Patent Publication Number: US-2021191096-A1

Title: Image pickup apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation application of International Application No. PCT/JP2019/004235 filed on Feb. 6, 2019, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates to an image pickup apparatus capable of isotropically acquiring spatial frequency information of a sample. 
     Description of the Related Art 
     An image pickup apparatus capable of acquiring spatial frequency information of a sample is disclosed in “Rapid 3D Refractive-Index Imaging of Live Cells in Suspension without Labeling Using Dielectrophoretic Cell Rotation”, Adv. Sci. 2017, 4, 1600205. 
     In “Rapid 3D Refractive-Index Imaging of Live Cells in Suspension without Labeling Using Dielectrophoretic Cell Rotation”, an image pickup apparatus includes a light source, a microscope objective lens, an imaging lens, and an image sensor. In this apparatus, a subject is rotated using two orthogonal axes as axes of rotation. The two axes of rotation are positioned in a plane orthogonal to an optical axis of the microscope objective lens. 
     It is possible to acquire spatial frequency information of a sample by rotating the sample about two axes. The acquisition of spatial frequency information will be described in detail later. 
     SUMMARY 
     An image pickup apparatus according to at least some embodiments of the present disclosure includes: 
     a signal acquisition unit and a rotation unit, wherein 
     the signal acquisition unit includes an illumination unit including a light source and configured to irradiate a sample with a light beam, a photodetector including a plurality of light-receiving portions two-dimensionally arranged, and a detection optical system configured to guide light having been irradiated from the illumination unit to the sample and passed through the sample, to the photodetector, 
     the rotation unit rotates the sample and the signal acquisition unit relative to each other, about a first axis intersecting an optical axis of the detection optical system in the sample, and 
     the illumination unit irradiates the sample with light beams at two or more incident angles, in a plane including the optical axis and the first axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an image pickup apparatus of the present embodiment; 
         FIG. 2  is a diagram illustrating an image pickup apparatus of the present embodiment; 
         FIG. 3A  and  FIG. 3B  are diagrams illustrating an example of a sample and an example of a scattering potential; 
         FIG. 4A  and  FIG. 4B  are diagrams illustrating the relation between the direction of measurement light and the position of a spherical shell of the Ewald sphere; 
         FIG. 5A ,  FIG. 5B , and  FIG. 5C  are diagrams illustrating movement of a spherical shell of the Ewald sphere according to a first method; 
         FIG. 6A  and  FIG. 6B  are diagrams illustrating a missing region; 
         FIG. 7A  and  FIG. 7B  are diagrams illustrating movement of a spherical shell of the Ewald sphere according to a second method; 
         FIG. 8A  and  FIG. 8B  are diagrams illustrating the relation between the direction of measurement light and the positions of spherical shells of the Ewald sphere; 
         FIG. 9  is a diagram illustrating a state of measurement light incident on a sample; 
         FIG. 10A ,  FIG. 10B , and  FIG. 10C  are diagrams illustrating a missing region; 
         FIG. 11A ,  FIG. 11B , and  FIG. 11C  are diagrams illustrating a missing region; 
         FIG. 12A  and  FIG. 12B  are diagrams illustrating a missing region; 
         FIG. 13A  and  FIG. 13B  are diagrams illustrating a ratio of the size of the missing region; 
         FIG. 14  is a diagram illustrating an illumination unit of the image pickup apparatus of the present embodiment; 
         FIG. 15  is a diagram illustrating an image pickup apparatus of the present embodiment; 
         FIG. 16  is a diagram illustrating an image pickup apparatus of the present embodiment; 
         FIG. 17  is a diagram illustrating an illumination unit of the image pickup apparatus of the present embodiment; 
         FIG. 18  is a diagram illustrating an image pickup apparatus of the present embodiment; 
         FIG. 19  is a diagram illustrating an image pickup apparatus of the present embodiment; 
         FIG. 20  is a diagram illustrating an image pickup apparatus of the present embodiment; 
         FIG. 21  is a diagram illustrating an image pickup apparatus of the present embodiment; 
         FIG. 22  is a diagram illustrating an image pickup apparatus of the present embodiment; 
         FIG. 23A ,  FIG. 23B ,  FIG. 23C ,  FIG. 23D ,  FIG. 23E ,  FIG. 23F ,  FIG. 23G ,  FIG. 23H , and  FIG. 23I  are diagrams illustrating an image illustrating an acquisition range of the scattering potential; 
         FIG. 24A ,  FIG. 24B ,  FIG. 24C ,  FIG. 24D ,  FIG. 24E ,  FIG. 24F ,  FIG. 24G ,  FIG. 24H ,  FIG. 24I ,  FIG. 24J ,  FIG. 24K , and  FIG. 24L  are diagrams illustrating an image of a sample; 
         FIG. 25A ,  FIG. 25B ,  FIG. 25C , and  FIG. 25D  are diagrams illustrating a flat sample and a distribution of the scattering potential; and 
         FIG. 26A ,  FIG. 26B ,  FIG. 26C , and  FIG. 26D  are diagrams illustrating a three-dimensional sample and a distribution of the scattering potential. 
     
    
    
     DETAILED DESCRIPTION 
     Prior to the explanation of examples, action and effect of embodiments according to certain aspects of the present disclosure will be described below. In the explanation of the action and effect of the embodiments concretely, the explanation will be made by citing concrete examples. However, similar to a case of the examples to be described later, aspects exemplified thereof are only some of the aspects included in the present disclosure, and there exists a large number of variations in these aspects. Consequently, the present disclosure is not restricted to the aspects that will be exemplified. 
     An image pickup apparatus of the present embodiment includes a signal acquisition unit and a rotation unit. The signal acquisition unit includes an illumination unit including a light source and configured to irradiate a sample with a light beam, a photodetector including a plurality of light-receiving portions two-dimensionally arranged, and a detection optical system configured to guide light having been irradiated from the illumination unit to the sample and passed through the sample, to the photodetector. The rotation unit rotates the sample and the signal acquisition unit relative to each other, about a first axis intersecting an optical axis of the detection optical system in the sample. The illumination unit irradiates the sample with light beams at two or more incident angles in a plane including the optical axis and the first axis. 
     The image pickup apparatus of the present embodiment is illustrated in  FIG. 1 . In a description of the image pickup apparatus of the present embodiment, a plurality of diagrams including  FIG. 1  are used. In each diagram, since the state of light rays is qualitatively drawn, the states of refraction of light rays and reflection of light rays are not always accurately drawn. An optical axis AX corresponds to the Z axis in an XYZ coordinate system. 
     An image pickup apparatus  1  includes a signal acquisition unit  2  and a rotation unit  3 . The signal acquisition unit  2  includes an illumination unit  4 , a detection optical system  5 , and a photodetector  6 . The rotation unit  3  will be described later. 
     The illumination unit  4  includes a light source  7  and irradiates a sample  8  with light. It is possible to use a light source that produces monochromatic light or a light source that produces quasi-monochromatic light for the light source  7 . The illumination unit  4  includes a mirror  9 . The mirror  9  will be described later. 
     The light source that produces monochromatic light is, for example, a laser. The light source that produces quasi-monochromatic light is, for example, a light-emitting diode (LED). It is possible to produce quasi-monochromatic light with a combination of a white light source and a narrow bandpass filter. 
     Light is irradiated from the illumination unit  4  to the sample  8 . The detection optical system  5  guides light passed through the sample  8  to the photodetector  6 . 
     The photodetector  6  includes a plurality of light-receiving portions two-dimensionally arranged. A CCD image sensor or a CMOS image sensor may be used for the photodetector  6 . It is possible to use a Si sensor or an InGaAs sensor as the photodetector  6  in accordance with the wavelength of light emitted from the light source  7 . 
     A parallel light beam or a substantially parallel light beam (hereinafter referred to as “parallel light beam”) is emitted from the light source  7 . The parallel light beam is incident on the mirror  9  and reflected by the mirror  9  toward the sample  8 . The sample  8  is irradiated with the parallel light beam reflected by the mirror  9 . As a result, the sample  8  is illuminated with the parallel light beam. 
     Light emerged from the sample  8  is incident on the detection optical system  5 . Light emerged from the detection optical system  5  is incident on the photodetector  6 . 
     In the image pickup apparatus  1 , an optical path is formed from the light source  7  to the photodetector  6 . If the number of optical paths is two, it is possible to obtain phase information directly from interference fringes. However, in the image pickup apparatus  1 , since there is only one optical path, it is not possible to obtain phase information directly from interference fringes. 
