Patent Publication Number: US-2023152563-A1

Title: Microscope system, imaging method, and imaging apparatus

Description:
FIELD 
     The present disclosure relates to a microscope system, an imaging method, and an imaging apparatus. 
     BACKGROUND 
     There is disclosed a technique of obtaining a captured image of a specimen by irradiating the specimen with light and receiving light emitted from the specimen. For example, there is disclosed a technique of obtaining a captured image having focus adjusted on a specimen by a contrast method of changing focus so as to maximize a contrast ratio of a captured image of the specimen. In addition, there is disclosed a technique of adjusting focus on a specimen using a phase difference obtained from a subject image on which a set of feature points included in a pupil-split image of an entire region including the specimen is extracted. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP 2014-123141 A 
     Patent Literature 2: JP 2014-178474 A 
     SUMMARY 
     Technical Problem 
     The conventional technique, however, has difficulty in achieving high-speed and high-accuracy focus adjustment. 
     In view of this, the present disclosure proposes a microscope system, an imaging method, and an imaging apparatus capable of achieving high-speed and high-accuracy focus adjustment. 
     Solution to Problem 
     In order to solve the above problem, a microscope system according to an aspect of the present disclosure includes: an irradiation unit that emits line illumination parallel to a first direction; a stage that supports a specimen and is movable in a second direction perpendicular to the first direction; a phase difference acquisition unit that acquires phase difference information regarding an image of light emitted from the specimen by being irradiated with the line illumination; an objective lens that focuses the line illumination on the specimen; a derivation unit that derives relative position information between the objective lens and the specimen based on the phase difference information; and a movement control unit that causes at least one of the objective lens and the stage to move in a third direction vertical to each of the first direction and the second direction based on the relative position information. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic diagram illustrating an example of a microscope system according to an embodiment of the present disclosure. 
         FIG.  2    is a schematic diagram illustrating an example of a plurality of two-dimensionally arranged light receiving units included in a pupil-split image capture unit according to the embodiment of the present disclosure. 
         FIG.  3    is a schematic diagram illustrating an example of a pupil-split picture according to the embodiment of the present disclosure. 
         FIG.  4    is a schematic diagram illustrating an example of a second captured image according to the embodiment of the present disclosure. 
         FIG.  5    is a diagram illustrating an example of a functional configuration of a control device according to the embodiment of the present disclosure. 
         FIG.  6 A  is a conceptual diagram of a measurement target region according to the embodiment of the present disclosure. 
         FIG.  6 B  is a schematic diagram illustrating an example of the pupil-split picture according to the embodiment of the present disclosure. 
         FIG.  6 C  is an explanatory diagram of selection of a unit region according to the embodiment of the present disclosure. 
         FIG.  7    is an explanatory diagram of a centroid according to the embodiment of the present disclosure. 
         FIG.  8    is a schematic diagram illustrating an example of a pupil-split picture according to the embodiment of the present disclosure. 
         FIG.  9    is an explanatory diagram of a light intensity change due to a change in a Z direction of the pupil-split picture according to the embodiment of the present disclosure. 
         FIG.  10    is an explanatory diagram of a phase difference according to the embodiment of the present disclosure. 
         FIG.  11    is a flowchart illustrating an example of a flow of information processing according to the embodiment of the present disclosure. 
         FIG.  12 A  is an explanatory diagram of a Z-stack image according to a modification of the present disclosure. 
         FIG.  12 B  is an explanatory diagram of a conventional Z-stack image. 
         FIG.  13    is a flowchart illustrating an example of a flow of focus map creation processing according to the modification of the present disclosure. 
         FIG.  14 A  is a schematic diagram of an emission surface of line illumination of a light source according to the modification of the present disclosure. 
         FIG.  14 B  is a schematic diagram illustrating an example of a pupil-split picture according to the modification of the present disclosure. 
         FIG.  15    is a diagram illustrating an example of a mechanism of imaging using phase difference AF according to the modification of the present disclosure. 
         FIG.  16    is an explanatory diagram for describing a difference in focus between a non-tissue part and a tissue part. 
         FIG.  17    is a diagram illustrating an example of a functional configuration of a control device  16  according to the modification of the present disclosure. 
         FIG.  18    is an explanatory diagram for describing processing of adding an offset to a focus position in the non-tissue part. 
         FIG.  19    is an explanatory diagram for describing processing of focusing by preferentially selecting fluorescence of the tissue part. 
         FIG.  20    is a diagram illustrating an example of a sensor region according to the modification of the present disclosure. 
         FIG.  21    is a diagram illustrating an example of luminance information of the sensor region according to the modification of the present disclosure. 
         FIG.  22    is an explanatory diagram for describing a moving speed of an objective lens at the time of phase difference AF. 
         FIG.  23    is a schematic block diagram of a microscope system according to an eighth modification of the present disclosure. 
         FIG.  24    is a view illustrating an example of an optical system in the microscope system according to the present disclosure. 
         FIG.  25    is a hardware configuration diagram according to the embodiment and the modification of the present disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present disclosure will be described below in detail with reference to the drawings. Note that in the following embodiments, the same parts are denoted by the same reference symbols, and a repetitive description thereof will be omitted. 
       FIG.  1    is a schematic diagram illustrating an example of a microscope system  1  according to the present embodiment. 
     The microscope system  1  is a system that irradiates a specimen T with line illumination LA and receives light emitted from the specimen T. Details of the line illumination LA and the specimen T will be described below. 
     The microscope system  1  includes an imaging apparatus  12 . The imaging apparatus  12  is communicably connected to a server device  10  via, for example, a wireless communication network or a wired communication network such as a network N. The server device  10  may be a computer. 
     In the present embodiment, the description will be given referring to a direction along a direction in which an objective lens  22  and the specimen T, which will be described below, approach each other and a direction in which the objective lens  22  and the specimen T separates from each other, as a Z-axis direction. In addition, the description will be given assuming that the Z-axis direction matches the thickness direction of the specimen T. In addition, in the present embodiment, the description will be given assuming that the Z-axis direction and an optical axis A2 of the objective lens  22  are parallel to each other. In addition, a stage  26  to be described below is a two-dimensional plane represented by two axes (an X-axis direction and a Y-axis direction) orthogonal to the Z-axis direction. A plane parallel to the two-dimensional plane of the stage  26  will sometimes be referred to as an XY plane. Details of these units will be described below. 
     The imaging apparatus  12  includes a measurement unit  14  and a control device  16 . The measurement unit  14  and the control device  16  are connected so as to be able to exchange data or signals. 
     The measurement unit  14  has an optical mechanism that measures light emitted from the specimen T included in a measurement target region  24 . The measurement unit  14  is applied to an optical microscope, for example. 
     The measurement unit  14  includes an irradiation unit  18 , a split mirror  20 , an objective lens  22 , a stage  26 , a half mirror  28 , an imaging optical unit  30 , a phase difference detection optical unit  36 , a first drive unit  44 , and a second drive unit  46 . The split mirror  20  selects a half mirror or a dichroic mirror according to a measurement method. 
     The irradiation unit  18  emits the line illumination LA and the area illumination LB. The irradiation unit  18  selectively performs irradiation by switching between the line illumination LA and the area illumination LB. 
     The line illumination LA is a light beam like a line long in a first direction. Specifically, the line illumination LA is a light beam in which a length of a light flux in the first direction in a two-dimensional plane orthogonal to an optical axis is several times or more a length in a direction orthogonal to the first direction. In the present embodiment, a case where the first direction being a longitudinal direction of the line illumination LA matches the X-axis direction in  FIG.  1    will be described as an example. Details of the X-axis direction will be described below. 
     The area illumination LB is light emitted at the time of imaging the specimen T to be described below. Specifically, the area illumination LB is a light beam applied to a region broader than in the Y-axis direction compared to the line illumination LA. 
     The irradiation unit  18  includes a light source unit  18 A and an imaging optical system  18 D. The light source unit  18 A includes a light source  18 B and an illumination optical system  18 C. The light source  18 B is a light source that selectively switches between the line illumination LA and the area illumination LB to emit light. Switching to the line illumination LA and the area illumination LB is performed under the control of the control device  16 . 
     For example, the light source  18 B has a configuration including a plurality of laser diodes being two-dimensionally arranged along a two-dimensional plane formed with the X-axis direction and the Y-axis direction orthogonal to the X-axis direction. For example, the light source  18 B emits light from each of the laser diodes arranged one-dimensionally in the X-axis direction to achieve emission of the line illumination LA. Furthermore, for example, the light source  18 B emits light from each of the laser diodes two-dimensionally arranged in the X-axis direction and the Y-axis direction to emit the area illumination LB. Furthermore, for example, the light source  18 B may irradiate the specimen T with the line illumination LA by emitting light through a slit long in the X-axis direction. 
     Here, in a magnifying device such as a microscope, when an image of the line illumination LA is formed at a point conjugate with the specimen T, a reduced image is projected at a reciprocal of the magnification. For example, when using a 20× objective lens  22  combined with a 1× condenser lens, magnification will be 1/20×. Therefore, for example, in order to achieve line illumination LA of 1 mm×5 um on the measurement target region  24 , a slit of 20 mm×0.1 mm long in the X-axis direction may be provided at a position conjugate with the specimen T in the optical path of the area illumination LB. In addition, when there is a need to prepare further higher luminance illumination, one possible way is to form a line of light at a point conjugate with the specimen T by using a cylinder lens or a Powell lens from collimated light beams. For example, the line illumination LA can be formed by using a one-dimensional cylinder lens array. In addition, the light source  18 B can be implemented by using a mercury lamp or a halogen lamp having a broad spectral band or a laser light source having a narrow band. 
     A case where the line illumination LA is emitted from the light source  18 B will be described as an example. Note that the optical path is similar to the case of the line illumination LA also when the area illumination LB is emitted from the light source  18 B. The line illumination LA emitted from the light source  18 B is substantially collimated by the illumination optical system  18 C, and then reaches the split mirror  20  via the imaging optical system  18 D. 
     The line of light indicates the shape of the illumination light with which the line illumination LA emitted from the light source  18 B irradiates the specimen T. 
     The light source  18 B may be a light source  18 B that selectively emits light in a wavelength region where the specimen T emits fluorescence. In addition, the irradiation unit  18  may be provided with a filter that selectively transmits light in the wavelength region. The present embodiment will describe, as an example, a mode in which the light source  18 B performs irradiation with the line illumination LA and the area illumination LB in a wavelength region corresponding to fluorescence emitted from the specimen T. In addition, the present embodiment allows the line illumination LA and the area illumination LB to be light of mutually different wavelength regions or the same wavelength region in a wavelength region corresponding to the fluorescence emitted from the specimen T. 
     The split mirror  20  reflects the line illumination LA and transmits light in a wavelength region other than the line illumination LA. In the present embodiment, the split mirror  20  transmits light emitted from the specimen T. The line illumination LA is reflected by the split mirror  20  and reaches the objective lens  22 . 
     The objective lens  22  also functions as a focus lens that condenses the line illumination LA on the specimen T. 
     The objective lens  22  includes the second drive unit  46 . The second drive unit  46  causes the objective lens  22  to move in the Z-axis direction in a direction toward or away from the specimen T. By adjusting an interval between the objective lens  22  and the specimen T, focus adjustment of the objective lens  22  is performed. 
     In addition, the first drive unit  44  causes the stage  26  to move at least in the Y-axis direction. With the movement of the stage  26 , the specimen T placed on the stage  26  moves relative to the objective lens  22  in the Y-axis direction. The Y-axis direction and the X-axis direction are directions orthogonal to the Z-axis direction. The Y-axis direction and the X-axis direction are directions orthogonal to each other. 
     The specimen T is an example of a measurement target in the microscope system  1 , and is placed in the measurement target region  24 . That is, the specimen T may be an object from which a captured image is obtained by the microscope system  1 . The present embodiment will describe, as an example, a mode in which the specimen T emits fluorescence by irradiation with the line illumination LA. The specimen T may contain a biological-origin sample. Examples of the biological-origin sample include microorganisms, cells, liposomes, red blood cells, white blood cells, and platelets in blood, vascular endothelial cells, minute cell pieces of epithelial tissue, and pathological tissue sections of various organs. 
     The specimen T may be, for example, a pair of glass members with a biological-origin sample placed between the pair of glass members. Furthermore, the specimen T may be a biological-origin sample placed on the glass member. An example of the glass member is a slide. The glass member may be referred to as a cover slip. The glass member may be any member on which a biological-origin sample can be placed, and is not limited to a member formed of glass. The glass member may be any member that transmits light emitted from the line illumination LA, the area illumination LB, and the specimen T. 
     In the specimen T, for example, a biological-origin sample in a state of being encapsulated with an encapsulant may be placed. The encapsulant may be formed with a known material that transmits each of the line illumination LA, the area illumination LB, and the light emitted from the specimen T incident on the measurement target region  24 . The encapsulant may be either liquid or solid. 
     The biological-origin sample that can be contained in the specimen T may be subjected to treatment such as staining or labeling. The treatment may be staining for demonstrating the form of the biological component or demonstrating a substance (such as a surface antigen) of the biological component, and examples thereof include Hematoxylin-Eosin (HE) staining and Immunohistochemistry staining. The biological-origin sample may be subjected to the treatment with one or more reagents, and the reagent may be a fluorescent dye, a coloring reagent, a fluorescent protein, or a fluorescently labeled antibody. 
     The specimen T may be prepared for the purpose of pathological diagnosis, clinical examination, or the like from a specimen or a tissue sample collected from a human body. In addition, the specimen T is not limited to a human body, and may be originated from an animal, a plant, or other materials. The specimen T has different properties depending on the type of tissue (for example, organ or cell) used, the type of target disease, the attribute of the subject (e.g., age, sex, blood type, race, etc.), the lifestyle of the subject (for example, dietary habits, exercise habits, and smoking habits), and the like. The sample may be managed with identification information (bar code information, QR code (registered trademark) information, or the like) by which each sample can be identified. 
     The light emitted from the specimen T may be, for example, fluorescence emitted from a fluorescent dye in the biological-origin sample by irradiation with the line illumination LA. Furthermore, the light emitted from the specimen T may be light emitted in a wavelength region other than fluorescence by irradiation with the line illumination LA, or may be specifically light produced as a result of scattering and reflection of the illumination light. Hereinafter, the fluorescence emitted from the specimen T by the irradiation of the line illumination LA may be simply referred to as light or a light beam. 
     The light emitted from the specimen T by the irradiation of the line illumination LA passes through the objective lens  22  and the split mirror  20 , which is represented by a dichroic mirror in the present embodiment, and reaches the half mirror  28 . The light emitted from the specimen T is fluorescence emitted by the specimen T by irradiation with the line illumination LA or the area illumination LB. The fluorescence includes scattered fluorescence components. 
     The half mirror  28  distributes a part of the light emitted from the specimen T to the imaging optical unit  30  and distributes the rest to the phase difference detection optical unit  36 . The distribution ratio of light to the imaging optical unit  30  and the phase difference detection optical unit  36  by the half mirror  28  may be the equal distribution ratio (for example, 50%-50%) or may be non-equal ratio. Therefore, a dichroic mirror or a polarizing mirror may be used instead of the half mirror  28 . 
     The light transmitted through the half mirror  28  reaches the imaging optical unit  30 . The light reflected by the half mirror  28  reaches the phase difference detection optical unit  36 . 
     Note that the line illumination LA formed by the irradiation unit  18  and the measurement target region  24  are assumed to have an optically conjugate relationship. In addition, it is assumed that the line illumination LA, the measurement target region  24 , an image capture unit  34  of the imaging optical unit  30 , and a pupil-split image capture unit  42  of the phase difference detection optical unit  36  are supposed to be in an optically conjugate relationship. 
     The imaging optical unit  30  includes a condenser lens  32 , a magnifying lens  35 , and the image capture unit  34 . The light transmitted through the half mirror  28  is condensed on the magnifying lens  35  by the condenser lens  32 , magnified by the magnifying lens  35 , and reaches the image capture unit  34 . The image capture unit  34  receives light emitted from the specimen T and obtains a captured image. That is, the image capture unit  34  obtains a captured image obtained by magnifying the imaging region of the specimen T to a predetermined magnification. The image capture unit  34  outputs a captured image of the received light to the control device  16 . The captured image is used for analysis of the type of the specimen T and the like. 
     The image capture unit  34  includes a plurality of light receiving units  33 . The light receiving unit  33  is an element that converts received light into a charge. The light receiving unit  33  is a photodiode, for example. For example, the image capture unit  34  has a configuration in which the plurality of light receiving units  33  is two-dimensionally arranged along a light receiving surface. For example, the image capture unit  34  has a configuration in which the plurality of light receiving units  33  is one-dimensionally arranged along the light receiving surface. In the present embodiment, the description will be given assuming that the light receiving unit  33  has a configuration in which the light receiving units  33  are two-dimensionally arranged along the light receiving surface. The light receiving surface of the light receiving unit  33  is a two-dimensional plane orthogonal to an optical axis of light incident on the image capture unit  34  via the condenser lens  32 . 
     The image capture unit  34  includes one or a plurality of imaging elements including a plurality of pixels arranged one-dimensionally or two-dimensionally, for example, a complementary metal-oxide semiconductor (CMOS) or a charge coupled device (CCD). The image capture unit  34  may include a low-resolution image acquisition imaging element and a high-resolution image acquisition imaging element, or may include a sensing imaging element for AF or the like and an image output imaging element for observation or the like. The imaging element may be a signal processing sensor including, in addition to the plurality of pixels, a signal processing unit (one, two, or three of a CPU, a DSP, and a memory device) that performs signal processing using a pixel signal from each pixel, and an output control unit that controls output of image data generated from the pixel signal and processing data generated by the signal processing unit. Furthermore, the imaging element may include an asynchronous event detection sensor that detects, as an event, that a luminance change of a pixel that photoelectrically converts incident light exceeds a predetermined threshold. The imaging element including the plurality of pixels, the signal processing unit, and the output control unit can be preferably configured as a one-chip semiconductor device. 
