that regulates a width of a light beam from the object in a second direction perpendicular to the first direction. system.   An imaging apparatus comprising: the optical system according to claim 1; and an imaging element that receives an image formed by the optical system.   The imaging apparatus according to claim 14, further comprising an image processing unit that generates an image based on image information obtained from the imaging element.   16. An imaging system comprising: the imaging apparatus according to claim 14; and a transport unit that changes a relative position between the imaging apparatus and the object.   The imaging system according to claim 16, wherein the transport unit moves the object in a direction perpendicular to the first direction.   The imaging system according to claim 16, wherein the transport unit moves the imaging device in a direction perpendicular to the first direction.   Image processing for generating an image of the object based on a plurality of pieces of image information acquired by causing the imaging device to perform imaging of the object while changing the relative positions of the imaging device and the object to the transport unit The imaging system according to claim 16, further comprising a unit.   The imaging system according to claim 19, wherein the image processing unit generates an image for each wavelength corresponding to the plurality of light beams.

Images