     Therefore, in the image pickup apparatus  1 , amplitude data of a wavefront is measured. The wavefront to be measured is a wavefront at a detection surface of the photodetector  6 . In measurement of amplitude data of a wavefront, measurement with a plurality of wavelengths or measurement by changing the illumination angle may be performed. In the measurement by changing the illumination angle, the illumination angle is changed by a minute angle. By using these measurement methods, it is possible to measure a data set necessary for phase estimation of a wavefront at the detection surface. 
     Therefore, in the image pickup apparatus  1 , a reference optical path is not necessary and thus an optical system can be more readily constructed. 
     An image pickup apparatus of the present embodiment is illustrated in  FIG. 2 . The same configuration as that in  FIG. 1  is denoted by the same numeral and a description thereof is omitted. 
     An image pickup apparatus  10  includes a signal acquisition unit  11  and a rotation unit  12 . The signal acquisition unit  11  includes an illumination unit  13 , the detection optical system  5 , and the photodetector  6 . The rotation unit  12  will be described later. 
     The illumination unit  13  includes a photocoupler  14 , an optical switch  15 , a lens  16 , and a mirror  17 . The optical switch  15  will be described later. 
     Light emitted from the light source  7  is incident on the photocoupler  14  through an optical system (not illustrated). The light incident on the photocoupler  14  is split into two light beams by the photocoupler  14 . The two light beams are emerged individually from an optical fiber  14   a  and an optical fiber  14   b.    
     The light emerged from the optical fiber  14   a  is incident on the lens  16  through the optical switch  15 . A parallel light beam is emerged from the lens  16 . The parallel light beam is incident on the mirror  17  and reflected by the mirror  17  toward the sample  8 . The sample  8  is irradiated with the parallel light beam reflected by the mirror  17 . As a result, the sample  8  is illuminated with the parallel light beam. 
     An output end of the optical fiber  14   b  is positioned in the vicinity of the photodetector  6 . The light emerged from the optical fiber  14   b  is converted to a parallel light beam by an optical system (not illustrated) and thereafter incident on the photodetector  6 . 
     An optical path is formed from the light source  7  to the photodetector  6  also in the image pickup apparatus  10 . However, unlike the image pickup apparatus  1 , the image pickup apparatus  10  has two optical paths. Accordingly, in the image pickup apparatus  10 , it is possible to obtain phase information directly from interference fringes. 
     Light emitted from the light source  7  is emitted from the output end of the optical fiber  14   b . That is, the light emitted from the optical fiber  14   b  does not pass through the sample  8 . In the image pickup apparatus  10 , a reference optical path is formed by the optical fiber  14   b.    
     It is possible to measure a data set that enables reconstruction of a wavefront from the interference fringes detected by the photodetector  6 . For the measurement of a data set, a phase shifting method or a Fourier fringe analysis method may be used. 
     The case of using interference fringes is described. In the following description, illumination light is referred to as measurement light. As described above, it is possible to detect interference fringes by the photodetector  6 . 
     By analyzing the interference fringes, it is possible to acquire a scattering potential. For example, a refractive index is obtained from the scattering potential. 
     The acquisition of the scattering potential is described. A space in which a sample is disposed (hereinafter referred to as “real space”) is a space in units of distance. The measurement light is a physical quantity in real space. The measurement light includes scattering light. Therefore, the scattering light is also a physical quantity in real space. 
     The real space is transformed by a Fourier transform into a space in units of frequency (hereinafter referred to as “frequency space”). The interference fringes can be considered as representation of information in frequency space. The interference fringes include information on physical quantity in real space, for example, information on scattering light. Scattering light in real space is represented by the scattering potential intersecting a spherical shell of the Ewald sphere in frequency space. 
     An example of the sample and an example of the scattering potential are illustrated in  FIG. 3A  and  FIG. 3B .  FIG. 3A  is a diagram illustrating a sample, and  FIG. 3B  is a diagram illustrating the scattering potential. 
     A sample  20  has colorless transparent spheres  21 . For example, the diameter of the sphere  21  is 10 μm, and the refractive index of the sphere  21  is 1.364. The surrounding of the sphere  21  is filled with a colorless transparent liquid. For example, the refractive index of the liquid is 1.334. Six spheres  21  are arranged in a row. 
     The scattering potential in real space is obtained from a refractive index distribution of the sample  20 . By performing Fourier transform for this scattering potential, a scattering potential in frequency space can be obtained. Physical information of the sample  20 , for example, the position, size, and refractive index all can be represented by numerical values. Thus, the scattering potential is obtained in a simulation. The scattering potential illustrated in  FIG. 3B  illustrates the result in a simulation. 
     The fy direction in frequency space corresponds to the y direction in real space. The fz direction in frequency space corresponds to the z direction in real space. As illustrated in  FIG. 3B , the scattering potential in frequency space is distributed in the fy direction and the fz direction. 
     As described above, scattering light is produced in the sample  20 . The direction in which scattering light is produced and the amplitude thereof depend on the irradiation angle of measurement light for the sample  20 . Therefore, when the irradiation angle of measurement light is determined, only the scattering light having a specific amplitude for each direction is incident on the photodetector  6 . That is, scattering light that can be detected is limited. 
     The scattering potential in frequency space corresponds to scattering light in real space. When scattering light that can be detected is limited, the scattering potential that can be acquired is also limited. In  FIG. 3B , the scattering potential is distributed in the fy direction and the fz direction. However, the scattering potential that can be acquired is a part of this. 
     The scattering potential that can be acquired depends on the irradiation angle of measurement light. The irradiation angle of measurement light is represented by the direction connecting the center of the sphere shell of the Ewald sphere and the origin in frequency space. 
     The relation between the direction of measurement light and the position of the spherical shell of the Ewald sphere is illustrated in  FIG. 4A  and  FIG. 4B .  FIG. 4A  is a diagram illustrating the direction of measurement light, and  FIG. 4B  is a diagram illustrating the position of the spherical shell of the Ewald sphere.  FIG. 4B  is an enlarged view of the central portion of  FIG. 3B . 
     In  FIG. 4A , the sample  20  is vertically irradiated with measurement light  22 . In this case, the position of the spherical shell of the Ewald sphere is as depicted by a curve  23  as illustrated in  FIG. 4B . 
     In  FIG. 4B , the curve  23  corresponds to the irradiation angle of the measurement light  22 . Thus, only the scattering potential at a portion intersecting the curve  23  is the scattering potential that can be actually acquired. 
     As illustrated in  FIG. 4B , the scattering potential is distributed in the fy direction and the fz direction. However, as illustrated in  FIG. 4B , the scattering potential that can be actually acquired is limited to the scattering potential at a portion intersecting the curve  23 . If the number of scattering potentials that can be acquired are few, it is difficult to calculate the refractive index at a high accuracy. 
     In order to increase the number of scattering potentials that can be acquired, the curve  23  may be moved. By moving the curve  23 , it is possible to acquire the scattering potential after moving the curve  23  in addition to the scattering potential before moving the curve  23 . As a result, it is possible to increase the number of scattering potentials that can be acquired. 
     The curve  23  depicts a cross section of the spherical shell of the Ewald sphere. Thus, it is possible to represent the movement of the curve  23  using the spherical shell of the Ewald sphere. A first method for moving the spherical shell of the Ewald sphere is described. 
     The movement of the spherical shell of the Ewald sphere according to the first method is illustrated in  FIG. 5A ,  FIG. 5B , and  FIG. 5C .  FIG. 5A  is a diagram illustrating a state before moving the spherical shell of the Ewald sphere,  FIG. 5B  is a diagram illustrating a state after moving the spherical shell of the Ewald sphere, and  FIG. 5C  is a diagram illustrating the two states superimposed on each other. 
     In the first method, a spherical shell  30  of the Ewald sphere (hereinafter referred to as “spherical shell  30 ”) is rotated around the fy axis. 
     The angle of rotation before moving the spherical shell  30  is 0 degrees. For example, as illustrated in  FIG. 5B , the spherical shell  30  is rotated from this state around the fy axis by 180 degrees. By doing so, as illustrated in  FIG. 5C , two spherical shells  30  are obtained. As a result, the number of scattering potentials that can be acquired is doubled. Strictly speaking, it is possible to acquire the scattering potential at 0 degrees and the scattering potential at 180 degrees simultaneously in one measurement. 
     Here, the spherical shell  30  is rotated around the fy axis by 180 degrees. However, the angle by which the spherical shell  30  is rotated around the fy axis is not limited to 180 degrees. The spherical shell  30  may be moved by an angle smaller than 180 degrees. For example, the spherical shell  30  may be rotated around the fy axis by 90 degrees. By doing so, it is possible to further increase the number of scattering potentials that can be acquired. 
     Even in the image pickup apparatus of the present embodiment, it is possible to further increase the number of scattering potentials that can be acquired, if the spherical shell  30  can be rotated around the fy axis by 180 degrees. 
