Optical system and image pickup apparatus

Abstract

[Problem] To provide an optical system that is compact and has high optical performance. [Solution] An optical system reflects light from an object side by a first reflection surface and further reflects the light by a second reflection surface to guide the light to an image side. The optical system includes a diffractive surface or metasurface having controlled wavelength dispersion characteristics. The abbe number of the diffraction surface or the super surface is satisfied under the following condition.

Description

Technical Field

[0001] The present invention relates to an optical system suitable for image acquisition devices such as digital cameras. Background Technology

[0002] Optical systems for compact image pickup devices are required to reduce the number of lenses in order to reduce the overall optical length. Patent Document 1 discloses an optical system that reduces the number of lenses by using planar lenses with diffractive surfaces and reduces the size by using two reflective surfaces to fold the optical path. Patent Document 2 discloses an optical system that uses two semi-transparent reflective surfaces to fold the optical path to reduce the optical length.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: U.S. Patent Application Publication No. 2022 / 0121016

[0006] Patent Document 2: Japanese Patent No. 6778823 Summary of the Invention

[0007] The problem the invention aims to solve

[0008] In the case of using a normal diffractive surface with uncontrolled wavelength dispersion characteristics, as in Patent Document 1, the Abbe number is -3.45, which corresponds to extremely high dispersion. Considering the chromatic aberration generated under such conditions, it is difficult to provide a large optical power (the reciprocal of the focal length) to the diffractive surface, and therefore there are limitations on further reducing the number of lenses and the optical length.

[0009] In Patent Document 2, since the semi-transparent reflective surface is configured as a planar surface for the purpose of reducing the back focal length of the lens, it is difficult to reduce the number of lenses in the optical system and achieve higher specifications.

[0010] The present invention aims to provide an optical system with reduced size and high optical performance by reducing the number of lenses and the total optical length.

[0011] Problem-solving methods

[0012] An optical device according to one aspect of the invention reflects light from the object side via a first reflective surface and further reflects the light via a second reflective surface to guide the light to the image side. The optical system includes a diffractive surface or metasurface having controlled wavelength dispersion characteristics. The Abbe number of the diffractive surface or the metasurface is such that the following condition is satisfied:

[0013] .

[0014] An image acquisition device having the above optical system also constitutes another aspect of the present invention.

[0015] The effects of the invention

[0016] This invention provides an optical system with reduced size and high optical performance by reducing the number of lenses and the total optical length. Attached Figure Description

[0017] Figure 1 It is a cross-sectional view of the optical system based on Example 1.

[0018] Figure 2 It is a longitudinal aberration diagram based on the optical system of Example 1.

[0019] Figure 3 It is a cross-sectional view of the optical system based on Example 2.

[0020] Figure 4 It is the longitudinal aberration diagram of the optical system based on Example 2.

[0021] Figure 5 It is a cross-sectional view of the optical system based on Example 3.

[0022] Figure 6 It is based on the longitudinal aberration diagram of the optical system in Example 3.

[0023] Figure 7 It is a cross-sectional view of the optical system based on Example 4.

[0024] Figure 8 It is the longitudinal aberration diagram of the optical system based on Example 4.

[0025] Figure 9 It is a cross-sectional view of the optical system based on Example 5.

[0026] Figure 10 It is based on the longitudinal aberration diagram of the optical system in Example 5.

[0027] Figure 11 It is a cross-sectional view of the optical system based on Example 6.

[0028] Figure 12 It is based on the longitudinal aberration diagram of the optical system in Example 6.

[0029] Figure 13 It is a cross-sectional view of the optical system based on Example 7.

[0030] Figure 14 It is a longitudinal aberration diagram based on the optical system in Example 7.

[0031] Figure 15It is a cross-sectional view of the optical system based on Example 8.

[0032] Figure 16 It is based on the longitudinal aberration diagram of the optical system in Example 8.

[0033] Figure 17 It is a cross-sectional view of the optical system based on Example 9.

[0034] Figure 18 It is a light spot diagram based on the optical system in Example 9.

[0035] Figure 19 The diagram illustrates the configuration using polarization.

[0036] Figure 20 The illustration shows an image pickup device including any one of the optical systems according to Examples 1 to 9. Detailed Implementation

[0037] In the following description, examples of the invention will be presented with reference to the accompanying drawings. Before describing examples 1 to 9, common aspects of each example will be explained.

[0038] Figure 1 , Figure 3 , Figure 5 , Figure 7 , Figure 9 , Figure 11 , Figure 13 , Figure 15 and Figure 17 The configurations of the optical systems according to Examples 1 through 9 are illustrated respectively. The optical systems according to each example are used as imaging optical systems in image acquisition devices such as digital cameras and film-based cameras, as well as in image acquisition devices installed in smartphones, tablets, etc. In each figure, the left side is the object side and the right side is the image side.

