Patent Publication Number: US-2022239850-A1

Title: Imaging device, imaging optical system, and imaging method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a Continuation of PCT International Application No. PCT/JP2020/040065 filed on Oct. 26, 2020 claiming priority under 35 U.S.C § 119(a) to Japanese Patent Application No. 2019-197778 filed on Oct. 30, 2019. Each of the above applications is hereby expressly incorporated by reference, in its entirety, into the present application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an imaging device, an imaging optical system, and an imaging method, and particularly relates to a technique for capturing an image of a plurality of wavelength ranges. 
     2. Description of the Related Art 
     In the related arts, a multi-band camera disclosed in JP2004-157059A has been proposed as this type of imaging device. 
     This multi-band camera includes an imaging optical system having an imaging lens system and a spectroscopic filter plate. The spectroscopic filter plate is inserted in an optical path passing through the imaging lens system, and a plurality of spectral regions which transmit light in a plurality of wavelength ranges different from each other have spectroscopic filters which are arranged substantially concentrically about an optical axis of the imaging lens system. 
     The multi-band camera captures a plurality of spectroscopic images of a subject in a surface-sequential manner by switching between a light-transmitting state and a light-blocking state in the plurality of spectroscopic regions of the spectroscopic filter plate. 
     In addition, imagery misregistration characteristics of chromatic aberration of the imaging lens system with respect to each of the plurality of wavelength ranges are adjusted so as to be substantially canceled by spherical aberration of the imaging lens system and the spectroscopic filter. 
     SUMMARY OF THE INVENTION 
     One embodiment according to a technique of the present disclosure provides an imaging device which can simultaneously capture images in different wavelength ranges and can improve a focusing accuracy of each image, an imaging optical system, and an imaging method. 
     An imaging device according to one aspect of the present invention includes an imaging optical system which has a first pupil region for passing light in a first wavelength range and a second pupil region for passing light in a second wavelength range different from the first wavelength range, in which an axial chromatic aberration of the imaging optical system due to a difference between the first wavelength range and the second wavelength range is reduced based on a relationship between an aberration other than the axial chromatic aberration of the imaging optical system and positions of the first pupil region and the second pupil region in the imaging optical system, an imaging element which includes a first pixel receiving the light passing through the first pupil region in the imaging optical system and a second pixel receiving the light passing through the second pupil region in the imaging optical system, and a signal processing unit which processes a signal output from the imaging element, and generates each of a first image of the first wavelength range and a second image of the second wavelength range based on an output signal of the first pixel and an output signal of the second pixel. 
     In the imaging device according to another aspect of the present invention, the aberration other than the axial chromatic aberration of the imaging optical system is a spherical aberration. 
     In the imaging device according to another aspect of the present invention, it is preferable that, in a case where the spherical aberration is negative, the first pupil region is a pupil region closer to an optical axis of the imaging optical system than the second pupil region, and a representative wavelength of the first wavelength range is shorter than a representative wavelength of the second wavelength range. 
     In the imaging device according to another aspect of the present invention, it is preferable that the first pupil region is a circular or annular pupil region divided by a concentric circle centered on the optical axis of the imaging optical system, and the second pupil region is an annular pupil region outside the first pupil region. 
     In the imaging device according to still another aspect of the present invention, the aberration other than the axial chromatic aberration of the imaging optical system is a coma aberration. 
     In the imaging device according to still another aspect of the present invention, it is preferable that an image-forming position of the light passing through the first pupil region on an optical axis of the imaging optical system is moved to an image side with respect to an image-forming position of the light passing through the second pupil region on the optical axis due to the coma aberration of the imaging optical system, and a representative wavelength of the first wavelength range is shorter than a representative wavelength of the second wavelength range. 
     In the imaging device according to still another aspect of the present invention, it is preferable that the imaging optical system has the coma aberration by which, among a first region and a second region which are located, with the optical axis in between, opposite to each other through a straight line intersecting the optical axis of the imaging optical system, an image-forming position of light incident on the first region on the optical axis is moved to the image side and an image-forming position of light incident on the second region on the optical axis is moved to an object side, and the first pupil region is a pupil region corresponding to the first region in the imaging optical system, and the second pupil region is a pupil region corresponding to the second region in the imaging optical system. 
     In the imaging device according to still another aspect of the present invention, the aberration other than the axial chromatic aberration of the imaging optical system is an astigmatism. 
     In the imaging device according to still another aspect of the present invention, it is preferable that an image-forming position of the light passing through the first pupil region on an optical axis of the imaging optical system is moved to an image side with respect to an image-forming position of the light passing through the second pupil region on the optical axis due to the astigmatism of the imaging optical system, and a representative wavelength of the first wavelength range is shorter than a representative wavelength of the second wavelength range. 
     In the imaging device according to still another aspect of the present invention, it is preferable that the imaging optical system has the astigmatism by which an image-forming position of light incident on a first region symmetrical to the optical axis of the imaging optical system on the optical axis is moved to the image side, and an image-forming position of light incident on a second region which is symmetrical to the optical axis and orthogonal to the first region on the optical axis is moved to an object side, and the first pupil region is a pupil region corresponding to the first region in the imaging optical system, and the second pupil region is a pupil region corresponding to the second region in the imaging optical system. 
     In the imaging device according to still another aspect of the present invention, it is preferable that the imaging optical system includes a wavelength selective filter unit having a first wavelength selective filter which allows the light in the first wavelength range to pass through the first pupil region, and a second wavelength selective filter which allows the light in the second wavelength range to pass through the second pupil region. 
     In the imaging device according to still another aspect of the present invention, it is preferable that the imaging optical system further has a third pupil region for passing light in a third wavelength range intermediate between the first wavelength range and the second wavelength range, the third pupil region is a pupil region between the first pupil region and the second pupil region, the imaging element further includes a third pixel receiving the light passing through the third pupil region, and the signal processing unit further generates a third image of the third wavelength range based on an output signal of the third pixel. 
     In the imaging device according to still another aspect of the present invention, it is preferable that the imaging optical system includes a first polarization filter polarizing the light passing through the first pupil region in a first direction, and a second polarization filter polarizing the light passing through the second pupil region in a second direction different from the first direction, and a third polarization filter polarizing incident light in the first direction is provided in the first pixel of the imaging element, and a fourth polarization filter polarizing incident light in the second direction is provided in the second pixel of the imaging element. 
     In the imaging device according to still another aspect of the present invention, it is preferable that a third wavelength selective filter for passing the light in the first wavelength range of incident light is provided in the first pixel of the imaging element, and a fourth wavelength selective filter for passing the light in the second wavelength range of incident light is provided in the second pixel of the imaging element. 
     In the imaging device according to still another aspect of the present invention, it is preferable that a first light blocking mask having a first microlens and a first opening corresponding to the first pupil region is provided in the first pixel of the imaging element, and a second light blocking mask having a second microlens and a second opening corresponding to the second pupil region is provided in the second pixel of the imaging element. 
     In the imaging device according to still another aspect of the present invention, it is preferable that the imaging optical system limits luminous flux incident on the imaging element only in a stop or only in the first pupil region and the second pupil region, which function as the stop. 
     In the imaging device according to still another aspect of the present invention, it is preferable that the imaging optical system has a front lens group disposed on an object side with respect to a stop, and a rear lens group disposed on an image side with respect to the stop, the first pupil region and the second pupil region are located between the front lens group and the rear lens group, and the front lens group has an angular magnification which makes an angle of off-axis principal ray incident on the first pupil region and the second pupil region close to perpendicular to a stop plane. 
     In the imaging device according to still another aspect of the present invention, it is preferable that the imaging optical system has a front lens group disposed on an object side with respect to a stop, and a rear lens group disposed on an image side with respect to the stop, the first pupil region and the second pupil region are located between the front lens group and the rear lens group, and the front lens group has a focal length which makes an angle of on-axis marginal ray incident on the first pupil region and the second pupil region close to perpendicular to a stop plane. 
     In the imaging device according to still another aspect of the present invention, it is preferable that the first pupil region and the second pupil region function as a stop, or are adjacent to a stop. 
     In the imaging device according to still another aspect of the present invention, it is preferable that in a case where n represents an integer of 2 or more, i represents a parameter which changes in a range of 1 to n, Qi represents an i-th pupil region in the imaging optical system, λ ij  represents a representative wavelength of light transmitted the i-th pupil region Ωi, m i  represents the number of representative wavelengths in the i-th pupil region Ωi, j represents a parameter which changes in a range of 1 to m, λ ij  represents a j-th representative wavelength in the i-th pupil region Ωi, f(λ) represents a paraxial focal length of the imaging optical system at a wavelength λ, g(x,y;λ) represents an on-axis longitudinal aberration of the wavelength λ in a pupil coordinate (x,y) of the pupil region in the imaging optical system, and SΩ i  represents an area of the i-th pupil region Ωi; 
     
       
         
           
             
               
                 
