Patent Publication Number: US-9906698-B2

Title: Imaging apparatus and imaging method

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The disclosure of Japanese Patent Applications enumerated below including specification, drawings and claims is incorporated herein by reference in its entirety: No. 2016-39258 filed Mar. 1, 2016. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a technique for imaging raw samples carried together with liquid in a sample container and particularly relates to the illumination of the raw samples. 
     2. Description of the Related Art 
     In medical and bioscience experiments, liquid or gel-like fluid (e.g. culture fluid, culture medium or the like) is poured into each well of a plate-like sample container (e.g. called a microplate, a microtiter plate or the like) on which a multitude of recesses, for example, also called wells are arranged, and cells and the like cultured here are observed and measured as samples. In recent years, samples have been imaged and converted into data by a CCD camera or the like, and various image processing techniques have been applied to this image data for observation and analysis. 
     In such an imaging apparatus, there is a problem that illumination light is refracted by the meniscus of a liquid surface and the brightness of an image becomes insufficient particularly at peripheral edge parts of the wells. To deal with this problem, in a technique described in JP 2015-118036A previously disclosed by the applicant of this application, an imaging optical system has an object-side hypercentric property, whereby light having a propagation direction bent in a direction away from an optical axis by refraction can be efficiently collected. 
     SUMMARY OF THE INVENTION 
     To observe finer structures, it is considered to set a higher imaging magnification of the imaging optical system. Further, the wells used come in various sizes. In these cases, there are cases where an area with the influence of the meniscus is included in an imaging field of view and cases where such an area is not included, particularly if an imaging field of view becomes smaller than one well. When the area without the influence of the meniscus is imaged, a light quantity is reduced in a peripheral edge part of the imaging field of view in imaging using the imaging optical system described above and an effective imaging field of view may become narrower. 
     This invention was developed in view of the above problem and aims to provide a technique capable of obtaining an image with good image quality by suppressing the influence of meniscus in a technique for imaging raw samples carried together with liquid in a sample container. 
     According to a first aspect of the disclosure, there is provided an imaging apparatus that images a raw sample as an imaging object carried together with liquid in a sample container with a bottom surface having optical transparency. The apparatus comprises: a holder that holds the sample container; an imaging optical system, arranged to face the sample container held by the holder, that has an object-side hypercentric property; a two-dimensional imaging element that images an image of the imaging object focused by the imaging optical system; a mover that relatively integrally moves the imaging optical system and the two-dimensional imaging element with respect to the imaging object in a direction orthogonal to an optical axis of the imaging optical system; an illuminator that illuminates the imaging object from a side opposite to the imaging optical system across the sample container held by the holder; and a controller that controls the two-dimensional imaging element, the mover and the illuminator, wherein: the illuminator includes a plurality of illumination optical systems that has mutually different exit pupil positions and coaxially emit lights toward the imaging object; the controller causes one of the plurality of illumination optical systems to emit light and causes the two-dimensional imaging element to image the imaging object when a relative position of the imaging optical system with respect to the imaging object reaches any one of a plurality of imaging positions determined in advance while causing the mover to relatively move the imaging optical system and the two-dimensional imaging element with respect to the imaging object; and the illumination optical system for emitting the light is switched according to the imaging position. 
     According to a second aspect of the disclosure, there is provided an imaging method that images a raw sample as an imaging object carried together with liquid in a sample container with a bottom surface having optical transparency. The method comprises: arranging an imaging optical system having an object-side hypercentric property and a two-dimensional imaging element for imaging an image of the imaging object focused by the imaging optical system to face the sample container and arranging an illuminator for illuminating the imaging object on a side opposite to the imaging optical system across the sample container; moving the imaging optical system and the two-dimensional imaging element with respect to the imaging object, relatively; and imaging the imaging object by the two-dimensional imaging element and emitting light from the illuminator when a relative position of the imaging optical system with respect to the imaging object reaches any one of a plurality of imaging positions determined in advance, wherein: the illuminator includes a plurality of illumination optical systems having mutually different exit pupil positions, emitted lights propagating toward the imaging object being coaxial with each other; and light is emitted selectively from one of the plurality of illumination optical systems and the illumination optical system for emitting the light is switched according to the imaging position in the imaging step. 
     In the invention thus configured, an image of an area of the imaging object including an area with the influence of meniscus can be imaged with good image quality as with the technique described in JP 2015-118036A by using the imaging optical system having the object-side hypercentric property. More microscopically, refraction by the meniscus is strongly present, for example, near a peripheral edge part of the sample container, whereas that influence is hardly present, for example, in a central part of the sample container. In this way, the presence of the influence of the meniscus differs depending on positions. Directions of chief rays of illumination light to be incident on the imaging object can be changed by providing the plurality of illumination optical systems having different exit pupil positions and switching them. Thus, imaging with the influence of the meniscus suppressed can be performed to correspond to a difference in the magnitude of refraction by the meniscus by switching the illumination optical systems according to the imaging position. 