[0039] In each example, Lm represents a diffractive surface with controlled wavelength dispersion characteristics (hereinafter simply referred to as a "dispersion-controlled diffractive surface"). M1 represents a first reflecting surface, and M2 represents a second reflecting surface. CG represents a glass block corresponding to a cover glass, low-pass filter, IR cutoff filter, etc. IM represents the image plane of the optical system. On the image plane IM, the imaging surface (light-receiving surface) of an imaging element such as a CCD sensor or CMOS sensor, or the photosensitive surface of a silver halide film, is arranged.

[0040] The optical system in each example is configured to guide light from the object side toward the image side by reflecting light on a first reflecting surface M1 and further reflecting light on a second reflecting surface M2, and includes a dispersion-controlled diffraction surface Lm. Using the first and second reflecting surfaces M1 and M2 to fold the optical path reduces the overall optical length of the optical system. In such an optical system, the Abbe number of the dispersion-controlled diffraction surface Lm is... The following condition, expressed by inequality (1), is satisfied:

[0041] (1)

[0042] Abbe number of dispersion-controlled diffraction surface Lm It is defined as follows. Here, the reference wavelength is the d-line ( =0.58756μm), and the principal dispersion is caused by the F line ( = 0.48613μm) and C line ( = 0.65627μm) is defined. The optical path difference function at each wavelength is , and The optical path difference dispersion at various wavelengths on the surface is... , and In this case, the following equation is satisfied:

[0043]

[0044] The condition of equation (1) is a condition concerning the wavelength dispersion characteristics of the dispersion-controlled diffraction surface Lm (corresponding to the Abbe number of the refractive lens), and indicates... The appropriate range is needed to achieve both achromaticity of the entire optical system and a reduction in the number of lenses, thereby achieving high optical performance. Below the lower limit of inequality (1), the wavelength dispersion characteristics of the diffraction surface Lm become excessively high due to the negative Abbe number, thereby increasing the chromatic aberration generated by the diffraction surface Lm. As a result, the optical power of the diffraction surface Lm cannot be increased, making it difficult to reduce the size of the optical system and the number of lenses, which is undesirable. When the wavelength dispersion characteristics of the diffraction surface Lm become higher than the upper limit of inequality (1), the positive Abbe number becomes excessively high, thus similarly increasing the chromatic aberration generated by the diffraction surface Lm. Therefore, it is not desirable to increase the optical power of the diffraction surface Lm. When the dispersion-controlled diffraction surface Lm is realized by a metasurface, it is difficult to design a superstructure with high diffraction efficiency to obtain high dispersion characteristics corresponding to the positive Abbe number, which is also undesirable.

[0045] More preferably, the numerical range of inequality (1) can be set as follows:

[0046] (1a)

[0047] Most preferably, the numerical range of inequality (1) can be set as follows:

[0048] (1b)

[0049] Each example appropriately sets the wavelength dispersion characteristics of the diffraction surface in an optical system that includes a dispersion-controlled diffraction surface and two reflective surfaces, thus reducing the number of lenses, reducing the total optical length, and achieving an optical system with reduced size and high optical performance.

[0050] The optical system according to each example preferably satisfies at least one of the following configurations and inequalities (2) to (5).

[0051] In each example, at least one of the first and second reflecting surfaces M1 and M2 is preferably a concave mirror with positive optical power. This configuration, by using a reflecting surface that does not generate chromatic aberration with a dispersion-controlled diffraction surface Lm, can suppress chromatic aberration throughout the optical system and reduce the size of the optical system. Specifically, the dispersion-controlled diffraction surface Lm is set to zero dispersion (…). In the case of a reflective surface, the optical system can be configured to produce virtually no chromatic aberration.

[0052] In each example, let fm be the focal length of the dispersion-controlled diffraction surface Lm, and let f be the focal length of the optical system. Thus, the following inequality (2) is preferably satisfied. In the case of providing multiple diffraction surfaces Lm, the focal length fm of the diffraction surface is the focal length of the diffraction surface with the strongest optical power.

[0053] (2)

[0054] Here, let U2 be the second-order coefficient of the optical path difference function of the surface at the wavelength (design wavelength). Then, the focal length fm of the dispersion-controlled diffraction surface Lm is calculated by the following equation (2).

[0055]

[0056] The conditions of Equation (2) constrain the appropriate focal length allocation of the dispersion-controlled diffractive surface Lm in the optical system. When f / |fm| becomes higher than the upper limit of Equation (2), the focal length of the diffractive surface Lm becomes too short relative to the focal length of the optical system, large monochromatic aberrations (such as spherical aberration) increase, and it becomes difficult to achieve high optical performance. This is not desirable. When f / |fm| becomes lower than the lower limit of Equation (2), the focal length of the diffractive surface Lm becomes too large relative to the focal length of the optical system, the total optical length of the optical system increases, and it becomes difficult to reduce the size of the optical system, which is also not desirable.