                   
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     In the imaging device according to still another aspect of the present invention, it is preferable that, in a case where a pixel pitch of the imaging element is defined as p, b in [expression 2] satisfies b≤8p. 
     The invention according to still another aspect is an imaging optical system constituting the above-described imaging device. 
     The invention according to still another aspect in an imaging optical system comprising: a first pupil region for passing light in a first wavelength range; and a second pupil region for passing light in a second wavelength range different from the first wavelength range, in which an axial chromatic aberration of the imaging optical system due to a difference between the first wavelength range and the second wavelength range is reduced based on a relationship between an aberration other than the axial chromatic aberration of the imaging optical system and positions of the first pupil region and the second pupil region in the imaging optical system, and in a case where n represents an integer of 2 or more, i represents a parameter which changes in a range of 1 to n, Ωi represents an i-th pupil region in the imaging optical system, λ ij  represents a representative wavelength of light transmitted the i-th pupil region Ωi, m i  represents the number of representative wavelengths in the i-th pupil region Ωi, j represents a parameter which changes in a range of 1 to m, λ ij  represents a j-th representative wavelength in the i-th pupil region Ωi, f(λ) represents a paraxial focal length of the imaging optical system at a wavelength λ, g(x,y;λ) represents an on-axis longitudinal aberration of the wavelength λ in a pupil coordinate (x,y) of the pupil region in the imaging optical system, and SΩ i  represents an area of the i-th pupil region Ωi, 
     
       
         
           
             
               
                 