     The rotation unit  3  and the rotation unit  12  are described. In the rotation unit  3  and the rotation unit  12 , relative rotation is performed around a first axis Y. The first axis Y is the axis intersecting the optical axis AX. The optical axis AX is, for example, the optical axis of the detection optical system  5 . 
     The intersection of the first axis Y and the optical axis AX coincides with the focus position of the detection optical system  5 . The relative rotation is performed in a state in which the sample  8  includes the focus position of the detection optical system  5  in its inside. 
     The image pickup apparatus  1  includes the rotation unit  3 . It is possible to rotate the sample  8  and the signal acquisition unit  2  relative to each other by the rotation unit  3 . In the image pickup apparatus  1 , the signal acquisition unit  2  is fixed, and the sample  8  rotates around the first axis Y. By rotating the sample  8  and the signal acquisition unit  2  relative to each other, it is possible to rotate the spherical shell  30 . Therefore, in the image pickup apparatus  1 , it is possible to increase the number of scattering potentials that can be acquired. 
     The image pickup apparatus  10  includes the rotation unit  12 . It is possible to rotate the sample  8  and the signal acquisition unit  11  relative to each other by the rotation unit  12 . In the image pickup apparatus  10 , the sample  8  is fixed, and the signal acquisition unit  11  rotates around the first axis Y. By rotating the sample  8  and the signal acquisition unit  11  relative to each other, it is possible to rotate the spherical shell  30 . Therefore, in the image pickup apparatus  10 , it is possible to increase the number of scattering potentials that can be acquired. 
     However, even when the sample  8  and the signal acquisition unit  2  are rotated relative to each other about the first axis Y, there still remains a region in which the scattering potential is unable to be acquired (hereinafter referred to as “missing region”). 
     The missing region is illustrated in  FIG. 6A  and  FIG. 6B .  FIG. 6A  is a diagram illustrating a state before rotating the spherical shell of the Ewald sphere one turn, and  FIG. 6B  is a diagram illustrating a state after rotating the spherical shell of the Ewald sphere one turn. The figure in  FIG. 6A  is a figure in which the figure in  FIG. 5A  is rotated by 90 degrees. Thus, a description of  FIG. 6A  is omitted. 
     When the spherical shell  30  is rotated one turn around the fy axis, as illustrated in  FIG. 6B , a region  31  and a region  32  are formed. The region  31  is a missing region. The region  32  is a region in which the scattering potential can be acquired. The region  32  is a region excluding the region  31  from the sphere. 
     The region  31  has two regions each having a shape similar to a cone. In each region, a line corresponding to the generatrix of the cone is a curve. 
     If the region  31  can be reduced, it is possible to increase the number of scattering potentials that can be acquired. In order to reduce the region  31 , the region  32  is further moved from the state illustrated in  FIG. 6B . A second method for moving the spherical shell of the Ewald sphere is described. 
     The movement of the spherical shell of the Ewald sphere according to the second method is illustrated in  FIG. 7A  and  FIG. 7B .  FIG. 7A  is a diagram illustrating a state before rotating the spherical shell of the Ewald sphere one turn, and  FIG. 7B  is a diagram illustrating a state after rotating the spherical shell of the Ewald sphere one turn. 
     In the second method, as illustrated in  FIG. 7A , the region  32  is rotated around the fx axis. By doing so, as illustrated in  FIG. 7B , a spherical region  33  is formed. The region  32  is positioned at a place where the region  31  has been formed, whereby the region  33  is formed. Therefore, the region  31  disappears in the region  33 . As just described, it is possible to eliminate a missing region by rotating the region  32  around the fx axis. 
     In order to rotate the region  32  around the fx axis, for example, it is conceivable that the sample  8  and the signal acquisition unit  2  are rotated relative to each other about a predetermined axis in the image pickup apparatus  1 . The predetermined axis is an axis passing through the intersection of the first axis Y and the optical axis AX and orthogonal to a plane including the first axis Y and the optical axis AX. 
     However, in a method that is a combination of the first method and the second method, rotation occurs around two orthogonal axes. For example, in the image pickup apparatus  1 , the sample  8  and the signal acquisition unit  2  are rotated around two orthogonal axes. Therefore, the apparatus is complicated and it also takes time to acquire information. Thus, it is not preferable to use the second method. 
     It is possible to move the position of the spherical shell  30 , that is, the position of the curve  23  by a third method. The third method for moving the spherical shell of the Ewald sphere is described. 
     The position of the curve  23  changes according to the irradiation angle of measurement light. Thus, the curve  23  can be moved by changing the irradiation angle of measurement light. 
     The relation between the direction of measurement light and the position of the spherical shell of the Ewald sphere is illustrated in  FIG. 8A  and  FIG. 8B .  FIG. 8A  is a diagram illustrating the direction of measurement light, and  FIG. 8B  is a diagram illustrating the positions of the spherical shells of the Ewald spheres. A curve  23 , a curve  25 , and a curve  27  are curves each depicting the spherical shell of the Ewald sphere. 
       FIG. 8A  illustrates a state in which the sample  20  is irradiated with measurement light from three directions. The measurement light  22  depicts measurement light emitted vertically to the sample  20 . In this case, the curve  23  intersects the scattering potential. 
     Measurement light  24  depicts light irradiated obliquely to the sample  20 . In this case, the curve  25  intersects the scattering potential. Measurement light  26  depicts light irradiated obliquely to the sample  20 . The measurement light  26  is irradiated at an angle larger than the measurement light  24  is. In this case, the curve  27  intersects the scattering potential. 
     When the irradiation angle of measurement light is changed from the angle of the measurement light  22  to the angle of the measurement light  26 , the spherical shell of the Ewald sphere changes from the position of the curve  23  to the position of the curve  27 . The scattering potential that can be acquired varies with positions. Thus, it is possible to widen the acquisition range of the scattering potential by widening a variable range of the irradiation angle of measurement light. 
     As just described, compared with irradiation only with the measurement light  22  (hereinafter referred to as “unidirectional irradiation”), irradiation with the measurement light  22 , the measurement light  24 , and the measurement light  26  (hereinafter referred to as “multidirectional irradiation”) can widen the acquisition range of the scattering potential. 
     In unidirectional irradiation, the sample is irradiated with illumination light from one direction. Thus, for example, irradiation only with the measurement light  24  and irradiation only with the measurement light  26  are also included in unidirectional irradiation. 
     In multidirectional irradiation, the sample is irradiated with illumination light from a plurality of directions. Thus, for example, irradiation only with the measurement light  22  and the measurement light  24  and irradiation only with the measurement light  24  and the measurement light  26  are also included in multidirectional irradiation. 
     Expansion of the range in which the scattering potential can be acquired means reduction of the missing region. It is possible to reduce the missing region by multidirectional irradiation. A case where a sample is irradiated with measurement light from two directions is described. 
     A state of measurement light incident on the sample is illustrated in  FIG. 9 . As illustrated in  FIG. 9 , by performing multidirectional irradiation, the incident angle of measurement light on the sample  8  changes. Since measurement light is a parallel light beam, an incident angle θILL of measurement light on the sample  8  is the angle formed between a central ray of measurement light and the optical axis AX. 
     In measurement light depicted by solid lines, a central ray L ILL  of measurement light intersects the optical axis AX. Therefore, θILL≠0°. In measurement light depicted by dotted lines, the central ray of measurement light coincides with the optical axis AX. Therefore, θILL=0°. 
     A state of the spherical shell of the Ewald sphere is illustrated in  FIG. 10A ,  FIG. 10B , and  FIG. 100 .  FIG. 10A  is a diagram illustrating a state before moving the spherical shell of the Ewald sphere,  FIG. 10B  is a diagram illustrating a state after moving the spherical shell of the Ewald sphere, and  FIG. 100  is a diagram illustrating the two states superimposed on each other. 
     In  FIG. 10A , a curve  34  and a curve  35  are curves each depicting the spherical shell of the Ewald sphere. The curve  34  is a curve when θILL=0°. The curve  35  is a curve when θILL=A° (where A≠0). 
     Measurement with θILL=0° and measurement with θILL=A° are not simultaneously performed. Therefore, the curve  34  and the curve  35  are not simultaneously generated. However, in data, as illustrated in  FIG. 10A , it is possible to superimpose the curve  34  and the curve  35  on each other. 
     In  FIG. 10B , a curve  34 ′ and a curve  35 ′ are curves each depicting the spherical shell of the Ewald sphere. The curve  34 ′ is a curve when θILL=0°. The curve  35 ′ is a curve when θILL=A° (where A≠0). 
       FIG. 10A  illustrates a state before moving the spherical shell of the Ewald sphere, where the angle of rotation is 0 degrees. In  FIG. 10B , the spherical shell of the Ewald sphere is rotated around the fy axis by 180 degrees. 