     The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawing. It is to be expressly understood, however, that the drawing is for purpose of illustration only and is not intended as a definition of the limits of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a schematic configuration of one embodiment of an imaging apparatus according to the invention. 
         FIG. 2  is a diagram showing detailed configurations and ray diagrams of the illumination optical systems. 
         FIGS. 3A and 3B  are diagrams showing illumination lights emitted from the first and second illumination optical systems. 
         FIGS. 4A and 4B  are diagrams showing a state at the time of imaging the well. 
         FIGS. 5 and 6  are diagrams showing a state of imaging when the size of the well is larger than that of the imaging field of view. 
         FIG. 7  is a diagram showing imaging using the second illumination optical system. 
         FIGS. 8A and 8B  are diagrams illustrating a method for dividing one well into a plurality of images. 
         FIG. 9  is a flow chart showing an imaging process in this embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a diagram showing a schematic configuration of one embodiment of an imaging apparatus according to the invention. This imaging apparatus is an apparatus for imaging raw samples such as cells, cell colonies and bacteria (hereinafter, referred to as “cells and the like”) cultured in liquid poured into recesses called wells W formed on the upper surface of a well plate WP. 
     The well plate WP is generally used in the fields of drug discovery and bioscience. The well plate WP has a flat plate shape and a plurality of tubular wells W having openings on the upper surface of this plate are arranged on the well plate WP. Each well W has, for example, a substantially circular cross-section and a bottom surface is transparent and flat and has optical transparency. The cross-section and the bottom surface shape of the well W are not limited to these. For example, the cross-section may be rectangular and the bottom surface may be curved. 
     The number of the wells W on the well plate WP is arbitrary. For example, a well plate WP having 96 (12×8 matrix array) wells can be used. A diameter and a depth of each well W are typically about several mm. Note that the size of a well plate and the number of wells used in this imaging apparatus  1  are arbitrary without being limited to these. For example, well plates having 6 to 384 wells are generally used. Further, without limitation to well plates having a plurality of wells, this imaging apparatus  1  can be used, for example, also for imaging of cells and the like cultured in a flat container called a dish. 
     A predetermined amount of liquid as a culture medium M is poured into each well of the well plate WP, and the cells and the like cultured under predetermined culture conditions in this liquid are imaging objects of this imaging apparatus  1 . The culture medium M may be added with appropriate reagents or may be gelled after being poured into the wells W in a liquid state. As described later, in this imaging apparatus  1 , cells and the like cultured on the inner bottom surfaces of the wells W can be, for example, imaging objects. About 50 to 200 microliters of the liquid is generally usually used. 
     The imaging apparatus  1  includes an illuminator  10  arranged in an upper part of the apparatus  1 , a holder  12  for holding the well plate WP below the illuminator  10 , an imager  13  arranged below the holder  12  and a controller  14  with a CPU  141  for controlling the operation of each of these parts. The holder  12  holds the well plate WP in a substantially horizontal posture by being held in contact with a peripheral edge part of the lower surface of the well plate WP carrying samples together with the medium M in each well W. 
     The illuminator  10  includes two light sources  101 ,  111 . For example, white LEDs (Light Emitting Diodes) can be used as the light sources  101 ,  111 . Light emitted from the light source  111  is incident on a beam splitter  104  via a collector lens  112 . On the other hand, light emitted from the light source  101  has an optical path reflected by a reflection mirror  102  and is incident on the beam splitter  104  via a collector lens  103 . A beam emitted from the beam splitter  104  has a propagation direction changed to a (−Z) direction, i.e. a vertically downward direction by a reflection mirror  105  and is emitted downward via a condenser lens  106 . The emitted light is incident on at least one well W from above the well plate WP supported on the holder  12  to illuminate imaging objects in the well W. 
     As just described, in the illuminator  10  of this embodiment, two illumination optical systems, i.e. a first illumination optical system  100  using the light source  101  as a light source thereof and composed of the reflection mirror  102 , the collector lens  103 , the beam splitter  104 , the reflection mirror  105 , the condenser lens  106  and the like and a second illumination optical system  110  using the light source  111  as a light source thereof and composed of the collector lens  112 , the beam splitter  104 , the reflection mirror  105 , the condenser lens  106  and the like coexist while sharing some constituent components. 
     The light sources  101 ,  111  are controlled to be turned on and off by a light source controller  146  provided in the controller  14  and selectively turned on in accordance with a control signal from the light source controller  146 . Thus, the illuminator  10  can cause first illumination light emitted from the first illumination optical system  100  including the light source  101  and second illumination light emitted from the second illumination optical system  110  including the light source  111  to be selectively incident on the well W. The first illumination light and the second illumination light are guided through the beam splitter  104  and these can be coaxially emitted. Specifically, center axes of the first illumination light and the second illumination light emitted from the condenser lens  106  coincide. 