                   
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     the imaging optical system has a combination of i1, i2, j1, and j2 satisfying [expression 1] and [expression 2] (where, a≥b). 
     An imaging method according to still another aspect of the present invention includes a step of preparing an imaging optical system which has a first pupil region for passing light in a first wavelength range and a second pupil region for passing light in a second wavelength range different from the first wavelength range, in which an axial chromatic aberration of the imaging optical system due to a difference between the first wavelength range and the second wavelength range is reduced based on a relationship between an aberration other than the axial chromatic aberration of the imaging optical system and positions of the first pupil region and the second pupil region in the imaging optical system, a step of dividing each of the light passing through the first pupil region in the imaging optical system and the light passing through the second pupil region in the imaging optical system into pupils to be incident on a first pixel and a second pixel included in an imaging element, and a step of processing, by a signal processing unit, a signal output from the imaging element, and generating each of a first image of the first wavelength range and a second image of the second wavelength range based on an output signal of the first pixel and an output signal of the second pixel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a schematic configuration of an embodiment of an imaging device according to the present invention. 
         FIG. 2  is a front view showing an embodiment of a bandpass filter unit shown in  FIG. 1 . 
         FIG. 3  is a graph showing transmission wavelength characteristics of each of a first bandpass filter and a second bandpass filter in the bandpass filter unit shown in  FIG. 2 . 
         FIGS. 4A and 4B  are diagrams used to explain a sign of a spherical aberration. 
         FIG. 5  is a front view of a polarization filter unit shown in  FIG. 1 . 
         FIG. 6  is a diagram showing a schematic configuration of an array of pixels of an imaging element shown in  FIG. 1 . 
         FIG. 7  a diagram showing a schematic configuration of the imaging element shown in  FIG. 1 . 
         FIG. 8  is a cross-sectional view showing a schematic configuration of one pixel (broken line portion in  FIG. 7 ) shown in  FIG. 7 . 
         FIG. 9  is a diagram showing an example of an arrangement pattern of polarization filter elements included in each pixel block of the imaging element. 
         FIG. 10  is a front view showing another embodiment of the bandpass filter unit. 
         FIG. 11  is a graph showing transmission wavelength characteristics of each of the four bandpass filters of the bandpass filter unit shown in  FIG. 10 . 
         FIG. 12  is a front view of a polarization filter unit provided corresponding to the bandpass filter unit shown in  FIG. 10 . 
         FIG. 13  is a diagram showing a schematic configuration of another imaging element applied to the imaging device according to the embodiment of the present invention. 
         FIG. 14  is a cross-sectional view showing a schematic configuration of one pixel (broken line portion in  FIG. 13 ) shown in  FIG. 13 . 
         FIG. 15  is a diagram showing an aberration type of the imaging optical system and a variation of a bandpass filter unit corresponding to the aberration type. 
         FIG. 16  is a diagram showing the aberration type of the imaging optical system and another variation of a bandpass filter unit corresponding to the aberration type. 
         FIG. 17  is a diagram showing the aberration type of the imaging optical system and still another variation of a bandpass filter unit corresponding to the aberration type. 
         FIG. 18  is a cross-sectional view showing an example of the imaging optical system to which the present invention is applied. 
         FIG. 19  is a chart showing lens data of the imaging optical system shown in  FIG. 18 . 
         FIG. 20  is a front view showing a configuration of a bandpass filter unit applied to the imaging optical system shown in  FIG. 19 . 
         FIG. 21  is a graph showing a spherical aberration, an astigmatism, a distortion, and a lateral chromatic aberration of the imaging optical system shown in  FIG. 18 . 
         FIG. 22  is a chart showing changes in axial chromatic aberration of the imaging optical system in a case where a bandpass filter unit is not provided in the imaging optical system shown in  FIG. 18  and in a case where a bandpass filter unit is provided in the imaging optical system shown in  FIG. 18 . 
         FIG. 23  is a diagram used to explain an off-axis luminous flux diameter on a stop. 
         FIGS. 24A and 24B  are diagrams used to explain the presence or absence of luminous flux regulation other than the stop. 
         FIG. 25  is a diagram used to explain an angle of off-axis principal ray passing through the stop. 
         FIG. 26  is a diagram used to explain an angle of on-axis marginal ray passing through the stop. 
         FIG. 27  is a diagram showing another example of an arrangement pattern of polarization filter elements included in each pixel block of the imaging element. 
         FIG. 28  is a diagram showing an example of one pixel of the imaging element having a light blocking mask. 
         FIG. 29  is a flowchart showing an embodiment of an imaging method according to the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of an imaging device, an imaging optical system, and an imaging method according to an embodiment of the present invention will be described with reference to the accompanying drawings. 
     [Configuration of Imaging Device] 
       FIG. 1  is a diagram illustrating a schematic configuration of an embodiment of an imaging device according to the present invention. 
     An imaging device  1  according to the present embodiment is a multispectral camera which captures a first image and a second image having different wavelength ranges, and includes an imaging optical system  10 , an imaging element  100 , and a signal processing unit  200 . 
     [Imaging Optical System] 
     The imaging optical system  10  is configured by combining a plurality of lenses  12 . The imaging optical system  10  has a bandpass filter unit  16  and a polarization filter unit  18  in the vicinity of a pupil thereof. In addition, the imaging optical system  10  has a focus adjustment mechanism (not shown). For example, the focus adjustment mechanism adjusts a focus by moving the entire imaging optical system  10  back and forth along an optical axis L. 
       FIG. 2  is a front view showing an embodiment of a bandpass filter unit shown in  FIG. 1 . 
     The bandpass filter unit  16  shown in  FIG. 2  is an example of a first pupil region for passing light in a first wavelength range λ 1  and a second pupil region for passing light in a second wavelength range λ 2  different from the first wavelength range, and functions as a wavelength selective unit which transmits light in a wavelength range different for each of the pupil regions of the first pupil region and the second pupil region. 
     The bandpass filter unit  16  is provided such that the center coincides with the optical axis L of the imaging optical system  10 , and is composed of a frame  16 A including a circular opening region  16 A 1  divided by a concentric circle centered on the optical axis L and an annular opening region  16 A 2  outside the opening region  16 A 1 , and two wavelength selective filters (bandpass filters)  16 B 1  and  16 B 2  provided in the frame  16 A. 
     In the following, as necessary, one opening region  16 A 1  provided in the frame  16 A is referred to as a first opening region  16 A 1  and the other opening region  16 A 2  is referred to as a second opening region  16 A 2  to distinguish between the two opening regions  16 A 1  and  16 A 2 . In addition, a bandpass filter  16 B 1  provided in the first opening region  16 A 1  is referred to as a first bandpass filter  16 B 1  and a bandpass filter  16 B 2  provided in the second opening region  16 A 2  is referred to as a second bandpass filter  16 B 2  to distinguish between the two bandpass filters  16 B 1  and  16 B 2 . 
     The frame  16 A has light shielding properties, and transmits the light in the first wavelength range λ 1  and the light in the second wavelength range λ 2  through the first bandpass filter  16 B 1  and the second bandpass filter  16 B 2  provided in the two opening regions  16 A 1  and  16 A 2 . The bandpass filter unit  16  functions as a stop. A stop (not shown) may be provided in the imaging optical system  10 , and the bandpass filter unit  16  may be installed adjacent to the stop. 
     In addition, as is clear from  FIG. 2 , a region (first pupil region) of the first bandpass filter  16 B 1  is a pupil region closer to the optical axis L of the imaging optical system  10  than a region (second pupil region) of the second bandpass filter  16 B 2 . 
       FIG. 3  is a graph showing transmission wavelength characteristics of each of the first bandpass filter  16 B 1  and the second bandpass filter  16 B 2 . 
     The first bandpass filter  16 B 1  transmits the light in the first wavelength range λ 1  and the second bandpass filter  16 B 2  transmits the light in the second wavelength range λ 2 . A representative wavelength of the first wavelength range λ 1  is shorter than a representative wavelength of the second wavelength range λ 2  transmitting the second bandpass filter  16 B 2 . 
     Therefore, the first bandpass filter  16 B 1  of the bandpass filter unit  16  transmits the light in the first wavelength range λ 1  from the first pupil region near the optical axis L of the imaging optical system  10 , and the second bandpass filter  16 B 2  transmits the light in the second wavelength range λ 2  from the second pupil region far from the optical axis L of the imaging optical system  10 . 
     By the way, due to dispersion characteristics of glass, a normal lens has a property (axial chromatic aberration) that an image is formed closer (on an object side) as a ray has a shorter wavelength. On the other hand, in a lens having various aberration characteristics, an image-forming position shifts depending on a position of the pupil. 
     For example, in a case of a lens having a positive spherical aberration as shown in  FIG. 4A  (in a case of (A)), as farther away from the optical axis of the lens in a peripheral direction, the image-forming position further shifts toward an image plane side. In addition, in a case of a lens having a negative spherical aberration as shown in  FIG. 4B  (in a case of (B)), as farther away from the optical axis of the lens in the peripheral direction, the image-forming position shifts toward the object side. 
     The imaging optical system  10  of the present example is a lens having a negative spherical aberration, and thus the image-forming position shifts toward the object side as farther away from the optical axis of the lens in the peripheral direction. In the imaging optical system  10 , this characteristic of spherical aberration is used to reduce the axial chromatic aberration as shown below. 
     (1) Due to the axial chromatic aberration of the imaging optical system  10 , an image-forming position of the light in the first wavelength range λ 1  shifts toward the object side with respect to an image-forming position of the light in the second wavelength range λ 2 . 
     (2) Due to the negative spherical aberration of the imaging optical system  10 , the image-forming position shifts toward the object side as luminous flux incident on the imaging optical system  10  moves away from the optical axis L in the peripheral direction. 
     Therefore, the bandpass filter unit  16  is configured such that the first bandpass filter  16 B 1  transmitting the light in the first wavelength range λ 1  is provided in the circular opening region  16 A 1  (region near the optical axis L) of the frame  16 A, and the second bandpass filter  16 B 2  transmitting the light in the second wavelength range λ 2  is provided in the annular opening region  16 A 2  (region far from the optical axis L) of the frame  16 A. 
     As a result, the imaging optical system  10  is realized in which the deviation of the image-forming position due to the aberrations of (1) and (2) described above is offset and the axial chromatic aberration is satisfactorily corrected. 
     In a case where the imaging optical system has a positive spherical aberration, contrary to the bandpass filter unit  16  shown in  FIG. 2 , the first bandpass filter  16 B 1  is provided in the annular opening region  16 A 2  far from the optical axis L, and the second bandpass filter  16 B 2  is provided in the circular opening region  16 A 1  near the optical axis L. 
       FIG. 5  is a front view of the polarization filter unit shown  18  in  FIG. 1 . 
     The polarization filter unit  18  constitutes a part of the pupil division optical system which divides light passing through two pupil regions (bandpass filters  16 B 1  and  16 B 2 ) into pupils and causes the light to be incident on two types of pixels (first pixel and second pixel) included in the imaging element  100 . 