     In  FIG. 100 , the missing region is illustrated. As described above, rotation of the spherical shell of the Ewald sphere is performed around the fy axis. Therefore, the missing region is formed along the fy axis. Then, an evaluation range in the fy direction of the missing region is set as an acquisition frequency range in the fy direction with θILL=0°. 
     As illustrated in  FIG. 100 , a missing region  36  is formed between the curve  34  and the curve  34 ′. The missing region  36  includes a region  36 ′ and a region  36 ″. A missing region  37  is formed between the curve  35  and the curve  35 ′. The missing region  37  includes a region  37 ′, a region  37 ″, and a region  37 ′″. 
     In a range fy(+), the region  36 ′ is formed between the curve  34  and the curve  34 ′. In a range fy(−), the region  36 ′ is formed between the curve  34  and the curve  34 ′. When θILL=0°, the range of the missing region and the shape of the missing region are the same in the range fy(+) and the range fy(−). 
     In the range fy(+), the region  37 ′ and the region  37 ″ are formed between the curve  35  and the curve  35 ′. In the range fy(−), the region  37 ′ ‘ is formed between the curve  35  and the curve  35 ’. When θILL=A°, the range of the missing region and the shape of the missing region are different in the range fy(+) and the range fy(−). 
     The missing region when measurement with θILL=0° and measurement with θILL=A° are performed is a region where the missing region when θILL=0° and the missing region when θILL=A° overlap each other. 
     In the range fy(+), the entire region  37 ′ overlaps the region  36 ′. In the region  37 ″, the curve  34  and the curve  34 ′ are positioned inside the region. In a region surrounded by the curve  34  and the curve  35  and a region surrounded by the curve  34 ′ and the curve  35 ′, it is possible to acquire the scattering potential. As just described, in the region  37 ″, a part of the region overlaps the region  36 ′. Thus, in the range fy(+), the region  37 ′ and a part of the region  37 ″ depict a missing region. 
     In the range fy(−), the entire region  36 ″ overlaps the region  37 ′″. Thus, in the range fy(−), the region  36 ″ depicts a missing region. 
     A state of the missing region is illustrated in  FIG. 11A ,  FIG. 11B , and  FIG. 11C .  FIG. 11A  is a diagram illustrating a missing region when θILL=0°,  FIG. 11B  is a diagram illustrating a missing region when θILL=A°, and  FIG. 11C  is a diagram illustrating a state in which the missing regions are superimposed on each other. 
     As illustrated in  FIG. 11A , the missing region  36  is formed when the curve  34  illustrated in  FIG. 10A  is rotated one turn around the fy axis. The missing region  36  includes the region  36 ′ and the region  36 ″. 
     As illustrated in  FIG. 11B , the missing region  37  is formed when the curve  35  illustrated in  FIG. 10A  is rotated one turn around the fy axis. The missing region  37  includes the region  37 ′, the region  37 ″, and the region  37 ′″. 
       FIG. 11C  illustrates a state in which the missing region  36  and the missing region  37  are superimposed on each other. As illustrated in  FIG. 11C , it is possible to reduce the missing region in the range fy(+) by irradiating the sample with measurement light with θILL=A°. 
     However, it is not possible to reduce the missing region in the range fy(−) even by irradiating the sample with measurement light with θILL=A°. In order to reduce the missing region in the range fy(−), the incident angle θILL is set to an angle different from A°, for example, to −A°. 
     A state of the missing region is illustrated in  FIG. 12A  and  FIG. 12B .  FIG. 12A  is a diagram illustrating a missing region when θILL=-A°, and  FIG. 12B  is a diagram illustrating a state in which the missing regions are superimposed on each other. 
     As illustrated in  FIG. 12A , a missing region  38  is formed by irradiating the sample with measurement light with θILL=−A°. The missing region  38  includes a region  38 ′, a region  38 ″, and a region  38 ′″. 
     When rotated around the fz axis by 180 degrees, the missing region  38  coincides with the missing region  37  illustrated in  FIG. 11B . Thus, the region  38 ′ corresponds to the region  37 ′ “, the region  38 ′ corresponds to the region  37 ”, and the region  38 ′″ corresponds to the region  37 ′. 
     As described above, the entire region  37 ′ overlaps the region  36 ′. Thus, the entire region  38 ′″ overlaps the region  36 ″. In the region  37 ″, a part of the region overlaps the region  36 ′. Thus, in the region  38 ″, a part of the region overlaps the region  36 ″. 
     When measurement with θILL=0° and measurement with θILL=-A° are performed, in the range fy(−), a part of the region  38 ″ and the region  38 ′″ depict a missing region. 
       FIG. 12B  illustrates a state in which the missing region  36 , the missing region  37 , and the missing region  38  are superimposed on each other. As illustrated in  FIG. 12B , it is possible to reduce the missing region in the range fy(+) by irradiating the sample with measurement light with θILL=A°, and it is possible to reduce the missing region in the range fy(−) by irradiating the sample with measurement light with θILL=−A°. 
     If the missing region can be reduced, it is possible to isotropically acquire spatial frequency information of a sample. 
     The mirror  9  and the optical switch  15  are described. 
     The image pickup apparatus  1  includes the mirror  9 . By moving the mirror  9 , it is possible to set the irradiation angle of measurement light to two or more angles. The irradiation angle is an incident angle of measurement light on the sample  8 . Thus, by moving the mirror  9 , it is possible to set the incident angle of measurement light on the sample  8  to two or more angles. In this way, multidirectional irradiation is possible in the image pickup apparatus  1 . 
     In the image pickup apparatus  1 , the mirror  9  is moved by rotation. The axis of rotation of the mirror  9  is orthogonal to a plane including the optical axis AX and the first axis Y. Thus, it is possible to set the incident angle of measurement light on the sample  8  to two or more angles in the plane including the optical axis AX and the first axis Y. 
     The image pickup apparatus  10  includes the optical switch  15 . Light emerged from the optical fiber  14   a  is incident on the optical switch  15 . The optical switch  15  includes a plurality of optical fibers  15   a  on the output side. In the optical switch  15 , light is emitted from any one optical fiber  15   a  among the optical fibers. By changing the optical fiber  15   a  that emits light, it is possible to set the incident angle of measurement light on the sample  8  to two or more angles. In this way, multidirectional irradiation is possible in the image pickup apparatus  10 . 
     In the image pickup apparatus  10 , a plurality of fibers are arranged in a row along an axis parallel to the optical axis AX. Thus, it is possible to set the incident angle of measurement light on the sample  8  to two or more angles in the plane including the optical axis AX and the first axis Y. 
     In multidirectional irradiation, it is desirable that the incident angle of measurement light can be changed to two or more angles. By doing so, it is possible to widen the acquisition range of the scattering potential. 
     In the image pickup apparatus  1 , the orientation of the mirror  9  changes over time. In the image pickup apparatus  10 , the optical fibers from which light is emerged are changed over time. Thus, the sample  8  is not irradiated simultaneously with a plurality of measurement light beams. That is, the sample  8  is irradiated with a plurality of measurement light beams separately in time. 
     As just described, in the image pickup apparatus  1  and the image pickup apparatus  10 , illumination by multidirectional irradiation is conditional on illumination. Then, measurement in a plurality of rotation states is performed under this illumination condition. 
     As described above, in the image pickup apparatus  1 , it is only necessary that the sample  8  rotates relative to the signal acquisition unit  2  about the first axis Y. In the image pickup apparatus  10 , it is only necessary that the signal acquisition unit  11  rotates relative to the sample  8 . That is, in the image pickup apparatus  1  and the image pickup apparatus  10 , unlike the conventional method, rotation does not occur around two orthogonal axes. Therefore, in the image pickup apparatus  1  and the image pickup apparatus  10 , it is possible to simplify the configuration of the apparatus. 
     Furthermore, in the image pickup apparatus  1 , the angle of light emerged from the mirror  9  changes, whereby the position of the spherical shell of the Ewald sphere changes. In the image pickup apparatus  10 , the optical fibers from which light is emerged are changed, whereby the position of the spherical shell of the Ewald sphere changes. Therefore, in the image pickup apparatus  1  and the image pickup apparatus  10 , it is possible to diminish the missing region in the spatial frequency domain desired to be acquired. As a result, it is possible to isotropically acquire spatial frequency information of the sample  8 . 
     Furthermore, by setting the incident angle of measurement light on the sample  8  to two or more angles, it is possible to complementarily reconstruct the spatial frequency of a sample. The complementary reconstruction of the spatial frequency of a sample means that the spatial frequencies of scattering potentials of the sample that are acquired independently from each other (hereinafter referred to as “measurement information”) are synthesized. In this reconstruction, measurement information may be synthesized directly in a spatial frequency domain, or the spatial frequency of the sample may be estimated by repeatedly performing comparison between estimation of a wavefront in the photodetector and the measurement information such that the difference from pieces of measurement information is reduced. 