     On the other hand, the phase difference detection optical unit  36  is an optical unit for obtaining a pupil-split image of light emitted from the specimen T irradiated with the line illumination LA. In the present embodiment, a case where the phase difference detection optical unit  36  is an optical unit for obtaining a pupil-split image using two separator lenses will be described as an example. 
     The phase difference detection optical unit  36  includes a field lens  38 , an aperture mask  39 , separator lenses  40  including a separator lens  40 A and a separator lens  40 B, and a pupil-split image capture unit  42 . The separator lenses  40  includes the separator lens  40 A and the separator lens  40 B. 
     The light emitted from the specimen T by the irradiation of the line illumination LA reaches the aperture mask  39  via the field lens  38 . The aperture mask  39  has a pair of apertures  39 A and  39 B at a target position with the optical axis of the field lens  38  as a boundary. The sizes of the pair of apertures  39 A and  39 B are adjusted such that the subject depths of the separator lens  40 A and the separator lens  40 B are wider than the subject depth of the objective lens  22 . 
     The aperture mask  39  divides light incident from the field lens  38  into two light fluxes by the pair of apertures  39 A and  39 B. The separator lens  40 A and the separator lens  40 B collect the light fluxes transmitted through the apertures  39 A and  39 B of the aperture mask  39  to the pupil-split image capture unit  42 . Therefore, the pupil-split image capture unit  42  receives the two light fluxes that have been split. 
     Note that the phase difference detection optical unit  36  may be a configuration not including the aperture mask  39 . In this case, the light that has reached the separator lens  40  via the field lens  38  is split into two light fluxes by the separator lens  40 A and the separator lens  40 B, and is collected to the pupil-split image capture unit  42 . 
       FIG.  2    is a schematic diagram illustrating an example of a plurality of two-dimensionally arranged light receiving units  41  included in the pupil-split image capture unit  42 . The pupil-split image capture unit  42  includes a plurality of light receiving units  41 . The light receiving unit  41  is an element that converts received light into a charge. The light receiving unit  41  is, for example, a photodiode.  FIG.  2    illustrates, as an example, the pupil-split image capture unit  42  in which the plurality of light receiving units  41  is two-dimensionally arranged along a light receiving surface  43  that receives light. 
     The light receiving surface  43  is a two-dimensional plane orthogonal to an optical axis of light incident on the pupil-split image capture unit  42  via the field lens  38 , the aperture mask  39 , and the separator lens  40 . The pupil-split image capture unit  42  includes one or a plurality of imaging elements including a plurality of pixels arranged one-dimensionally or two-dimensionally, for example, a CMOS or a CCD. The pupil-split image capture unit  42  may include a low-resolution image acquisition imaging element and a high-resolution image acquisition imaging element, or may include a sensing imaging element for AF or the like and an image output imaging element for observation or the like. The imaging element may be a signal processing sensor including, in addition to the plurality of pixels, a signal processing unit (one, two, or three of a CPU, a DSP, and a memory device) that performs signal processing using a pixel signal from each pixel, and an output control unit that controls output of image data generated from the pixel signal and processing data generated by the signal processing unit. Furthermore, the imaging element may include an asynchronous event detection sensor that detects, as an event, that a luminance change of a pixel that photoelectrically converts incident light exceeds a predetermined threshold. The imaging element including the plurality of pixels, the signal processing unit, and the output control unit can be preferably configured as a one-chip semiconductor device. 
     In the present embodiment, a case where the pupil-split image capture unit  42  has a configuration in which a plurality types of unit regions  37  is arranged in plurality along the light receiving surface  43  will be described as an example. Each of the plurality of types of unit regions  37  includes one or a plurality of light receiving units  41 . The plurality of types of unit regions  37  have mutually different exposure setting values of the light receiving unit  41  included in individual regions. 
     The exposure setting value can be controlled by at least one of a gain and an exposure time. The gain indicates at least one of an analog-to-digital conversion gain and an amplification gain. The exposure time indicates a charge accumulation time per output of a fluorescence signal in a case where the pupil-split image capture unit  42  is a charge accumulation type such as CMOS or CCD. 
     That is, the plurality of unit regions  37  is regions in which at least one of the gain and the exposure time of the included light receiving unit  41  is different from each other. Note that the exposure setting values of the plurality of light receiving units  41  included in one unit region  37  are assumed to be the same value. 
     Each of the plurality of light receiving units  41  is to have a predetermined light sensitivity that has been set for each type of the unit region  37  to which the light receiving unit  41  belongs. Therefore, the light receiving unit  41  is to be implemented by using the light receiving unit  41  that can set the light sensitivity to any value. 
       FIG.  2    illustrates, as an example, a mode in which the pupil-split image capture unit  42  has a configuration in which a unit region  37 A and a unit region  37 B are alternately arranged as the two types of unit regions  37 . The unit region  37 A and the unit region  37 B are the unit regions  37  having mutually different exposure setting values. For example, high light sensitivity is preset in the light receiving unit  41  included in the unit region  37 A. The high exposure setting value can be set by changing at least one of the gain and the exposure time. In addition, low light sensitivity is preset in the light receiving unit  41  included in the unit region  37 B. The low exposure setting value can be set by changing at least one of the gain and the exposure time. The gain and the exposure charge accumulation time may be preset. 
     Note that the pupil-split image capture unit  42  may have a configuration arranging three or more types of unit regions  37  having mutually different exposure setting values, and is not limited to the two types of unit regions  37 . Furthermore, in the pupil-split image capture unit  42 , the exposure setting values of all the included light receiving units  41  may be the same. 
     In the present embodiment, a mode in which the pupil-split image capture unit  42  has a configuration in which two types of unit regions  37  are arranged in plurality along the light receiving surface  43  will be described as an example. 
     Return to  FIG.  1   , and description will continue. As described above, the pupil-split image capture unit  42  receives two light fluxes split by two pupils (separator lens  40 A and separator lens  40 B). By receiving two light fluxes, the pupil-split image capture unit  42  can capture an image including an image of a set of light fluxes. Here, the pupil-split image capture unit  42  acquires split two light fluxes as a pupil-split image. The pupil-split image may include a light intensity distribution corresponding to each of the two split light fluxes. This makes it possible to calculate the phase difference in a subsequent deriving step in the derivation unit described below. 
       FIG.  3    is a schematic diagram illustrating an example of a pupil-split picture  70  acquired by the pupil-split image capture unit  42 . The pupil-split picture  70  includes a pupil-split image  72  that is a set of an image  72 A and an image  72 B. 
     The pupil-split picture  70  is an image corresponding to the position and brightness of light received by each of the plurality of light receiving units  41  provided in the pupil-split image capture unit  42 , and includes a light intensity distribution. Hereinafter, the brightness of the light received by the light receiving unit  41  may be referred to as a light intensity value. 
     Hereinafter, description will be given with reference to  FIGS.  2  and  3   . In this case, the pupil-split picture  70  is a picture in which the light intensity value is defined for each pixel corresponding to each of the unit regions  37  having a plurality of mutually different exposure setting values. In this case, the light intensity value is represented by the gradation of the pixel, although the relationship between the gradation and the light intensity is different in each of the unit regions  37 . 
     The image  72 A and the image  72 B included in the pupil-split picture  70  are light receiving regions, and are regions having a larger light intensity value than other regions. As described above, the irradiation unit  18  irradiates the specimen T with the line illumination LA. Therefore, the light emitted from the specimen T irradiated with the line illumination LA becomes a line of light. Therefore, the image  72 A and the image  72 B constituting the pupil-split image  72  are images of lines each long in a predetermined direction. This predetermined direction is a direction optically corresponding to the X-axis direction which is the longitudinal direction of the line illumination LA. 
     Specifically, the vertical axis direction (YA-axis direction) of the pupil-split picture  70  illustrated in  FIG.  3    optically corresponds to the Y-axis direction in the measurement target region  24  of the pupil-split image  72  included in the pupil-split picture  70 . Furthermore, the horizontal axis direction (XA-axis direction) of the pupil-split picture  70  illustrated in  FIG.  3    optically corresponds to the X-axis direction in the measurement target region  24 . As described above, the X-axis direction is the longitudinal direction of the line illumination LA. 
     Note that the phase difference detection optical unit  36  only needs to be an optical unit for obtaining a change in the pupil-split image  72  (set of image  72 A and image  72 B), and the pupil-split image  72  (set of image  72 A and image  72 B) is not limited to the twin-lens pupil-split image. The phase difference detection optical unit  36  may be, for example, an optical unit that obtains a pupil-split image of triple lenses or more by receiving three or more light fluxes obtained by splitting light emitted from the specimen T. 
     Return to  FIG.  1   , and description will continue. In the present embodiment, the measurement unit  14  causes the first drive unit  44  to drive the stage  26  on which the specimen T is placed, and irradiates the specimen T with the line illumination LA while performing relative movement of the measurement target region  24  with respect to the line illumination LA in the Y-axis direction. That is, in the present embodiment, the Y-axis direction is a scanning direction of the measurement target region  24 . A scanning method of the line illumination LA is not limited. Examples of the scanning method include a method of scanning in a direction (Y-axis direction) orthogonal to the longitudinal direction (X-axis direction) of the line illumination LA, and a method of moving at least a part of the configuration other than the measurement target region  24  in the measurement unit  14  in the Y-axis direction with respect to the measurement target region  24 . In addition, a deflection mirror may be disposed between the split mirror  20  and the objective lens  22 , and the line illumination LA may be scanned in the Y-axis direction by the deflection mirror. 
     By executing imaging by the image capture unit  34  while scanning the measurement target region  24  in the Y-axis direction leads to acquisition of a captured image of the specimen T. 
       FIG.  4    is a schematic diagram illustrating an example of a second captured image  74  acquired by the image capture unit  34 . The second captured image  74  is a captured image obtained by the image capture unit  34  when the measurement target region  24  is irradiated with the line illumination LA. In other words, the second captured image  74  is a captured image obtained by capturing, by the image capture unit  34 , the light emitted from the specimen T irradiated with the line illumination LA. The second captured image  74  includes a subject image  75  like a line. 
     The subject image  75  included in the second captured image  74  is a light receiving region, and is a region having a larger light intensity value than other regions. 
     Specifically, the vertical axis direction (YB-axis direction) of the second captured image  74  illustrated in  FIG.  4    optically corresponds to the Y-axis direction in the measurement target region  24 . The horizontal axis direction (XB-axis direction) of the second captured image  74  in  FIG.  4    optically corresponds to the X-axis direction in the measurement target region  24 . As described above, the X-axis direction is the longitudinal direction of the line illumination LA. The depth direction (ZA-axis direction) of the second captured image  74  illustrated in  FIG.  4    optically corresponds to a Z-axis direction which is a thickness direction of the measurement target region  24 . 
     Note that the image capture unit  34  similarly obtains a captured image when the specimen T is irradiated with the area illumination LB. Hereinafter, a captured image obtained by the image capture unit  34  when the measurement target region  24  is irradiated with the area illumination LB will be described as a first captured image. When the first captured image and the second captured image  74  are collectively described, they are simply referred to as captured images. 
     Return to  FIG.  1   , and description will continue. Next, the control device  16  will be described. 
     The control device  16  is a type of information processing device. The control device  16  is connected to each of the light source  18 B, the image capture unit  34 , the phase difference detection optical unit  36 , the first drive unit  44 , and the second drive unit  46  so as to be able to exchange data or signals. 
     The control device  16  acquires, from the pupil-split image capture unit  42 , the pupil-split picture  70  of light emitted from the specimen T irradiated with the line illumination LA, and executes focus adjustment on the picture base on light intensity distributions of the image  72 A and the image  72 B which are the pupil-split images  72  included in the pupil-split picture  70 . 
       FIG.  5    is a diagram illustrating an example of a functional configuration of the control device  16 . Note that, in  FIG.  5   , the light source  18 B, the pupil-split image capture unit  42 , the image capture unit  34 , the first drive unit  44 , and the second drive unit  46  are also illustrated for the sake of explanation. 
     The control device  16  includes a control unit  60 , a storage unit  62 , and a communication unit  64 . The control unit  60  is connected to the storage unit  62  and the communication unit  64  so as to be able to exchange data and signals. The storage unit  62  is a storage medium that stores various types of data. The storage unit  62  is, for example, a hard disk drive, an external memory device, or the like. The communication unit  64  communicates with an external device such as the server device  10  via the network N or the like. 
     The control unit  60  includes a light source control unit  60 A, a captured image acquisition unit  60 B, a reference focus unit  60 C, a pupil-split image acquisition unit  60 D, a derivation unit  60 E, and a movement control unit  60 F. The derivation unit  60 E includes a selection unit  60 H, a phase difference acquisition unit  60 I, and a relative distance derivation unit  60 J. 
     Some or all of the light source control unit  60 A, the captured image acquisition unit  60 B, the reference focus unit  60 C, the pupil-split image acquisition unit  60 D, the derivation unit  60 E, the movement control unit  60 F, an output control unit  60 G, the selection unit  60 H, the phase difference acquisition unit  60 I, and the relative distance derivation unit  60 J may be implemented by execution of a program by a processing device such as a central processing unit (CPU) that is, by software, may be implemented by hardware such as an integrated circuit (IC), or may be implemented by using software and hardware in combination. 
     The light source control unit  60 A controls the light source  18 B to selectively emit the line illumination LA or the area illumination LB. Under the control of the light source control unit  60 A, the line illumination LA or the area illumination LB is selectively emitted from the light source  18 B. 
     The captured image acquisition unit  60 B acquires, from the image capture unit  34 , a captured image of light emitted from the specimen T irradiated with the line illumination LA or the area illumination LB. That is, the captured image acquisition unit  60 B acquires the second captured image  74  or the first captured image. 
     Here, the twin-lens phase difference method is not a method of performing image evaluation such as the maximum contrast ratio or the minimum spot size which are performed in the contrast method or the operation concentric circle method. Therefore, in the twin-lens phase difference method, when the products of the refractive index and the distance, which is referred to an optical distance, are the same, it is determined that the focusing amounts are the same. For example, the optical distance between the objective lens  22  and the specimen T is greatly different between a case where the specimen T is disposed in a medium having a high refractive index and a case where the specimen T is exposed on the air surface, even with the same physical distance. This causes a difference in optical aberration and chromatic aberration. In order to correct the difference, the reference focus is measured. 
     Here is an assumable case where that the specimen T has a thickness of several um and the cover slip has a thickness of several hundred um like a microscope slide. In this case, even with the same optical distance between the objective lens  22  and the specimen T, the physical distance to the optimum focus position with respect to the specimen T varies depending on the thickness of the cover slip corresponding to the measurement target region  24 . 
     Therefore, the reference focus unit  60 C adjusts the initial relative position between the objective lens  22  and the specimen T. As described above, the specimen T is included in the measurement target region  24 , and the measurement target region  24  is placed on the stage  26 . Therefore, by adjusting the relative position between the objective lens  22  and the stage  26 , the relative position between the objective lens  22  and the specimen T is adjusted. 
     The relative position is a relative position of one of the objective lens  22  and the specimen T with respect to the other. The relative position is determined by the distance between the objective lens  22  and the specimen T in the Z-axis direction, for example. For example, the relative position is represented by the movement direction and the movement amount of at least one of the objective lens  22  and the specimen T with respect to the current position of each of the objective lens  22  and the specimen T. 
     The initial relative position indicates a relative position for pre-adjustment before obtaining a captured image for use in analysis of the specimen T or the like in the microscope system  1 . That is, the reference focus unit  60 C executes reference focus processing for pre-adjustment. 
     For example, the reference focus unit  60 C calculates a contrast ratio of the light intensity values between the included adjacent pixels using the first captured image acquired by the captured image acquisition unit  60 B. That is, the reference focus unit  60 C calculates the contrast ratio using the first captured image which is a captured image of the light emitted from the specimen T irradiated with the area illumination LB. Subsequently, by repeating the control of the movement control unit  60 F and the calculation of the contrast ratio, the reference focus unit  60 C adjusts the initial relative position to a position where the contrast ratio is maximized. Actually, the reference focus may be determined by a method other than the contrast ratio. Although the reference focus may be determined by any method, this example is a case where the contrast method is used. 
     The movement control unit  60 F controls the movement of the first drive unit  44  and the second drive unit  46 . Under the control of the movement control unit  60 F, at least one of the objective lens  22  and the stage  26  is driven such that the objective lens  22  and the specimen T are moved in a direction of approaching or separating from each other in the Z-axis direction. That is, the relative position of the objective lens  22  and the specimen T in the Z-axis direction changes. In addition, the movement control unit  60 F causes the stage  26  to move in the Y-axis direction which is the scanning direction of the area illumination LB. With the movement of the stage  26 , the specimen T placed on the stage  26  is moved in the Y-axis direction, allowing the irradiation region of the line illumination LA to be scanned in the scanning direction (Y-axis direction) of the specimen T. 
     Every time the distance between the objective lens  22  and the specimen T in the Z-axis direction is changed by the movement control unit  60 F, the reference focus unit  60 C repeats the calculation of the contrast ratio using the first captured image. By repeating calculation of the contrast ratio while performing stepwise reduction of the movement amount of the objective lens  22  in the Z-axis direction via the movement control unit  60 F, the reference focus unit  60 C specifies the relative position at which the contrast ratio is maximized within the imaging range of the image capture unit  34  as the initial relative position. The reference focus unit  60 C then ends the control by the movement control unit  60 F at the specified initial relative position. Through these steps of processing, the reference focus unit  60 C executes the reference focus processing by the contrast method to adjust the initial relative position. 
     The reference focus unit  60 C adjusts the initial relative position using the contrast method, leading to achievement of the initial focus adjustment with high accuracy on the specimen T in the measurement target region  24 . However, depending on the situation of the specimen T and the observation purpose, the reference focus may be determined using the design value of the pupil-split image  72  without performing the reference focus detection. 