     Similar to the bandpass filter unit  16 , the polarization filter unit  18  is provided such that the center thereof consists with the optical axis L of the imaging optical system  10 , and is composed of a frame  18 A and two polarization filters  18 B 1  and  18 B 2 . 
     The frame  18 A of the polarization filter unit  18  has the same projected shape as the frame  16 A of the bandpass filter unit  16 , and includes a circular opening region  18 A 1  divided by a concentric circle centered on the optical axis L and an annular opening region  18 A 2 . 
     The polarization filter  18 B 1  is disposed in the circular opening region  18 A 1  of the frame  18 A, and the polarization filter  18 B 2  is disposed in the annular opening region  18 A 2 . 
     Here, the polarization filter  18 B 1  (first polarization filter) polarizes the light passing through the first pupil region in a first direction, and the polarization filter  18 B 2  (second polarization filter) polarizes the light passing through the second pupil region in a second direction different from the first direction. That is, the two polarization filters  18 B 1  and  18 B 2  differ in the direction of polarization axes by 90° from each other, and in  FIG. 5 , a direction of the polarization axis of the polarization filter  18 B 1  is a left-right direction (x-axis direction) on an xy plane orthogonal to the optical axis L (z-axis), and a direction of the polarization axis of the polarization filter  18 B 2  is a vertical direction (y-axis direction). 
     In the following, as necessary, one opening region  18 A 1  provided in the frame  18 A is referred to as a first opening region  18 A 1  and the other opening region  18 A 2  is referred to as a second opening region  18 A 2  to distinguish between the two opening regions  18 A 1  and  18 A 2 . In addition, the polarization filter  18 B 1  provided in the first opening region  18 A 1  is referred to as a first polarization filter  18 B 1  and the polarization filter  18 B 2  provided in the second opening region  18 A 2  is referred to as a second polarization filter  18 B 2  to distinguish between the two polarization filters  18 B 1  and  18 B 2  having different polarization axes. 
     In the imaging optical system  10  having the above-described configuration, the pupil region is divided into two regions by the bandpass filter unit  16  and the polarization filter unit  18 . That is, it is divided into the first pupil region defined by the first opening region  16 A 1  of the bandpass filter unit  16  and the first opening region  18 A 1  of the polarization filter unit  18  and the second pupil region defined by the second opening region  16 A 2  of the bandpass filter unit  16  and the second opening region  18 A 2  of the polarization filter unit  18 . 
     Light having different characteristics is emitted from the first pupil region and the second pupil region. That is, from the first pupil region, the first light in the first wavelength range λ 1 , having the polarization direction of x direction, is emitted, and from the second pupil region, the second light in the second wavelength range λ 2 , having the polarization direction of y direction, is emitted. 
     [Imaging Element] 
       FIG. 6  is a diagram showing a schematic configuration of an array of pixels of the imaging element  100  shown in  FIG. 1 . 
     As shown in the figure, the imaging element  100  has a plurality of types (two types) of pixels P 1  and P 2  on a light-receiving surface thereof. The pixels P 1  and P 2  are regularly arranged at a constant pitch along a horizontal direction (x-axis direction) and a vertical direction (y-axis direction). 
     In the imaging element  100  according to the present example, one pixel block PB(X,Y) is composed of four adjacent (2×2) pixels P 1  and P 2 , and the pixel blocks PB(X,Y) are regularly arranged along the horizontal direction (x-axis direction) and the vertical direction (y-axis direction). Hereinafter, as necessary, the pixel P 1  is referred to as a first pixel P 1  and the pixel P 2  is referred to as a second pixel P 2  to distinguish between the pixels P 1  and P 2 . The pixels P 1  and P 2  have different optical characteristics. 
       FIG. 7  a diagram showing a schematic configuration of the imaging element  100  shown in  FIG. 1 , and  FIG. 8  is a cross-sectional view showing a schematic configuration of one pixel (broken line portion in  FIG. 7 ) shown in  FIG. 7 . 
     The imaging element  100  is a complementary metal-oxide semiconductor (CMOS) type image sensor, and includes a pixel array layer  110 , a polarization filter element array layer  120 , and a microlens array layer  140 . Each layer is arranged in the order of the pixel array layer  110 , the polarization filter element array layer  120 , and the microlens array layer  140  from the image plane side to the object side. The imaging element  100  is not limited to the CMOS type, and may be an XY address type or charge coupled device (CCD) type image sensor. 
     The pixel array layer  110  is composed of a large number of photodiodes  112  arranged two-dimensionally. One photodiode  112  constitutes one pixel. The photodiodes  112  are regularly arranged along the horizontal direction (x direction) and the vertical direction (y direction). 
     The polarization filter element array layer  120  is composed of two types of polarization filter elements  122 A and  122 B having different polarization directions from each other, which are arranged two-dimensionally. 
     Hereinafter, as necessary, the polarization filter element  122 A is referred to as a first polarization filter element  122 A and the polarization filter element  122 B is referred to as a second polarization filter element  122 B to distinguish between the polarization filter elements  122 A and  122 B. Each of the polarization filter elements  122 A and  122 B is disposed at the same interval as the photodiodes  112  and is included in each pixel. 
       FIG. 9  is a diagram showing an example of an arrangement pattern of polarization filter elements included in each pixel block of the imaging element  100 . 
     As shown in the figure, the two polarization filters  18 B 1  and  18 B 2  differ in direction of polarization axes by 90° from each other, and in  FIG. 9 , a direction (first polarization direction Θ 1 ) of the polarization axis of the first polarization filter element  122 A is a left-right direction (x-axis direction), and a direction of the polarization axis of the second polarization filter element  122 B is a vertical direction (y-axis direction). In each pixel block PB(X,Y), the polarization filter elements  122 A and  122 B are regularly arranged. 
     The first polarization direction Θ 1  of the first polarization filter element  122 A coincides with the polarization direction of the first polarization filter  18 B 1  in the polarization filter unit  18  ( FIG. 5 ), and the second polarization direction Θ 2  of the second polarization filter element  122 B coincides with the polarization direction of the second polarization filter  18 B 2  in the polarization filter unit  18 . 
     Therefore, the first pixel P 1  having the polarization filter element  122 A in the imaging element  100  receives only the first light in the first wavelength range λ 1  transmitted through the first pupil region having the first polarization filter  18 B 1 , and the second pixel P 2  having the polarization filter element  122 B receives only the second light in the second wavelength range λ 2  transmitted through the second pupil region having the second polarization filter  18 B 2 . 
     In a case where the imaging device  1  receives a shooting instruction input from a shutter release switch or the like, the imaging device  1  performs exposure control in the imaging element  100 . An optical image of a subject captured on the light-receiving surface of the imaging element  100  by the exposure control is converted into an electric signal by the imaging element  100 . In each of the pixels (first pixel and second pixel) of the imaging element  100 , charges corresponding to an amount of light incident on the photodiode  112  are accumulated, and an electric signal corresponding to an amount of charge accumulated in each pixel is read out from the imaging element  100  as an image signal, and the image signal is output. 
     Returning to  FIG. 1 , the signal processing unit  200  processes the signal output from the imaging element  100 , and generates each of a first image of the first wavelength range λ 1  and a second image of the second wavelength range λ 2  based on the output signal of the first pixel P 1  and the output signal of the second pixel P 2 . 
     As a result, the imaging device  1  can simultaneously capture the first image of the first wavelength range λ 1  and the second image of the second wavelength range λ 2  (multispectral image), and the first image and the second image are images having different wavelength ranges from each other, but are captured as images in which the axial chromatic aberration of the imaging optical system  10  is improved. 
     [Another Embodiment of Bandpass Filter Unit] 
       FIG. 10  is a front view showing another embodiment of the bandpass filter unit. 
     A bandpass filter unit  16 - 2  shown in  FIG. 10  is an example of a first pupil region for passing light in a first wavelength range λ 11 , a second pupil region for passing light in a second wavelength range λ 12 , a third pupil region for passing light in a third wavelength range λ 21 , and a fourth pupil region for passing light in a fourth wavelength range λ 22 , and functions as a wavelength selective unit which transmits light in a wavelength range different for each of the pupil regions of the first pupil region to the fourth pupil region. 
     The bandpass filter unit  16 - 2  is provided such that the center coincides with the optical axis of the imaging optical system, and is composed of a frame  16 - 2 A including a circular opening region divided by a concentric circle centered on the optical axis and three annular opening regions, and a first wavelength selective filter to fourth wavelength selective filter (four bandpass filters)  16 - 2 B 1  to  16 - 2 B 4  provided in the frame  16 - 2 A. 
       FIG. 11  is a graph showing transmission wavelength characteristics of each of the four bandpass filters  16 - 2 B 1  to  16 - 2 B 4  of the bandpass filter unit  16 - 2  shown in  FIG. 10 . 
     The bandpass filter  16 - 2 B 1  transmits the light in the first wavelength range λ 11 , the bandpass filter  16 - 2 B 2  transmits the light in the second wavelength range λ 12 , the bandpass filter  16 - 2 B 3  transmits the light in the third wavelength range λ 21 , and the bandpass filter  16 - 2 B 4  transmits the light in the fourth wavelength range λ 22 . 
     Here, the bandpass filter  16 - 2 B 1  which transmits the light in the first wavelength range λ 11 , which has the shortest representative wavelength of the four wavelength ranges, is provided in the circular pupil region of the bandpass filter unit  16 - 2  closest to the optical axis, and the bandpass filters  16 - 2 B 2  to  16 - 2 B 4  which transmit the light in the second wavelength range λ 12 , the third wavelength range λ 21 , and the fourth wavelength range λ 22 , in which the representative wavelengths are sequentially lengthened, are provided in the three annular pupil regions sequentially distant from the optical axis. That is, the bandpass filters  16 - 2 B 1  to  16 - 2 B 4  which transmit the light in the first wavelength range λ 11  to the fourth wavelength range λ 22 , in which the representative wavelengths are sequentially lengthened as the distance from the optical axis increases, are provided in the bandpass filter unit  16 - 2 . 
     Due to the axial chromatic aberration of the imaging optical system, as a wavelength of subject light is shorter, an image-forming position shifts toward an object side. However, since the imaging optical system has a negative spherical aberration, and the bandpass filters  16 - 2 B 1  to  16 - 2 B 4  which transmit the light in the first wavelength range λ 11  to the fourth wavelength range λ 22 , in which the representative wavelengths are sequentially lengthened, are provided in the first pupil region to fourth pupil region sequentially distant from the optical axis L, an imaging optical system capable of satisfactorily correcting the axial chromatic aberration is configured by offsetting the deviation of an image-forming position due to the axial chromatic aberration by the deviation of an image-forming position due to the spherical aberration. 
       FIG. 12  is a front view of a polarization filter unit  18 - 2  provided corresponding to the bandpass filter unit  16 - 2  shown in  FIG. 10 . 
     The polarization filter unit  18 - 2  constitutes a part of the pupil division optical system which divides light passing through four pupil regions (bandpass filters  16 - 2 B 1  to  16 - 2 B 4 ) into pupils and causes the light to be incident on four types of pixels (first pixel to fourth pixel) included in an imaging element  100 - 2 . 
     Similar to the bandpass filter unit  16 - 2 , the polarization filter unit  18 - 2  is provided such that the center thereof consists with the optical axis of the imaging optical system, and is composed of a frame  18 - 2 A and four polarization filters  18 - 2 B 1  to  18 - 2 B 4 . 