[0057] More preferably, the numerical range of inequality (2) is set as follows:

[0058] (2a)

[0059] Most preferably, the numerical range of inequality (2) is set as follows:

[0060] (2b)

[0061] In each example, let fm be the focal length of the dispersion-controlled diffraction surface Lm, and let fr be the focal length of the reflecting surface with the strongest optical power among the first and second reflecting surfaces M1 and M2. Thus, the following inequality (3) is preferably satisfied:

[0062] (3)

[0063] The condition of inequality (3) limits the appropriate power distribution between the dispersion-controlled diffractive surface Lm and the reflecting surface with the strongest optical power. When |fm / fr| becomes higher than the upper limit of inequality (3), the optical power of the diffractive surface Lm becomes too strong relative to the optical power of the reflecting surface, generating large monochromatic aberrations (such as spherical aberration) on the diffractive surface Lm, and making it difficult to obtain high optical performance. This is not desirable.

[0064] More preferably, the numerical range of inequality (3) is set as follows:

[0065] (3a)

[0066] More preferably, the numerical range of inequality (3) is set as follows:

[0067] (3b)

[0068] In each example, the dispersion-controlled diffraction surface Lm is preferably deployed on the object side of the first reflecting surface M1 (closer to the object than the first reflecting surface M1). In this case, let D1 be the effective optical radius of the first reflecting surface M1, let f be the focal length of the optical system, and let Fno be the full aperture ratio of the optical system. Thus, the following inequality (4) is preferably satisfied. The effective optical radius is the radius (mm) of the area on the image plane through which the light rays contributing to imaging pass. The full aperture ratio is the aperture ratio (F number) when the aperture stop is fully open.

[0069] (4)

[0070] Inequality (4) defines the appropriate relationship between the entrance pupil diameter and the effective optical radius of the first reflecting surface M1. Deploying the dispersion-controlled diffraction surface Lm on the object side of the first reflecting surface M1 (closer to the object than the first reflecting surface M1) to converge the incident beam onto the first reflecting surface M1 reduces the effective optical radius of the first reflecting surface M1 and the size of the optical system. This configuration is particularly advantageous in telescope optical systems with large entrance pupil diameters and large aperture ratios. When D1 / (f / Fno) is below the lower limit of inequality (4), the effective optical radius of the first reflecting surface M1 becomes too small relative to the entrance pupil diameter, requiring the diffraction surface Lm to have excessive optical power and increasing monochromatic aberrations (such as spherical aberration), which is undesirable. When D1 / (f / Fno) becomes above the upper limit of inequality (4), the effective optical radius of the first reflecting surface M1 becomes too large, increasing the size of the optical system, which is also undesirable.

[0071] More preferably, the numerical range of inequality (4) is set as follows:

[0072] (4a)

[0073] Most preferably, the numerical range of inequality (4) is set as follows:

[0074] (4b)

[0075] In each example, let OAL be the total optical length of the optical system, and let f be the focal length of the optical system. Thus, the following inequality (5) is preferably satisfied. The total optical length OAL is the distance along the optical axis from the lens surface closest to the object (the foremost surface) to the image plane. The portion corresponding to a glass block such as a cover glass CG is converted to an air equivalent length.

[0076] (5)

[0077] The condition of inequality (5) limits the appropriate telephoto ratio of the optical system. When OAL / f becomes below the lower limit of inequality (5), the total optical length becomes too short relative to the focal length of the optical system. As a result, even when the light path is folded through the first and second reflecting surfaces M1 and M2, the dispersion-controlled diffraction surface Lm and each reflecting surface bear a strong optical power. Therefore, it becomes difficult to correct monochromatic aberrations (such as spherical aberration) and field curvature. When OAL / f becomes above the upper limit of inequality (5), the total optical length becomes too long relative to the focal length, and the size of the optical system increases, which is undesirable.

[0078] More preferably, the numerical range of inequality (5) is set as follows:

[0079] (5a)

[0080] More preferably, the numerical range of inequality (5) is set as follows:

[0081] (5b)

[0082] As in each example, the dispersion-controlled diffraction surface Lm preferably has positive optical power. In the case of providing multiple diffraction surfaces Lm, at least one diffraction surface Lm can have positive optical power. By providing positive optical power, the diffraction surface Lm shares a portion of the positive optical power of the optical system, and the size of the optical system can be reduced. In particular, by setting the diffraction surface Lm with positive optical power to low dispersion, the traditional configuration requiring two lenses (a positive lens and a negative lens) for achromatic purposes can be replaced by a single diffraction surface Lm.

[0083] As in Examples 1 to 7, both the first and second reflective surfaces M1 and M2 can be semi-transparent reflective surfaces. In this case, preferably, light from the object can pass through the second reflective surface M2, be reflected by the first reflective surface M1, be reflected by the second reflective surface M2, and then pass through the first reflective surface M1 to be guided to the image plane. Using two semi-transparent reflective surfaces in this way to fold the optical path can reduce the total optical length. The polarization utilization configuration described later can prevent unwanted light that is not reflected by either of the two semi-transparent reflective surfaces from reaching the image plane.