     From the viewpoint of reducing the processing time, the reference focus unit  60 C may adjust the initial relative position by a contrast method using the second captured image  74  (refer to  FIG.  4   ) being a captured image of light emitted from the specimen T irradiated with the line illumination LA. In this case, the light receiving region is smaller than that in the case of using the first captured image, the adjustment time of the initial relative position can be reduced. 
     Furthermore, the reference focus unit  60 C may adjust the initial relative position by a contrast method using the image  72 A or the image  72 B included in the pupil-split picture  70  acquired by the pupil-split image acquisition unit  60 D described below. 
     Furthermore, the reference focus unit  60 C may adjust the initial relative position using a group of a plurality of Z-stack images obtained by stacking the imaging region of the measurement target region  24  in the Z-axis direction. In this case, for example, the reference focus unit  60 C may adjust the initial relative position by using the algorithm described in Journal of Biomedical Optics 17(3), 036008 (March 2012), a combination thereof, or the like. 
     Next, the pupil-split image acquisition unit  60 D will be described. The pupil-split image acquisition unit  60 D acquires the pupil-split picture  70  of the light emitted from the specimen T irradiated with the line illumination LA. The pupil-split image acquisition unit  60 D acquires the pupil-split picture  70  from the pupil-split image capture unit  42 , thereby acquiring the image  72 A and the image  72 B, which are the pupil-split image  72  included in the pupil-split picture  70 . 
     The derivation unit  60 E derives relative position information between the objective lens  22  and the specimen T based on the light intensity distributions of the image  72 A and the image  72 B. In other words, using the light intensity distributions of the image  72 A and the image  72 B, the derivation unit  60 E derives the relative position information regarding the relative position at which the objective lens  22  is focused on the specimen T, that is, the relative position at which the focus is adjusted to the specimen T. 
     As described with reference to  FIG.  3   , the pupil-split picture  70  includes the set of images  72 A and  72 B. In the present embodiment, the derivation unit  60 E derives the relative position information between the objective lens  22  and the specimen T based on an interval YL representing the phase difference between the image  72 A and the image  72 B. 
     Return to  FIG.  5   , and description will continue. The derivation unit  60 E will be described in detail. The derivation unit  60 E includes a selection unit  60 H, a phase difference acquisition unit  60 I, and a relative distance derivation unit  60 J. 
     As described above, the pupil-split image capture unit  42  of the present embodiment has a configuration in which the plurality of types of unit regions  37  having different exposure setting values for the included light receiving unit  41  are arranged along the light receiving surface  43 . Therefore, it is preferable that the derivation unit  60 E derive the relative position information based on the light intensity distribution of the pupil-split image  72  received by the light receiving unit  41  included in the unit region  37  having a specific exposure setting value. 
     Therefore, the selection unit  60 H selects the unit region  37  including the light receiving unit  41  to which a specific light sensitivity is set from among the plurality of types of unit regions  37 . 
       FIG.  6 A  is a conceptual diagram of the measurement target region  24  including the specimen T. The measurement target region  24  is irradiated with the irradiation light LA as a line of light. The description will be given assuming a case where the specimen T included in the measurement target region  24  is an object such as a cell labeled with a fluorescent dye that fluoresces by irradiation with the line illumination LA. In this case, in the irradiation region of the line illumination LA in the measurement target region  24 , the intensity of the light emitted from a region PB where the specimen T exists is higher than the intensity of the light emitted from the region PA where the specimen T does not exist. 
       FIG.  6 B  is a schematic view illustrating only one side of a pupil-split picture  70 C. This figure is a schematic diagram illustrating an example. The pupil-split picture  70 C is an example of the pupil-split picture  70 .  FIG.  6 B  illustrates only the light intensity distribution of the image  72 A on one side in the pupil-split picture  70 C. The similar applies to the light intensity distribution of the image  72 B. 
     In the pupil-split picture  70 C, a region EA corresponding to the region PA where the specimen T does not exist has a lower intensity value of the light received by the light receiving unit  41  compared to a region EB corresponding to the region PB where the specimen T exists. Therefore, it is preferable to perform, for a region EA, information processing using the intensity value of light received by the light receiving unit  41  having a high exposure setting value. Moreover, it is preferable to perform, for a region EB, information processing using the intensity value of fluorescence received by the light receiving unit  41  having a low exposure setting value. 
     Therefore, from among a plurality of types of unit regions  37  included in the pupil-split image capture unit  42 , the selection unit  60 H selects the unit region  37  including the light receiving unit  41  to which a specific light sensitivity has been set. The selection unit  60 H selects the unit region  37  using the pupil-split picture  70  acquired by the pupil-split image acquisition unit  60 D. Specifically, the selection unit  60 H selects the unit region  37  including the light receiving unit  41  having a light intensity value within a predetermined range. For example, it is assumed that the light intensity value is represented by a gradation value of 0 to 255. In this case, the selection unit  60 H specifies a region in the pupil-split picture  70 , in which the gradation value representing the light intensity value is within a predetermined range. Subsequently, the selection unit  60 H selects the unit region  37  including the light receiving unit  41  corresponding to the specified region. For example, the selection unit  60 H selects, as the predetermined range, the unit region  37  including the light receiving unit  41  that has output the light intensity value in the range in which the gradation value is 10 or more and 250 or less. 
       FIG.  6 C  is an explanatory diagram of selection of the unit region  37 . Through the selection processing, the selection unit  60 H selects, within the pupil-split picture  70 , the unit region  37 A (unit regions  37 A 1 ,  37 A 2 ,  37 A 3 , and  37 A 4 ) in which high light sensitivity is set in the included light receiving unit  41  for the region EA corresponding to the region PA where the specimen T does not exist. Furthermore, the selection unit  60 H selects, within the pupil-split picture  70 , the unit region  37 B (unit regions  37 B 4  and  37 B 5 ) in which low light sensitivity is set in the included light receiving unit  41  for the region EB corresponding to the region PB where the specimen T exists. 
     Subsequently, within the pupil-split picture  70  acquired by the pupil-split image acquisition unit  60 D, the selection unit  60 H outputs, to the phase difference acquisition unit  60 I, the pupil-split picture  70  including the image  72 A and the image  72 B having the phase difference including the light intensity value of the light receiving unit  41  included in the selected unit region  37 . Therefore, the selection unit  60 H can output, to the phase difference acquisition unit  60 I, the pupil-split picture  70  including the pupil-split image  72  having the phase difference in which saturation or signal insufficiency is suppressed. Note that the derivation unit  60 E may be a configuration not including the selection unit  60 H. 
     Return to  FIG.  5   , and description will continue. The phase difference acquisition unit  60 I calculates the interval YL representing the phase difference between the set of images  72 A and  72 B constituting the pupil-split image  72  included in the pupil-split picture  70 . In the present embodiment, the phase difference acquisition unit  60 I calculates a phase difference obtained from the interval YL between the image  72 A and the image  72 B included in the pupil-split picture  70  received from the selection unit  60 H. 
     In the present embodiment, the phase difference acquisition unit  60 I calculates the interval between the centroid of the image  72 A and the centroid of the image  72 B as the interval YL between the image  72 A and the image  72 B. 
     The centroid represents the centroid of the light intensity distribution of each of the image  72 A and the image  72 B. 
       FIG.  7    is an explanatory diagram of a centroid g. Among the image  72 A and the image  72 B,  FIG.  7    illustrates the image  72 A as an example. The centroid g means the centroid of the light intensity distribution in the YA-axis direction in the image  72 A as a line long in the XA-axis direction. As described above, the image  72 A is an image of line long in the XA-axis direction. Therefore, in the pupil-split picture  70 , the centroid g is represented by a line running in the XA-axis direction which is an extending direction of the image  72 A. 
       FIG.  8    is a schematic diagram illustrating an example of a pupil-split picture  70 B. The pupil-split picture  70 B is an example of the pupil-split picture  70 . The phase difference acquisition unit  60 I calculates the interval YL between a centroid ga which is the centroid g of the image  72 A and a centroid gb which is the centroid g of the image  72 B. 
     Specifically, for example, the phase difference acquisition unit  60 I calculates the interval YL using the following Formulas (1) to (3). 
     
       
         
           
             
               
                 
                   
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     In Formula (1), [Ytt, Ytb] indicates a range R in the YA-axis direction of the light intensity distribution of the image  72 A (refer to  FIG.  7   ). In Formulas (1) to (3), Ytt represents an upper end R1 of the light intensity distribution of the image  72 A in the YA-axis direction (refer to  FIG.  7   ). Ytb represents a lower end R2 in the YA-axis direction of the light intensity distribution of the image  72 A (refer to  FIG.  7   ). In Formulas (1) to (3), W represents the pixel width of the pupil-split picture  70 . The pixel width indicates a width of one pixel in the X-axis direction or the Y-axis direction of the imaging range of the pupil-split picture  70  in the measurement target region  24 . In the present embodiment, the description will be given assuming that the widths of the imaging range of the pupil-split picture  70  for one pixel in the X-axis direction and the Y-axis direction are the same. 
     In Formulas (1) to (3), A black  is a black level average pixel value of a region other than the light receiving regions of the images  72 A and  72 B in the pupil-split picture  70 . In Formulas (1) to (3), A dif  is a noise level of a region other than the region of  72 A and  72 B in the pupil-split picture  70 . 
     The phase difference acquisition unit  60 I calculates the range R of image  72 A by using the above Formula (1). In addition, the phase difference acquisition unit  60 I calculates the range R of the image  72 B similarly to the case of the image  72 A. 
     Next, the phase difference acquisition unit  60 I calculates centroid g of image  72 A by using Formula (2). In Formula (2), Ytc represents the centroid ga of the image  72 A. In addition, the phase difference acquisition unit  60 I calculates the centroid gb of the image  72 B in the same manner as the image  72 A. 
     Subsequently, the phase difference acquisition unit  60 I calculates an interval YL between the centroid ga of the image  72 A representing the phase difference and the centroid gb of the image  72 B using Formula (3). In Formula (3), Y phase  represents a phase difference, representing the interval YL between the centroid ga of the image  72 A and the centroid gb of the image  72 B. Ybc indicates the centroid gb of the image  72 B. Ytc represents the centroid ga of the image  72 A. 
     The phase difference acquisition unit  60 I outputs the calculated interval YL to the relative distance derivation unit  60 J. 
     As described above, the image  72 A and the image  72 B constituting the pupil-split picture  70  are images obtained when the specimen T is irradiated with the line illumination LA. Therefore, the image  72 A and the image  72 B constituting the pupil-split picture  70  are images as lines each long in a predetermined direction. As described above, the predetermined direction is a direction optically corresponding to the X-axis direction which is the longitudinal direction of the line illumination LA. 
     The interval YL representing the phase difference between the pair of images  72 A and  72 B may include positions at different intervals depending on the positions in the XA-axis direction which is the longitudinal direction of these images. In addition, the image  72 A and the image  72 B are not limited to the image as a perfect strain line, and may be an image as a line including partially bent regions. The width (thickness) of each of the image  72 A and the image  72 B as lines also varies depending on the focus shift. The width of each of the image  72 A and the image  72 B represents the length of each of the image  72 A and the image  72 B in the YA-axis direction. 
     Therefore, as described above, it is preferable that the phase difference acquisition unit  60 I calculate, as the interval YL, the distance between the centroid ga in the XA-axis direction and the YA-axis direction of the image  72 A and the centroid gb in the XA-axis direction and the YA-axis direction of the image  72 B. 
     Additionally, the phase difference acquisition unit  60 I may calculate the interval YL by the following method. 
     For example, after adjusting the brightness and contrast of the pupil-split picture  70  by adjusting the light intensity value of the pupil-split picture  70 , the phase difference acquisition unit  60 I may calculate the centroid g and the interval YL of each of the image  72 A and the image  72 B similarly to the method described above. 
     The phase difference acquisition unit  60 I may calculate the centroid g and the interval YL of each of the image  72 A and the image  72 B in the same manner as described above. 
     For example, among individual positions in the XA-axis direction which is the direction optically corresponding to the longitudinal direction of the line illumination LA in each of the centroid g of the image  72 A and the image  72 B, the phase difference acquisition unit  60 I specifies the position of the centroid g having the light intensity value being the first threshold or more and the second threshold or less. The phase difference acquisition unit  60 I may then calculate an interval between the centroid g representing the phase difference of the image  72 A at the specified position and the centroid g of the image  72 B, as the interval YL. 
     The first threshold and the second threshold may be preset to values larger than the minimum value of the light intensity value that can be output by the light receiving unit  41  and smaller than the maximum value. In addition, the second threshold needs to be a value larger than the first threshold. 
     Furthermore, it is allowable to configure such that, for example, after adjusting the light intensity value of the pupil-split picture  70  by weighting, the phase difference acquisition unit  60 I specifies the centroid g of each of the image  72 A and the image  72 B and calculates the interval YL. 
     In this case, the phase difference acquisition unit  60 I corrects the light intensity value, which is the gradation value of each pixel constituting the pupil-split picture  70 , such that the higher the light intensity value, the higher weighting is to be applied. Subsequently, the phase difference acquisition unit  60 I calculates the centroid g for each position in the XA-axis direction, which is a direction optically corresponding to the longitudinal direction of the line illumination LA, for the image  72 A and the image  72 B included in the corrected pupil-split picture  70 . The phase difference acquisition unit  60 I may then calculate, as the interval YL, the distance between the centroid g at the position where the light intensity value is a fifth threshold or more in the XA-axis direction of the image  72 A and the image  72 B. The fifth threshold may be determined in advance. 
     In addition, for example, the phase difference acquisition unit  60 I divides each of the image  72 A and the image  72 B into a plurality of split regions in the XA-axis direction which is a direction optically corresponding to the longitudinal direction of the line illumination LA. Subsequently, the phase difference acquisition unit  60 I specifies the centroid g of each of the included images  72 A and  72 B for each split region. Furthermore, for each of the image  72 A and the image  72 B, the phase difference acquisition unit  60 I specifies a position indicating an average value of the maximum light intensity values of each of the split regions. The phase difference acquisition unit  60 I may then calculate the interval in the YA-axis direction between the positions in the pupil-split picture  70  as the interval YL. 
     Furthermore, for example, the phase difference acquisition unit  60 I fits the width direction of each of the image  72 A and the image  72 B in the YA-axis direction to a quadratic function or a Gaussian for each pixel unit in the XA-axis direction. Subsequently, the phase difference acquisition unit  60 I may calculate, as the interval YL, the distance between the modes of the peaks of the image  72 A and the image  72 B corresponding to the phase difference after the fitting. 
     Note that the phase difference acquisition unit  60 I may use any of the above methods as a method of calculating the interval YL. For example, the phase difference acquisition unit  60 I may specify the method of calculating the interval YL according to the type of the specimen T as a measurement target. The phase difference acquisition unit  60 I may then calculate the interval YL by the specified calculation method. The method of calculating the interval YL by the phase difference acquisition unit  60 I is not limited to the above method. For example, the phase difference acquisition unit  60 I may calculate, as interval YL, a distance between the centers of the line widths of image  72 A and image  72 B formed in a region having a light intensity value being a certain threshold or more. 
     Return to  FIG.  5   , and description will continue. The relative distance derivation unit  60 J calculates the relative position between the objective lens  22  and the specimen T using the interval YL received from the phase difference acquisition unit  60 I. Specifically, the relative distance derivation unit  60 J calculates, as the relative position, the relative movement amount and the relative movement direction according to a difference between the interval YL and a reference interval. 
     The reference interval is the interval YL between the image  72 A and the image  72 B when the objective lens  22  is focused on the specimen T. Regarding the reference interval, the present embodiment uses the interval YL between the image  72 A and the image  72 B when the objective lens  22  and the specimen T on the stage  26  are adjusted to the initial relative positions by the reference focus unit  60 C, as the reference interval. 
     Here, the interval YL corresponding to the phase difference between the image  72 A and the image  72 B constituting the pupil-split image  72  having the phase difference is proportional to a focal length between the objective lens  22  and the specimen T. This allows the relative distance derivation unit  60 J to calculate the relative position by using the difference between the interval YL and the reference interval. As described above, the relative position is the relative position of one of the objective lens  22  and the specimen T with respect to the other. The relative position is represented by the movement direction and the movement amount of at least one of the objective lens  22  and the specimen T with respect to the current position of each of the objective lens  22  and the specimen T. The movement direction and the movement amount are represented by, for example, a displacement amount ΔZ in the Z-axis direction of the focus position of the objective lens  22 . 
     The twin-lens phase difference method is a method of calculating the displacement amount ΔZ in the Z-axis direction with respect to the reference position from the phase difference of the image. That is, the calculation of the relative position means the calculation of the phase difference of the image, which represents the calculation of the displacement amount ΔZ. In the present embodiment, by calculating the displacement amount ΔZ, the relative distance derivation unit  60 J calculates the relative position displacement between the objective lens  22  and the specimen T. 
     The displacement amount ΔZ represents the relative movement amount and the relative movement direction of the objective lens  22  and the specimen T. That is, an absolute value |ΔZ| of the displacement amount ΔZ represents the relative movement amount, and a positive/negative sign of the displacement amount ΔZ represents a relative movement direction. 
       FIG.  9    illustrates an example in which 17 pupil-split pictures  70  acquired while changing the distance between the objective lens and the specimen by line illumination are arranged in a strip shape for comparison. It can be seen that the line interval on the right side is wider than that on the left side. In this manner, the phase change amount of the image can be acquired from the pupil-split image  72  as the relative distance change between the image  72 A and the image  72 B. 
     In the control unit  60  of the present embodiment, the initial relative position is adjusted by the reference focus unit  60 C before the derivation unit  60 E derives the displacement amount ΔZ. Subsequently, the relative distance derivation unit  60 J uses the interval between the image  72 A and the image  72 B when the objective lens  22  and the specimen T are adjusted to the initial relative positions by the reference focus unit  60 C, as a reference interval YL′. 