     The frame  18 - 2 A of the polarization filter unit  18 - 2  has the same projected shape as the frame  16 - 2 A of the bandpass filter unit  16 - 2 , and includes a circular opening region divided by a concentric circle centered on the optical axis and three annular opening regions. 
     The polarization filter  18 - 2 B 1  is disposed in the circular opening region of the frame  18 - 2 A, and the three polarization filters  18 - 2 B 2  to  18 - 2 B 4  are arranged in the three annular opening regions. 
     Here, in  FIG. 12 , among four polarization filters  18 - 2 B 1  to  18 - 2 B 4 , polarization axes of two polarization filters  18 - 2 B 1  and  18 - 2 B 3  are in a left-right direction (x-axis direction), and polarization axes of the remaining two polarization filters  18 - 2 B 2  and  18 - 2 B 4  are in a vertical direction (y-axis direction). In addition, in  FIG. 12 , the polarization filters  18 - 2 B 1  and  18 - 2 B 3  and the polarization filters  18 - 2 B 2  and  18 - 2 B 4  differ in the direction of polarization axes by 90° from each other. 
     The bandpass filter unit  16 - 2  shown in  FIG. 10  transmits light in the four wavelength ranges λ 11 , λ 12 , λ 21 , and λ 22  from the four pupil regions, respectively, and by the polarization filter unit  18 - 2 , the light in the four wavelength ranges λ 11 , λ 12 , λ 21 , and λ 22  is emitted as a first light in the wavelength ranges λ 11  and λ 21  in which the polarization direction is the x-direction, and as a second light in the wavelength ranges λ 12  and λ 22  in which the polarization direction is the y-direction. 
       FIG. 13  is a diagram showing a schematic configuration of another imaging element applied to the imaging device according to the embodiment of the present invention.  FIG. 14  is a cross-sectional view showing a schematic configuration of one pixel (broken line portion in  FIG. 13 ) shown in  FIG. 13 . Components shown in  FIGS. 13 and 14  common to the portion of the imaging element shown in  FIGS. 7 and 8  will be denoted by the same reference numerals, and the detailed description thereof will be omitted. 
     The imaging element  100 - 2  shown in  FIGS. 13 and 14  differs from the imaging element  100  shown in  FIG. 7  in that a spectral filter element array layer  130  which functions as a first wavelength selective filter and a second wavelength selective filter is added. 
     The spectral filter element array layer  130  is composed of two types of spectral filter elements  132 A and  132 B (first wavelength selective filter and second wavelength selective filter) having different transmission wavelength characteristics from each other, which are arranged two-dimensionally. Hereinafter, as necessary, the spectral filter element  132 A is referred to as a first spectral filter element  132 A and the spectral filter element  132 B is referred to as a second spectral filter element  132 B to distinguish between the spectral filter elements  132 A and  132 B. Each of the spectral filter elements  132 A and  132 B is disposed at the same interval as the photodiodes  112  and is included in each pixel. 
       FIG. 11  is a graph including an example of transmission wavelength characteristics of each spectral filter element. 
     As shown in  FIG. 11 , the first spectral filter element  132 A transmits light in a wavelength range kA, and the second spectral filter element  132 B transmits light in a wavelength range B. 
     The first light in the wavelength ranges λ 11  and λ 21  in which the polarization direction is the x-direction and the second light in the wavelength ranges λ 12  and λ 22  in which the polarization direction is the y-direction are incident on the imaging element  100 - 2 . 
     On the other hand, among pixels of the imaging element  100 - 2 , in a case where a pixel having the first spectral filter element  132 A and the first polarization filter element  122 A is defined as a pixel P 1 , a pixel having the first spectral filter element  132 A and the second polarization filter element  122 B is defined as a pixel P 2 , a pixel having the second spectral filter element  132 B and the first polarization filter element  122 A is defined as a pixel P 3 , and a pixel having the second spectral filter element  132 B and the second polarization filter element  122 B is defined as a pixel P 4 , in the imaging element  100 - 2 , one pixel block PB(X,Y) is composed of four adjacent (2×2) pixels P 1 , P 2 , P 3 , and P 4 , and the pixel blocks PB(X,Y) are regularly arranged along the horizontal direction (x-axis direction) and the vertical direction (y-axis direction). 
     Hereinafter, as necessary, the pixels P 1 , P 2 , P 3 , and P 4  are referred to as a first pixel P 1 , a second pixel P 2 , a third pixel P 3 , and a fourth pixel P 4  to distinguish between the four types of pixels P 1 , P 2 , P 3 , and P 4 . 
     By the combination of the polarization filter elements  122 A and  122 B and the spectral filter elements  132 A and  132 B, the four types of pixels P 1  to P 4  receive only light in any one of the four wavelength ranges λ 11  to λ 22 . That is, the first pixel P 1  receives only the light in the wavelength range λ 11 , the second pixel P 2  receives only the light in the wavelength range λ 12 , the third pixel P 3  receives only the light in the wavelength range λ 21 , and the fourth pixel P 4  receives only the light in the wavelength range λ 22 . 
     In a case where the imaging device receives a shooting instruction input from a shutter release switch or the like, the imaging device performs exposure control in the imaging element  100 - 2 . An optical image of a subject captured on the light-receiving surface of the imaging element  100 - 2  by the exposure control is converted into an electric signal by the imaging element  100 - 2 . In each of the pixels (pixels P 1  to P 4 ) of the imaging element  100 - 2 , charges corresponding to an amount of light incident on the photodiode  112  are accumulated, and an electric signal corresponding to an amount of charge accumulated in each pixel is read out from the imaging element  100 - 2  as an image signal, and the image signal is output. 
     The signal processing unit  200  shown in  FIG. 1  processes the signal output from the imaging element  100 - 2 , and generates each of a first image of the first wavelength range λ 11 , a second image of the second wavelength range λ 12 , a third image of the third wavelength range λ 21 , and a fourth image of the fourth wavelength range λ 22 , based on the output signal of the first pixel P 1 , the output signal of the second pixel P 2 , the output signal of the third pixel P 3 , and the output signal of the fourth pixel P 4 . 
     As a result, the imaging device can simultaneously capture the first image of the first wavelength range λ 11 , the second image of the second wavelength range λ 12 , the third image of the third wavelength range λ 21 , and the fourth image of the fourth wavelength range λ 22  (multispectral image), and the first image to the fourth image are images having different wavelength ranges from each other, but are captured as images in which the axial chromatic aberration of the imaging optical system is improved. 
     [Variation of Bandpass Filter Unit Corresponding to Aberration Type of Imaging Optical System] 
       FIGS. 15 to 17  are diagrams showing aberration types of the imaging optical system and variations of a bandpass filter unit corresponding to the aberration type, respectively. 
     As the aberration types of the imaging optical system applicable to the correction of the axial chromatic aberration of the imaging optical system, a spherical aberration, a coma aberration, and an astigmatism can be considered. 
     In order to correct the axial chromatic aberration of the imaging optical system, the imaging optical system may be designed so that aberrations other than the axial chromatic aberration of the imaging optical system are positively generated. 
     As a method of realizing an imaging optical system having the spherical aberration as the aberrations other than the axial chromatic aberration, it is considered that, for an imaging optical system in which the spherical aberration is well corrected, a surface spacing or curvature near the stop is changed to generate the spherical aberration. 
     In addition, as a method of realizing an imaging optical system having the coma aberration, it is considered that the coma aberration is generated by eccentricity of lens near the stop, and as a method of realizing an imaging optical system having the astigmatism, it is considered that the astigmatism is generated by providing an anamorphic lens near the stop. 
     &lt;Principle of Correction of Axial Chromatic Aberration&gt; 
     1) Due to dispersion characteristics of glass, a normal lens has a property that an image is formed closer to an object side as a ray has a shorter wavelength (axial chromatic aberration). 
     2) In a lens with residual aberration, an image-forming position shifts depending on a position of the pupil. 
     As shown in “Aberration characteristics” of  FIG. 15(A) , it is assumed that the spherical aberration, the coma aberration, or the astigmatism remains in the lens of the imaging optical system. 
     In the “Aberration characteristics” of  FIG. 15(A) , a region a 1  is a region in which a phase advances to a region b 1  of the imaging optical system, and assuming that a region el is a region el in which the phase is delayed, the image-forming position approaches the image plane side in the order of the regions a 1 , b 1 , and c 1 . 
     Therefore, in order to correct the axial chromatic aberration, as shown in “Wavelength allocation image” of  FIG. 15(B)  corresponding to the “Aberration characteristics” of FIG.  15 (A), bandpass filters having different wavelength ranges are assigned to each of pupil regions a 2 , b 2 , and c 2  corresponding to the regions a 1 , b 1 , and c 1 . That is, bandpass filters having a longer wavelength, in which light transmits in the order of the regions a 2 , b 2 , and c 2 , are arranged. 
     In a case where the bandpass filters having different wavelength ranges transmitted are appropriately arranged corresponding to the “Wavelength allocation image” of  FIG. 15(B) , it is possible to realize an imaging optical system in which the deviation of the image-forming position due to the influences of 1) and 2) is offset and the axial chromatic aberration is satisfactorily corrected. 
     &lt;Bandpass Filter Unit in which Spherical Aberration is Used to Reduce Axial Chromatic Aberration&gt; 
     As a variation of the bandpass filter unit in which the spherical aberration is used to reduce the axial chromatic aberration, six bandpass filter units shown in “Concentric circle type” of  FIG. 15(C) , “Circular opening vertical 3-hole type” of  FIG. 15(D) , “Circular opening cross 5-hole type” of  FIG. 15(E) , “Circular opening vertical 5-hole type” of  FIG. 16(F) , “Circular opening cross 9-hole type” of  FIG. 16(G) , and “Circular opening 8-direction and 9-hole type” of  FIG. 16(H)  are shown in  FIGS. 15 and 16 . 
     In a case where the imaging optical system having the spherical aberration has the characteristics (characteristics of negative spherical aberration) shown in the “Aberration characteristics” of  FIG. 15(A) , in order to correct the axial chromatic aberration, as shown in the “Wavelength allocation image” of  FIG. 15(B) , the bandpass filters having a longer wavelength, in which light transmits in the order of the regions a 2 , b 2 , and c 2  sequentially distant from the optical axis, are arranged. 
     The “Concentric circle type” of  FIG. 15(C)  is a bandpass filter unit of the same type as the bandpass filter unit  16  shown in  FIG. 2  and the bandpass filter unit  16 - 2  shown in  FIG. 10 . 
     The bandpass filter units shown in the “Circular opening vertical 3-hole type” of  FIG. 15(D) , “Circular opening cross 5-hole type” of  FIG. 15(E) , and “Circular opening 8 direction and 9-hole type” of  FIG. 16(H)  have a circular opening with a visible white filter or no filter in the circular opening on the optical axis, and the bandpass filters transmitting light in a wavelength range in which a representative wavelength is longer than a representative wavelength of visible white are arranged in a plurality of circular openings equidistant from the optical axis. According to these bandpass filter units, it is possible to simultaneously capture an image in the wavelength range of visible white and an image in the wavelength range having a longer representative wavelength than the representative wavelength of visible white, and to improve focusing accuracy of each image even for each image having different wavelength ranges. 
     In the “Circular opening vertical 5-hole type” of  FIG. 16(F)  and “Circular opening cross 9-hole type” of  FIG. 16(G) , a bandpass filter transmitting light in the first wavelength range with a short wavelength is disposed in the center, a bandpass filter transmitting light in the second wavelength range with a long wavelength is disposed on the outermost surface, and in the middle thereof, a third pupil region transmitting light in the third wavelength range intermediate between the first wavelength range and the second wavelength range is provided. 