       FIG. 2  is a diagram showing detailed configurations and ray diagrams of the illumination optical systems. In  FIG. 2 , the first and second illumination optical systems  100 ,  110  sharing some constituent components as described above are separately shown to clearly show optical paths. Further, for the same purpose, the optical axes bent by the reflection mirrors and the beam splitter is shown to be straight. Thus, the reflection mirrors  102 ,  105  and the beam splitter  104  having a function of bending the optical axes are not shown. 
     In the first illumination optical system  100 , light emitted from the light source  101  is collected by the collector lens  103  and emitted toward a sample surface, where the cells and the like as imaging objects are present, via the condenser lens  106 . The sample surface is a well bottom surface Wb (see  FIGS. 3A and 3B , etc.) in nocinal use. The collector lens  103  focuses an image of the light source  101  between the collector lens  103  and the condenser lens  107 . Specifically, a light source image C 1  is formed between the collector lens  103  and the condenser lens  106 . Further, the collector lens  103  and the condenser lens  106  are arranged such that the light source image C 1  is located at a front (light source-side) focus position of the condenser lens  106  and chief rays propagating from the condenser lens  106  toward the sample surface are parallel to an optical axis shown by a chain line. Specifically, the first illumination optical system  100  constitutes telecentric illumination. 
     An aperture stop  107  is arranged on a light emitting surface of the light source  101  if necessary to specify the size of the light source image C 1  formed by the collector lens  103 . An NA (numerical aperture) of illumination can be adjusted by the aperture stop  107 . Further, a field stop  108  is arranged behind the collector lens  103  and before a conjugate point if necessary. In this way, only a range necessary for imaging can be illuminated to prevent the occurrence of flare in the imaging optical system. 
     In the second illumination optical system  110 , light emitted from the light source  111  is collected by the collector lens  112  and emitted toward the sample surface via the condenser lens  106 . The collector lens  112  is imparted with such a refraction characteristic that a light source image C 2  of the light source  111  is located behind the condenser lens  106  and before the sample surface. 
     An aperture stop  113  is arranged on a light emitting surface of the light source  111  if necessary to specify the size of the light source image C 2  formed by the collector lens  112  and the condenser lens  106 . An NA of illumination can be adjusted by the aperture stop  113 . Further, a field stop  114  is arranged between the collector lens  112  and the condenser lens  106  if necessary. In this way, only a range necessary for imaging can be illuminated to prevent the occurrence of flare in the imaging optical system. 
     The two illumination optical systems  100 ,  110  share the condenser lens  106 . The beam splitter  104  for enabling this is provided between the collector lenses and the condenser lenses of the illumination optical systems  100 ,  110 . More specifically, the beam splitter  104  is arranged at a position to satisfy conditions that the beam splitter  104  is behind the collector lens  103  (field stop  108  if the field stop  108  is provided) and before the condenser lens  106  in the first illumination optical system  100  and behind the collector lens  112  (field stop  114  if the field stop  114  is provided) and before the condenser lens  106  in the second illumination optical system  110 . 
     Referring back to  FIG. 1 , the description of the imaging apparatus  1  is continued. The imager  13  for imaging the imaging objects such as cells and the like in the wells W is provided below the well plate WP held by the holder  12 . In the imager  13 , an objective lens  131  is arranged at a position right below the well plate WP. An optical axis of the objective lens  131  extends in a vertical downward direction and is coaxial with the optical axes of the first and second illumination optical systems  100 ,  110 . Light emitted from the illuminator  10  and incident on the liquid surface from above the well W illuminates the imaging objects and light transmitted downward from the bottom surface of the well W is incident on the objective lens  131 . A low magnification afocal system  132  and a high magnification afocal system  133  are switchably provided below the objective lens  131 . 
     Specifically, the low magnification afocal system  132  and the high magnification afocal system  133  are integrally movable in a horizontal direction by an unillustrated drive mechanism, and one thereof is selectively positioned at a position right below the objective lens  131 . In a state where the high magnification afocal system  133  is positioned at the position right below the objective lens  131  as shown in solid line in  FIG. 1 , a high magnification imaging optical system including the objective lens  131  and the high magnification afocal system  133  is configured. At this time, a relatively narrow range of the imaging objects can be imaged at a high magnification. On the other hand, in a state where the low magnification afocal system  132  is positioned at the position right below the objective lens  131  as shown in dotted line in  FIG. 1 , a low magnification imaging optical system including the objective lens  131  and the low magnification afocal system  132  is configured. At this time, a relatively wide range of the imaging objects can be imaged at a low magnification. 
     Light emitted from the low magnification afocal system  132  or the high magnification afocal system  133  is reflected by a reflecting mirror  134  and incident on an imaging element  136  via a focusing lens  135 . As described later, the imaging optical system composed of the objective lens  131 , the low magnification afocal system  132 , the focusing lens  135  and the like has an object-side hypercentric optical property. On the other hand, the imaging optical system composed of the objective lens  131 , the high magnification afocal system  133 , the focusing lens  135  and the like has an object-side telecentric optical property. 