[0084] As in Example 8, the first reflective surface M1 may have a non-reflective portion (e.g., an aperture through which light passes). In this case, preferably, light from the object can be reflected by the reflective portion of the first reflective surface M1, reflected by the second reflective surface M2, and passed through or through the non-reflective portion of the first reflective surface M1 to be guided to the image plane. Using two reflective surfaces in this way to fold the optical path can reduce the total optical length.

[0085] As in each example, at least one refractive lens can be deployed in the optical system. By providing a refractive lens, monochromatic aberrations that cannot be fully corrected by the dispersion-controlled diffraction surface Lm and the first and second reflecting surfaces M1 and M2 can be corrected, and high optical performance can be achieved. The Petzval term generated by the dispersion-controlled diffraction surface Lm is zero. Therefore, when optical power is assigned to the reflecting surfaces to reduce the size of the optical system, the Petzval term generated by the reflecting surfaces can be compensated by the Petzval term generated by the refractive lens, and the image field curvature can be properly corrected.

[0086] As in Examples 2 through 9, the optical system may include at least one positive refractive lens and one negative refractive lens. By including both positive and negative refractive lenses, monochromatic aberrations and chromatic aberrations that cannot be fully corrected by the diffraction surface Lm and the first and second reflecting surfaces M1 and M2 can be corrected, and high optical performance can be achieved.

[0087] In the case where the optical system includes positive and negative refractive lenses, let Let be the optical power of the i-th refractive lens counted from the object side, and let be the optical power of the i-th refractive lens. Let be the Abbe number of the i-th refractive lens based on the d-line, and let It is for all refractive lenses. The sum of these, and let f be the focal length of the optical system. Thus, preferably, the following inequality (6) is satisfied. Based on the Abbe number of the d-line. Defined as Nd, NF, and NC are the refractive indices at wavelengths of the d-line (587.6 nm), F-line (486.1 nm), and C-line (656.3 nm) in the Fraunhofer lines, respectively.

[0088] (6)

[0089] The condition of inequality (6) defines the appropriate achromatic state of the refractive lenses included in the optical system. Since the first and second reflecting surfaces M1 and M2 do not generate chromatic aberration, high optical performance can be achieved by satisfactorily correcting the chromatic aberration of the optical system to zero or low dispersion by setting the dispersion characteristics of the dispersion-controlled diffraction surface Lm to zero dispersion or low dispersion and by appropriately arranging refractive lenses suitable for achromatic purposes. If the chromatic aberration becomes higher than the upper limit of inequality (6), the chromatic aberration generated by the refracting lens becomes too large and leaves an undesirable chromatic aberration.

[0090] More preferably, the numerical range of inequality (6) is set as follows:

[0091] (6a)

[0092] Most preferably, the numerical range of inequality (6) is set as follows:

[0093] (6b)

[0094] Polarization Utilization Configuration

[0095] Figure 19 The illustrations depict polarization-based optical configurations in optical systems according to Examples 1 through 7. In these examples, both the first and second reflective surfaces M1 and M2 are semi-transmissive reflective surfaces, and at least one of them is configured with a polarization-selective semi-transmissive reflective element. Figure 19 In this configuration, the first reflective surface M1 is a semi-reflective mirror (HM) C, and the second reflective surface M2, deployed on the object OBJ side of the first reflective surface M1, is a polarization-selective semi-transmissive reflective element A. The semi-reflective mirror C is formed from dielectric multilayer films, metal deposition, etc., and acts as a semi-transmissive reflective surface in the wavelength region of the light to be imaged. Examples of polarization-selective semi-transmissive reflective elements A include wire grid elements such as WGF (registered trademark) manufactured by Asahi Kasei Corporation, reflective linear polarizers such as IQP-E manufactured by 3M Corporation, and circularly polarized light reflective elements using cholesteric liquid crystals. Figure 19 In the configuration, polarization-selective semi-transmissive reflective element A transmits linearly polarized light with a polarization direction parallel to the plane of the paper and reflects linearly polarized light with a polarization direction perpendicular to the plane of the paper.

[0096] A first quarter-wave plate B is positioned between two semi-transmissive reflective surfaces (A and C). The first quarter-wave plate B is positioned such that its slow axis is tilted at 45° relative to the polarization transmission axis of the polarization-selective semi-transmissive reflective element A. When using a circularly polarized light reflective element as the polarization-selective semi-transmissive reflective element A, the first quarter-wave plate B can be omitted.