     In these six bandpass filter units, in a case where the spherical aberration is negative, the bandpass filters having a longer wavelength are arranged as they are sequentially distant from the optical axis. In this way, by arranging the bandpass filters for each wavelength range in consideration of the spherical aberration of the imaging optical system, the deviation of the image-forming position for each wavelength due to the axial chromatic aberration is reduced, and focusing accuracy of a plurality of images captured at the same time is improved. 
     In a case of an imaging optical system having a positive spherical aberration, which is different from the imaging optical system having the negative spherical aberration characteristics shown in  FIG. 15(A) , bandpass filters having a shorter wavelength, in which light transmits in the order of regions sequentially distant from the optical axis, are arranged. 
     &lt;Bandpass Filter Unit in which Coma Aberration is Used to Reduce Axial Chromatic Aberration&gt; 
     As a variation of the bandpass filter unit in which the coma aberration is used to reduce the axial chromatic aberration, ten bandpass filter units shown in “Circular opening vertical 3-hole type” of  FIG. 15(D) , “Circular opening cross 5-hole type” of  FIG. 15(E) , “Circular opening vertical 5-hole type” of  FIG. 16(F) , “Circular opening cross 9-hole type” of  FIG. 16(G) , “Circular opening 8-direction and 9-hole type” of  FIG. 16(H) , “Square opening 2×2 type” of  FIG. 16(J) , “Square opening 8-hole rhombus type” of  FIG. 17(K) , “Square opening 3×3 type” of  FIG. 17(L) , “Square opening cross type” of  FIG. 17(M) , “Fan-shaped 4-split type” of  FIG. 17(N) , and “Fan-shaped 8-split type” of  FIG. 17(O)  are shown in  FIGS. 15 to 17 . 
     In a case where the imaging optical system having the coma aberration has the characteristics shown in the “Aberration characteristics” of  FIG. 15(A) , among a region a 2  (first region) and a region c 2  (second region) which are located, with the optical axis in between, opposite to each other through a straight line intersecting the optical axis of the imaging optical system, an image-forming position of light incident on the region a 2  on the optical axis is moved to an image side, an image-forming position of light incident on the region c 2  on the optical axis is moved to an object side. 
     Therefore, in order to correct the axial chromatic aberration, as shown in the “Wavelength allocation image” of  FIG. 15(B) , bandpass filters having a longer wavelength, in which light transmits in the order of the regions a 2 , b 2 , and c 2  from bottom to top in  FIG. 15 , are arranged. 
     In the bandpass filter unit of the “Circular opening vertical 3-hole type” of  FIG. 15(D) , in which the coma aberration is used to reduce the axial chromatic aberration, bandpass filters having a longer wavelength, in which light transmits into three circular openings from bottom to top in  FIG. 15 , are arranged. 
     According to the “Wavelength allocation image” of  FIG. 15(B) , due to the influence of the coma aberration, an image-forming position shifts toward an object side from bottom to top, but by applying a bandpass filter unit having a bandpass filter in which the wavelength is longer sequentially from the bottom to the top, it is possible to offset the deviation of the image-forming position due to the axial chromatic aberration and obtain a good image in which the deviation of the image-forming position for each wavelength is reduced. 
     Similarly, in the other bandpass filter units, bandpass filters having a longer transmitted wavelength is sequentially arranged in each opening from the bottom to the top. 
     In a case of an imaging optical system having a coma aberration, which is different from the imaging optical system having the coma aberration characteristics shown in  FIG. 15(A) , it is necessary to determine the placement position of a plurality of bandpass filters having different transmitted wavelengths according to the characteristics of the coma aberration. 
     &lt;Bandpass Filter Unit in which Astigmatism is Used to Reduce Axial Chromatic Aberration&gt; 
     As a variation of the bandpass filter unit in which the astigmatism is used to reduce the axial chromatic aberration, ten bandpass filter units shown in “Circular opening cross 5-hole type” of  FIG. 15(E) , “Circular opening cross 9-hole type” of  FIG. 16(G) , “Circular opening 8-direction and 9-hole type” of  FIG. 16(H) , “Circular opening astigmatism special type” of  FIG. 16(I) , “Square opening 2×2 type” of  FIG. 16(J) , “Square opening 8-hole rhombus type” of  FIG. 17(K) , “Square opening 3×3 type” of  FIG. 17(L) , “Square opening cross type” of  FIG. 17(M) , “Fan-shaped 4-split type” of  FIG. 17(N) , and “Fan-shaped 8-split type” of FIG.  17 (O) are shown in  FIGS. 15 to 17 . 
     In a case where the imaging optical system having the astigmatism has the characteristics shown in the “Aberration characteristics” of  FIG. 15(A) , an image-forming position of light incident on left and right regions (first region), which is symmetrical with respect to the optical axis of the imaging optical system, on the optical axis is moved to an image side, and an image-forming position of light incident on upper and lower regions (second region), which are symmetrical with respect to the optical axis and orthogonal (including a case of being approximately orthogonal) to the left and right regions, on the optical axis is moved to an object side. 
     Therefore, in order to correct the axial chromatic aberration, as shown in the “Wavelength allocation image” of  FIG. 15(B) , bandpass filters having a longer wavelength, in which light transmits in the order of the left and right regions a 2 , a central region b 2 , and the upper and lower regions c 2  in  FIG. 15 , are arranged. 
     In the bandpass filter unit of the “Circular opening cross 5-hole type” of  FIG. 15(E) , in which the astigmatism is used to reduce the axial chromatic aberration, with respect to bandpass filters arranged in the central (on the optical axis) circular opening in  FIG. 15 , bandpass filters having a shorter wavelength are arranged in the left and right circular openings, and bandpass filters having a shorter wavelength are arranged in the upper and lower circular openings. 
     In a case of the “Circular opening cross 9-hole type” of  FIG. 16(G) , as the distance from the center to the left and right directions increases, bandpass filters having a shorter transmitted wavelength are sequentially arranged than the central bandpass filter, and as distance from the center to the upper and lower directions increases, bandpass filters having a longer transmitted wavelength are sequentially arranged than the central bandpass filter. 
     In a case of the “Circular opening 8-direction and 9-hole type” of  FIG. 16(H) , five circular openings in an oblique direction, including the center, are different from the “Circular opening cross 5-hole type” of  FIG. 15(E)  in that the same bandpass filter is disposed as the bandpass filter disposed in the central circular opening. 
     In a case of the “Circular opening astigmatism special type” of  FIG. 16(I) , the bandpass filter is not disposed in the center, and in twelve circular openings formed along sides of a rhombus, bandpass filters having a shorter wavelength are sequentially arranged as they move away from a straight line in a vertical direction passing through the optical axis in a horizontal direction. 
     The bandpass filter units shown in the “Square opening 2×2 type” of  FIG. 16(J)  and “Fan-shaped 4-split type” of  FIG. 17(N)  have an opening with a visible white filter or no filter in the left and right openings, and the bandpass filters transmitting light in a wavelength range in which a representative wavelength is longer than a representative wavelength of visible white are arranged in the upper and lower openings. 
     Similarly, in the other bandpass filter units, bandpass filters having a shorter transmitted wavelength are arranged as the distance increases in the left and right direction, and bandpass filters having a longer transmitted wavelength are arranged as the distance increases in the upper and lower direction. 
     In a case of an imaging optical system having an astigmatism, which is different from the imaging optical system having the astigmatism characteristics shown in  FIG. 15(A) , it is necessary to determine the position and region of a plurality of bandpass filters having different transmitted wavelengths according to the characteristics of the astigmatism. 
     Example 
       FIG. 18  is a cross-sectional view showing an example of the imaging optical system to which the present invention is applied. 
     An imaging optical system  20  shown in  FIG. 18  includes a front lens group  20 A disposed closer to an object side than a position of a stop represented by a plane number  8 , and a rear lens group  20 B disposed closer to an image side than the position of the stop. 
     In  FIG. 18, 1 to 7  represent plane numbers for four lenses constituting the front lens group  20 A, and  9  to  16  represent plane numbers for four lenses constituting the rear lens group  20 B and plane numbers for a parallel flat plate. 
       FIG. 19  is a chart showing lens data of the imaging optical system  20  shown in  FIG. 18 . 
       FIG. 20  is a front view showing a configuration of a bandpass filter unit applied to the imaging optical system  20  shown in  FIG. 19 . 
     A bandpass filter unit  16 - 3  shown in  FIG. 20  is disposed between the front lens group  20 A and the rear lens group  20 B of the imaging optical system  20  shown in  FIG. 18 , and for example, the bandpass filter unit  16 - 3  can be disposed on the position of the stop shown in  FIG. 18  and functions as a fixed stop, or can be disposed on a position adjacent to the stop. 
     The bandpass filter unit  16 - 3  is a bandpass filter unit is a “Concentric circle type” bandpass filter unit same as the bandpass filter unit  16 - 2  shown in  FIG. 10 , and is composed of a bandpass filter transmitting light having a representative wavelength  111  (blue: 475 nm), a bandpass filter transmitting light having a representative wavelength λ 12  (green: 560 nm), a bandpass filter transmitting light having a representative wavelength λ 21  (red: 668 nm), and a bandpass filter transmitting light having a representative wavelength λ 22  (red edge: 717 nm), which are provided in one circular opening region and three annular opening regions, divided by four concentric circles having diameters of 6 mm, 6√2 mm, 6√3 mm, and 12 mm, respectively. 
     The imaging optical system  20  of the present example includes a polarization filter unit (corresponding to the polarization filter unit  18 - 2  shown in  FIG. 12 ) corresponding to the bandpass filter unit  16 - 3 . 
       FIG. 21  is a graph showing a spherical aberration, an astigmatism, a distortion, and a lateral chromatic aberration of the imaging optical system  20  shown in  FIG. 18 . 
       FIG. 22  is a chart showing changes in axial chromatic aberration of the imaging optical system  20  in a case where the bandpass filter unit  16 - 3  is not provided in the imaging optical system  20  and in a case where the bandpass filter unit  16 - 3  is provided in the imaging optical system  20 . 
     The axial chromatic aberration shown in  FIG. 22  indicates a deviation amount of image-forming positions of other wavelengths with respect to the wavelength λ 12  (560 nm), and a negative value indicates a deviation amount toward the object side. In addition, these deviation amounts are measured based on an image-forming position in which a modulation transfer function (MFT) at a spatial frequency of 73 [lp/mm] takes a peak value. 
     As shown in  FIG. 22 , by disposing the bandpass filter unit  16 - 3  in the imaging optical system  20 , it can be seen that the axial chromatic aberration due to the imaging optical system  20  is reduced. 
     [Features of Imaging Optical System] 
     Next, features of the imaging optical system including the imaging device according to the present invention, particularly having various aberrations will be quantitatively described. 
     First, various variables, parameters, and the like related to the imaging optical system are defined as follows. 
     n represents an integer of 2 or more, i represents a parameter which changes in a range of 1 to n, Ωi represents an i-th pupil region in the imaging optical system, λ ij  represents a representative wavelength of light transmitted the i-th pupil region Ωi, m i  represents the number of representative wavelengths in the i-th pupil region Ωi, j represents a parameter which changes in a range of 1 to m, λ ij  represents a j-th representative wavelength in the i-th pupil region Ωi, f(λ) represents a paraxial focal length of the imaging optical system at a wavelength λ, g(x,y;λ) represents an on-axis longitudinal aberration of the wavelength λ in a pupil coordinate (x,y) of the pupil region in the imaging optical system, and SΩ i  represents an area of the i-th pupil region Ωi. 
     Based on the above-described definition, 
     