     The imaging element  136  is an area image sensor having a two-dimensional light receiving surface and a CCD sensor or a CMOS sensor can be, for example, used. An image of imaging objects focused on the light receiving surface of the imaging element  136  by the focusing lens  135  is imaged by the imaging element. The imaging element  136  converts the received optical image into an electrical signal and outputs it as an image signal. In such an imaging method, cells and the like as the imaging objects can be imaged in a non-contact, non-destructive and non-invasive manner and damage on the cells and the like by imaging can be suppressed. The operation of each part of the imager  13  is controlled by an imaging controller  143  provided in the controller  14 . 
     The image signal output from the imaging element  136  is send to the controller  14 . Specifically, the image signal is input to an AD converter (A/D)  144  provided in the controller  14  and converted into digital image data. The CPU  141  performs appropriate image processings based on the received image data. The controller  14  further includes an image memory  147  for storing and saving image data and a memory  148  for storing and saving programs to be executed by the CPU  141  and data generated by the CPU  141 , but these may be integrated. Further, the image memory  147  and the memory  148  may be realized by appropriately combining a high-capacity storage and a semiconductor memory. The CPU  141  performs various imaging processings and arithmetic processings by executing a control program stored in the memory  148  and operating each part of the apparatus. 
     The imager  13  is moved in the horizontal and vertical directions by a mechanism controller  145  provided in the controller  14 . Specifically, the mechanism controller  145  operates a drive mechanism  15  based on a control command from the CPU  141  to move the imager  13  in the horizontal direction, whereby the imager  13  moves in the horizontal direction relative to the wells W. Further, focusing is performed by moving the imager  13  in the vertical direction. When imaging is performed with one well W entirely contained in an imaging field of view, the mechanism controller  145  positions the imager  13  in the horizontal direction such that the optical axis of the objective lens  131  coincides with a center of this well W. 
     Further, the drive mechanism  15  relatively moves the illuminator  10  integrally with the imager  13  as shown by dotted line arrows in  FIG. 1  when the imager  13  is moved in the horizontal direction. Specifically, the illuminator  10  is arranged such that a center of emitted light substantially coincides with the optical axis of the objective lens  131 . When the imager  13  moves in the horizontal direction, the illuminator  10  moves in conjunction with the imager  13 . In this way, a center of a beam of light from the illuminator  10  is constantly located on the optical axis of the objective lens  131  regardless of which well W is to be imaged, wherefore imaging conditions can be maintained to be satisfactory by making illumination conditions constant for each well W. 
     Besides, the controller  14  is provided with an interface (IF) unit  142 . The interface unit  142  has a function of performing data exchange with an external apparatus connected via a communication line besides a user interface function of receiving an operation input from a user and presenting information such as processing results to the user. Although not shown, an input receiver for receiving the operation input from the user and a display for displaying and outputting a message, a processing result and the like from the user are connected to the interface unit  142  to realize the user interface function. 
     Note that the configuration of the controller  14  mentioned here is basically the same as that of a general computer device. Accordingly, the controller  14  provided in the imaging apparatus  1  may be a dedicated device with the hardware described above or may be a general-purpose processing device such as a personal computer or a work station incorporated with a control program for realizing a processing function to be described later. Specifically, a general-purpose computer device can be utilized as the controller  14  of this imaging apparatus  1 . In the case of using the general-purpose computer device, it is sufficient to provide the imaging apparatus  1  with control functions minimum necessary to operate each part such as the imager  13 . 
       FIGS. 3A and 3B  are diagrams showing illumination lights emitted from the first and second illumination optical systems. More specifically,  FIG. 3A  shows first illumination light L 1  emitted from the first illumination optical system  100  and  FIG. 3B  shows second illumination light L 2  emitted from the second illumination optical system  110 . As shown in  FIG. 3A , the first illumination light L 1  emitted from the condenser lens  106  in the first illumination optical system  100  using the light source  101  as a light source is incident in a state where chief rays are incident in a parallel state on the bottom surface Wb of the well W as the sample surface on which the imaging objects are distributed. Specifically, the first illumination optical system  100  constitutes telecentric illumination whose exit pupil position is at infinity. 
     On the other hand, the second illumination light L 2  emitted from the condenser lens  106  in the second illumination optical system  110  using the light source  111  as a light source propagates in a direction toward the optical axis of the second illumination optical system  110  and intersects with the optical axis above the well bottom surface Wb, i.e. on a front side when viewed from the illumination optical system. Specifically, an exit pupil position Pp where an image of the light source  111  (more strictly, aperture stop  113 ) is focused on the optical path of the second illumination optical system L 2  is located at a position closer than the well bottom surface Wb as the sample surface on which the imaging objects are distributed when viewed from the second illumination optical system  110 . 