[0097] exist Figure 19 In this configuration, the second quarter-wave plate D is deployed on the image side of the semi-transparent mirror C. The second quarter-wave plate D is deployed such that its slow axis is tilted at 45° relative to the polarization transmission axis of the polarization-selective semi-transparent reflective element A. The first quarter-wave plate B and the second quarter-wave plate D are preferably arranged such that their slow axes are tilted at 90° relative to each other. Due to this arrangement, when light passes through the first quarter-wave plate B and the second quarter-wave plate D, the wavelength dispersion characteristics of the quarter-wave plates cancel each other out.

[0098] exist Figure 19In this configuration, an absorptive linear polarizer E is deployed on the image side of the second quarter-wave plate D. With the slow axes of the first and second quarter-wave plates B and D oriented as described above, the polarization transmission axis of the absorptive linear polarizer E extends in a direction perpendicular to the plane of the paper.

[0099] Although each of the above elements is omitted in the figures of Examples 1 to 7, the method of placing each element may include bonding the film element to the optical surface of the lens, molding the wire grid structure integrally with the lens substrate during the molding of the resin lens, or another method.

[0100] The above polarization configuration can suppress the reduction in the amount of light following the normal optical path from object OBJ to image plane IM, and reduce unwanted light (ghost light or stray light) that does not reach image plane IM after being reflected by either of the two semi-transparent reflective surfaces (A and C).

[0101] The optical path of the normal imaging light in the above-described polarization utilization configuration will be described. Imaging light, incident from object OBJ as unpolarized light, passes through polarization-selective semi-transmissive reflective element A and is converted into linearly polarized light with a polarization direction parallel to the plane of the paper. The linearly polarized light is converted into circularly polarized light by a first quarter-wave plate B, and the circularly polarized light enters the semi-reflective mirror C.

[0102] A portion of the circularly polarized light incident on the semi-reflecting mirror C is converted into linearly polarized light with a polarization direction parallel to the plane of the paper by the second quarter-wave plate D. This linearly polarized light is absorbed by the absorptive linear polarizer E. Conversely, the remaining portion of the circularly polarized light incident on the semi-reflecting mirror C is reflected by the mirror and converted into circularly polarized light rotating in the opposite direction. This rotating circularly polarized light returns to the second quarter-wave plate D and is converted into linearly polarized light with a polarization direction perpendicular to the plane of the paper. This linearly polarized light enters the polarization-selective semi-transmissive reflective element A and is selectively reflected by the polarization of the element.

[0103] Linearly polarized light reflected by polarization-selective semi-transmissive reflective element A is converted by a first quarter-wave plate B into circularly polarized light with a rotation direction opposite to that of the initially circularly polarized light generated by the first quarter-wave plate B, and the circularly polarized light enters the semi-reflective mirror C. The circularly polarized light that has passed through the semi-reflective mirror C is converted by a second quarter-wave plate D into linearly polarized light with a polarization direction perpendicular to the plane of the paper. The linearly polarized light passes through an absorptive linear polarizer E and reaches the image plane IM.

[0104] Therefore, normal imaging light is reflected by the semi-reflective mirror C, reflected by the polarization-selective semi-transmissive reflective element A, and guided to the image plane IM through the semi-reflective mirror C. When using a reflective linear polarizer as the polarization-selective semi-transmissive reflective element A, an absorptive polarizer having a polarization transmission axis in the same direction as the polarization transmission axis of the reflective linear polarizer A is preferably deployed on the object side of the polarization-selective semi-transmissive reflective element A (closer to the object than the polarization-selective semi-transmissive reflective element A). In this configuration, the linearly polarized light component in the polarization direction perpendicular to the paper plane, which is incident on the polarization-selective semi-transmissive reflective element A from the object OBJ and reflected by the reflective linear polarizer A, can be absorbed by the absorptive polarizer.

[0105] The optical system, according to each example, may further include a focusing function using a known configuration and an image stabilization function for correcting image blur caused by camera shake, etc. The focusing function is achieved, for example, by moving the entire or a portion of the optical system or the image sensor along the optical axis. The image stabilization function is achieved, for example, by eccentrically moving the entire or a portion of the optical system or the image sensor relative to the optical axis. Both the focusing and image stabilization functions can be implemented using optical elements whose refractive power is variable through mechanical or electrical action (such as shape-variable lenses using pressure or electrowetting, or liquid crystal lenses).

[0106] In the case where an image of an object formed by an optical system according to each example is photoelectrically converted (captured) by an image sensor to generate image data, various aberrations such as distortion aberrations can be electronically corrected by image processing of the image data.

[0107] The optical path difference function equivalent to the optical path difference function of the diffraction surface Lm can be realized by a superlens with a so-called single-layer metasurface consisting of one layer, or by a so-called multilayer metasurface consisting of multiple layers.

[0108] Example 1

[0109] Figure 1 The optical system illustrated in the figure according to Example 1 (numerical example 1) is a medium telescope optical system with a diagonal half-angle of approximately 10° and an aperture ratio of approximately 2.