       FIG.  10    is an explanatory diagram of a step of acquiring a phase difference.  FIG.  10    illustrates the reference interval YL′ as a distance between an image  72 A 1  and an image  72 B 1  when a position  80 A being the position of the specimen T is in focus, that is, when the focus adjustment is achieved. The image  72 A 1  and image  72 A 2  are examples of the image  72 A and the image  72 B. 
     Here is an assumable case where, by scanning the line illumination LA in the scanning direction (Y-axis direction) after the initial relative position is adjusted by the reference focus unit  60 C, the imaging region of the image capture unit  34  in the measurement target region  24  has been changed. This might cause a change in the distance between the specimen T and the objective lens  22 . With this change, the focus position changes by the displacement amount ΔZ in the Z-axis direction. Due to this change in the displacement amount ΔZ, the interval YL between the image  72 A and the image  72 B is different from the reference interval YL′. 
     For example, when the position of the specimen T has been changed to a position  80 B shifted by the displacement amount ΔZ in the Z-axis direction from the actual position  80 A of the specimen T, the interval YL is an interval YL2 different from the reference interval YL′. The interval YL2 is an example of the interval YL, and is an interval between the image  72 A 2  and the image  72 B 2 . The image  72 A 2  and the image  72 B 2  are examples of the image  72 A and image  72 B, respectively. 
     In addition, in a case where there is a specimen at a position  80 C shifted from the position  80 A in the X-axis direction, the interval YL1 is the same as the reference interval YL′. The interval YL1 is an example of the interval YL, and is an interval between an image  72 A 3  and an image  72 B 3 . The image  72 A 3  and the image  72 B 3  are examples of the image  72 A and image  72 B, respectively. 
     Therefore, the relative distance derivation unit  60 J may calculate the relative position between the objective lens  22  and the specimen T by inversely calculating the displacement amount ΔZ from a difference ΔYL between the reference interval YL′ and the interval YL between the image  72 A and the image  72 B constituting the pupil-split picture  70  acquired by the pupil-split image acquisition unit  60 D. 
     Note that, as described above, an absolute value |ΔZ| of the displacement amount ΔZ represents the relative movement amount, and a positive/negative sign of the displacement amount ΔZ represents a relative movement direction. Accordingly, the relative distance derivation unit  60 J calculates the relative movement amount |ΔZ| and the positive/negative values of AZ being the relative movement direction, as the relative position according to the displacement amount ΔZ corresponding to the difference ΔYL between the interval YL and the reference interval YL′. 
     The following Formulas (3) and (4) are paraxial calculations for  FIG.  10   . 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     In Formulas (3) and (4), Δyi represents a difference ΔYL between the interval YL and the reference interval YL′. m is an imaging magnification of the image  72 A 1  and the image  72 B 1  when the specimen T is at the position  80 A. m′ is an imaging magnification of the image  72 A 1  and the image  72 B 1  when the specimen T is at the position  80 B. Si is a distance in the Z direction from the image  72 A 1  to the separator lens  40 A or from the image  72 B 1  to the separator lens  40 B. So is a distance in the Z direction from the position  80 A of the specimen T to the separator lens  40 A or  40 B. ΔSo is the amount of change in the Z direction of the specimen T and is equal to ΔZ in  FIG.  10   . In addition, Δyi represents a difference ΔYL between the interval YL and the reference interval YL′. Yo is a half of the interval YL. 
     As illustrated in  FIG.  10   , here is an assumable case where the position  80 B is in focus, the position  80 B being the position shifted from the position  80 A of the specimen T in the Z-axis direction by the displacement amount ΔZ. In this case, the distance between the separator lens  40  and the pupil-split image capture unit  42  does not change. However, the magnification of the pupil-split image  72  having the phase difference and formed on the pupil-split image capture unit  42  changes from m to m′. At this time, the positions of the image  72 A and the image  72 B, which are images of the specimen T formed on the pupil-split image capture unit  42 , change to positions having the interval YL2 obtained by adding ΔY to the reference interval YL′ corresponding to the focal shift. 
     The relative distance derivation unit  60 J calculates the difference ΔYL between the interval YL2 and the reference interval YL′ using the two-dimensional coordinates of the light receiving surface  43  of the pupil-split image capture unit  42 . Subsequently, the relative distance derivation unit  60 J calculates the displacement amount ΔZ in the focus using the difference ΔYL. 
     As indicated in the above Formulas (3) and (4), in a range where the displacement amount ΔZ is small, a proportional relationship of the displacement amount ΔZ∝difference ΔYL is established. Therefore, the relative distance derivation unit  60 J can obtain the displacement amount ΔZ from the difference ΔYL. Alternatively, the displacement amount ΔZ and the difference ΔYL have a certain correlation. Therefore, the relative distance derivation unit  60 J may create in advance a function or a lookup table representing the correlation between the displacement amount ΔZ and the difference ΔYL. In this case, the relative distance derivation unit  60 J can calculate the displacement amount ΔZ using the difference ΔYL between the interval YL2 and the reference interval YL′ and using the function or the lookup table. 
     The relative distance derivation unit  60 J may calculate the relative position by calculating the displacement amount ΔZ using the reference interval YL′ stored in advance in the storage unit  62 . 
     However, in the same measurement target region  24 , the relationship between the optical distance and the physical distance is considered to be substantially constant. Therefore, before obtaining a captured image for use in analysis or the like of the specimen T in the microscope system  1 , it is preferable to adjust the initial relative position by the reference focus unit  60 C and derive the reference interval YL′. 
     Return to  FIG.  5   , and description will continue. The movement control unit  60 F causes at least one of the objective lens  22  and the specimen T to move to the relative position derived by the derivation unit  60 E. The movement control unit  60 F controls the movement of at least one of the first drive unit  44  and the second drive unit  46  such that at least one of the objective lens  22  and the stage  26  moves in the Z-axis direction. For example, the movement control unit  60 F controls the movement of the first drive unit  44  such that the stage  26  moves in a relative movement direction according to the relative movement amount |ΔZ| represented by the displacement amount ΔZ derived as the relative position and the positive/negative values of the displacement amount ΔZ. Under the control of the movement control unit  60 F, the objective lens  22  and the specimen T are moved in a direction of approaching each other or a direction of separating from each other in the Z-axis direction. That is, the relative position in the Z-axis direction between the objective lens  22  and the specimen T is adjusted to be the relative position derived by the derivation unit  60 E, so as to achieve a focus adjustment state in which the specimen T is in focus. 
     In synchronization with the movement control performed by the movement control unit  60 F, the captured image acquisition unit  60 B acquires a captured image of light emitted from the specimen T from the image capture unit  34 . 
     The output control unit  60 G outputs the captured image acquired by the captured image acquisition unit  60 B to an external device such as the server device  10  via the communication unit  64 . The output control unit  60 G may store the captured image acquired by the captured image acquisition unit  60 B in the storage unit  62 . Furthermore, the output control unit  60 G may output the captured image to a display connected to the control unit  60 . 
     By analyzing the captured image acquired by the captured image acquisition unit  60 B using the known method, the output control unit  60 G may analyze the type or the like of the specimen T and output the analysis result to the server device  10  or the like. 
     Next, an example of a flow of information processing executed by the control device  16  according to the present embodiment will be described. 
       FIG.  11    is a flowchart illustrating an example of a flow of information processing executed by the control device  16 . It is assumed that the measurement target region  24  including the specimen T is placed on the stage  26  before the control device  16  executes the following information processing. The placement of the measurement target region  24  on the stage  26  may be performed manually or may be automatically controlled using a loader, a manipulator, or the like. 
     The light source control unit  60 A controls the light source  18 B to turn off the line illumination LA and turn on the area illumination LB (step S 100 ). With the control in step S 100 , the line illumination LA is emitted from the light source  18 B. 
     The captured image acquisition unit  60 B acquires, from the image capture unit  34 , the first captured image of light emitted from the specimen T irradiated with the area illumination LB (step S 102 ). 
     Using the first captured image acquired in step S 102 , the reference focus unit  60 C performs reference focus processing (step S 104 ). In step S 104 , the reference focus unit  60 C calculates the contrast ratio of the first captured image acquired in step S 102 . The reference focus unit  60 C repeats the movement of the objective lens  22  in the Z-axis direction under the control of the movement control unit  60 F and the calculation of the contrast ratio of the first captured image acquired from the image capture unit  34 . With this repetition processing, the reference focus unit  60 C adjusts the initial relative position between the objective lens  22  and the specimen T to a position where the contrast ratio is maximized. 
     Next, the light source control unit  60 A controls the light source  18 B to turn off the area illumination LB (step S 106 ). Then, the light source control unit  60 A controls the light source  18 B to turn on the line illumination LA (step S 108 ). With the control in step S 108 , the line illumination LA is emitted from the light source  18 B. 
     The pupil-split image acquisition unit  60 D acquires the pupil-split picture  70  from the pupil-split image capture unit  42  to acquire the pupil-split image  72  being an image of light emitted from the specimen T irradiated with the line illumination LA (step S 110 ). 
     Next, the selection unit  60 H selects the unit region  37  including the light receiving unit  41  set to a specific light sensitivity from among the plurality of types of unit regions  37  (step S 112 ). The selection unit  60 H selects the unit region  37  including the light receiving unit  41  that has output the light intensity value within a predetermined gradation value range (for example, the gradation value ranges of 10 or more and 250 or less) in the pupil-split picture  70  acquired in step S 110 . The selection unit  60 H then outputs the pupil-split picture  70  including the image  72 A and the image  72 B having the light intensity value of the light receiving unit  41  included in the selected unit region  37  in the pupil-split picture  70  acquired in step S 110  to the phase difference acquisition unit  60 I. 
     Next, the phase difference acquisition unit  60 I specifies the centroid g of each of the set of images  72 A and  72 B constituting the pupil-split image  72  received from the selection unit  60 H (step S 114 ). The phase difference acquisition unit  60 I then calculates the interval between the specified centroids g as a reference interval YL′ between the image  72 A and the image  72 B (step S 116 ). 
     The reference focus processing is performed by the processing of steps S 100  to S 116 , and the reference interval YL′ is calculated. 
     Next, the movement control unit  60 F controls the movement of the first drive unit  44  such that the irradiation position of the line illumination LA becomes the initial position in the scanning direction (Y-axis direction) of the measurement target region  24  (step S 118 ). 
     Next, the pupil-split image acquisition unit  60 D acquires the pupil-split picture  70  from the pupil-split image capture unit  42 , thereby acquiring the pupil-split image  72  being the image  72 A and the image  72 B of the light emitted from the specimen T irradiated with the line illumination LA (step S 120 ). 
     Next, similarly to step S 112 , the selection unit  60 H selects the unit region  37  including the light receiving unit  41  set to a specific light sensitivity among the plurality of types of unit regions  37  (step S 122 ). The selection unit  60 H then outputs, to the phase difference acquisition unit  60 I, the pupil-split picture  70  including the image  72 A and the image  72 B having the light intensity value of the light receiving unit  41  included in the selected unit region  37  in the pupil-split picture  70  acquired in step S 120 . 
     The phase difference acquisition unit  60 I specifies (step S 124 ) the centroid g of each of the set of images  72 A and  72 B constituting the pupil-split image  72  included in the pupil-split picture  70  received from the selection unit  60 H in step S 122 . The phase difference acquisition unit  60 I then calculates the interval between the specified centroids g as an interval YL between the image  72 A and the image  72 B (step S 126 ). 
     Next, the relative distance derivation unit  60 J calculates a difference ΔYL between the interval YL calculated in step S 126  and the reference interval YL′ calculated in step S 116  (step S 128 ). 
     Next, by inversely calculating the displacement amount ΔZ from the difference ΔYL calculated in step S 128 , the relative distance derivation unit  60 J calculates relative position information indicating the relative position between the objective lens  22  and the specimen T (step S 130 ). 
     By controlling the movement of at least one of the first drive unit  44  and the second drive unit  46 , the movement control unit  60 F causes at least one of the objective lens  22  and the stage  26  to move in the Z-axis direction (step S 132 ). Specifically, the movement control unit  60 F causes at least one of the objective lens  22  and the specimen T to move in the relative movement direction according to the relative movement amount |ΔZ| represented by the displacement amount ΔZ derived as the relative position in step S 130  and the positive/negative values of the displacement amount ΔZ. For example, the movement control unit  60 F controls the movement of at least one of the first drive unit  44  and the second drive unit  46  such that at least one of the objective lens  22  and the stage  26  moves along the Z-axis direction. Specifically, the movement control unit  60 F controls the movement of the first drive unit  44  such that the stage  26  moves in a relative movement direction according to the relative movement amount |ΔZ| represented by the displacement amount ΔZ derived as the relative position and the positive/negative values of the displacement amount ΔZ. 
     With the control of step S 132 , the objective lens  22  and the specimen T are moved in a direction of approaching each other or a direction of separating from each other in the Z-axis direction. That is, the relative position in the Z-axis direction between the objective lens  22  and the specimen T is adjusted to be the relative position calculated in step S 130 , so as to achieve a focus adjustment state in which the specimen T is in focus. 
     Next, the captured image acquisition unit  60 B acquires the second captured image  74  of the light emitted from the specimen T from the image capture unit  34  (step S 134 ). The second captured image  74  acquired in step S 134  is a captured image at a position in the scanning direction (Y-axis direction) of the measurement target region  24 . 
     The control unit  60  determines whether to end the acquisition of the second captured image  74  (step S 136 ). The control unit  60  discerns whether the line illumination LA has been scanned from one end to the other end in the scanning direction in the measurement target region  24 , thereby making the determination in step S 136 . When a negative determination is made in step S 136  (step S 136 : No), the processing proceeds to step S 138 . 
     In step S 138 , the movement control unit  60 F controls the movement of the first drive unit  44  so as to move the stage  26  in the scanning direction (Y-axis direction) by the width of the line illumination LA (step S 138 ). With the processing in step S 138 , the irradiation position of the line illumination LA in the scanning direction (Y-axis direction) of the measurement target region  24  is moved in the scanning direction by the width of the line illumination LA. Subsequently, the processing returns to step S 120  described above. 
     In step S 136 , the control unit  60  may discern whether the line illumination LA has been scanned from one end to the other end in the scanning direction in the measurement target region  24  and whether the line illumination LA has been scanned from one end side to the other end side in the X-axis direction in the measurement target region  24  to make the determination in step S 136 . In this case, in step S 138 , the movement control unit  60 F may shift the irradiation position of the line illumination LA in the X-axis direction every time the scanning of the line illumination LA is completed from one end to the other end in the scanning direction of the measurement target region  24 , and may then return to step S 120 . 
     Furthermore, the irradiation of the line illumination LA may be turned off during the movement of the stage  26  by the processing of step S 138 . It is also allowable to configure such that, when the movement of the stage  26  is stopped, the line illumination LA will be turned on again, the processing will be executed after returning to step S 120 . 
     In contrast, when an affirmative determination is made in step S 136  (step S 136 : Yes), the processing proceeds to step S 140 . In step S 140 , the output control unit  60 G stores the second captured image  74  from one end to the other end in the scanning direction of the measurement target region  24  in the storage unit  62  as a captured image of the specimen T included in the measurement target region  24  (step S 140 ). This completes the present routine. 
     As described above, the microscope system  1  of the present embodiment includes the irradiation unit  18 , the stage  26 , the pupil-split image acquisition unit  60 D, the objective lens  22 , the derivation unit  60 E, and the movement control unit  60 F. The irradiation unit  18  emits the line illumination LA parallel to the first direction. The stage  26  supports the specimen T and is movable in a second direction perpendicular to the first direction. The pupil-split image acquisition unit  60 D acquires the phase difference of the image of the light emitted from the specimen T by being irradiated with the line illumination LA. The objective lens  22  condenses the line illumination LA on the specimen T. The derivation unit  60 E derives relative position information between the objective lens  22  and the specimen T based on the phase difference (for example, phase difference increase/decrease information and the like). Based on the relative position information, the movement control unit  60 F causes at least one of the objective lens  22  and the stage  26  to move in a third direction vertical to the first direction and the second direction. 
     Here, there is a technique, disclosed as a conventional technique, of performing focus adjustment to bring a specimen T in focus by a contrast method of changing a focus to maximize the contrast ratio of a captured image of the entire region including the specimen T. 
     The contrast method, however, is a method using a hill-climbing method that performs repetition control accompanied with physical movement such as movement of the objective lens  22 , acquisition of a captured image, and acquisition and comparison of a plurality of captured images before and after the focus position. For this reason, the conventional technique takes time to adjust the focus in some cases. 
     In addition, there is a technique, disclosed as a conventional technique, of performing focus adjustment on a specimen T using a set of subject images included in a pupil-split picture of the entire region including the specimen T. In this technique, however, each of the set of subject images is a subject image of the entire region including the specimen T. Because of this, the conventional technique has had a need to further specify some feature points from within the specimen T in the subject image and to perform focus adjustment using these specified regions. Therefore, in order to extract some feature points from the pupil-split two-dimensional image of the specimen T, it may take additional time for the focus adjustment by the extraction time in the conventional technique. 
     Furthermore, there is a technique, disclosed as a conventional technique, having a configuration in which focusing image sensors are arranged in front and rear of the focus position. In this conventional technique, focus adjustment is performed by analyzing captured images obtained by the plurality of image sensors. Although the use of this method is considered to be able to achieve high-speed focus adjustment, it is necessary to align the positions of the two independent image sensors with high accuracy, making it difficult to achieve high-accuracy focus adjustment. 
     Compared to these techniques, the microscope system  1  of the present embodiment irradiates the specimen T with the line illumination LA, and acquires the phase difference of the image of the light emitted from the specimen T irradiated with the line illumination LA. In addition, the microscope system  1  derives relative position information between the objective lens  22  and the specimen T based on the light intensity information regarding the image  72 A and the image  72 B, and causes at least one of the objective lens  22  and the stage  26  to move in the third direction. 
     In this manner, the microscope system  1  of the present embodiment derives the relative position information between the objective lens  22  and the stage  26  based on the light intensity distribution of the pupil-split image  72  obtained by the irradiation of the line illumination LA. Therefore, since the relative position information represented by the displacement amount ΔZ can be derived without searching for the focus position, the focus can be adjusted to the specimen T at high speed. 