     More specifically, under illumination by the second illumination optical system  110 , the image of the light source  111  is focused between an output end of the condenser lens  106  emitting the illumination light L 2  and the objective lens  131  of the imaging optical system. That is, a conjugate point for the light source  111  is located at this position. The holder  12  holds the well plate WP such that the well bottom surface Wb as the sample surface on which the imaging objects are distributed is located between this conjugate point and the objective lens  131 . Thus, a chief ray of the illumination light incident on the well bottom surface Wb has a direction component in a direction away from the optical axes of the second illumination optical system  110  and the objective lens  131 . In the second illumination optical system  110 , the exit pupil position, the position of the image of the light source  111  and the position of the conjugate point for the light source  111  coincide. 
     As just described, the exit pupil positions differ from each other in the two illumination optical systems  100 ,  110  and these are properly used according to need in imaging the well W as described later. Note that stroboscopic illumination is used in imaging. That is, the illumination light is emitted only for a short time when imaging by the imager  13  is performed. Thus, the light source controller  146  can realize the switch of the illumination light by selecting which of the two light sources  101 ,  111  is to be turned on. A specific mode of properly using one of these illumination optical systems  100 ,  110  is described below. 
       FIGS. 4A and 4B  are diagrams showing a state at the time of imaging the well. More specifically,  FIG. 4A  is a diagram showing a propagation path of light at the time of imaging the well and  FIG. 4B  is a diagram showing a relationship of the well and the imaging field of view. The medium M poured in a liquid state is in the well W carrying the cells and the like as the imaging objects. Thus, the illumination light L incident from above the well W is incident on the well bottom surface Wb (sample surface), where the imaging objects are present, via the liquid surface of the medium M. The liquid surface generally forms an upward concave meniscus, whereby the illumination light L is refracted and bent outwardly from the center of the well W. Refraction is small near the center of the well W and becomes larger toward the peripheral edge part of the well W. 
     The imaging optical system including the objective lens  131  constitutes an object-side hypercentric optical system and has a function of efficiently collecting the light bent outwardly in this way and guiding it to the imaging element  136 . Specifically, light incident obliquely outwardly can be focused on the imaging element  136  at a position distant from the optical axis of the lens. Thus, this imaging optical system is suitable when one well W is imaged while being entirely contained in an imaging field of view V as shown in  FIG. 4B . This point is as disclosed also in JP 2015-118036A. 
     One well W can be entirely contained in the imaging field of view V as shown in  FIG. 4B  when the well W having a relatively small opening diameter is imaged at a low magnification. On the other hand, such as in the case of imaging imaging objects carried in a well having a large opening diameter (e.g. well on a well plate with six wells), the size of an area to be imaged becomes relatively larger than that of the imaging field of view and it may not be possible to contain the entire well W in the imaging field of view V. 
       FIGS. 5 and 6  are diagrams showing a state of imaging when the size of the well is larger than that of the imaging field of view. More specifically,  FIG. 5  shows imaging when the well peripheral edge part is not contained in the imaging field of view and  FIG. 6  shows imaging when the well peripheral edge part is contained in the imaging field of view. Here is described a case where the well W having a larger opening diameter than that described thus far is imaged at a low magnification. For example, a similar way of thinking can be applied when the size of an area to be imaged is larger than that of the imaging field of view V such as when imaging objects carried in a shallow container having a large opening diameter called a dish are imaged. 
     When the area to be imaged is wider than the imaging field of view V, it is considered to generate an image representing the entire area to be imaged by imaging and dividing this area into a plurality of images and combining those images by image processings. In this case, the individual images before synthesis need to have better quality to ensure desired image quality of the generated image. Points of concern for this are described next. 
     When the imaging field of view V contains only a central area distant from a peripheral edge part Wp of the well W as shown in  FIG. 5 , the influence of the meniscus on the surface of the medium M on the optical path is sufficiently small. Thus, in telecentric illumination, light incident near the optical axis of the objective lens  131  is collected and incident on the imaging element  136 , whereas a mismatch occurs due to a difference in inclination between the incident light and the chief rays of the optical system at positions distant from the optical axis. 
     Specifically, at the positions distant from the optical axis, light whose chief rays are inclined outwardly as shown in dotted line in  FIG. 5  is received, assuming refraction on the liquid surface on the side of the objective lens  131 , whereas the light transmitted through the well W propagates straight without being subjected to refraction by the meniscus, wherefore the inclinations of the chief rays in the incident light and those of the chief rays on a light receiving side do not coincide. How the image looks may change due to this, particularly due to the degradation of image quality in the peripheral edge part of the imaging field of view V, specifically due to a dark image or the incidence of the illumination light only in partial directions of a light collection range of the objective lens. 
     On the other hand, when the imaging field of view V is set in an area including the peripheral edge part Wp of the well W or an area very close to this as shown in  FIG. 6 , the inclinations of the chief rays of the light refracted by the meniscus and those of the chief rays on the light receiving side substantially coincide and light can be efficiently collected near the well peripheral edge part Wp. Contrary to this, in a part near the well central part on a side opposite to the well peripheral edge part Wp across the optical axis of the objective lens  131 , image quality is, of course, degraded since the light propagating straight is incident on the objective lens  131  having the inclinations of the chief rays assuming refraction by the meniscus as in the case of  FIG. 5 . 