[0110] The optical system includes a dispersion-controlled diffractive surface Lm positioned closest to the object, and a second reflective surface M2 and a first reflective surface M1 positioned sequentially toward the image side. The first and second reflective surfaces M1 and M2 are used to fold the optical path, reducing the overall optical length. Providing positive refractive force to the dispersion-controlled diffractive surface Lm reduces the effective optical radius of the first reflective surface M1, and configuring the second reflective surface M2 as a concave mirror with positive optical power reduces the size of the optical system. High optical performance is achieved by deploying refractive lenses with aspherical surfaces in the optical system and by appropriately distributing monochromatic aberration correction among the aspherical surfaces, the diffractive surface Lm, and the first and second reflective surfaces M1 and M2.

[0111] Example 2

[0112] Figure 3 The optical system illustrated in the figure according to Example 2 (numerical example 2) has a similar basic configuration and optical specifications to Example 1, but differs in the wavelength dispersion characteristics and optical path difference function of the diffraction surface Lm, as well as the shapes of the first and second reflecting surfaces M1 and M2 and the refractive lens.

[0113] Example 2 achieves achromaticity in the refractive system by placing a positive refractive lens and a negative refractive lens in the optical system, and achieves achromaticity in the entire optical system by combining the zero-dispersion diffraction surface Lm with the first and second reflective surfaces M1 and M2.

[0114] Example 3

[0115] Figure 5 The optical system illustrated in the figure according to Example 3 (numerical example 3) is a large-aperture standard optical system with a diagonal half-angle of approximately 23° and an aperture ratio of approximately 0.8.

[0116] The basic configuration of the optical system in this example is similar to that in Example 2, but it differs from Example 2 in optical specifications, wavelength dispersion characteristics and optical path difference function of the diffraction surface Lm, and the shapes of the first and second reflective surfaces M1 and M2 and the refractive lens.

[0117] Example 4

[0118] Figure 7 The optical system illustrated in the figure according to Example 4 (numerical example 4) is a large-aperture wide-angle optical system with a diagonal half-angle of approximately 35° and an aperture ratio of approximately 0.8.

[0119] The basic configuration of the optical system in this example is similar to that in Example 2, but it differs from Example 2 in optical specifications, wavelength dispersion characteristics and optical path difference function of the diffraction surface Lm, and the shapes of the first and second reflective surfaces M1 and M2 and the refractive lens.

[0120] Example 5

[0121] Figure 9 The optical system illustrated in the figure according to Example 5 (numerical example 5) has similar optical specifications to Example 4, but the difference is that a dispersion-controlled diffraction surface Lm is deployed between the first and second reflective surfaces M1 and M2, and the wavelength dispersion characteristics of the diffraction surface Lm are different from those according to Example 4.

[0122] In this example, the imaging light passes through the diffraction surface Lm three times. Therefore, the characteristics of the diffraction surface Lm, which has zero petzvar and controllable wavelength dispersion, can be utilized more effectively.

[0123] Example 6

[0124] Figure 11 The optical system illustrated in the figure according to Example 6 (numerical example 6) has optical specifications similar to those according to Example 4, but the difference is that the dispersion-controlled diffraction surface Lm is deployed on the image side of the first and second reflective surfaces M1 and M2, and the wavelength dispersion characteristics of the diffraction surface Lm are different from those of Example 4.

[0125] This example places the diffraction surface Lm at a height high on the principal paraxial ray and allows selective control of lateral chromatic aberration and exit pupil position without significantly affecting longitudinal chromatic aberration or spherical aberration.

[0126] Example 7

[0127] Figure 13 The optical system illustrated in the figure according to Example 7 (numerical example 7) is a supertelescope optical system with a diagonal half-angle of approximately 2.5° and an aperture ratio of approximately 2.8. The basic configuration of the optical system according to this example and the wavelength dispersion characteristics of the diffraction surface Lm are similar to those in Example 2. In this example, the diffraction surface Lm is deployed on the object side of the first and second reflective surfaces M1 and M2, and the optical specifications of the optical system, the optical path difference function of the diffraction surface Lm, and the shapes of the first and second reflective surfaces M1 and M2 and the refractive lens differ from those in Example 2.

[0128] Example 8

[0129] Figure 15 The optical system illustrated in the figure according to Example 8 (numerical example 8) is a supertelescope optical system with a diagonal half-angle of approximately 2.5° and an aperture ratio of approximately 5.6. The basic configuration of the optical system according to this example and the wavelength dispersion characteristics of the diffraction surface Lm are similar to those in Example 7. This example uses a so-called catadioptric configuration in which the optical path is folded by a combination of a first reflecting surface M1 having a non-reflective portion (aperture portion) and a second reflecting surface M2, thereby reducing the total optical length.