       
         
           
             
               
                 
                   
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     the imaging optical system has a combination of i1, i2, j1, and j2 satisfying [expression 1] and [expression 2] (where, a≥b). 
     [expression 1] means that the imaging optical system has an axial chromatic aberration exceeding a. 
     [expression 2] means that, by transmitting light at an appropriate wavelength for each position and region of the pupils of the imaging optical system, the axial chromatic aberration is reduced by the on-axis longitudinal aberration based on various aberrations of the imaging optical system, and the axial chromatic aberration exceeding a is less than b. 
     Here, in a case where a pixel pitch of the imaging element is defined as p, it is preferable that b in [expression 2] satisfies b≤8p. 
     Grounds for the 8p described above are as follows. 
     In a case where a permissible circle of confusion is defined as  6  and an F number is defined as F, a focal depth of the imaging optical system can be expressed by ±SF. Here, in a case where F is 4, the focal depth is ±46. 
     In the permissible circle δ of confusion, in a case where a pixel pitch p of the imaging element is a half-Nyquist, δ=2p. Therefore, as the focal depth of the imaging optical system, ±4δ=±8p. 
     Therefore, by setting b≤8p, the axial chromatic aberration of the imaging optical system can be reduced within the focal depth of the imaging optical system. 
     In the permissible circle δ of confusion, in a case where the pixel pitch p of the imaging element is a full-Nyquist, it is more preferable that b≤4p. 
     In addition, in the pixel pitch of the imaging element used in the example, p=3.45 μm, and in this case, 8p=27.6 μm and 4p=13.8 μm. The axial chromatic aberration with a filter shown in  FIG. 22  satisfies b≤4p. 
     [Regarding Off-Axis Luminous Flux Diameter on Stop] 
       FIG. 23  is a diagram used to explain an off-axis luminous flux diameter on the stop. 
     In a bandpass filter unit  16 - 4  shown in  FIG. 23(A) , in a case where off-axis luminous flux passes only inside a circle  17  as shown by a dotted line in  FIG. 23(A) , light at a wavelength assigned to the outermost shell at this angle of view cannot reach. 
     In this case, as shown in  FIG. 23(B) , an image of the wavelength assigned to the outermost shell is not captured in an edge part with a high image height. That is, an image circle is smaller. 
     Therefore, it is preferable to use an imaging optical system with a large image circle, so that the off-axis luminous flux can cover the outermost annular region, whereby, as shown in  FIG. 23(C) , the image of the wavelength assigned to the outermost shell is also captured up to the outermost edge part. 
     [Regarding Presence or Absence of Luminous Flux Regulation Other than Stop] 
       FIGS. 24A and 24B  are diagrams used to explain the presence or absence of luminous flux regulation other than the stop. 
       FIG. 24A  is a diagram showing an imaging optical system  30 - 1  in which luminous flux is regulated by a stop  32 , and  FIG. 24B  is a diagram showing an imaging optical system  30 - 2  in which luminous flux is regulated in a point other than the stop  32 . 
     As shown in  FIG. 24B , in a case where there are regulation points  11 A and  11 B which regulate the luminous flux in addition to the stop  32 , the off-axis luminous flux on the stop  32  is smaller than a stop diameter. 
     By removing (or minimizing) the regulation of the luminous flux other than the stop  32 , the off-axis luminous flux diameter on the stop  32  can cover almost the entire area of the stop  32 , and an image size can be the same for each annular region (for each wavelength range). 
     In a normal optical design, aberration is reduced by inserting a ray cut (luminous flux regulation other than the stop) to improve the performance. 
     As shown in  FIG. 24A , by preventing ray from being cut except for the stop  32 , normally, it is difficult to reduce the aberration, but in the present invention, since the axial chromatic aberration is corrected by utilizing the aberration, the desired performance can be realized even in a case where the ray cut is limited. 
     [Regarding Regulation  1  of Angle on Stop] 
       FIG. 25  is a diagram used to explain an angle of off-axis principal ray passing through the stop. 
     An imaging optical system  40  shown in  FIG. 25  includes a front lens group  40 A and a rear lens group  40 B, and a stop plane  42  is disposed between the front lens group  40 A and the rear lens group  40 B. 
     As shown in  FIG. 25 , an angle α of off-axis principal ray passing through the stop plane  42  is determined by an angle of the ray and an angular magnification of the front lens group  40 A. 
     In a case where the angular magnification of the front lens group  40 A is small, the angle α of the off-axis principal ray passing through the stop plane  42  approaches perpendicular to the stop plane  42 . As a result, an input angle of light incident on the stop plane  42  or a bandpass filter (not shown) disposed adjacent to the stop plane  42  can be reduced, and a wavelength shift due to a large incidence angle can be reduced. 
     [Regarding Regulation  2  of Angle on Stop] 
       FIG. 26  is a diagram used to explain an angle of on-axis marginal ray passing through the stop. 
     An imaging optical system  50  shown in  FIG. 26  includes a front lens group  50 A and a rear lens group  50 B, and a stop plane  52  is disposed between the front lens group  50 A and the rear lens group  50 B. 
     As shown in  FIG. 26 , an angle  3  of on-axis marginal ray passing through the stop plane  52  is determined by a diameter of entrance pupil and a focal length of the front lens group  50 A. 
     Therefore, the front lens group  50 A has a focal length which makes the angle β of the on-axis marginal ray incident on the stop plane  52  close to perpendicular to the stop plane  52 . 
     In a case where the focal length of the front lens group  50 A is large, the angle β of the on-axis marginal ray passing through the stop plane  52  approaches perpendicular to the stop plane  52 . As a result, the angle β (input angle) of the on-axis marginal ray incident on the stop plane  52  or a bandpass filter (not shown) disposed adjacent to the stop plane  52  can be reduced, and a wavelength shift due to a large incidence angle can be reduced. 
     [Other Methods for Acquiring Plurality of Images by Pupil Division] 
     &lt;Method of Acquiring Three Images Using Polarization Filter&gt; 
       FIG. 27  is a diagram showing another example of an arrangement pattern of polarization filter elements included in each pixel block of the imaging element. 
     As shown in the figure, polarization filter elements  122 A to  122 D having different transmission polarization directions are provided in four pixels P 1  to P 4  constituting one pixel block PB(X,Y). Specifically, the first pixel P 1  includes a first polarization filter element  122 A (first polarization filter) transmitting light in a polarization direction Θ 1  (for example, 90°). The second pixel P 2  includes a second polarization filter element  122 B (second polarization filter) transmitting light in a polarization direction Θ 2  (for example, 0°). The third pixel P 3  includes a third polarization filter element  122 C (third polarization filter) transmitting light in a polarization direction Θ 3  (for example, 45°). The fourth pixel P 4  includes a fourth polarization filter element  122 D (fourth polarization filter) transmitting light in a polarization direction Θ 4  (for example, 135°). 
     In a case where the number of wavelength ranges selected by the bandpass filter unit is 3, a polarization filter unit having three types of polarization directions (for example, Θ 1 , Θ 2 , and Θ 3 ) is provided corresponding to each pupil region of the bandpass filter unit. 
     Four image signals D 1  to D 4  are generated by separating and extracting pixel signals of the first pixel P 1 , the second pixel P 2 , the third pixel P 3 , and the fourth pixel P 4  from each pixel block PB(X,Y) of the imaging element. However, interference (crosstalk) occurs in the four image signals. That is, since light for each wavelength range is incident on each of the pixels P 1  to P 4 , the generated image is an image in which images of each wavelength range are mixed at a predetermined ratio. 
     Therefore, the signal processing unit  200  ( FIG. 1 ) performs interference elimination processing to generate the image signal for each wavelength range. 
     Hereinafter, the interference elimination processing performed in the signal processing unit  200  will be described. 
     In each pixel block PB(X,Y), a pixel signal (signal value) obtained from the first pixel P 1  is defined as x1, a pixel signal obtained from the second pixel P 2  is defined as x2, and a pixel signal obtained from the third pixel P 3  is defined as x3. The three pixel signals x1 to x3 are obtained from each pixel block PB(X,Y). The signal processing unit  200  calculates three pixel signals X1 to X3 corresponding to light in each wavelength range (for example, λ 1 , λ 2 , and λ 3 ) from the three pixel signals x1 to x3 to eliminate interference. Specifically, by [expression 4] in which a matrix A of [expression 3] is used, the three pixel signals X1 to X3 corresponding to light in each of the wavelength ranges λ 1 , λ 2 , and λ 3  to eliminate interference. 
     