     If an area capable of ensuring necessary image quality out of the physical imaging field of view V of the imager  13  is considered to be an effective imaging field of view, the effective imaging field of view becomes narrower when the area in the imaging field of view is not affected by the meniscus as described above in the combination of telecentric illumination and the hypercentric imaging optical system. Accordingly, in this embodiment, the second illumination optical system  110  capable of obtaining good image quality in the absence of the influence of the meniscus is provided separately from the first illumination optical system  100  constituting telecentric illumination. 
       FIG. 7  is a diagram showing imaging using the second illumination optical system. As described above, the illumination light L 2  emitted from the condenser lens  106  in the second illumination optical system  110  is such that the chief rays are inclined in the directions toward the optical axis. Thus, in the absence of the influence of the meniscus, the chief rays of the illumination light incident on the bottom surface Wb of the well W are not parallel to each other. Since the exit pupil position Pp of the second illumination optical system  110  is located before the well bottom surface Wb when viewed from the second illumination optical system  110  in this embodiment, the chief rays spread outwardly from the optical axis of the objective lens  131  in the illumination light L 2  incident on the well bottom surface Wb. 
     If the inclinations of the chief rays at this time are set to coincide with those of the chief rays on the side of the objective lens  131 , the light transmitted through the well bottom surface Wb is collected by the objective lens  131  and finally guided to the imaging element  136  as shown in  FIG. 7 . Thus, imaging can be performed with good image quality in the entire imaging field of view V and the entire physical imaging field of view V can be utilized as the effective imaging field of view. 
     In terms of adjusting an incident direction of the illumination light in accordance with the properties of the imaging optical system, it is, in principle, possible to obtain similar effects, for example, by arranging a point light source at the exit pupil position of the objective lens  131 . However, this is not realistic since problems such as the interference of the light source and the well plate WP and the alteration of the samples due to heat emitted from the light source may occur due to the necessity to arrange the point light source at a position right above the sample surface. 
     If the above illumination is constituted by the illumination optical system for relaying the light emitted from the light source  111  by a plurality of lenses as in the above embodiment, it is possible to realize an illumination optical system optimized according to the properties of the imaging optical system by appropriately combining the lenses, the stops and the like. For example, if the conjugate point of the light source  111  in the illumination optical system  110  and the exit pupil position of the objective lens  131  are set to coincide, the illumination light propagating in the direction away from the optical axis beyond the conjugate point can be efficiently guided to the imaging element  136  by the objective lens  131 . 
     As just described, in the imaging apparatus  1  of this embodiment, the first illumination optical system  100  suitable for imaging the area subjected to the influence of the meniscus and the second illumination optical system  110  suitable for imaging the area free from the influence of the meniscus are equipped as the illumination optical systems to be combined with the imaging optical system having the hypercentric property. When it is necessary to image an area wider than the imaging field of view V of the imager  13 , a plurality of images are imaged by properly using the illumination optical systems for the areas with and without the influence of the meniscus, whereby the quality of an image obtained by combining these images can be made satisfactory. 
       FIGS. 8A and 8B  are diagrams illustrating a method for dividing one well into a plurality of images. More specifically,  FIG. 8A  shows an example of allocating images in dividing the well W into a plurality of images and  FIG. 8B  is a diagram showing a scanning path of the imager  13  to obtain such images. In  FIG. 8A , each of the plurality of images covering the entire well W by partially overlapping each other is shown by a solid-line rectangle, diagonals of each rectangle are shown in dotted line and a centroid position thereof is shown by a black circle to make the plurality of rectangles easily distinguishable. The centroid position of the rectangle corresponds to a position where the optical axis of the objective lens  131  intersects with the well bottom surface Wb during imaging. 
     In the allocation example shown in  FIG. 8A , the entire well W is divided into eleven images. In the central part of the well W, the arrangement of the image is set not to include the well peripheral edge part Wp in the imaging field of view. The image not including the well peripheral edge part Wp in this way is supposed to be imaged using the second illumination optical system  110 . On the other hand, the image imaged to include the well peripheral edge part Wp in the imaging field of view is arranged such that at least a centroid of the image is located in the well W. Imaging is supposed to be performed using the first illumination optical system  100 . By doing so, a difference in the inclinations of the chief rays can be reduced between the illumination side and the light receiving side at an outer side of the centroid position (side close to the well peripheral edge part Wp) and image quality degradation can be suppressed. 
       FIG. 8B  shows an example of a movement path of the imager  13  in dividing and imaging the well W into the plurality of images. In the imaging apparatus  1  of this embodiment, the imager  13  and the illuminator  10  integrally horizontally move relative to the well plate WP placed on the holder  12 . The plurality of images shown in  FIG. 8A  can be obtained by performing imaging at appropriate relative positions while moving the imager  13  in the X and Y directions along the bottom surface of the well plate WP.  FIG. 8B  shows the scanning movement path of the imager  13  at this time, more precisely a locus of the intersection of the optical axis of the objective lens  131  provided in the imager  13  and the well bottom surface Wb. 