     In addition, in the microscope system  1  of the present embodiment, the specimen T is irradiated with the line illumination LA, and the focus adjustment is performed using the pupil-split image  72  of the light emitted from the specimen T irradiated with the line illumination LA. Therefore, in the microscope system  1  of the present embodiment, focus adjustment can be performed without specifying a plurality of feature points from the specimen T included in the pupil-split image  72 . This makes it possible for the microscope system  1  of the present embodiment to perform focus adjustment at high speed regardless of the size of the specimen T, the number of tissues of the specimen T, and the like. In addition, as compared with a case where focus adjustment is performed using the pupil-split image  72  of light emitted from the specimen T irradiated with the area illumination LB, the microscope system  1  of the present embodiment only needs to process data of a data amount smaller by one digit or more, leading to achievement of high-speed focus adjustment of the specimen T. 
     Therefore, the microscope system  1  of the present embodiment can achieve high-speed and highly accurate focus adjustment. 
     In addition, the microscope system  1  of the present embodiment performs focus adjustment using the pupil-split image  72  of light emitted from the specimen T irradiated with the line illumination LA. Therefore, the microscope system  1  of the present embodiment can achieve robust and stable focus adjustment with a simple configuration in addition to the above effects. 
     In addition, in the microscope system  1  of the present embodiment, the line illumination LA is used as illumination to be emitted at acquisition of the pupil-split image  72 . This makes it possible to shorten the light irradiation time with respect to the specimen T as compared with the case without the use of the line illumination. Therefore, the microscope system  1  of the present embodiment can suppress fading of the specimen T in addition to the above effects. 
     The present embodiment has described, as an example, a mode in which the specimen T is irradiated with the line illumination LA at acquisition of the pupil-split image  72  and the specimen T is also irradiated with the line illumination LA at acquisition of the second captured image  74 . However, at acquisition of the second captured image  74 , the specimen T may be irradiated with the area illumination LB. In this case, for example, it is also allowable to use, as the area illumination LB, light in a wavelength region different from that of the line illumination LA. In addition, it is allowable to use a configuration including, in the half mirror  28 , a filter that selectively reflects the light emitted from the specimen T by the irradiation of the line illumination LA to the phase difference detection optical unit  36  and transmits the light emitted from the specimen T by the irradiation of the area illumination LB to the imaging optical unit  30 . 
     Furthermore, the optical axis of the image capture unit  34  and the optical axis of the pupil-split image capture unit  42  may be in alignment or non-alignment with each other. Still, it is preferable to adjust the arrangement of the pupil-split image capture unit  42  and the pupil-split image capture unit  34  such that the incidence of light on the pupil-split image capture unit  34  precedes the incidence of light on the pupil-split image capture unit  42  in the scanning direction (Y-axis direction) of the line illumination LA. 
     Furthermore, in the present embodiment, the case where the phase difference detection optical unit  36  is an optical unit for obtaining a twin-lens pupil-split image has been described as an example. However, the phase difference detection optical unit  36  only needs to be an optical unit for obtaining the phase difference of the pupil-split image  72 , and may be an optical unit for obtaining the pupil-split image  72  of triple lenses or more as described above. Furthermore, the phase difference detection optical unit  36  may be an off-axis optical system offset from the optical axis. The off-axis optical system is an optical system using one or more lenses off the optical axis. In this case, the control unit  60  may derive the relative position information by calculating the difference ΔYL from the optical system using the off-axis lens. 
     (First Modification) 
     The above-described embodiment has described a mode in which the microscope system  1  acquires the second captured image  74  every time at least one of the objective lens  22  and the specimen T is caused to move to the relative position represented by the displacement amount ΔZ derived by the derivation unit  60 E. That is, the embodiment described above has described a mode in which the second captured image  74  is acquired every time the focus is adjusted onto the specimen T at each position in the scanning direction of the measurement target region  24 . 
     Alternatively, the microscope system  1  may acquire the Z-stack image as the second captured image  74  at each position in the scanning direction of the measurement target region  24 . 
     In this case, the control unit  60  may execute the following processing every time at least one of the objective lens  22  and the specimen T is caused to move to the relative position represented by the displacement amount ΔZ derived by the derivation unit  60 E. 
     For example, the position, in the Z-axis direction, of the objective lens  22  and the specimen T located at the relative position derived by the derivation unit  60 E is defined as a position where the displacement amount ΔZ is “0”. Here is an assumable case where nine Z-stack images are acquired at intervals of 0.5 μm in the range of ±2 μm in the Z-axis direction with respect to the specimen T. In this case, the control unit  60  may execute the following control. 
     Specifically, it is assumed that the value of the difference ΔYL at which the displacement amount ΔZ is 0.5 um is a difference ΔYL0. The movement control unit  60 F causes the objective lens  22  to move stepwise in the Z-axis direction so that the difference ΔYL is as follows. ΔYL=4ΔYL0,3ΔYL0,2ΔYL0,1ΔYL0,0,−ΔYL0,−2ΔYL0,−3ΔYL0,−4ΔYL0 
     Subsequently, the captured image acquisition unit  60 B acquires the second captured image  74  for each movement of the objective lens  22  in the Z-axis direction, thereby acquiring nine Z-stack images. 
     Through these steps of processing, the control unit  60  can acquire nine Z-stack images as the second captured image  74  every time at least one of the objective lens  22  and the specimen T is caused to move to the relative position represented by the displacement amount ΔZ derived by the derivation unit  60 E. The number of Z-stack images is not limited to nine. 
       FIG.  12 A  is an explanatory diagram of a Z-stack image obtained by the processing of the present modification.  FIG.  12 B  is an explanatory diagram of a conventional Z-stack image. 
     As illustrated in  FIG.  12 B , in the conventional technique, a Z-stack image  76 ′ has been acquired in parallel to the two-dimensional plane of the stage  26 . On the other hand, in the present modification, as illustrated in  FIG.  12 A , even when the specimen T is placed on the stage  26  so as to be inclined with respect to the two-dimensional plane, a Z-stack image  76  at each position before and after the displacement amount ΔZ can be acquired from the surface of the specimen T, the bottom surface of the specimen T, the surface between the surface and the bottom surface, and the like. 
     (Second Modification) 
     The above-described embodiment has described, as an example, a mode in which the microscope system  1  executes real-time focusing of performing focus adjustment based on the interval YL between the image  72 A and the image  72 B constituting the pupil-split image  72  included in the pupil-split picture  70  every time the pupil-split picture  70  is acquired. 
     However, the microscope system  1  may create a focus map for adjusting the focus to each position of the specimen T and may adjust the focus to each position of the specimen T according to the focus map. In addition, it is also allowable to use the microscope system  1  to acquire, instead of a focus map, a three-dimensional surface conformational structure including also the Z-axis direction of the specimen T. 
     In the case of acquiring the focus map, the control unit  60  may execute the following processing. In addition, also in the case of acquiring the three-dimensional surface conformational structure, the processing is the same. 
     Similarly to the above-described embodiment, the phase difference acquisition unit  60 I calculates the interval YL being an inter-image interval between a set of images  72 A and  72 B constituting the pupil-split image  72 . At this time, the phase difference acquisition unit  60 I calculates the interval YL for each position in the extending direction (X-axis direction) between the set of images  72 A and  72 B constituting the pupil-split image  72 . Regarding the calculation of the interval YL, it is sufficient to calculate the interval YL between the centroid ga of the image  72 A and the centroid gb of the image  72 B for each position in the X-axis direction. 
     By calculating the displacement amount ΔZ for each position in the X-axis direction using the interval YL received from the phase difference acquisition unit  60 I, the relative distance derivation unit  60 J may preferably calculate the relative position between the objective lens  22  and the specimen T. 
     The phase difference acquisition unit  60 I and the relative distance derivation unit  60 J repeatedly execute the above processing each time the irradiation position of the line illumination LA in the measurement target region  24  is changed in the scanning direction (Y-axis direction) or the X-axis direction. Through these steps of processing, the derivation unit  60 E can calculate the displacement amount ΔZ at each position over the two-dimensional plane defined by two axes in the X-axis direction and the Y-axis direction of the measurement target region  24 . 
     Thereafter, the derivation unit  60 E only needs to register the position coordinates of each position on the two-dimensional plane defined from two axes in the X-axis direction and the Y-axis direction of the measurement target region  24  and the calculated displacement amount ΔZ to the focus map in association with each other. Note that the derivation unit  60 E only needs to register, to the focus map, at least one of the displacement amount ΔZ and the relative position represented by the displacement amount ΔZ. 
     At the acquisition of the second captured image  74  by the image capture unit  34 , the derivation unit  60 E specifies, from the focus map, the displacement amount ΔZ corresponding to the position of the imaging region of the second captured image  74  in the measurement target region  24 . Thereafter, the control unit  60  only needs to adjust the relative position between the objective lens  22  and the specimen T to the relative position represented by the specified displacement amount ΔZ so as to acquire the second captured image  74 . 
       FIG.  13    is a flowchart illustrating an example of a flow of focus map creation processing. 
     First, the reference focus unit  60 C executes reference interval YL′ calculation processing (step S 200 ). The processing of step S 200  is similar to steps S 100  to S 116  described with reference to  FIG.  11   . 
     Next, the movement control unit  60 F controls the movement of the first drive unit  44  such that the irradiation position of the line illumination LA becomes the initial position in the scanning direction (Y-axis direction) of the measurement target region  24  (step S 202 ). 
     Next, the pupil-split image acquisition unit  60 D acquires the pupil-split picture  70  from the pupil-split image capture unit  42 , thereby acquiring the image  72 A and the image  72 B of the light emitted from the specimen T irradiated with the line illumination LA (step S 204 ). 
     Next, similarly to step S 112 , the selection unit  60 H selects the unit region  37  including the light receiving unit  41  to which the specific light sensitivity is set, from among the plurality of types of unit regions  37  (step S 206 ). The selection unit  60 H then outputs the pupil-split picture  70  including the image  72 A and the image  72 B having the light intensity value of the light receiving unit  41  included in the selected unit region  37  in the pupil-split picture  70  acquired in step S 204  to the phase difference acquisition unit  60 I. 
     The phase difference acquisition unit  60 I specifies (step S 208 ) the centroid g of each of the set of images  72 A and  72 B included in the pupil-split picture  70  received from the selection unit  60 H in step S 206 . At this time, the phase difference acquisition unit  60 I specifies centroid g at each position of each of the image  72 A and the image  72 B in the X-axis direction. 
     Subsequently, the phase difference acquisition unit  60 I calculates an interval between the centroids g of each position in the X-axis direction of the image  72 A and the image  72 B as an interval YL between the each position (step S 210 ). 
     Next, the relative distance derivation unit  60 J calculates the difference ΔYL between the interval YL calculated in step S 210  and the reference interval YL′ calculated in step S 200  for each position in the X-axis direction between the image  72 A and the image  72 B (step S 212 ). 
     Next, the relative distance derivation unit  60 J inversely calculates the displacement amount ΔZ from the difference ΔYL calculated in step S 212 . With this processing, the relative distance derivation unit  60 J calculates the displacement amount ΔZ for each position in the X-axis direction at the current irradiation position of the line illumination LA in the scanning direction (Y-axis direction), and calculates the relative position between the objective lens  22  and the specimen T for each position (step S 214 ). 
     Subsequently, the relative distance derivation unit  60 J registers the displacement amount ΔZ calculated in step S 214  and the relative position to the focus map in association with the position coordinates of each corresponding position in the measurement target region  24 . With this processing, the relative distance derivation unit  60 J updates the focus map (step S 216 ). 
     The control unit  60  determines whether to end the focus map update processing (step S 218 ). For example, the control unit  60  discerns whether the line illumination LA has been scanned from one end to the other end in the scanning direction in the measurement target region  24 , thereby making the determination in step S 218 . When a negative determination is made in step S 218  (step S 218 : No), the processing proceeds to step S 220 . 
     In step S 220 , the movement control unit  60 F controls the movement of the first drive unit  44  to move the stage  26  in the scanning direction (Y-axis direction) by the width of the line illumination LA (step S 220 ). With the processing in step S 220 , the irradiation position of the line illumination LA in the scanning direction (Y-axis direction) of the measurement target region  24  is moved in the scanning direction by the width of the line illumination LA. The processing returns to step S 204  described above. 
     The determination of step S 218  may be performed by the discernment made by the control unit  60  as to whether the line illumination LA has been scanned from one end portion to the other end portion in the scanning direction in the measurement target region  24  and whether the line illumination LA has been scanned from one end side to the other end side in the X-axis direction in the measurement target region  24  in step S 218 . In this case, processing can proceed such that, when the scanning of the line illumination LA is completed from one end to the other end in the scanning direction of the measurement target region  24 , the movement control unit  60 F shifts the irradiation position of the line illumination LA in the X-axis direction in step S 220 , and then returns to step S 204 . 
     When an affirmative determination is made in step S 218  (step S 218 : Yes), the present routine is ended. The processing of steps S 200  to S 218  creates the focus map for the specimen T. 
     As described above, the microscope system  1  may create a focus map in advance and adjust the focus to each position of the specimen T according to the focus map. 
     As described above, creation of the focus map uses the light intensity distributions of the image  72 A and the image  72 B of the light emitted from the specimen T irradiated with the line illumination LA. 
     This makes it possible for the microscope system  1  of the present modification to create a high-speed and highly accurate focus map, similarly to the above-described embodiment. 
     (Third Modification) 
     The embodiment described above is a case where the line illumination LA having one wavelength is emitted, and the relative position information is derived based on the light intensity distributions of the image  72 A and the image  72 B of the light emitted from the specimen T irradiated with the line illumination LA. 
     Alternatively, the microscope system  1  may derive the relative position information for each of a plurality of line illuminations LA. 
     In this case, the light source unit  18 A of the irradiation unit  18  can be preferably configured to emit the line illumination LA, which is a plurality of non-coaxial line illuminations LA having different wavelengths and parallel to the first direction (X-axis direction). The light source control unit  60 A can control the light source unit  18 A such that the light source unit  18 A emits the plurality of line illuminations LA. 
       FIG.  14 A  is a schematic diagram of an emission surface of the line illumination LA of the light source  18 B. For example, the light source  18 B includes a light source  18 B-R, a light source  18 B-Y, a light source  18 B-G, and a light source  18 B-B. The light source  18 B-R, the light source  18 B-Y, the light source  18 B-G, and the light source  18 B-B each produce the line illumination LA as lines long in the X-axis direction and at least some wavelength regions of which do not overlap each other. The light source  18 B-R, the light source  18 B-Y, the light source  18 B-G, and the light source  18 B-B are arranged in parallel at different positions in the Y-axis direction. 
     The light source  18 B-R emits a line illumination LA in a wavelength region that excites the specimen T labeled with a fluorescent dye that emits red fluorescence, for example. The light source  18 B-Y emits a line illumination LA in a wavelength region that excites the specimen T labeled with a fluorescent dye that emits yellow fluorescence, for example. The light source  18 B-G emits a line illumination LA in a wavelength region that excites the specimen T labeled with a fluorescent dye that emits green fluorescence, for example. The light source  18 B-B emits a line illumination LA in a wavelength region that excites the specimen T labeled with a fluorescent dye that emits blue fluorescence, for example. 
       FIG.  14 B  is a schematic diagram illustrating an example of a pupil-split picture  70 F. The pupil-split picture  70 F is an example of the pupil-split picture  70 . The pupil-split picture  70  is the pupil-split picture  70 F of light emitted from the specimen T by being irradiated with the line illumination LA from each of the light source  18 B-R, the light source  18 B-Y, the light source  18 B-G, and the light source  18 B-B. 
     As illustrated in  FIG.  14 B , the pupil-split picture  70 F includes an image  72 A and an image  72 B corresponding to each of the plurality of line illuminations LA. 
     Specifically, the pupil-split picture  70 F includes an image  72 A-R and an image  72 B-R. The image  72 A-R and the image  72 B-R are a set of images corresponding to the line illumination LA emitted from the light source  18 B-R. The pupil-split picture  70 F further includes an image  72 A-Y and an image  72 B-Y The image  72 A-Y and the image  72 B-Y are a set of images corresponding to the line illumination LA emitted from the light source  18 B-Y. 
     The pupil-split picture  70 F further includes an image  72 A-G and an image  72 B-G. The image  72 A-G and the image  72 B-G are a set of images corresponding to the line illumination LA emitted from the light source  18 B-G. The pupil-split picture  70 F further includes an image  72 A-B and an image  72 B-B. The image  72 A-B and the image  72 B-B are a set of images corresponding to the line illumination LA emitted from the light source  18 B-B. 
     The derivation unit  60 E may derive the relative position information corresponding to each of the plurality of line illuminations LA based on the light intensity distribution of each of the pupil-split images  72  (image  72 A-R and image  72 B-R′, the image  72 A-Y and the image  72 B-Y, the image  72 A-G and the image  72 B-G, the image  72 A-B and the image  72 B-B), which are images having a plurality of types of phase differences each corresponding to each of the plurality of line illuminations LA, similarly to the above-described embodiment. The movement control unit  60 F may perform focusing for each of the plurality of line illuminations LA similarly to the above embodiment. 
     In this manner, the microscope system  1  may derive the relative position information for each of the plurality of line illuminations LA. In this case, in addition to the effects of the above embodiment, the microscope system  1  can perform focus adjustment with higher accuracy according to the type of the specimen T. 
     Note that the light source  18 B may selectively emit the line illumination LA from each of the light source  18 B-R, the light source  18 B-Y, the light source  18 B-G, and the light source  18 B-B. In this case, the control unit  60  may derive the relative position information from the pupil-split image  72  every time the line illumination LA having the wavelength region in which at least a part does not overlap is emitted from the light source  18 B. 