     As just described, in the image pickup apparatus  1  and the image pickup apparatus  10 , it is possible to isotropically acquire spatial frequency information of a sample, with a simple configuration. 
     In the image pickup apparatus of the present embodiment, it is preferable that the illumination unit can change the incident angle to an angle that satisfies the following Conditional Expression (1): 
       0 &lt;NAILL&lt;NA   (1)
 
     where 
     NA is an optical numerical aperture of the detection optical system, and 
     NAILL is a numerical aperture of light incident on the sample. 
     It is possible to determine the numerical aperture NAILL of light incident on the sample by NAILL=sin θILL. It is possible to determine the optical numerical aperture of the detection optical system  5  by NA=sin θ. 
     The ratio of the size of the missing region in multidirectional irradiation to the size of the missing region in unidirectional irradiation (hereinafter referred to as “the ratio of the size of the missing region”) is illustrated in  FIG. 13A  and  FIG. 13B .  FIG. 13A  is a diagram illustrating the ratio of the size of the missing region using NAILL and NA, and  FIG. 13B  is a diagram illustrating the ratio of the size of the missing region using NAILL/NA. 
     As described above, the following relation holds for θILL and θ. Thus, it is possible to describe the ratio of the size of the missing region using NAILL and NA. The parameter n is the refractive index of a medium. 
         NAILL=n  sin θ ILL  
 
         NA=n  sin θ
 
     In  FIG. 13A , NA is used for the horizontal axis, NAILL is used for the vertical axis, and the ratio of the size of the missing region is represented by brightness. The darker region has a smaller ratio of the size of the missing region. 
     In  FIG. 13B , NAILL/NA is used for the horizontal axis, and the ratio of the size of the missing region is used for the vertical axis. Thus, the ratio of the size of the missing region is represented by a numerical value. Each curve depicts the ratio of the size of the missing region on a linear line illustrated in  FIG. 13A . 
     For example, the change in ratio of the size of the missing region on a straight line L 01  is depicted by a curve denoted by 0.1 (hereinafter referred to as “curve  01 ”) in  FIG. 13B . Change in brightness in the straight line L 01  and change in numerical value in the curve  01  illustrate change in ratio of the size of the missing region when the numerical aperture of illumination light is changed from 0 to 0.1 in the detection optical system with a numerical aperture of 0.1. 
     A straight line L 02  corresponds to a curve denoted by 0.2, a straight line L 03  corresponds to a curve denoted by 0.3, a straight line L 04  corresponds to a curve denoted by 0.4, and a straight line L 05  corresponds to a curve denoted by 0.5. A straight line corresponding to a curve denoted by 0.6 and a straight line corresponding to a curve denoted by 0.7 are not illustrated in  FIG. 13A . 
     As illustrated in  FIG. 13A  and  FIG. 13B , the ratio of the size of the missing region changes between values of NAILL/NA of 0 and 1. When NAILL/NA=0, it is equivalent to unidirectional irradiation, and when NAILL/NA=1, the ratio of the size of the missing region is the same as that when NAILL/NA=0. This means that the effect of multidirectional irradiation is not produced when NAILL/NA=1. 
     Even when the value of the numerical aperture of the detection optical system varies, it is possible to reduce the ratio of the size of the missing region. The position at which the ratio of the size of the missing region is smallest does not depend on the value of numerical aperture of the detection optical system. 
     In the image pickup apparatus of the present embodiment, Conditional Expression (1) is satisfied. Therefore, it is possible to diminish the missing region. As a result, it is possible to isotropically acquire spatial frequency information of the sample  8 . 
     In the image pickup apparatus of the present embodiment, it is preferable that the illumination unit change the incident angle to an angle that satisfies the following Conditional Expression (1′). 
       0.13× NA 0 &lt;NAILL&lt; 0.7× NA   (1′)
 
     By changing the incident angle to the angle that satisfies Conditional Expression (1′), it is possible to set the ratio of the size of the missing region to 50% or smaller. 
     In the image pickup apparatus of the present embodiment, it is preferable that the illumination unit include an illumination optical system configured to irradiate the sample with light and a first optical deflection element disposed at a position conjugate to an intersection of the optical axis and the first axis, and the incident angle be changed to two or more angles by the first optical deflection element. 
     An illumination unit of an image pickup apparatus of the present embodiment is illustrated in  FIG. 14 . The same configuration as that in  FIG. 1  is denoted by the same numeral and a description thereof is omitted. 
     An illumination unit  40  includes the light source  7 , the mirror  9 , and an illumination optical system  41 . The illumination optical system  41  includes a lens  42  and a lens  43 . 
     The illumination unit  40  includes the first optical deflection element. It is possible to change the angle of the emerged light ray to two or more angles by the first optical deflection element. In the illumination unit  40 , the angle of the emerged light ray is changed to two or more angles by the mirror  9 . Thus, the mirror  9  is the first optical deflection element. 
     For example, it is possible to use a galvanometer scanner, a polygon scanner, or an acousto-optic deflector (AOD) as the optical deflection element. 
     The size of the galvanometer scanner and the size of the acousto-optic deflector is smaller than the size of the polygon scanner. Thus, it is possible to make the image pickup apparatus compact when the galvanometer scanner or the acousto-optic deflector is used. 
     In the galvanometer scanner, a large deflection angle can be obtained. However, it is difficult to deflect light at high speed. It is possible to achieve deflection of light not only by a mirror but by a half mirror. 
     In the polygon scanner, it is possible to obtain a large deflection angle and in addition, deflect light at high speed. In the acousto-optic deflector (AOD), it is possible to deflect light at high speed. However, the deflection angle is small. 
     It is preferable that the sample  8  be illuminated with a parallel light beam. In the illumination unit  40 , the parallel light beam is incident on the mirror  9 . Therefore, the mirror  9  is disposed at a position conjugate to the intersection of the optical axis AX and the first axis Y. As a result, it is possible to illuminate the sample  8  with the parallel light beam. 
     In addition, by deflecting the mirror  9 , it is possible to change the incident angle of measurement light on the sample  8  and irradiate the sample  8  with measurement light. 
     It is preferable that the image pickup apparatus of the present embodiment further include a first beam splitter, a second optical deflection element, and a second beam splitter, and a measurement optical path passing through the sample and a reference optical path be positioned between the light source and the photodetector. The first beam splitter and the second beam splitter each have an optical surface having an optical film. In the first beam splitter, light transmitted in a first direction and light reflected in a second direction is generated from incident light by the optical film. The measurement optical path is positioned in the first direction and the reference optical path is positioned in the second direction, or the reference optical path is positioned in the first direction and the measurement optical path is positioned in the second direction. The first optical deflection element is disposed on the measurement optical path, the second optical deflection element is disposed on the reference optical path, and an incident light ray is deflected and emerged. The second beam splitter is disposed at an intersection of the measurement optical path deflected by the first optical deflection element and the reference optical path deflected by the second optical deflection element. A light ray on the measurement optical path reflected by the second beam splitter and a light ray on the reference optical path transmitted through the second beam splitter is incident on the photodetector. 
     An image pickup apparatus of the present embodiment is illustrated in  FIG. 15 . The same configuration as that in  FIG. 1  and  FIG. 14  is denoted by the same numeral and a description thereof is omitted. 
     An image pickup apparatus  50  includes a beam splitter  51 , a mirror  52 , and a beam splitter  53 . The beam splitter  51 , the mirror  52 , and the beam splitter  53  are disposed, for example, in a signal acquisition unit  54 . 
     A measurement optical path OPm passing through the sample  8  and a reference optical path OPr are positioned between the light source  7  and the photodetector  6 . 
     In the measurement optical path OPm, the illumination unit  4  and a detection optical system  55  are disposed. In the reference optical path OPr, an optical path length adjustor  56 , the mirror  52 , and an optical system  57  are disposed. The optical path length adjustor  56  and the optical system  57  are disposed if necessary. 
     The beam splitter  51  has an optical surface having an optical film. In the beam splitter  51 , light transmitted in a first direction and light reflected in a second direction are generated from incident light by the optical film. Thus, the beam splitter  51  is the first beam splitter. 
     In the image pickup apparatus  50 , the measurement optical path OPm is positioned in the first direction, and the reference optical path OPr is positioned in the second direction. However, the reference optical path OPr may be positioned in the first direction, and the measurement optical path OPm may be positioned in the second direction. 