     Points P 1  to P 11  indicated by black circles respectively correspond to the centroid positions of the plurality of images shown in  FIG. 8A . Further, since the centroid position of each image is also the position of the optical axis of the objective lens  131  when this image is imaged, necessary images can be obtained by a series of scanning movements if a scanning movement recipe of the imager  13  (and the illuminator  10 ) is prepared such that the optical axis of the objective lens  131  successively passes through these points P 1  to P 11  and imaging by the imager  13  is performed when the optical axis position of the objective lens  131  reaches these points P 1  to P 11 . In this sense, the positions of the imager  13  corresponding to the points P 1  to P 11  are referred to as “imaging positions”. If the allocation of the images is determined, the imaging positions can be set according to that. Further, Ps and Pe denote a start point and an end point of the optical axis position of the objective lens  131  in the scanning movement of the imager  13 , respectively. 
       FIG. 9  is a flow chart showing an imaging process in this embodiment. The CPU  141  performs the imaging process shown in  FIG. 9  by causing each part of the apparatus to perform a predetermined operation based on the control program prepared in advance, whereby a plurality of images, for example, shown in  FIG. 8A  are obtained. First, the imager  13  is positioned at a predetermined start position by the drive mechanism  15  that operates according to a control command from the mechanism controller  145  (Step S 101 ). The start point Ps shown in  FIG. 8B  corresponds to the optical axis position of the objective lens  131  at this time. Note that although only the scanning movement of the imager  13  is described here, the illumination light  10  also moves according to the movement of the imager  13  such that the center of the illumination light and the optical axis of the objective lens  131  constantly coincide as described above. 
     Subsequently, the scanning movement of the imager  13  relative to the well W is started based on the scanning movement recipe set in advance (Step S 102 ). When the imager  13  reaches an end position corresponding to the end point Ps, the process is finished (Step S 103 ). Until the end position is reached, Steps S 105  to S 107  are performed for imaging every time the imager  13  reaches any one of the imaging positions corresponding to the points P 1  to P 11  (Step S 104 ). At which position the imager  13  is located can be detected, for example, based on an output signal from a position sensor (not shown) mounted in the imager  13 . 
     Specifically, the illumination optical system is selected according to the imaging position where the imager  13  is currently located (Step S 105 ). Specifically, the first illumination optical system  100  is selected if the well peripheral edge part Wp is included in the imaging field of view V at the current position of the imager  13  and the second illumination optical system  110  is selected if the well peripheral edge part Wp is not included. In the example shown in  FIGS. 8A and 8B , the second illumination optical system  110  is selected at the imaging positions corresponding to the points P 5  to P 7  and the first illumination optical system  100  is selected at the imaging positions corresponding to the other points P 1  to P 4  and P 8  to P 11 . As just described, the selective arrangement of the illumination optical systems  100 ,  110  corresponds to an example of an “arranging step” of the invention. 
     Subsequently, the imaging objects are stroboscopically illuminated by lighting the light source of the selected illumination optical system for a predetermined time and, simultaneously with this, the imaging element  136  performs imaging, whereby one image is obtained (Step S 106 : imaging step). Image data obtained by digitizing an image signal output from the imaging element  136  by the AD converter  144  is stored in the image memory  147  (Step S 107 ). By repeating the above process until the imager  13  reaches the end position, imaging is performed at the imaging position corresponding to each of the points P 1  to P 11  and eleven images are obtained. 
     Since imaging is performed under stroboscopic illumination, the scanning movement of the imager  13  needs not be temporarily stopped for imaging and the drive mechanism  15  may scan and move the imager  13  at a constant speed in accordance with the scanning movement recipe. By optimizing the scanning movement recipe to minimize a length of the path connecting the respective points P 1  to P 11 , a time required for imaging can be shortened. 
     Out of the plurality of images thus obtained, those including the well peripheral edge part Wp are imaged under illumination by the first illumination optical system and those not including the well peripheral edge part Wp are imaged under illumination by the second illumination optical system  110 . Thus, in each image, a reduction of image quality due to the presence or absence of the meniscus and a mismatch with the illumination optical system is suppressed. The image of the entire well W can be generated with good quality by extracting and combining parts with good image quality from the respective images. In an area where the plurality of images overlap, the image having better image quality may be adopted in view of the position of this area in the well W and the used illumination optical system. 
     As described above, in this embodiment, two illumination optical systems  100 ,  110  are provided. the illumination optical system  100  is corresponding to the case where the influence of the meniscus is present in the imaging field of view V of the imager  13  while the illumination optical system  110  is corresponding to the case free from the influence of the meniscus. Imaging is performed while those illumination optical systems are switched according to the imaging position. By doing so, images with good image quality can be obtained in both the areas with the influence of the meniscus and those without the influence of the meniscus out of the areas to be imaged. 