     (Fourth Modification) 
     In the above embodiment, regarding the issue of the difference in the fluorescence intensity between a region in which the cellular tissue is present (hereinafter, also referred to as a tissue part) and a region in which the cellular tissue is not present (hereinafter, also referred to as a non-tissue part or a sealing part) in an imaging range of the specimen T, in order to prevent a state of pixel value saturation in the pixel included in the region in which the tissue part is imaged in the pupil-split image  72  and/or a state of noise becoming dominant in the region in which the non-tissue part is imaged, the sensor region (refer to  FIG.  2   ) of the pupil-split image capture unit  42  is split into a plurality of types of unit regions  37 A and  37 B having mutually different exposure setting values, and the phase difference is calculated from the interval YL of the centroids g of the images  72 A and  72 B captured in the unit region  37 A in which the exposure setting value with high light sensitivity is set for the non-tissue part, and the phase difference is calculated from the interval YL of the centroids g of the images  72 A and  72 B captured in the unit region  37 B in which the exposure setting value with low photosensitivity is set for the tissue part. 
     However, in a case where, as in the above embodiment, a plurality of simultaneously emitted line illuminations (for example, line illuminations LA and LB) shares one objective lens  22 , and when the height is different between the tissue part where the cellular tissue of the specimen T exists and the non-tissue part where the cellular tissue does not exist (for example, the sealing part using a sealant), there would be a situation in which the tissue part is not in focus while the non-tissue part is focused, leading to blurring in a part of the captured image of the specimen T. 
     To handle this, an example as a fourth modification will be given as a case of enabling acquisition of a captured image of a focused specimen T (hereinafter, also referred to as a tissue image or a fluorescence image) even when the non-tissue part and the tissue part are different in height. 
     In normal imaging of tissue cells, a reference interval YL′ between the image  72 A and the image  72 B is determined in the tissue part, and the position of the objective lens  22  in the height direction (Z direction) is adjusted based on the determined reference interval YL′. Therefore, for example, when the tissue part and the non-tissue part are different in height, there may be a case where the interval YL between the image  72 A and the image  72 B is different in the non-tissue part even they are in a same place. In this case, when the tissue part and the non-tissue part are in the same field of view, while it is possible, by controlling the objective lens  22  based on the reference interval YL′, to acquire a focused tissue image in the tissue part, there may be a case where an out-of-focus tissue image is acquired in the non-tissue part by controlling the objective lens  22  based on the reference interval YL′. This is caused, for example, in a case where the height of the focus position of the non-tissue part is different from the height of the focus position of the tissue part. For example, the height of the focus position of the non-tissue part may be higher or lower than the height of the focus position of the tissue part. 
       FIG.  15    is a diagram illustrating an example of a mechanism of imaging using phase difference AF according to the fourth modification. In the present description, the configuration of the microscope system  1  and the basic operation thereof may be similar to those of the above-described embodiment or its modifications. 
     As illustrated in  FIG.  15 ( a ) , the fourth modification generates a captured image of the specimen T by simultaneously using four types of line illumination, namely, line illuminations LA1 to LA4. The line illuminations LA1 to LA4 may be, for example, laser beams of 638 nm, 532 nm, 488 nm, and 405 nm. The spots of lines formed by the line illuminations LA1 to LA4 may be, for example, parallel to each other and arranged in a direction perpendicular to the longitudinal direction of each spot. 
     In the acquisition of the captured image, by movement of the stage  26  in a predetermined direction (for example, the arrangement direction of the spots individually formed by the line illuminations LA1 to LA4) with respect to the objective lens  22 , the specimen T in the measurement target region  24  placed on the stage  26  is sequentially scanned by each of the line illuminations LA1 to LA4. This leads to acquisition of a captured image of the specimen T in the image capture unit  34  (refer to  FIG.  1   ). 
       FIG.  15 ( b )  is a schematic diagram when the stage  26  and the objective lens  22  are viewed in the X-axis direction. Although  FIG.  15 ( b )  uses dots to illustrate images of the line illuminations LA1 to LA4 condensed on a focus plane SR1, the images in practice may be images of lines extending in a depth direction (x-axis direction) perpendicular to the surface of the page. A position PT1 on the focus plane SR1 indicates a position at which the line illumination LA1 positioned at the head in the Y direction is condensed, and a position PT2 indicates a position at which the line illumination LA4 positioned at the tail end in the Y direction is condensed. In the case of the reciprocating scanning, the Y direction and the −Y direction may be alternated as the scanning direction. When the scanning direction is the Y direction, the line illumination LA1 is positioned at the head in the scanning direction, and when the scanning direction is the −Y direction, the line illumination LA1 is positioned at the tail end in the scanning direction. 
       FIG.  15    is a case where all of the line illuminations LA1 to LA4 irradiate the specimen through the common objective lens  22 , making it difficult to obtain the focus in each of the line illuminations LA1 to LA4. Still, since the line illuminations LA1 to LA4 are arranged close to each other so as to fall within one field of view of the objective lens  22 , even when the position of the objective lens  22  is determined to bring one of the line illuminations LA1 to LA4 into focus, the focus of the other line illuminations is not considered to be greatly shifted. In view of this, the fourth modification will exemplify a case where focusing is performed using the line illumination LA4. Note that one of the other line illuminations LA1 to LA3 may be used, not limited to the line illumination LA4. 
     Here, in general, in the scanning of the specimen T, the operation is started from a sealing part (that is, a non-tissue part) outside the tissue of the specimen T. Therefore, when the objective lens  22  is focused using the line illumination LA4 positioned at the tail end when the scanning direction is the Y direction, the line illumination LA1 to LA3 ahead of the line illumination LA1 maintain a state in which the non-tissue part is focused until the tissue part is focused by the line illumination LA4 even though the scanning of the tissue part is started before the line illumination LA4. This might result in an occurrence of a problem of blur in an image of a tissue part in a region adjacent to the non-tissue part (hereinafter, also referred to as a boundary region) in a captured image obtained by the irradiation with each of the line illuminations LA1 to LA3 (hereinafter, also referred to as a captured images with the line illuminations LA1 to LA3) 
       FIG.  16    is an explanatory diagram for describing a difference in focus between the non-tissue part and the tissue part, and is a diagram for describing blur that occurs in a boundary region of a captured image captured by another line illumination (in the present example, the line illumination LA2) when focusing is performed using the line illumination LA4 located at the tail end.  FIG.  16 ( a )  illustrates an image captured with the line illumination LA4.  FIG.  16 ( b )  illustrates a captured image captured with the line illumination LA2.  FIG.  16 ( c )  illustrates an image obtained by enlarging a region of a dotted line portion including a boundary region in the captured image captured with the line illumination LA2. In  FIG.  16   , a position PT3 indicates a position at which imaging of the line illumination LA4 is switched from a non-tissue part HB1 to a tissue part SB1. Since the line illumination LA2 starts imaging the tissue part SB1 before the line illumination LA4, the position of switching from the non-tissue part HB1 to the tissue part SB1 in  FIG.  16 ( b )  is on the left side of the position PT3. 
     In addition, as illustrated in  FIGS.  16 ( b ) and  16 ( c ) , in the tissue part SB1 in the range (boundary region) from the position PT4 to the position PT5 before the imaging of the line illumination LA4 is switched to the tissue part SB1, the captured image obtained with the line illumination LA2 is out of focus and blurred. In addition, in the tissue part SB1 after the position PT5 after the imaging of the line illumination LA4 is switched to the tissue part SB1, the captured image obtained with the line illumination LA2 is in focus. In this manner, there may be a case where the image of the boundary region of the tissue part SB1 captured with the line illuminations LA1 to LA3 ahead of the line illumination LA4 is out of focus. 
     To handle this, in the present modification, when the line illumination being used for focusing is scanning the non-tissue part (hereinafter, it is also referred to as a case of focusing on the non-tissue part), the position obtained by adding an offset to the position actually in focus is set as a target focus position. At that time, the target focus position may be, for example, a position at the same height as the tissue part. With this setting, even when focusing is performed on the non-tissue part, the focus position of the objective lens  22  can be adjusted to the height of the tissue part, making it possible to suppress blurring of the image of the boundary region of the tissue part in the captured image obtained with each of the line illuminations LA1 to LA4. 
     Note that the offset may be added to the difference ΔYL between the interval YL between the image  72 A and the image  72 B constituting the pupil-split picture  70  acquired by the pupil-split image acquisition unit  60 D and the reference interval YL′, or may be added to the displacement amount ΔZ inversely calculated from the difference ΔYL, for example. In either case, the position of the objective lens  22  after focusing can be adjusted by the offset. Offset assignment is not limited thereto, and the offset may be assigned to various parameters as long as the focus position of the target can be adjusted to a position at the same height as the tissue part. 
       FIG.  17    is a diagram illustrating an example of a functional configuration of a control device  16  according to the present modification. As illustrated in  FIG.  17   , in the control device  16  according to the present modification, the derivation unit  60 E further includes a discerning unit  60 K in a configuration similar to the functional configuration described with reference to  FIG.  5    in the above embodiment. 
     The discerning unit  60 K discerns whether the line illumination (in the present example, the line illumination LA4) used for focusing is scanning the tissue part SB1 or the non-tissue part HB1. When having discerned that the non-tissue part HB1 is being scanned, the discerning unit  60 K adds an offset to the focus position. 
     Specifically, for example, the discerning unit  60 K inputs the pupil-split picture  70  acquired by the pupil-split image acquisition unit  60 D, and discerns whether the part the line illumination LA4 is scanning is the tissue part SB1 or the non-tissue part HB1 based on the input pupil-split picture  70 . For example, normally, the amount of fluorescence emitted by the irradiation from the line illumination LA4 is larger in the tissue part SB1 than in the non-tissue part HB1. Therefore, the discerning unit  60 K may discern that the line illumination LA4 is scanning the tissue part SB1 in a case where the luminance value of the pupil-split picture  70  in the pupil-split picture  70  is higher than a threshold set in advance, and may discern that the line illumination LA4 is scanning the non-tissue part HB1 in a case where the luminance value is lower than the threshold. The discerning method is not limited thereto and may be flexibly varied, that is, the discerning unit  60 K may, for example, discern whether the line illumination LA4 is scanning the tissue part SB1 or scanning the non-tissue part HB1 based on the captured image captured by the area illumination LB among the captured images acquired by the captured image acquisition unit  60 B. 
     In the addition of the offset to the focus position, for example, the discerning unit  60 K may input to the relative distance derivation unit  60 J the offset amount to be added to the difference ΔYL or the displacement amount ΔZ calculated by the relative distance derivation unit  60 J, or may instruct the relative distance derivation unit  60 J to add the offset amount held in advance by the relative distance derivation unit  60 J to the difference ΔYL or the displacement amount ΔZ. 
       FIG.  18    is an explanatory diagram for describing processing of adding an offset to a focus position in the non-tissue part. In  FIG.  18   , the focus position of the non-tissue part HB1 is higher than the focus position of the tissue part SB1.  FIG.  18    illustrates a case where the measurement target region  24  moves from right to left, that is, the objective lens  22  scans from left to right. As illustrated in  FIG.  18   , when it is discerned that the non-tissue part HB1 is being focused, the discerning unit  60 K gives an offset to the focus position (for example, the difference ΔYL or the displacement amount ΔZ) so as to bring the tissue part SB1 in focus. In the example illustrated in  FIG.  18   , an offset is given so that the focus position in the case of focusing in the non-tissue part HB1 becomes lower by the height of a. The height a may be a difference in height between the assumed non-tissue part HB1 and the tissue part SB1. 
     When having discerned that the scanning position of the line illumination LA4 used for focusing has been switched from the non-tissue part HB1 to the tissue part SB1, the discerning unit  60 K may cancel assignment of the offset. Alternatively, when having discerned that the scanning position has been switched from the non-tissue part HB1 to the tissue part SB1, the discerning unit  60 K may assign an offset different from that in a case where the scanning position has been discerned to be the non-tissue part HB1. 
     Although  FIG.  18    illustrates a case where the discerning unit  60 K assigns an offset to the focus position in the non-tissue part HB1 and adjusts the focus position to the tissue part SB1, the focus position adjustment is not limited to this example. For example, the microscope system  1  may provide an offset to the focus position in the tissue part SB1 to adjust the focus position to the non-tissue part HB1. 
     Here, as described above, the non-tissue part HB1 and the tissue part SB1 may be discerned based on the luminance information regarding the light emitted from the specimen, for example, the luminance information regarding each pixel in the pupil-split picture  70  or the captured image. This is because, although the fluorescence intensity of the non-tissue part HB1 is weak, the fluorescence intensity tends to be strong in the tissue part SB1 because autofluorescence is also present in addition to fluorescence attributed to the fluorescent dye. 
     In addition, the discerning unit  60 K may discern whether the tissue part SB1 is irradiated with at least a part of the line illumination LA4 used for focusing. When having discerned that at least a part of the line illumination LA4 is applied to the tissue part SB1, the discerning unit  60 K may instruct, for example, the selection unit  60 H (phase difference acquisition unit  60 I and/or relative distance derivation unit  60 J as necessary) to perform focusing using the pupil-split picture  70  or the captured image of the region irradiated with the line illumination LA4 in the tissue part SB1. 
     Note that the discerning unit  60 K may discern whether at least a part of the line illumination LA4 is applied to the tissue part SB1 based on not only the luminance information but also various types of information as long as the non-tissue part HB1 and the tissue part SB1 can be discerned from each other. For example, the discerning unit  60 K may determine whether there is a structure from the pattern of the pixel values of the captured image, and determine that at least a part of the line illumination LA4 is emitted to the tissue part SB1 in a case where there is a structure. In addition, for example, the discerning unit  60 K may determine whether at least a part of the region irradiated with the current line illumination LA4 is the tissue part SB1 based on a low resolution image (For example, thumbnail images) obtained by imaging the entire specimen T. 
       FIG.  19    is an explanatory diagram for describing the processing of focusing based on fluorescence from the tissue part SB1 when at least a part of the line illumination LA4 is being applied to the tissue part SB1. RT1 indicates the range of the line irradiated with the laser.  FIG.  19 ( a )  illustrates a case where RT1 is moved in a direction KD1 to approach the tissue part SB1.  FIG.  19 ( b )  illustrates a case where the movement of RT1 in the direction KD1 allows a part of RT1 to be included in the tissue part SB1. In this manner, the switching of the line irradiation of RT1 from the non-tissue part HB1 to the tissue part SB1 may be performed as gradual switching instead of simultaneous switching of the line irradiation to the tissue part SB1. 
     In  FIG.  19 ( b ) , a part of RT1 is included in the tissue part SB1, and the other part of RT1 is included in the non-tissue part HB1. In such a case, the discerning unit  60 K may instruct the selection unit  60 H (phase difference acquisition unit  60 I and/or relative distance derivation unit  60 J as necessary) to execute processing of preferentially selecting the fluorescence of the tissue part SB1 in execution of focusing. In  FIG.  19 ( b ) , by further moving RT1 in the direction KD1, RT1 is completely included in the tissue part SB1. 
     In addition, the discerning unit  60 K may determine whether to focus based on the fluorescence from the tissue part SB1 according to the ratio of the tissue part SB1 to the irradiation region of the line illumination LA4. For example, when having discerned that the proportion occupied by the tissue part SB1 is high such that 80% in the irradiation region is the tissue part SB1 and the remaining 20% is the non-tissue part HB1, the discerning unit  60 K may discern the execution of the focusing processing based on the fluorescence from the tissue part SB1, specifically, based on the region of the tissue part SB1 in the irradiation region of the line illumination LA4 in the pupil-split picture  70  or the captured image. Furthermore, for example, in a case where it is determined that the proportion occupied by the tissue part SB1 is low such that 10% in the irradiation region is the tissue part SB1 and the remaining 90% is the non-tissue part, the discerning unit  60 K may determine to execute the focusing processing based on the fluorescence from the non-tissue part HB1, specifically, based on the non-tissue part HB1 in the irradiation region of the line illumination LA4 in the pupil-split picture  70  or the captured image. 
     In this manner, the discerning unit  60 K may discern whether to use the fluorescence of the tissue part SB1 or the fluorescence of the non-tissue part HB1 for focusing according to the ratio of each of the irradiation regions of the line illumination LA4. 
       FIG.  18    illustrates a case where focusing processing is performed by assigning an offset to the focus position in the non-tissue part. Furthermore,  FIG.  19    illustrates a case where focusing processing is performed by preferentially selecting fluorescence of a tissue part. Here, the discerning unit  60 K may perform focusing processing by using the processing of  FIG.  18    and the processing of  FIG.  19    in combination. Hereinafter, processing in a case where the discerning unit  60 K uses two types of processing in combination will be described. 
       FIG.  20    is a diagram illustrating an example of a sensor region of the pupil-split image capture unit  42  according to the fourth modification. As illustrated in  FIG.  20   , the fourth modification uses a configuration in which each unit region  37  in the sensor region of the pupil-split image capture unit  42  is further split into two regions  371  and  372  in a direction (X direction) perpendicular to the scanning direction (Y direction). In the present modification, there is no distinction between the unit region  37 A and the unit region  37 B. Furthermore, in each unit region  37  in the present modification, the region  371  is a region in which an exposure setting value for high light sensitivity is set, while the region  372  is a region in which an exposure setting value for low light sensitivity is set. 