[0130] Example 9

[0131] Figure 17 The optical system illustrated in the middle, according to Example 9, is a medium telescope optical system with a half-angle viewing angle of approximately 7° and an aperture ratio of approximately 4.

[0132] In the optical system according to this example, the arrangement of the diffraction surface Lm and the wavelength dispersion characteristics are similar to those in Example 7.

[0133] In this example, the first reflecting surface M1 and the second reflecting surface M2 are freeform (aspherical) surfaces and form an optical path along an eccentric optical axis, thereby reducing the overall volume of the optical system.

[0134] In the coaxial folding configurations according to Examples 1 to 7, employing the aforementioned polarization utilization configuration, the reduction in T-number is approximately third-order, and in the catadioptric configuration according to Example 8, it is approximately first-order. In the freeform mirror eccentric configuration according to Example 9, no reduction in T-number occurs. Although the coaxial folding configuration utilizing polarization results in a relatively large reduction in T-number, it enables a folded optical path configuration that reduces the total optical length over a wide range from the wide-angle end to the mid-telephoto end, and is advantageous in achieving large apertures, particularly in the wide-angle to mid-telephoto range. In the catadioptric configuration, problems such as ring blurring caused by pupil shading and shading of imaging light in wide-angle to standard optical systems occur; however, in mid-telephoto optical systems, a good balance can be achieved between the reduction in total optical length and the reduction in T-number. The freeform mirror eccentric configuration is advantageous in suppressing the reduction in T-number, but the reflective surface is difficult to manufacture, and the effect of reducing the total optical length is not as good as that of the coaxial folding configuration.

[0135] Taking these factors into account and selecting one of the three optical configurations above based on the required optical specifications can provide an optical system with reduced size and high optical performance.

[0136] Numerical examples 1 through 9 will be described below. In each numerical example, surface number i indicates the order of the surfaces counted from the object side. r represents the radius of curvature (mm) of the i-th surface counted from the object side, and d represents the on-axis lens thickness or air gap (mm) between the i-th and (i+1)-th surfaces. nd represents the refractive index of the optical material between the i-th and (i+1)-th surfaces with respect to the d-line. As defined above, The Abbe number based on the d-line represents the optical material between the i-th and (i+1)-th surfaces. The effective diameter (mm) corresponds to the above optical effective diameter and indicates the radius (mm) of the area on the i-th surface through which the light rays contributing to imaging pass.

[0137] BF represents the back focal length (mm). The back focal length is defined as the distance along the optical axis from the surface of the optical system closest to the image plane (the rearmost surface) to the paraxial image plane, expressed as the equivalent length in air. The total lens length corresponds to the total optical length above and is defined as the length obtained by adding the back focal lengths to the distance along the optical axis from the frontmost surface of the optical system to the rearmost surface of the optical system.

[0138] An asterisk (*) attached to the surface number indicates that the surface has an aspherical shape. Let x be the displacement from the surface vertex along the optical axis, let h be the height from the optical axis in a direction orthogonal to the optical axis (positive for the direction of light propagation), let R be the paraxial radius of curvature, let k be the conic constant, and let A4 to A10 be the aspherical coefficients. The shape of the aspherical surface is expressed by the following equation. The "e±M" for the conic constant and aspherical coefficients signifies ×10-10. ±M .

[0139] Let U2 to U10 be the coefficients of the surface's optical path difference function. Then, the surface's optical path difference function at the design wavelength is expressed by the following equation:

[0140]

[0141] The “(Diffraction)” appended to the surface number indicates an optically designed surface that uses the optical path difference function.

[0142] Table 1 summarizes the values ​​of inequalities (1) to (6) for numerical examples 1 to 9. As can be understood from Table 1, the optical systems according to numerical examples 1 to 8 satisfy the conditions of inequalities (1) to (6). The optical system according to numerical example 9 satisfies the conditions of inequalities (1), (2) and (6).

[0143] Figure 2 , Figure 4 , Figure 6 , Figure 8 , Figure 10 , Figure 12 , Figure 14 and Figure 16 The diagrams illustrate longitudinal aberration maps (spherical aberration, astigmatism, distortion, and lateral chromatic aberration) of the optical systems of Examples 1 to 8 corresponding to the numerical examples 1 to 8, under the focusing state on an object at infinity (hereinafter referred to as the infinity focusing state). In the spherical aberration map, the vertical axis Fno indicates the F-number, and in the astigmatism, distortion, and chromatic aberration maps, the vertical axis... Indicates the half field of view (°). The horizontal axis represents each aberration.

[0144] In the spherical aberration diagram, the solid line represents the spherical aberration for the d-line (wavelength 587.6 nm), and the alternating long and two short dashed lines represent the spherical aberration for the g-line (wavelength 435.8 nm). In the astigmatism diagram, the solid line S represents the astigmatism on the sagittal image plane, and the dashed line M represents the astigmatism on the meridional image plane. The distortion diagram represents the distortion for the d-line. The lateral chromatic aberration diagram represents the lateral chromatic aberration for the g-line.