     As described above, the mirror  9  is the first optical deflection element. Thus, the first optical deflection element is disposed on the measurement optical path OPm. The mirror  52  is disposed on the reference optical path OPr and deflects an incident light ray and the deflected light ray is emerged. Thus, the mirror  52  is the second optical deflection element. 
     The beam splitter  53  has an optical surface having an optical film. The beam splitter  53  is disposed at the intersection of the measurement optical path OPm deflected by the mirror  9  and the reference optical path OPr deflected by the mirror  52 . Thus, the beam splitter  53  is the second beam splitter. 
     The optical path length adjustor  56  is disposed between the beam splitter  51  and the mirror  52 . The optical path length adjustor  56  includes, for example, a piezo stage and four mirrors. Two mirrors are placed on the piezo stage. By moving the two mirrors, it is possible to change the optical path length in the reference optical path OPr. 
     The optical system  57  is disposed between the mirror  52  and the beam splitter  53 . By disposing the optical system  57 , it is possible to overlap a light ray incident on the photodetector  6  from the reference optical path OPr on a light ray incident on the photodetector  6  from the measurement optical path OPm. 
     In the image pickup apparatus  50 , a light ray on the measurement optical path OPm reflected by the beam splitter  53  and a light ray on the reference optical path OPr transmitted through the beam splitter  53  are incident on the photodetector  6 . Thus, in the image pickup apparatus  50 , it is possible to obtain interference fringes. It is possible to measure a data set that enables reconstruction of a wavefront from the interference fringes detected by the photodetector  6 . 
     In the image pickup apparatus  50 , the signal acquisition unit  54  is fixed, and the sample  8  rotates around the first axis Y. Furthermore, multidirectional irradiation can be performed by moving the mirror  9 . In this way, in the image pickup apparatus  50 , it is possible to isotropically acquire spatial frequency information of a sample, with a simple configuration. 
     An image pickup apparatus of the present embodiment is illustrated in  FIG. 16 . The same configuration as that in  FIG. 1  and  FIG. 15  is denoted by the same numeral and a description thereof is omitted. 
     An image pickup apparatus  60  includes the beam splitter  51 , a mirror  61 , and a beam splitter  62 . The beam splitter  51 , the mirror  61 , and the beam splitter  62  are disposed, for example, in a signal acquisition unit  63 . 
     On the measurement optical path OPm, the illumination unit  4 , the detection optical system  5 , and the mirror  61  are disposed. On the reference optical path OPr, the optical path length adjustor  56  is disposed. The optical path length adjustor  56  is disposed if necessary. 
     The mirror  61  deflects an incident light ray and the deflected light ray is emerged. Thus, the mirror  61  is an optical deflection element. As described above, the mirror  9  is the first optical deflection element. In the image pickup apparatus  60 , the two optical deflection elements are disposed on the measurement optical path OPm. 
     Since light is deflected by the mirror  9 , light incident on the mirror  61  moves. The mirror  61  is an optical deflection element as described above and therefore, it is possible to change a deflection angle and a deflection direction. By appropriately setting the deflection angle and the deflection direction in the mirror  61 , it is possible to cancel the deflection of light caused by the mirror  9  with the deflection by the mirror  61 . Thus, in the image pickup apparatus  60 , a light ray emerged from the mirror  61  does not move. 
     The beam splitter  62  has an optical surface having an optical film. The beam splitter  62  is disposed at the intersection of the measurement optical path OPm and the reference optical path OPr. 
     In the image pickup apparatus  60 , a light ray on the measurement optical path OPm transmitted through the beam splitter  62  and a light ray on the reference optical path OPr reflected by the beam splitter  62  are incident on the photodetector  6 . Thus, in the image pickup apparatus  60 , it is possible to obtain interference fringes. It is possible to measure a data set that enables reconstruction of a wavefront from the interference fringes detected by the photodetector  6 . 
     In the image pickup apparatus  60 , the signal acquisition unit  63  is fixed, and the sample  8  rotates around the first axis Y. Furthermore, multidirectional irradiation can be performed by moving the mirror  9 . In this way, in the image pickup apparatus  60 , it is possible to isotropically acquire spatial frequency information of a sample, with a simple configuration. 
     In the image pickup apparatus of the present embodiment, it is preferable that the illumination unit include an illumination optical system configured to irradiate the sample with light and the illumination unit include a plurality of light-emerging portions arranged in an array and capable of being independently controlled. The light-emerging portions is disposed at a position conjugate to the intersection of the optical axis and the first axis. The illumination unit change the incident angle to two or more angles by performing control of changing the light-emerging portion from which light is emerged among the light-emerging portions. 
     The illumination unit of the image pickup apparatus of the present embodiment is illustrated in  FIG. 17 . The same configuration as that in  FIG. 14  is denoted by the same numeral and a description thereof is omitted. 
     An illumination unit  70  includes a light source  71  and the illumination optical system  41  that irradiates the sample  8  with light. The illumination unit  70  includes a plurality of light-emerging portions  72 . In the light-emerging portion  72 , it is possible to dispose a light source itself, for example, a light-emitting diode (LED) or a semiconductor laser (LD). 
     The light-emerging portions  72  are arranged in an array. When a light source itself is disposed in the light-emerging portion  72 , it is possible to control light emitting condition and non-emitting condition independently in the light-emerging portions  72 . 
     It is preferable that the sample  8  be illuminated with the parallel light beam. In the illumination unit  70 , the area of the light-emerging portions  72  is small to a degree that can be considered as a point light source. Therefore, the light-emerging portions  72  are disposed at a position conjugate to a pupil position  73  of the illumination optical system. 
     The illumination unit  70  includes a lens  74 . The lens  74  is disposed between the light source  71  and the lens  42 . With the lens  74  and the lens  42 , it is possible to conjugate the position of the light source  71  and the position of the pupil position  73 . When it is possible to ensure a sufficient space in the surrounding of the pupil position  73 , the light-emerging portions  72  may be disposed at the pupil position  73 . 
     In the illumination unit  70 , the light source  71  is controlled. In this control, among the light-emerging portions  72 , the light-emerging portion  72  from which light is emitted is changed. Multidirectional irradiation can be performed with this control. 
     Instead of using the light source  71 , it is possible to use the light source  7 , the photocoupler  14 , and the optical switch  15  as illustrated in  FIG. 2 . In this case, an output end surface of the optical fiber  15   a  is located at the position of the light-emerging portion  72 . Light emission does not occur at the output end surface of the optical fiber  15   a . However, the function is the same as when a light source itself is disposed at the light-emerging portion  72 , in that light is emitted. Thus, it is possible to consider the output end surface of the optical fiber  15   a  as the light-emerging portion  72 . 
     It is preferable that the image pickup apparatus of the present embodiment include a sample holder configured to hold the sample, and the rotation unit fix the signal acquisition unit and rotate the sample holder relative to the signal acquisition unit. 
     An image pickup apparatus of the present embodiment is illustrated in  FIG. 18 . The same configuration as that in  FIG. 2  and  FIG. 16  is denoted by the same numeral and a description thereof is omitted. 
     An image pickup apparatus  80  includes a signal acquisition unit  81  and the rotation unit  3 . The signal acquisition unit  81  includes the illumination unit  13 , the detection optical system  5 , and the photodetector  6 . 
     On the measurement optical path, the illumination unit  13 , the detection optical system  5 , and a mirror  82  are disposed. On the reference optical path, the optical fiber  14   b , a lens  83 , a mirror  84 , and the optical path length adjustor  56  are disposed. 
     The mirror  82  is a fixed mirror. Thus, in the image pickup apparatus  80 , a light ray emerged from the mirror  82  moves. Since the light flux diameter in the reference optical path is sufficiently large, it is possible to form interference fringes even when the light ray emerged from the mirror  82  moves. Furthermore, if the mirror  82  is disposed at a position conjugate to the sample  8  with the detection optical system  5  interposed therebetween, it is possible to suppress movement of a light flux on the photodetector  6 . 
     In the image pickup apparatus  80 , a light ray on the measurement optical path transmitted through the beam splitter  62  and a light ray on the reference optical path reflected by the beam splitter  62  are incident on the photodetector  6 . Thus, in the image pickup apparatus  80 , it is possible to obtain interference fringes. It is possible to measure a data set that enables reconstruction of a wavefront from the interference fringes detected by the photodetector  6 . 
     In the image pickup apparatus  80 , the signal acquisition unit  81  is fixed, and the sample  8  rotates around the first axis Y. Furthermore, multidirectional irradiation can be performed by changing the optical fibers  15   a  that light is emerged. In this way, in the image pickup apparatus  80 , it is possible to isotropically acquire spatial frequency information of a sample, with a simple configuration. 
     An image pickup apparatus of the present embodiment is illustrated in  FIG. 19 . The same configuration as that in  FIG. 1  and  FIG. 17  is denoted by the same numeral and a description thereof is omitted. 