     As described above, in the imaging apparatus  1  of this embodiment, the illuminator  10  functions as an “illuminator” of the invention, the holder  12  functions as a “holder” of the invention, the controller  14  functions as a “controller” of the invention and the drive mechanism  15  functions as a “mover” of the invention. Further, the reflection mirror  102 , the collector lens  103 , the beam splitter  104 , the reflection mirror  105  and the condenser lens  106  integrally function as one “illumination optical system” of the invention. Further, the collector lens  112 , the beam splitter  104 , the reflection mirror  105  and the condenser lens  106  integrally function as another “illumination optical system” of the invention. 
     Further, the objective lens  131 , the low magnification afocal system  132 , the high magnification afocal system  133 , the reflection mirror  134  and the focusing lens  135  integrally function as an “imaging optical system” of the invention, and the imaging element  136  functions as a “two-dimensional imaging element” of the invention. Further, the well plate WP corresponds to a “sample container” of the invention in the above embodiment. 
     Note that the invention is not limited to the above embodiment and various changes other than those described above can be made without departing from the gist of the invention. For example, in the illuminator  10  of the above embodiment includes two illumination optical systems, i.e. the first illumination optical system  100  constituting telecentric illumination and having the exit pupil position at infinity and the second illumination optical system  110  having the exit pupil position Pp before the well bottom surface Wb. However, the exit pupil positions in the illumination optical systems are not limited to these and it is sufficient to provide two or more illumination optical systems having different exit pupil positions. The effects are more remarkable as a difference between the exit pupil positions becomes larger. Further, three or more illumination optical systems may be provided. For example, an illumination optical system having a more limited irradiation range may be further provided for high magnification imaging. 
     Further, although two light sources  101 ,  111  are provided in correspondence with the two illumination optical systems  100 ,  110  in the above embodiment, two illumination optical systems may be configured by a single light source and two optical systems. Further, although some of the members constituting the two illumination optical systems are shared in the above embodiment, completely independent two illumination optical systems may be provided. 
     Further, the allocation of the images in the above imaging process is merely for description, and an actual arrangement is not limited to this. Further, although the case of imaging only one well is described in the above allocation example, the scanning movement recipe may be so configured that a plurality of wells can be collectively imaged by a series of scanning movements. 
     As described above, according to the invention, imaging is performed by switching a plurality of illumination optical systems having mutually different exit pupil positions, thus having mutually different directions of chief rays of illumination light incident on imaging objects, according to an imaging position, whereby an image with good image quality can be obtained by suppressing the influence of meniscus. 
     Further, as described by way of the specific embodiment, one of the illumination optical systems may constitute telecentric illumination for causing light to be incident on the imaging objects such that chief rays are parallel to an optical axis of an imaging optical system in the invention. In such an illumination optical system, the inclinations of the chief rays approach those of the chief rays of the imaging optical system having a hypercentric property by bending the chief rays outwardly by refraction on a liquid surface, wherefore good image quality can be obtained when the influence of the meniscus is present in the imaging field of view. 
     In this case, the plurality of illumination optical systems may further include an illumination optical system whose exit pupil position is located closer than the imaging objects when viewed from the illumination optical system. In such an illumination optical system, illumination light having inclinations equivalent to the inclinations of the chief rays of the imaging optical system having the hypercentric property can be caused to be incident on the imaging objects when not being subjected to the influence of the meniscus. 
     Further, for example, the lights emitted from the plurality of illumination optical systems may be coaxial with the optical axis of the imaging optical system. If the emitted lights are coaxial with the optical axis of the imaging optical system, the inclinations of the chief rays can be easily matched between the illumination optical system and the imaging optical system at each position on and around the optical axis. 
     Further, for example, the imaging field of view on the bottom surface of the sample container may be smaller than the bottom surface of the sample container. Even if the imaging field of view is smaller than the bottom surface of the sample container and the entire bottom surface of the sample container cannot be contained in the imaging field of view, the entire interior of the sample container can be imaged by dividing the bottom surface into a plurality of images. In this case, the influence of the meniscus is possibly present or absent depending on the imaging position. In the invention, an image with good quality can be obtained both when the influence of the meniscus is present and when the influence of the meniscus is absent by switchingly using the plurality of illumination optical systems. Thus, particularly remarkable effects can be exhibited when an area to be imaged is divided and imaged into a plurality of images in this way. 
     Further, for example, the holder may hold the sample container in a substantially horizontal posture, the illuminator may be arranged above the sample container held by the holder to illuminate the imaging objects from above, and the two-dimensional imaging element may be arranged below the sample container held by the holder and perform imaging by causing light incident from the illuminator via the liquid surface and emitted from the bottom surface of the sample container to be incident on the imaging optical system. According to such a configuration, the optical axis in a direction perpendicular to the substantially horizontal liquid surface can be set and each part of the apparatus is easily supported and positioned. 
     This invention can be suitably applied to imaging apparatuses for imaging various raw samples such as cells, cell colonies, spheroids, bacteria, lesion tissues and microorganisms. 
     Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiment, as well as other embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.