[0145] Figure 18 It is a spot pattern for the d-line and g-line based on the optical system of Example 9 corresponding to Example 9 in the infinity focusing state.

[0146]

[0147]

[0148]

[0149]

[0150]

[0151]

[0152]

[0153]

[0154]

[0155]

[0156]

[0157]

[0158]

[0159]

[0160]

[0161]

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[0173]

[0174]

[0175] Table 1

[0176]

[0177]

[0178] Image pickup device

[0179] Figure 20 The illustration depicts a digital still camera as an image-capturing device using an optical system according to any of the examples above as an imaging optical system. Reference numeral 20 denotes the camera body, and reference numeral 21 denotes an imaging optical system including any of the optical systems according to Examples 1 to 9. Reference numeral 22 denotes an image sensor, such as a CCD sensor or a CMOS sensor, which is integrated into the camera body 20 and captures the optical image (i.e., object image) formed by the imaging optical system 21. Reference numeral 23 denotes a recorder that records image data generated by processing the imaging signal output from the image sensor 22, and reference numeral 24 denotes a rear display unit that displays the image data.

[0180] The optical system for each example can provide a camera with reduced size and high optical performance. The camera can be a single-lens reflective camera with a fast-rotating mirror, or it can be a mirrorless camera without a fast-rotating mirror.

[0181] While preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications and changes can be made within the scope of the spirit of the present invention.

Claims

1. An optical system that reflects light from an object side via a first reflective surface and further reflects the light via a second reflective surface to guide the light to an image side. The optical system includes a diffractive surface with controlled wavelength dispersion characteristics. Where the following equation is satisfied: ， in The Abbe number of the diffraction surface is given, the reference wavelength is the d-line, and the principal dispersions are the F-line and C-line. , ,as well as It is the optical path difference function at the d-line, F-line, and C-line, and , ,as well as The surface optical path difference dispersion at the d-line, F-line, and C-line is subject to the following condition: 。 2. The optical system of claim 1, wherein at least one of the first reflective surface and the second reflective surface comprises a concave mirror and has positive optical power.

3. The optical system according to claim 1 or 2, wherein the following condition is satisfied: ， Where f is the focal length of the optical system and fm is the focal length of the diffraction surface.

4. The optical system according to any one of claims 1 to 3, wherein the following condition is satisfied: ， Where fm is the focal length of the diffractive surface, and fr is the focal length of the reflective surface with the strongest optical power among the first and second reflective surfaces.

5. The optical system according to any one of claims 1 to 4, wherein the diffractive surface is deployed closer to the object than the first reflective surface, and The following conditions are met: ， Where D1 is the optically effective diameter of the first reflective surface, f is the focal length of the optical system, and Fno is the total aperture ratio of the optical system.

6. The optical system according to any one of claims 1 to 5, wherein the following condition is satisfied: ， Where OAL is the total optical length of the optical system, and f is the focal length of the optical system.

7. The optical system according to any one of claims 1 to 6, wherein the diffractive surface has positive optical power.

8. The optical system according to any one of claims 1 to 7, wherein both the first reflective surface and the second reflective surface are semi-transmissive reflective surfaces, and The light from the object side is directed to the image side such that the light passes through the first reflective surface, is reflected by the second reflective surface, is reflected again by the first reflective surface, and passes through the second reflective surface.

9. The optical system according to any one of claims 1 to 7, wherein the first reflective surface has a non-reflective portion that allows the light to pass through, and The light from the object side is directed to the image side such that the light is reflected by the first reflective surface, reflected by the second reflective surface, and passes through the non-reflective portion.

10. The optical system according to any one of claims 1 to 9, wherein the optical system comprises at least one refractive lens.

11. The optical system of claim 10, wherein the refractive lens comprises a positive lens and a negative lens.

12. The optical system according to any one of claims 1 to 11, wherein the following condition is satisfied: ， Among all the refractive lenses included in the optical system It is the optical power of the i-th refractive lens, counted from the object side. It is the Abbe number based on the d-line of the i-th refractive lens. It is for all refractive lenses. The sum of these two values, where f is the focal length of the optical system.

13. An optical system that reflects light from an object side via a first reflective surface and further reflects the light via a second reflective surface to guide the light to an image side. The optical system described herein includes a metasurface with controlled wavelength dispersion characteristics. The following conditions must be met: ， in The Abbe number of the metasurface is given, the reference wavelength is the d-line, and the principal dispersions are the F-line and C-line. , ,as well as It is the optical path difference function at the d-line, F-line, and C-line, and , ,as well as The surface optical path difference dispersion at the d-line, F-line, and C-line is subject to the following condition: 。 14. An image acquisition device, comprising: The optical system according to any one of claims 1 to 13; as well as An image sensor configured to capture an object through the optical system.

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