     An image pickup apparatus  90  includes a signal acquisition unit  91  and the rotation unit  3 . The signal acquisition unit  91  includes the illumination unit  70 , the detection optical system  5 , and the photodetector  6 . 
     In the image pickup apparatus  90 , since there is only one optical path, it is not possible to obtain interference fringes. Therefore, in the image pickup apparatus  90 , amplitude data of a wavefront is measured. The measurement method is as described above. 
     In the image pickup apparatus  90 , the signal acquisition unit  91  is fixed, and the sample  8  rotates around the first axis Y. Furthermore, multidirectional irradiation can be performed by changing the light-emerging portions  72  from which light is emitted. In this way, in the image pickup apparatus  90 , it is possible to isotropically acquire spatial frequency information of a sample, with a simple configuration. 
     It is preferable that the image pickup apparatus of the present embodiment include a sample holder configured to hold the sample, and the rotation unit fix the sample holder and rotate the signal acquisition unit relative to the sample holder. 
     In the image pickup apparatus of the present embodiment, the rotation unit fixes the holder and rotates the signal acquisition unit relative to the holder. Since the signal acquisition unit rotates, it is unnecessary to rotate the sample. Therefore, when a sample is held in an aqueous solution, it becomes possible to conduct measurement without considering the position of the sample and the angle followability. 
     An image pickup apparatus of the present embodiment is illustrated in  FIG. 20 . The same configuration as that in  FIG. 2  and  FIG. 16  is denoted by the same numeral and a description thereof is omitted. 
     An image pickup apparatus  100  includes a signal acquisition unit  101  and the rotation unit  12 . The signal acquisition unit  101  includes the illumination unit  4 , the detection optical system  5 , and the photodetector  6 . 
     On the measurement optical path OPm, the illumination unit  4 , the detection optical system  5 , and the mirror  82  are disposed. On the reference optical path OPr, the optical path length adjustor  56  is disposed. 
     The mirror  82  is a fixed mirror. Thus, in the image pickup apparatus  100 , a light ray emerged from the mirror  82  moves. Since the light flux diameter in the reference optical path OPr is sufficiently large, it is possible to form interference fringes even when the light ray emerged from the mirror  82  moves. Furthermore, if the mirror  82  is disposed at a position conjugate to the sample  8  with the detection optical system  5  interposed therebetween, it is possible to suppress movement of a light flux on the photodetector  6 . 
     In the image pickup apparatus  100 , a light ray on the measurement optical path OPm transmitted through the beam splitter  62  and a light ray on the reference optical path OPr reflected by the beam splitter  62  are incident on the photodetector  6 . Thus, in the image pickup apparatus  100 , it is possible to obtain interference fringes. It is possible to measure a data set that enables reconstruction of a wavefront from the interference fringes detected by the photodetector  6 . 
     In the image pickup apparatus  100 , the sample  8  is fixed, and the signal acquisition unit  101  rotates around the first axis Y. Furthermore, multidirectional irradiation can be performed by moving the mirror  9 . In this way, in the image pickup apparatus  100 , it is possible to isotropically acquire spatial frequency information of a sample, with a simple configuration. 
     An image pickup apparatus of the present embodiment is illustrated in  FIG. 21 . The same configuration as that in  FIG. 1  and  FIG. 2  is denoted by the same numeral and a description thereof is omitted. 
     An image pickup apparatus  110  includes a signal acquisition unit  111  and the rotation unit  12 . The signal acquisition unit  111  includes the illumination unit  4 , the detection optical system  5 , and the photodetector  6 . 
     In the image pickup apparatus  110 , since there is only one optical path, it is not possible to obtain interference fringes. Therefore, in the image pickup apparatus  110 , amplitude data of a wavefront is measured. The measurement method is as described above. 
     In the image pickup apparatus  110 , the sample  8  is fixed, and the signal acquisition unit  111  rotates around the first axis Y. Furthermore, multidirectional irradiation can be performed by moving the mirror  9 . In this way, in the image pickup apparatus  110 , it is possible to isotropically acquire spatial frequency information of a sample, with a simple configuration. 
     An image pickup apparatus of the present embodiment is illustrated in  FIG. 22 . The same configuration as that in  FIG. 2  and  FIG. 19  is denoted by the same numeral and a description thereof is omitted. 
     An image pickup apparatus  120  includes a signal acquisition unit  121  and the rotation unit  12 . The signal acquisition unit  121  includes the illumination unit  70 , the detection optical system  5 , and the photodetector  6 . 
     In the image pickup apparatus  120 , since there is only one optical path, it is not possible to obtain interference fringes. Therefore, in the image pickup apparatus  120 , amplitude data of a wavefront is measured. The measurement method is as described above. 
     In the image pickup apparatus  120 , the sample  8  is fixed, and the signal acquisition unit  121  rotates around the first axis Y. Furthermore, multidirectional irradiation can be performed by changing the light-emerging portions  72  from which light is emitted. In this way, in the image pickup apparatus  120 , it is possible to isotropically acquire spatial frequency information of a sample, with a simple configuration. 
     In the image pickup apparatuses described above, it is possible to isotropically acquire spatial frequency information of a sample. That is, it is possible to widen the acquisition range of the scattering potential. As a result, it is possible to increase the number of scattering potentials that can be acquired. With increase in number of scattering potentials, it is possible to generate an image of a sample more accurately. 
     In order to reduce a shape change of incident wavefronts involved with rotation, the sample holder may be shaped like a cylinder. In order to reduce a wavefront change at the time of incidence on the sample holder, a gap between the sample holder and the apparatus may be filled with a matching solution. Furthermore, an optical system that applies a wavefront change in advance to cancel a wavefront change at the time of incidence on the sample holder may be further included. 
     It is also possible to obtain the scattering potential in a simulation. It is possible to generate an image of a sample based on the scattering potential obtained in a simulation. 
     An image illustrating the acquisition range of the scattering potential is illustrated in  FIG. 23A ,  FIG. 23B ,  FIG. 23C ,  FIG. 23D ,  FIG. 23E ,  FIG. 23F ,  FIG. 23G ,  FIG. 23H , and  FIG. 23I . An image of a sample is illustrated in  FIG. 24A ,  FIG. 24B ,  FIG. 24C ,  FIG. 24D ,  FIG. 24E ,  FIG. 24F ,  FIG. 24G ,  FIG. 24H ,  FIG. 24I ,  FIG. 24J ,  FIG. 24K , and  FIG. 24L . All the images are images obtained in a simulation. 
     In a simulation, a rod array is used as a sample. In the rod array, a plurality of rods are arranged at regular intervals. The diameter of one rod is 4 μm. The rod array is irradiated with light having a wavelength of 1.3 μm. 
     The simulation is performed in a case where unidirectional irradiation is used and a case where multidirectional irradiation is used. In unidirectional irradiation, the incident angle of light on the sample is 0°. In multidirectional irradiation, the incident angles of light on the sample are 0° and 7.5°. Furthermore, the numerical aperture of the detection optical system is 0.5. Thus, the condition for the angle is as follows. 
     Unidirectional irradiation: θILL=0° 
     Multidirectional irradiation: θILL=0°, 7.5° 
     NA=0.5 
     In  FIG. 23A ,  FIG. 23B , and  FIG. 23C , the acquisition range of the scattering potential in a plane including the fy axis and the fx axis is illustrated. In  FIG. 23D ,  FIG. 23E , and  FIG. 23F , the acquisition range of the scattering potential in a plane including the fz axis and the fy axis is illustrated. In  FIG. 23G ,  FIG. 23H , and  FIG. 23O , the acquisition range of the scattering potential in a plane including the fx axis and the fz axis is illustrated. 
     In  FIG. 23A ,  FIG. 23D , and  FIG. 23G , the acquisition range of the scattering potential in unidirectional irradiation is illustrated. In  FIG. 23B ,  FIG. 23E , and  FIG. 23H , the acquisition range of the scattering potential in multidirectional irradiation is illustrated. In  FIG. 23C ,  FIG. 23F , and  FIG. 23I , a difference is illustrated. The difference is the difference between the acquisition range of the scattering potential in unidirectional irradiation and the acquisition range of the scattering potential in multidirectional irradiation. 
     As can be understood from the comparison between  FIG. 23A  and  FIG. 23B  and the comparison between  FIG. 23D  and  FIG. 23E , the acquisition range of the scattering potential in multidirectional irradiation is wider than the acquisition range of the scattering potential in unidirectional irradiation. The acquisition range of the scattering potential extends in the fy-axis direction. This is because the incident angle of measurement light on the sample is changed in a plane including the Y axis and the Z axis. 