     For the unit region  37  discerned to be the region corresponding to the tissue part SB1 by the discerning unit  60 K, the relative distance derivation unit  60 J in the derivation unit  60 E calculates the displacement amount ΔZ based on the difference ΔYL between the interval YL calculated by the phase difference acquisition unit  60 I from the pupil-split image  72  acquired in the region  371  in which the exposure setting value representing high light sensitivity is set and the reference interval YL′; for the unit region  37  discerned to be the region corresponding to the non-tissue part HB1 by the discerning unit  60 K, the relative distance derivation unit  60 J either calculates the displacement amount ΔZ based on a value obtained by adding an offset to the difference ΔYL between the interval YL calculated by the phase difference acquisition unit  60 I and the reference interval YL′ from the pupil-split image  72  acquired in the region  372  in which the exposure setting value representing low light sensitivity is set, or assigns an offset to the displacement amount ΔZ calculated based on the difference ΔYL. The displacement amount ΔZ given from the derivation unit  60 E to the movement control unit  60 F may be, for example, an average value of the displacement amount ΔZ calculated from the tissue part SB1 and the displacement amount ΔZ (including offset) calculated from the non-tissue part HB1. At this time, each displacement amount ΔZ may be weighted based on the ratio of each of the tissue part SB1 and the non-tissue part HB1 in the irradiation region of the line illumination LA4. For example, when the ratio of the tissue part SB1 is 20% and the ratio of the non-tissue part HB1 is 80%, the sum of the value obtained by multiplying the displacement amount ΔZ calculated from the tissue part SB1 by 0.2 and a value obtained by multiplying the displacement amount ΔZ (including the offset) calculated from the non-tissue part HB1 by 0.8 may be set as the displacement amount ΔZ to be input to the movement control unit  60 F. 
     Alternatively, the relative distance derivation unit  60 J may calculate the displacement amount ΔZ only based on the unit region  37  discerned to be the region corresponding to the tissue part SB1 by the discerning unit  60 K. In this case, the phase difference acquisition unit  60 I may calculate the interval YL between the centroids g of the image  72 A and the image  72 B using the pupil-split image  72  acquired in the region  371  in the unit region  37  corresponding to the tissue part SB1. In that case, even when the fluorescence intensity in the non-tissue part HB1 is very weak and noise is dominant in the pupil-split image  72  of the non-tissue part HB1, it is still possible to suppress degradation in focusing accuracy. 
     (Fifth Modification) 
     The above-described embodiment is the case where the phase difference acquisition unit  60 I calculates the phase difference (interval YL) based on the distance between the centroids g of the image  72 A and the image  72 B. However, excessively bright line illumination would cause saturation in the pixels (that is, the pixel value of each pixel in the pupil-split picture  70 ) of the pupil-split image capture unit  42 , leading to a failure in appropriately specifying the position of the centroid g of each of the images  72 A and  72 B. On the other hand, excessively dark line illumination would make noise dominant in the pupil-split picture  70 , leading to a failure in appropriately specifying the position of the centroid g of the line illumination. 
     In view of this, it is allowable, in a fifth modification, to discern whether the signal (corresponding to the pixel value; hereinafter also referred to as luminance) read from each light receiving unit  41  (corresponding to a pixel) in each unit region  37  of the pupil-split image capture unit  42  is in an appropriate range, and to perform focusing based on the pupil-split image  72  acquired in the unit region  37  in which luminance of all pixels has been discerned to be within an appropriate range. 
     Note that the functional configuration of the control device  16  according to the present modification may be similar to the configuration example described with reference to  FIG.  17    in the fourth modification, and thus, the description will be given with reference to  FIG.  17   . Also in the present modification, the discerning unit  60 K may execute the operation exemplified in the fourth modification. 
       FIG.  21    is a diagram illustrating focusing operation according to the present modification.  FIG.  21 ( a )  is a diagram illustrating a relationship between a signal level (x-axis) and a signal frequency (y-axis), and  FIG.  21 ( b )  is a diagram illustrating an example of a pupil-split image  72  (one of an image  72 A and an image  72 B) obtained in each unit region  37 . In  FIG.  21 ( a ) , HG1 represents the signal distribution of the background (corresponding to the black portion in  FIG.  21 ( b ) ), and HG2 represents the signal distribution of the image  72 A or  72 B (corresponding to the gray to white portion in  FIG.  21 ( b ) ). A first threshold HF1 indicates a lower limit threshold for discerning whether the luminance is within an appropriate range, and a second threshold HF2 indicates an upper limit threshold for discerning whether the luminance is within an appropriate range. 
     In the present modification, for example, in each unit region  37 , when the peak of the distribution of the pixel value (luminance) of the light receiving unit  41  as a constituent is lower than the first threshold HF1, the pixel value in the pupil-split picture  70  is not dominant with respect to the noise level, deteriorating the SN ratio. This makes it difficult, as a result, to accurately calculate the position of the centroids g of the images  72 A and  72 B in the focusing, leading to degradation of the focusing accuracy. On the other hand, for example, in each unit region  37 , when at least one pixel value (luminance) of the light receiving unit  41  constituting each unit region is higher than the second threshold HF2, this causes blown-out highlight in the pixel in the pupil-split picture  70 , making it difficult to specify the accurate position of the centroids g of the images  72 A and  72 B, leading to degradation of focusing accuracy. 
     Therefore, in the present modification, for example, the discerning unit  60 K may specify a region acquired in the unit region  37  in which the peak of the distribution of the pixel values is the first threshold HF1 or more and all the pixel values are the second threshold HF2 or less, among the plurality of unit regions  37  constituting the pupil-split image capture unit  42  in the input pupil-split picture  70 , and may instruct the selection unit  60 H (phase difference acquisition unit  60 I and/or relative distance derivation unit  60 J as necessary) to perform focusing using the specified region, for example. 
     Specifically, as illustrated in  FIG.  21 ( c ) , when there is a unit region  37  in which the peak of the distribution of the pixel values has been judged to be larger than the first threshold HF1 or at least one pixel value has been judged to be larger than the second threshold HF2, the discerning unit  60 K instructs the selection unit  60 H to perform focusing in a region other than a region corresponding to an unit region  27  in the pupil-split picture  70 . 
     The operation to perform when there is the unit region  37  in which the peak of the distribution of the pixel values has been judged to be larger than the first threshold HF1 or at least one pixel value has been judged to be larger than the second threshold HF2 is not limited to the above operation, and for example, it is also allowable to perform processing of stopping scanning and re-imaging or processing of stopping scanning and outputting a warning (alert). 
     (Sixth Modification) 
     Furthermore, in the above-described embodiment or the modification thereof, a sudden change in the height (height in the Z direction) of the objective lens  22  at the time of controlling the focus position would cause, in the acquired captured image and the pupil-split picture  70 , a sudden change in the brightness of the image before and after the change of the height of the objective lens  22 . To handle this, the present modification uses a gentle change of speed when changing the height of the objective lens  22 .  FIG.  22    is a diagram illustrating the moving speed of the objective lens  22  according to the present modification.  FIG.  22 ( a )  illustrates an example of a captured image acquired when the height of the objective lens  22  is suddenly changed,  FIG.  22 ( b )  illustrates a case where the height of the objective lens  22  is suddenly changed, and  FIG.  22 ( c )  illustrates a case where the height of the objective lens  22  is gently changed. 
     As illustrated in  FIG.  22 ( b ) , when the moving speed of the objective lens  22  is too high, as illustrated in  FIG.  22 ( a ) , there is a case with an occurrence of differences in brightness HJ1 to HJ3 that appear as switching of images in a captured image. 
     Therefore, in the present modification, as illustrated in  FIG.  22 ( b ) , the objective lens  22  is controlled to move slowly. Specifically, for example, the movement control unit  60 F performs control such that the movement amount of the objective lens  22  in the height direction (Z direction) while an imaging line travels by one step in the scanning direction (Y direction) is the movement amount corresponding to the focal depth of the objective lens  22  or less. Note that the imaging line may be a band-shaped range irradiated with the line illumination LA or LB during each imaging cycle (also referred to as a frame rate) of the image capture unit  34  or the pupil-split image capture unit  42 . 
     In this manner, by setting the movement amount in the height direction in which the objective lens  22  moves in one imaging cycle to the amount corresponding to the focal depth of the objective lens  22 , it is possible to sufficiently reduce the difference in brightness between the captured images (or the pupil-split picture  70 ) acquired in consecutive imaging cycles. This makes it possible to improve the accuracy of analysis using the captured image, the accuracy of focusing using the pupil-split picture  70 , and the like. 
     (Seventh Modification) 
     The above-described embodiment and the modifications thereof have illustrated an exemplary case where the exposure installation values of the unit regions  37 A and  37 B (or the regions  371  and  372 ) are set values determined in advance, determination of the value is not limited thereto. For example, the high exposure setting value assuming the non-tissue part HB1 and the low exposure setting value assuming the tissue part SB1 may be determined based on a low-resolution image such as a thumbnail image obtained by imaging the specimen T in advance. For example, the light source control unit  60 A may determine a high exposure setting value assuming the non-tissue part HB1 based on luminance information regarding the non-tissue part HB1 in a low-resolution image acquired in advance, and may determine a low exposure setting value assuming the tissue part SB1 based on luminance information regarding the tissue part SB1. This makes it possible to perform scanning based on appropriate gain setting, leading to acquisition of the pupil-split picture  70  with a better focus state. 
     (Eighth Modification) 
     In addition, the technique according to the above-described embodiment and the modifications thereof can also be applied to a system referred to as a non-coaxial excitation scanner type microscope system that irradiates a pathological specimen (corresponding to the specimen T) with a plurality of line illuminations having different wavelengths and non-coaxially arranged in parallel. 
       FIG.  23    is a schematic block diagram of a microscope system according to an eighth modification, and  FIG.  24    is a diagram illustrating an example of an optical system in the microscope system. 
     [Overall Configuration] 
     As illustrated in  FIG.  23   , a microscope system  100  according to the eighth modification includes an observation unit  101 . The observation unit  101  includes: an excitation unit  110  that irradiates a pathological specimen (pathological sample) with a plurality of line illuminations having different wavelengths and non-coaxially arranged in parallel; a stage  120  that supports the pathological specimen; and a spectral imaging unit  130  that acquires a fluorescence spectrum (spectral data) of the pathological specimen excited as a line. 
     Here, non-axial arrangement in parallel indicates the state including a plurality of line illuminations arranged non-axially in parallel. The state of non-axial means that the illuminations are not coaxial, with a distance between the axes not particularly limited. The term “parallel” is not limited to parallel in a strict sense, and includes a state of being substantially parallel. For example, the term permits distortion due to an optical system such as a lens or deviation from a parallel state due to manufacturing tolerance, and the cases of these are also regarded as parallel. 
     The microscope system  100  further includes a processing unit  102 . Based on the fluorescence spectrum of the pathological specimen (hereinafter, it is also referred to as a sample S) acquired by the observation unit  101 , the processing unit  102  typically forms an image of the pathological specimen or outputs a distribution of the fluorescence spectrum. The image referred to herein refers to an image converted into red, green, and blue (RGB) colors from a constituent ratio or a waveform regarding dyes constituting the spectrum or autofluorescence originated from the sample, or refers to a luminance distribution in a specific wavelength band, and the like. 
     The excitation unit  110  and the spectral imaging unit  130  are connected to the stage  120  via an observation optical system  140  such as an objective lens  144 . The observation optical system  140  has a function of optimum focus tracking by using a focus mechanism  160 . The observation optical system  140  may be connected to a non-fluorescence observation unit  170  for dark field observation, bright field observation, or the like. 
     The microscope system  100  may be connected to a control unit  180  that controls an excitation unit (LD/shutter control), an XY stage which is a scanning mechanism, a spectral imaging unit (camera), a focus mechanism (a detector and a Z stage), a non-fluorescence observation unit (camera), and the like. 
     The excitation unit  110  includes a plurality of excitation light sources L1, L2, . . . that can output light of a plurality of excitation wavelengths Ex1, Ex2, . . . . The plurality of excitation light sources is typically configured with a light emitting diode (LED), a laser diode (LD), a mercury lamp, and the like, and light from each device is emitted as line illumination and applied to the sample S of the stage  120 . 
     The sample S (corresponding to the specimen T) is typically composed of a slide including an observation target such as a tissue section, but it is needless to say that the sample S may be composed of others. The sample S (observation target) is stained with a plurality of fluorescent dyes. The observation unit  101  magnifies and observes the sample S at a desired magnification. The excitation unit  110  has arranged a plurality of line illuminations (for example, line illuminations LA and LB), and the imaging area of the spectral imaging unit  130  is arranged so as to overlap with each illumination area. The two line illuminations LA and LB are individually parallel to the Z-axis direction, and are disposed away from each other by a predetermined distance (Δy) in the Y-axis direction. 
     The imaging area corresponds to each slit part of an observation slit  131  ( FIG.  24   ) in the spectral imaging unit  130 . That is, the number of slit parts arranged in the spectral imaging unit  130  is the same as the number of line illuminations. Regarding the magnitude relationship between the line width and the slit width of the illumination, either may be larger. When the line width of the illumination is larger than the slit width, it is possible to increase an alignment margin of the excitation unit  110  with respect to the spectral imaging unit  130 . 
     The wavelength forming a first line illumination Ex1 and the wavelength forming a second line illumination Ex2 are different from each other. The line of fluorescence excited by the line illuminations Ex1 and Ex2 is observed in the spectral imaging unit  130  via the observation optical system  140 . 
     The spectral imaging unit  130  includes: an observation slit  131  having a plurality of slit parts that allows the passage of the fluorescence excited by a plurality of line illuminations; and at least one imaging element  132  capable of individually receiving the fluorescence passing through the observation slit  131 . The imaging element  132  can be implemented by employing a two-dimensional imager such as a CCD or a CMOS. With the observation slit  131  disposed on the optical path, the fluorescence spectra excited in the respective lines can be detected without overlapping. 
     The spectral imaging unit  130  acquires, from each of the line illuminations Ex1 and Ex2, spectral data (x, λ) of fluorescence using a pixel array in one direction (for example, a vertical direction) of the imaging element  132  as a channel of a wavelength. The obtained spectral data (x, λ) is recorded in the processing unit  102  in a state where the spectral data is associated with excitation wavelength as origination of the spectral data. 
     The processing unit  102  can be implemented by hardware elements used in a computer, such as a CPU, RAM, and ROM, and necessary software. Instead of or in addition to the CPU, a programmable logic device (PLD) such as a field programmable gate array (FPGA), a digital signal processor (DSP), an application specific integrated circuit (ASIC), or the like may be used. 
     The processing unit  102  includes a storage unit  121  that stores spectral data indicating the correlation between the wavelengths of the plurality of line illuminations Ex1 and Ex2 and the fluorescence received by the imaging element  132 . The storage unit  121  uses a storage device such as nonvolatile semiconductor memory or a hard disk drive, and stores in advance a standard spectrum of autofluorescence related to the sample S and a standard spectrum of a single dye staining the sample S. The spectral data (x, λ) received by the imaging element  132  is stored in the storage unit  121 . In the present modification, the storage unit that stores the autofluorescence of the sample S and the standard spectrum of the single dye and the storage unit that stores the spectral data (measurement spectrum) of the sample S acquired by the imaging element  132  are actualized by the same storage unit  121 . The storage unit is not limited thereto, and may be actualized by separate storage units. 
     As illustrated in  FIG.  24   , a dichroic mirror  142  and a band pass filter  145  are inserted in the middle of the optical path so as to prevent the excitation light (E×1 and E×2) from reaching the imaging element  132 . In this case, a non-continuous portion occurs in the fluorescence spectrum formed on the imaging element  132 . By excluding such non-continuous portions from a reading region, the frame rate can be further improved. 
     As illustrated in  FIG.  24   , the imaging element  132  may include a plurality of imaging elements  132   a  and  132   b  capable of receiving fluorescence that has passed through the observation slit  131 , individually. In this case, the fluorescence spectra excited by the line illuminations Ex1 and Ex2 are acquired on the imaging elements  132   a  and  132   b  and stored in the storage unit  121  in association with the excitation light. 
     The line illuminations Ex1 and Ex2 are not limited to the configuration with a single wavelength, and each may have a configuration with a plurality of wavelengths. When each of the line illuminations Ex1 and Ex2 includes a plurality of wavelengths, the fluorescence excited by each of the line illuminations Ex1 and Ex2 also includes a plurality of spectra. In this case, the spectral imaging unit  130  includes a wavelength distribution element for separating the fluorescence into a spectrum originated from the excitation wavelength. The wavelength distribution element includes a diffraction grating, a prism, or the like, and is typically disposed on an optical path between the observation slit  131  and the imaging element  132 . 
     The observation unit  101  further includes a scanning mechanism  150  that scans the stage  120  with the plurality of line illuminations Ex1 and Ex2 in the Y-axis direction, that is, in the arrangement direction of the line illuminations Ex1 and Ex2. With the scanning mechanism  150 , dye spectra (fluorescence spectra) spatially separated by Δy and excited at different excitation wavelengths on the sample S (observation target) can be continuously recorded in the Y-axis direction. In this case, for example, the imaging region is split into a plurality of parts in the X-axis direction, and an operation of scanning the sample S in the Y-axis direction, then moving in the X-axis direction, and further performing scanning in the Y-axis direction is repeated. With a single scan, a spectral image based on the sample excited by several excitation wavelengths can be obtained. 
     In the scanning mechanism  150 , the stage  120  is typically scanned in the Y-axis direction. Alternatively, scanning may be performed in the Y-axis direction with a plurality of line illuminations Ex1 and Ex2 by a galvanometer mirror disposed in the middle of the optical system. Finally, three-dimensional data of (X, Y, X) is acquired for each of the plurality of line illuminations Ex1 and Ex2. The three-dimensional data originated from each of the line illuminations Ex1 and Ex2 is data whose coordinates are shifted by Δy with respect to the Y-axis, and thus is corrected and output based on a value Δy recorded in advance or a value Δy calculated from the output of the imaging element  132 . 
     Although the above example uses two line illuminations as the excitation light beam, the number of line illuminations as the excitation light beam may be three, four, or five or more, not limited to two. In addition, each line illumination may include a plurality of excitation wavelengths selected to suppress degradation of color separation performance as much as possible. In addition, even with one line illumination, by using the excitation light source formed with a plurality of excitation wavelengths and recording the individual excitation wavelengths in association with Row data obtained by the imaging element, it is still possible to obtain a polychromatic spectrum although it is not possible to obtain separability equal to the case using non-axial parallel beams. 
     [Observation Unit] 
     Next, details of the observation unit  101  will be described with reference to  FIG.  24   . Here, an example in which the observation unit  101  is configured with the configuration example 2 in  FIG.  10    will be described. 