       
         
           
             
               
                 
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     Hereinafter, a reason why the interference can be eliminated using [expression 4] described above will be described. 
     The interference occurs in a case where light from the pupil region of each wavelength range of each of the pixels P 1  to P 3  is mixed. A ratio (interference amount (also referred to as an interference rate)) in which the light incident on the pupil region of each wavelength range is received by each of the pixels (P 1  to P 3 ) is uniquely determined from a relationship between the polarization direction of the polarization filter unit provided in the pupil region for each wavelength range and the polarization direction of the polarization filter elements  122 A to  122 C provided in each of the pixels P 1  to P 3 . 
     Here, in a case where a ratio (interference amount) in which the light incident on the first pupil region is received by the first pixel P 1  is defined as b11, a ratio in which the light incident on the second pupil region is received by the first pixel P 1  is defined as b12, and a ratio in which the light incident on the third pupil region is received by the first pixel P 1  is defined as b13, X1, X2, X3, and x1 satisfy the following relationship. 
         b 11* X 1+ b 12* X 2+ b 13* X 3= x 1  [expression 5]
 
     In addition, in a case where a ratio in which the light incident on the first pupil region is received by the second pixel P 2  is defined as b21, a ratio in which the light incident on the second pupil region is received by the second pixel P 2  is defined as b22, and a ratio in which the light incident on the third pupil region is received by the second pixel P 2  is defined as b23, X1, X2, X3, and x2 satisfy the following relationship. 
         b 21* X 1+ b 22* X 2+ b 23* X 3= x 2  [expression 6]
 
     In addition, in a case where a ratio in which the light incident on the first pupil region is received by the third pixel P 3  is defined as b31, a ratio in which the light incident on the second pupil region is received by the third pixel P 3  is defined as b32, and a ratio in which the light incident on the third pupil region is received by the third pixel P 3  is defined as b33, X1, X2, X3, and x3 satisfy the following relationship. 
         b 31* X 1+ b 32* X 2+ b 33* X 3= x 3  [expression 7]
 
     For X1, X2, and X3, the simultaneous equations of [expression 4] to [expression 6] can be solved to acquire a pixel signal of an original image, that is, the pixel signals X1, X2, and X3 of the images passing through the three pupil regions. 
     Here, the simultaneous equations can be represented using [expression 9] in which a matrix B of [expression 8] is used. 
     
       
         
           
             
               
                 
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                     ] 
                   
                 
               
               
                 
                   [ 
                   
                     expression 
                     ⁢ 
                     
                         
                     
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                     8 
                   
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                               b 
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                   [ 
                   
                     expression 
                     ⁢ 
                     
                         
                     
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                     9 
                   
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     X1, X2, and X3 are calculated by the following expression by multiplying both sides of [expression 9] by an inverse matrix B −1  of the matrix B. 
     
       
         
           
             
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     As described above, the pixel signals X1, X2, and X3 of the images obtained in each pupil region can be calculated from the pixel signals x1, x2, and x3 of each of the pixels P 1  to P 3 , based on the ratio (interference amount) in which the light incident on each pupil region is received by each of the pixels P 1  to P 3 . The matrix A in [expression 3] described above is the inverse matrix B −1  of the matrix B (A=B −1 ). 
     &lt;Method of Dividing Pupils Using Light Blocking Mask&gt; 
     A first light blocking mask having a first microlens and a first opening corresponding to the first pupil region is provided in the first pixel of the imaging element, and a second light blocking mask having a second microlens and a second opening corresponding to the second pupil region is provided in the second pixel of the imaging element. As a result, by using the light blocking masks (first light blocking mask and second light blocking mask) to divide the first pupil region and the second pupil region, luminous flux passing through each pupil region can be incident on the first pixel and the second pixel of the imaging element. 
       FIG. 28  is a diagram showing an example of one pixel of the imaging element having the light blocking mask. 
     In  FIG. 28, 112  is a photodiode and  142  is a microlens, and a light blocking mask M is disposed between the photodiode  112  and the microlens  142 . 
     In addition, the light blocking mask M has an opening corresponding to a shape of each pupil region. The photodiode  112  receives only light passing through a desired pupil region by the light blocking mask M. 
     Therefore, by configuring the pixel block of the imaging element with a plurality of types of pixels provided with the light blocking mask M having an opening corresponding to the shape of each pupil region, images of each wavelength range passing through each pupil region can be acquired. 
     [Imaging Method] 
       FIG. 29  is a flowchart showing an embodiment of an imaging method according to the present invention. The imaging method shown in  FIG. 29  corresponds to the imaging device  1  shown in  FIG. 1 , and in a case of performing the imaging method, the imaging optical system  10  according to the embodiment of the present invention is prepared. 
     In  FIG. 29 , the imaging device  1  determines whether or not there is a shooting instruction input for images in a plurality of wavelength ranges by, for example, an instruction input from a shutter release switch (step S 10 ). 
     In a case where the imaging device  1  receives an instruction input for shooting, the imaging device  1  controls exposure to the imaging element  100  (step S 12 ). As a result, in each of the pixels (first pixel P 1  and second pixel P 2 ) of the imaging element  100 , charges corresponding to an amount of light incident on the photodiode are accumulated, and an electric signal corresponding to an amount of charge accumulated in each pixel is read out from the imaging element  100  as an image signal, and the image signal is output. 
     The signal processing unit  200  acquires the signal output from the imaging element  100  (step S 14 ), and generates a first image in the first wavelength range based on the output signal of the first pixel P 1  and generates a second image in the second wavelength range λ 2  based on the output signal of the second pixel P 2 . 
     As a result, the imaging device  1  can simultaneously capture the first image of the first wavelength range λ 1  and the second image of the second wavelength range λ 2  (multispectral image), and the first image and the second image are images having different wavelength ranges from each other by the bandpass filter unit  16 . However, in the imaging device  10 , since the axial chromatic aberration of the imaging optical system  10  due to the difference between the first wavelength range λ 1  and the second wavelength range λ 2  is reduced based on the relationship between the aberrations (in the embodiment of  FIG. 1 , spherical aberration) other than the axial chromatic aberration of the imaging optical system  10  and the positions of the first pupil region and the second pupil region of the imaging optical system  10 , a multispectral image with improved axial chromatic aberration is captured. 
     [Others] 
     The imaging device according to the embodiment of the present invention is not limited to an imaging device for capturing a still image in a plurality of wavelength ranges, and may be an imaging device for simultaneously capturing a motion picture in a plurality of wavelength ranges. 
     In addition, the present invention is not limited to the imaging device, and includes an imaging optical system constituting the imaging device. For example, in a case of an imaging device in which the imaging optical system can be exchanged, by exchanging with an imaging optical system designed according to the wavelength range, it is possible to capture a multispectral image in a desired wavelength range. 
     The present invention is not limited to the above embodiments and can be subjected to various modifications without departing from the spirit of the present invention. 
     EXPLANATION OF REFERENCES 
     
         
         
           
               1 : imaging device 
               10 ,  20 ,  30 - 1 ,  30 - 2 ,  40 ,  50 : imaging optical system 
               11 A,  11 B: regulation point 
               12 : lens 
               16 ,  16 - 2 ,  16 - 3 ,  16 - 4 : bandpass filter unit 
               16 A,  16 - 2 A,  18 A,  18 - 2 A: frame 
               16 B 1 ,  16 B 2 ,  16 - 2 B 1 ,  16 - 2 B 2 ,  16 - 2 B 3 ,  16 - 2 B 4 : bandpass filter 
               16 A 1 ,  16 A 2 ,  18 A 1 ,  18 A 2 : opening region 
               18 ,  18 - 2 : polarization filter unit 
               18 B 1 ,  18 B 2 ,  18 - 2 B 1 ,  18 - 2 B 2 ,  18 - 2 B 3 : polarization filter 
               20 A,  40 A,  50 A: front lens group 
               20 B,  40 B,  50 B: rear lens group 
               32 : stop 
               42 ,  52 : stop plane 
               100 ,  100 - 2 : imaging element 
               110 : pixel array layer 
               112 : photodiode 
               120 : polarization filter element array layer 
               122 A,  122 B,  122 C,  122 D: polarization filter element 
               130 : spectral filter element array layer 
               132 A,  132 B: spectral filter element 
               140 : microlens array layer 
               142 : microlens 
               200 : signal processing unit 
             L: optical axis 
             M: light blocking mask 
             P 1 , P 2 , P 3 , P 4 : pixel 
             PB: pixel block 
             S 10 : step 
               512 : step 
               514 : step 
             Θ 1 , Θ 2 , Θ 3 , Θ 4 : polarization direction 
             Ωi: pupil region 
             α: angle of off-axis principal ray 
             β: angle of on-axis marginal ray