     When  FIG. 23G  and  FIG. 23H  are compared, the acquisition range of the scattering potential does not change. This is because the incident angle of measurement light on the sample is not changed in a plane including the X axis and the Z axis. 
     In  FIG. 24A ,  FIG. 24B ,  FIG. 24C , and  FIG. 24D , an image of a sample in a plane including the Y axis and the X axis is illustrated. In  FIG. 24E ,  FIG. 24F ,  FIG. 24G , and  FIG. 24H , an image of a sample in a plane including the Z axis and the Y axis is illustrated. In  FIG. 24I ,  FIG. 24J ,  FIG. 24K , and  FIG. 24L , an image of a sample in a plane including the X axis and the Z axis is illustrated. 
     In  FIG. 24A ,  FIG. 24E , and  FIG. 24I , an input image is illustrated. In  FIG. 24B ,  FIG. 24F , and  FIG. 24J , an output image in unidirectional irradiation is illustrated. In  FIG. 24C ,  FIG. 24G , and  FIG. 24K , an output image in multidirectional irradiation is illustrated. In  FIG. 24D ,  FIG. 24H , and  FIG. 24I , a differential image is illustrated. The differential image is the difference between the output image in unidirectional irradiation and the output image in multidirectional irradiation. 
     As can be understood from the comparison between  FIG. 24B  and  FIG. 24C  and the comparison between  FIG. 24F  and  FIG. 24G , the output image in multidirectional irradiation is sharper than the output image in unidirectional irradiation. The output image is sharper in the Y-axis direction. This is because the incident angle of measurement light on the sample is changed in a plane including the Y axis and the Z axis. 
     When  FIG. 24J  and  FIG. 24K  are compared, there is a slight difference in brightness but the sharpness does not change. This is because the incident angle of measurement light on the sample is not changed in a plane including the X axis and the Z axis. 
     As just described, it is understood that with increase in number of scattering potentials, it is possible to generate an image of a sample more accurately, also in a simulation. 
     The overview of generation of an image is described. A flat sample and a three-dimensional sample are described. 
     A flat sample and a distribution of the scattering potential are illustrated in  FIG. 25A ,  FIG. 25B ,  FIG. 25C , and  FIG. 25D .  FIG. 25A  is a diagram illustrating a sample,  FIG. 25B  is a diagram illustrating a distribution of the scattering potential in real space,  FIG. 25C  is a diagram illustrating a distribution of the scattering potential in frequency space, and  FIG. 25D  is a diagram illustrating a frequency distribution that can be acquired by an optical system. 
     As illustrated in  FIG. 25A , in the flat sample, a plurality of transparent phase objects are distributed in the XY plane. Therefore, as illustrated in  FIG. 25B , the distribution of the scattering potential in real space is also flat. 
     In this case, in the distribution of the scattering potential in frequency space, as illustrated in  FIG. 25C , brightness varies in the fy-axis direction but brightness does not vary in the fz-axis direction. 
     The variation in brightness means that frequencies of different values exist. Thus, it follows that frequencies of different values exist in the fy-axis direction but only a frequency of one value exists in the fz-axis direction. 
     In frequency space, the scattering potential is distributed as illustrated in  FIG. 25C . In acquisition of the scattering potential, it is preferable that all the scattering potential be acquired. However, as illustrated in  FIG. 25D , the frequency that can be acquired by the optical system is limited. The distribution illustrated in  FIG. 25D  is a frequency distribution in unidirectional irradiation. 
     In this case, a frequency distribution C 2D  of the acquired scattering potential is given by the following Expression (1): 
         C   2D   =A   2D   ×B   (1)
 
     where 
     A 2D  is a distribution of the scattering potential in frequency space, and 
     B is a frequency distribution that can be acquired by the optical system. 
     Furthermore, a spatial distribution D 2D  of the acquired scattering potential is given by the following Expression (2): 
         D   2D   =FFT ( C   2D )  (2)
 
     where FFT( ) is a Fourier transform. 
     In a flat sample, as illustrated in  FIG. 25A , there is little thickness in the Z-axis direction. Therefore, when an image of a flat sample is to be generated, it is only necessary to acquire an image of D 2D  (x,y,z=0). Thus, even if no scattering potential is obtained in the fz-axis direction, this does not hinder generation of an image. 
     As illustrated in  FIG. 25C , the brightness varies in the fy-axis direction. Furthermore, as illustrated in  FIG. 25D , a curve exists in the fy-axis direction. Thus, the scattering potential can be obtained in the fy-axis direction. As a result, even for a transparent phase object, it is possible to obtain an image of D 2D  (x,y,z=0), that is, an image of a flat sample. 
     In the image pickup apparatus of the present embodiment, it is possible to rotate the sample and the signal acquisition unit relative to each other. Thus, the scattering potential can be obtained in the fz-axis direction. However, since it does not matter if no scattering potential is obtained in the fz-axis direction, rotation is not necessarily performed. 
     In the image pickup apparatus of the present embodiment, illumination by multidirectional irradiation is possible. Thus, even for a transparent phase object, it is possible to obtain an image of a flat sample more accurately. 
     A three-dimensional sample and a distribution of the scattering potential are illustrated in  FIG. 26A ,  FIG. 26B ,  FIG. 26C , and  FIG. 26D .  FIG. 26A  is a diagram illustrating a sample,  FIG. 26B  is a diagram illustrating a distribution of the scattering potential in real space,  FIG. 26C  is a diagram illustrating a distribution of the scattering potential in frequency space, and  FIG. 26D  is a diagram illustrating a frequency distribution that can be acquired by an optical system. 
     As illustrated in  FIG. 26A , in a three-dimensional sample, a plurality of transparent phase objects are distributed not only in the XY plane but also distributed in the Z direction. Therefore, as illustrated in  FIG. 26B , a distribution of the scattering potential in real space is also three-dimensional. 
     In this case, in a distribution of the scattering potential in frequency space, as illustrated in  FIG. 26C , the brightness varies not only in the fy-axis direction but also in the fz-axis direction. 
     As described above, the variation in brightness means that frequencies of different values exist. Thus, it follows that frequencies of different values exist not only in the fy-axis direction but also in the fz-axis direction. 
     In frequency space, the scattering potential is distributed as illustrated in  FIG. 26C . In acquisition of the scattering potential, it is preferable that all the scattering potential be acquired. However, as illustrated in  FIG. 26D , the frequency that can be acquired by the optical system is limited. The distribution illustrated in  FIG. 26D  is a frequency distribution in unidirectional irradiation. 
     In this case, a frequency distribution C 3D  of the acquired scattering potential is given by the following Expression (3): 
         C   3D   =A   3D   ×B   (3)
 
     where 
     A 3D  is a distribution of the scattering potential in frequency space, and 
     B is a frequency distribution that can be acquired by the optical system. 
     Furthermore, a spatial distribution D 3D  of the acquired scattering potential is given by the following Expression (4): 
         D   3D   =FFT ( C   3D )  (4)
 
     where FFT( ) is a Fourier transform. 
     As described above, in a three-dimensional sample, as illustrated in  FIG. 26C , the brightness varies even in the fz-axis direction. Thus, the scattering potential has a distribution not uniform for the spatial frequency even in the fz-axis direction. 
     In a three-dimensional sample, as illustrated in  FIG. 26A , there is a thickness in the Z-axis direction. Therefore, when an image of a three-dimensional sample is to be generated, it is necessary to obtain not only an image of D 3D  (x,y,z=0) but also an image of D 3D  (x,y,z≠0). Thus, if no scattering potential is obtained in the fz-axis direction, this hinders generation of an image. 
     As illustrated in  FIG. 26C , since the brightness varies in the fy-axis direction and the fz-axis direction, it is preferable that the scattering potential can be acquired in the fz-axis direction and the fz-axis direction. 
     As illustrated in  FIG. 26D , a curve exists in the fy-axis direction. Thus, the scattering potential can be obtained in the fy-axis direction. Furthermore, in the image pickup apparatus of the present embodiment, it is possible to rotate the sample and the signal acquisition unit relative to each other. Thus, the scattering potential can be obtained even in the fz-axis direction. As a result, even for a transparent phase object, it is possible to obtain an image of D 3D  (x,y,z=0) and an image of D 3D  (x,y,z≠0), that is, an image of a three-dimensional sample. 
     In addition, in the image pickup apparatus of the present embodiment, illumination by multidirectional irradiation is possible. Thus, even for a transparent phase object, it is possible to obtain an image of a three-dimensional sample more accurately. 
     According to the present disclosure, it is possible to provide an image pickup apparatus capable of isotropically acquiring spatial frequency information of a sample, with a simple configuration. 
     As described above, the present disclosure is suitable for an image pickup apparatus capable of isotropically acquiring spatial frequency information of a sample, with a simple configuration.