     The excitation unit  110  includes a plurality of (four in the present example) excitation light sources L1, L2, L3, and L4. Each of the excitation light sources L1 to L4 includes a laser light source that outputs laser light having a wavelength of 405 nm, 488 nm, 561 nm, and 645 nm, respectively. 
     The excitation unit  110  further includes: a plurality of collimator lenses  111  and a plurality of laser line filters  112  each of which corresponding to the respective excitation light sources L1 to L4; dichroic mirrors  113   a ,  113   b , and  113   c ; a homogenizer  114 ; a condenser lens  115 ; and an incident slit  116 . 
     The laser light emitted from the excitation light source L1 and the laser light emitted from the excitation light source L3 are collimated by the collimator lens  111 , transmitted through the laser line filter  112  for cutting out-of-band components of each wavelength band, and is formed into a coaxial beam by the dichroic mirror  113   a . The two coaxial laser beams undergoes further beam-shaping by the homogenizer  114  such as a fly-eye lens and the condenser lens  115  so as to be the line illumination Ex1. 
     Similarly, the laser light emitted from the excitation light source L2 and the laser light emitted from the excitation light source L4 are formed into a coaxial beam by the dichroic mirrors  113   b  and  113   c  so as to be the line illumination, namely, the line illumination Ex2 which is non-coaxial to the line illumination Ex1. The line illuminations Ex1 and Ex2 form non-coaxial line illumination (primary image) separated by Δy in the incident slit  116  (slit conjugate) having a plurality of slit parts each permitting the passage of each of the line illumination Ex1 and Ex2. 
     The sample S on the stage  120  is irradiated with the primary image through the observation optical system  140 . The observation optical system  140  includes a condenser lens  141 , dichroic mirrors  142  and  143 , an objective lens  144 , a band pass filter  145 , and a condenser lens  146 . The line illuminations Ex1 and Ex2 are collimated by the condenser lens  141  paired with the objective lens  144 , reflected by the dichroic mirrors  142  and  143 , transmitted through the objective lens  144 , and applied to the sample S. 
     The fluorescence excited on the surface of the sample S is condensed by the objective lens  144 , reflected by the dichroic mirror  143 , transmitted through the dichroic mirror  142  and the band pass filter  145  that cuts off the excitation light, condensed again by the condenser lens  146 , and incident on the spectral imaging unit  130 . 
     The spectral imaging unit  130  includes an observation slit  131 , imaging elements  132  ( 132   a  and  132   b ), a first prism  133 , a mirror  134 , a diffraction grating  135  (wavelength distribution element), and a second prism  136 . 
     The observation slit  131  is disposed at the focal point of the condenser lens  146  and has the same number of slit parts as the number of excitation lines. The fluorescence spectra based on the two excitation lines that have passed through the observation slit  131  are separated by the first prism  133  and reflected by the grating surface of the diffraction grating  135  via the mirror  134 , so as to be further separated into fluorescence spectra of individual excitation wavelengths. The four fluorescence spectra thus separated are incident on the imaging elements  132   a  and  132   b  via the mirror  134  and the second prism  136 , and developed into (x, λ) information as spectral data. 
     The pixel size (nm/Pixel) of the imaging elements  132   a  and  132   b  is not particularly limited, and is set to 2 nm or more and 20 nm or less, for example. This variance value may be achieved optically or by setting a pitch of the diffraction grating  135 , or may be achieved by using hardware binning of the imaging elements  132   a  and  132   b.    
     The stage  120  and the scanning mechanism  150  constitute an X-Y stage, and cause the sample S to move in the X-axis direction and the Y-axis direction in order to acquire a fluorescence image of the sample S. In whole slide imaging (WSI), an operation of scanning the sample S in the Y-axis direction, then moving the sample S in the X-axis direction, and further performing scanning in the Y-axis direction is repeated. 
     The non-fluorescence observation unit  170  includes a light source  71 , a dichroic mirror  143 , an objective lens  144 , a condenser lens  172 , an imaging element  173 , and the like. In the non-fluorescence observation system,  FIG.  24    illustrates an observation system by dark field illumination. 
     The light source  71  is disposed below the stage  120 , and irradiates the sample S on the stage  120  with illumination light from the side opposite to the line illuminations Ex1 and Ex2. In the case of dark field illumination, an excitation light source  171  illuminates from the outside of the numerical aperture (NA) of the objective lens  144 , and the light (dark field image) diffracted by the sample S is imaged by the imaging element  173  via the objective lens  144 , the dichroic mirror  143 , and the condenser lens  172 . By using dark field illumination, even an apparently transparent sample such as a fluorescently-stained sample can be observed with contrast. 
     Note that this dark field image may be observed simultaneously with fluorescence and used for real-time focusing. In this case, the illumination wavelength can be determined by selecting a wavelength that would not affect fluorescence observation. Not limited to the observation system that acquires the dark field image, the non-fluorescence observation unit  170  may include an observation system that can acquire a non-fluorescence image such as a bright field image, a phase difference image, a phase image, and an in-line hologram image. For example, examples of an applicable method of acquiring a non-fluorescence image include various observation methods such as a Schlieren method, a phase difference contrast method, a polarization observation method, and an epi-illumination method. The illumination light source need not be located below the stage, and may be located above the stage or around the objective lens. Furthermore, not only a method of performing focus control in real time, but also another method such as a prefocus map method of recording focus coordinates (Z coordinates) in advance may be adopted. 
     In the above configuration, the control unit  180  in  FIG.  23    can correspond to the control unit  60  according to the above-described embodiment or its modifications. In addition, the excitation unit  110  can correspond to the light source  18 B, the spectral imaging unit  130  can correspond to the image capture unit  34  and the pupil-split image capture unit  42 , the scanning mechanism  150  can correspond to the first drive unit  44 , the focus mechanism  160  can correspond to the second drive unit  46 , the sample stage  120  can correspond to the stage  26 , and the observation optical system  140  can correspond to the optical system including the objective lens  22 . 
     (Hardware Configuration) 
       FIG.  25    is a hardware configuration diagram illustrating an example of a computer  1000  that implements the functions of the control device  16  according to the embodiment and its modifications. 
     The computer  1000  includes a CPU  1100 , RAM  1200 , read only memory (ROM)  1300 , a hard disk drive (HDD)  1400 , a communication interface  1500 , and an input/output interface  1600 . Individual components of the computer  1000  are interconnected by a bus  1050 . 
     The CPU  1100  operates based on a program stored in the ROM  1300  or the HDD  1400  so as to control each of components. For example, the CPU  1100  develops the program stored in the ROM  1300  or the HDD  1400  into the RAM  1200  and executes processing corresponding to the programs. 
     The ROM  1300  stores a boot program such as a basic input output system (BIOS) executed by the CPU  1100  when the computer  1000  starts up, a program dependent on hardware of the computer  1000 , or the like. 
     The HDD  1400  is a non-transitory computer-readable recording medium that records a program executed by the CPU  1100 , data used by the program, or the like. Specifically, the HDD  1400  is a recording medium that records a focus adjustment program according to the present disclosure, which is an example of program data  1450 . 
     The communication interface  1500  is an interface for connecting the computer  1000  to an external network  1550  (for example, the Internet). For example, the CPU  1100  receives data from other devices or transmits data generated by the CPU  1100  to other devices via the communication interface  1500 . 
     The input/output interface  1600  is an interface for connecting between an input/output device  1650  and the computer  1000 . For example, the CPU  1100  receives data from an input device such as a keyboard or a mouse via the input/output interface  1600 . In addition, the CPU  1100  transmits data to an output device such as a display, a speaker, or a printer via the input/output interface  1600 . Furthermore, the input/output interface  1600  may function as a media interface for reading a program or the like recorded on predetermined recording media. Examples of the media include optical recording media such as a digital versatile disc (DVD) or a phase change rewritable disk (PD), a magneto-optical recording medium such as a magneto-optical disk (MO), a tape medium, a magnetic recording medium, and semiconductor memory. 
     For example, when the computer  1000  functions as the control device  16  according to the above-described embodiment, the CPU  1100  of the computer  1000  executes the program loaded on the RAM  1200  to implement the functions of the light source control unit  60 A, the captured image acquisition unit  60 B, the reference focus unit  60 C, the pupil-split image acquisition unit  60 D, the derivation unit  60 E, the movement control unit  60 F, the output control unit  60 G, the selection unit  60 H, the phase difference acquisition unit  60 I, the relative distance derivation unit  60 J, and the like. The HDD  1400  stores the program and the data according to the present disclosure. While the CPU  1100  executes program data  1450  read from the HDD  1400 , the CPU  1100  may acquire these programs from another device via the external network  1550 , as another example. 
     Note that the present technique can also have the following configurations. 
     (1) 
     A microscope system including: 
     an irradiation unit that emits line illumination parallel to a first direction; 
     a stage that supports a specimen and is movable in a second direction perpendicular to the first direction; 
     a phase difference acquisition unit that acquires phase difference information regarding an image of light emitted from the specimen by being irradiated with the line illumination; 
     an objective lens that focuses the line illumination on the specimen; 
     a derivation unit that derives relative position information between the objective lens and the specimen based on the phase difference information; and 
     a movement control unit that causes at least one of the objective lens and the stage to move in a third direction vertical to each of the first direction and the second direction based on the relative position information. 
     (2) 
     The microscope system according to (1), 
     wherein a plurality of the line illuminations is a plurality of the line illuminations parallel to the first direction and coaxially arranged with mutually different wavelengths. 
     (3) 
     The microscope system according to (1) or (2), 
     wherein the phase difference acquisition unit includes a plurality of lenses and acquires a pupil-split image of light emitted from the specimen as the phase difference information. 
     (4) 
     The microscope system according to any one of (1) to (3), 
     wherein the phase difference acquisition unit acquires the phase difference information based on a light intensity distribution of a pupil-split image of light emitted from the specimen. 
     (5) 
     The microscope system according to (4), 
     wherein the phase difference acquisition unit calculates centroid positions of the light intensity distribution and acquires the phase difference information by comparing the centroid positions. 
     (6) 
     The microscope system according to any one of (3) to (5), 
     wherein the derivation unit includes: 
     a first calculation unit that calculates an interval of the pupil-split image; and 
     a second calculation unit that calculates a relative movement amount and a relative movement direction according to a difference between the interval and a reference interval, as the relative position information. 
     (7) 
     The microscope system according to any one of (3) to (6), 
     wherein the derivation unit derives the relative position information for each position of the specimen based on an interval between positions in an extending direction of the image between the pupil-split images. 
     (8) 
     The microscope system according to any one of (1) to (7), 
     wherein the movement control unit performs focusing for each of a plurality of line illuminations parallel to the first direction and coaxially arranged with mutually different wavelengths. 
     (9) 
     The microscope system according to any one of (3) to (7), 
     wherein the phase difference acquisition unit acquires the pupil-split image from an image capture unit, the image capture unit including a plurality of light receiving units that receive light and having a configuration in which a plurality of types of unit regions having exposure setting values of the light receiving units included being different from each other is arranged along a light receiving surface, and 
     the derivation unit 
     includes a selection unit that selects the unit region including the light receiving unit having a specific exposure setting value among the plurality of types of unit regions, and 
     measures a phase difference based on a light intensity distribution of the pupil-split image received by the light receiving unit included in the selected unit region and derives the relative position information. 
     (10) 
     The microscope system according to (9), 
     wherein the exposure setting value includes at least one of a gain and an exposure time of the light receiving unit. 
     (11) 
     An imaging method executed by a computer for controlling a measurement unit including an irradiation unit that emits line illumination parallel to a first direction, a stage that supports a specimen and is movable in a second direction perpendicular to the first direction, and an objective lens that focuses the line illumination on the specimen, the imaging method including: 
     a step of acquiring phase difference information regarding an image of light emitted from the specimen by being irradiated with the line illumination; 
     a step of deriving relative position information between the objective lens and the specimen based on the phase difference information; and 
     a step of causing at least one of the objective lens and the stage to move in a third direction vertical to each of the first direction and the second direction based on the relative position information. 
     (12) 
     An imaging apparatus including: a measurement unit; and software used to control an operation of the measurement unit, 
     wherein the software is installed in an imaging apparatus, 
     the measurement unit includes: 
     an irradiation unit that emits line illumination parallel to a first direction; 
     a stage that supports a specimen and is movable in a second direction perpendicular to the first direction; and 
     an objective lens that focuses the line illumination on the specimen, and 
     the software 
     acquires phase difference information regarding an image of light emitted from the specimen by being irradiated with the line illumination, 
     derives relative position information between the objective lens and the specimen based on the phase difference information, and 
     causes at least one of the objective lens and the stage to move in a third direction vertical to each of the first direction and the second direction based on the relative position information. 
     (13) 
     A microscope system including: 
     an irradiation unit that emits line illumination parallel to a first direction; 
     a stage that supports a specimen and is movable in a second direction perpendicular to the first direction; 
     a phase difference acquisition unit that acquires phase difference information regarding an image of light emitted from the specimen by being irradiated with the line illumination; 
     an objective lens that focuses the line illumination on the specimen; and 
     a derivation unit that derives relative position information between the objective lens and the specimen based on the phase difference information, and registers, based on the relative position information, each position in the first direction and the second direction to a focus map in association with a displacement amount of a focus position of the objective lens in a third direction vertical to each of the first direction and the second direction. 
     (14) 
     The microscope system according to any one of (1) to (10) and (13), further including 
     a discerning unit that discerns whether at least a part of a region irradiated with the line illumination is a tissue region in which tissue of the specimen exists or a non-tissue region in which the tissue does not exist based on luminance information regarding the light emitted from the specimen. 
     (15) 
     The microscope system according to (14), 
     wherein the non-tissue region is a region in which a sealant is disposed. 
     (16) 
     The microscope system according to (14) or (15), 
     wherein the discerning unit discerns a ratio of the tissue region in the region irradiated with the line illumination. 
     (17) 
     The microscope system according to any one of (14) to (16), 
     wherein the discerning unit discerns whether at least a part of the region irradiated with the line illumination is the tissue region or the non-tissue region based on an image obtained by capturing an entire image of the specimen. 
     (18) 
     The microscope system according to any one of (14) to (17), 
     wherein the discerning unit discerns whether at least a part of the region irradiated with the line illumination is the tissue region or the non-tissue region based on luminance information regarding fluorescence emitted from the specimen. 
     (19) 
     The microscope system according to any one of (14) to (18), 
     wherein the discerning unit discerns whether the region irradiated with the line illumination is the tissue region or the non-tissue region for each of a plurality of types of unit regions having mutually different exposure setting values of a plurality of light receiving units that receive light. 
     (20) 
     The microscope system according to any one of (14) to (19), 
     wherein the discerning unit discerns whether at least a part of the region irradiated with the line illumination is the tissue region or the non-tissue region based on a level of luminance obtained from luminance information regarding the light emitted from the specimen. 
     (21) 
     The microscope system according to any one of (14) to (20), 
     wherein the phase difference acquisition unit acquires the phase difference information based on the image of the light originated from the tissue region discerned by the discerning unit. 
     (22) 
     The microscope system according to any one of (14) to (20), 
     wherein the phase difference acquisition unit acquires a phase difference of the image of the light based on the image of the light originated from the non-tissue region discerned by the discerning unit, and acquires the phase difference information by correcting the acquired phase difference. 
     (23) 
     The microscope system according to (21) or (22), 
     wherein the derivation unit derives relative position information between the objective lens and the specimen based on the phase difference information acquired by the phase difference acquisition unit, and generates the relative position information by correcting the derived position information. 
     (24) 
     The microscope system according to any one of (14) to (20), 
     wherein the derivation unit derives the relative position information between the objective lens and the specimen based on the phase difference information originated from the tissue region discerned by the discerning unit. 
     (25) 
     The microscope system according to any one of (14) to (20), 
     wherein the derivation unit derives relative position information between the objective lens and the specimen based on the phase difference information originated from the non-tissue region discerned by the discerning unit, and generates the relative position information by correcting the derived position information. 
     (26) 
     The microscope system according to any one of (14) to (20), 
     wherein the derivation unit either derives relative position information between the objective lens and the specimen based on the phase difference information originated from the tissue region and originated from the non-tissue region according to a ratio of the tissue region in the region irradiated with the line illumination discerned by the discerning unit and generates the relative position information by correcting the derived position information, or derives relative position information between the objective lens and the specimen based on the phase difference information originated from the tissue region and generates the relative position information by correcting the derived position information. 
     (27) 
     The microscope system according to (20), 
     wherein the discerning unit controls to stop derivation of the relative position information performed by the derivation unit when it is discerned that either the level of the luminance is lower than a preset threshold or the luminance is saturated. 
     (28) 
     The microscope system according to any one of (1) to (10) and (13) to (27), 
     wherein the derivation unit controls such that a distance by which the objective lens moves perpendicularly to a measurement surface of the specimen while the objective lens moves parallel to the measurement surface of the specimen by a predetermined distance is a focal depth of the objective lens or less. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  MICROSCOPE SYSTEM 
               12  IMAGING APPARATUS 
               14  MEASUREMENT UNIT 
               18  IRRADIATION UNIT 
               22  OBJECTIVE LENS 
               34  IMAGE CAPTURE UNIT 
               37  UNIT REGION 
               41  LIGHT RECEIVING UNIT 
               42  PUPIL-SPLIT IMAGE CAPTURE UNIT 
               44  FIRST DRIVE UNIT 
               46  SECOND DRIVE UNIT 
               60 B CAPTURED IMAGE ACQUISITION UNIT 
               60 D PUPIL-SPLIT IMAGE ACQUISITION UNIT 
               60 E DERIVATION UNIT 
               60 H SELECTION UNIT 
               60 I PHASE DIFFERENCE ACQUISITION UNIT 
               60 J RELATIVE DISTANCE DERIVATION UNIT 
               60 K DISCERNING UNIT 
               70  PUPIL-SPLIT PICTURE 
               72 A,  72 B IMAGE 
               371 ,  372  REGION 
             T SPECIMEN