Abstract:
In an image acquisition device, an optical path difference generating member can form an optical path length difference of a second optical image without splitting light in a second optical path. This can suppress the quantity of light required for the second optical path to obtain information of the focal position, whereby a quantity of light can be secured for a first imaging device to capture an image. The image acquisition device synchronizes the movement of a predetermined part of a sample within a field of an objective lens with rolling readout such that each pixel column of a second imaging device is exposed to an optical image of the predetermined part in the sample.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an image acquisition device and a method and system for acquiring focus information of a sample. 
       BACKGROUND ART 
       [0002]    In an image acquisition device for observing a sample such as a tissue cell, when the distance between the sample on the stage and an objective lens is kept constant, irregularities on a surface of the sample may have an out-of-focus region in an image. Therefore, image acquisition devices employing various focusing methods such as a dynamic focus scheme which captures an image of the sample while acquiring focus information and a prefocus scheme which acquires focus information before capturing the image of the sample have been developed. 
       CITATION LIST 
     Patent Literature 
       [0003]    Patent Literature 1: Japanese Patent Application Laid-Open No, 2012-108184 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0004]    The above-mentioned conventional device can grasp in-focus regions which are in focus and out-of-focus regions which are out of focus in images captured by an image pickup element. This makes it possible to determine focal position information from the position of the stage at the time when a pixel column corresponding to an in-focus region captures an image. In this technique, however, the position (two-dimensional position) of the sample to be subjected to imaging varies among the pixel columns, whereby focus position information of parts slightly different from each other is acquired in practice. The above-mentioned device is a microscope device which performs imaging at a high magnification of 20× to 40×, for example, and thus has such a small depth of field that the field of the microscope optical system is very small as compared with the sample. Therefore, for acquiring focal position information of the sample as a whole, it is necessary to perform imaging while moving the field of the microscope optical system, which seems to complicate operations in the device that does not drive the stage. 
         [0005]    For solving the problem mentioned above, it is an object of the present invention to provide an image acquisition device and a method and system for acquiring focus information of a sample, which can acquire focus information of samples rapidly and accurately. 
       Solution to Problem 
       [0006]    For solving the above-mentioned problem, the image acquisition device in accordance with one aspect of the present invention comprises a stage for mounting a sample; a light source for emitting light to the sample; a lightguide optical system including an objective lens arranged so as to oppose the sample on the stage; an image pickup element for capturing an optical image of the sample guided by the lightguide optical system; a focus calculation unit for calculating focus information of the sample according to image data from the image pickup element; a first drive unit for moving a field position of the objective lens with respect to the sample; a second drive unit for changing a focal position of the objective lens with respect to the sample; and a controller for controlling the image pickup element, first drive unit, and second drive unit; the image pickup element is a two-dimensional image pickup element, adapted to perform rolling readout, having a plurality of pixel columns; the controller synchronizes movement of a predetermined part of the sample within a field of the objective lens caused by the first drive unit with the rolling readout of the image pickup element such that each pixel column of the image pickup element is exposed to an optical image of the predetermined part in the sample, while causing the second drive unit to change the focal position of the objective lens with respect to the sample. 
         [0007]    The system for acquiring focus information of a sample in accordance with one aspect of the present invention comprises a stage for holding the sample; a lightguide optical system including an objective lens arranged so as to oppose the sample on the stage; an image pickup element, constituted by a two-dimensional image pickup element, adapted to perform rolling readout, having a plurality of pixel columns, for capturing an optical image of the sample guided by the lightguide optical system; and a focus calculation unit for calculating focus information of the sample according to image data from the image pickup element; the system synchronizes movement of a predetermined part of the sample within a field of the objective lens with the rolling readout of the image pickup element such that each pixel column of the image pickup element is exposed to an optical image of the predetermined part in the sample, while changing a focal position of the objective lens with respect to the sample. 
         [0008]    The above-mentioned image acquisition device and system use as an image pickup element a two-dimensional image pickup element which is adapted to perform rolling readout while having a plurality of pixel columns. The rolling readout scheme, which varies image data readout timings among pixel columns and thus may distort images when used for movable objects, is typically employed for objects which stand still. In contrast, by utilizing a delay in image data readout timings among pixel columns in the rolling readout, the above-mentioned image acquisition device and system synchronize the movement of a predetermined part (the same part) of the sample within the field of the objective lens with the rolling readout such that each pixel column of the image pickup element is exposed to an optical image of the predetermined part in the sample. As a consequence, image data from each pixel column includes contrast information obtained when the focal position of the objective lens is changed in the same part of the sample, whereby the focus information can be calculated rapidly and accurately according to the contrast information. 
         [0009]    The controller may control the first drive unit such that the predetermined part of the sample is moved at a fixed speed within the field of the objective lens. This can easily control the synchronization of the movement of the predetermined part of the sample within the field of the objective lens with the rolling readout. 
         [0010]    The controller may start exposing each pixel column of the image pickup element after a lapse of a predetermined time since the first drive unit starts moving the field position of the objective lens with respect to the sample. This can perform exposure favorably. 
         [0011]    A plurality of divisional regions where the image pickup element performs imaging may be set, while the predetermined part of the sample may be set so as to be located in a region other than end parts of the divisional regions. When set in an end part of divisional regions, the predetermined part of the sample is more susceptible to acceleration at the time of being moved by the first drive unit. Therefore, setting the predetermined part of the sample in a region other than end parts of the divisional regions makes it possible to control the synchronization of the movement of the predetermined part of the sample within the field of the objective lens with the rolling readout more easily. 
         [0012]    The focus calculation unit may calculate the focus information when the first drive unit moves the field position of the objective lens between the divisional regions. This can acquire the focus information during the movement of the field position of the objective lens, whereby imaging of the sample can be executed rapidly. 
         [0013]    The controller may control the first drive unit such that the field position of the objective lens is moved with respect to the sample over at least three divisional regions. Targeting imaging lines over three or more divisional regions can shorten the time required for imaging, while simplifying control. 
         [0014]    The image pickup element may be adapted to switch readout directions of the rolling readout. This can extend the degree of freedom in moving directions of the predetermined part of the sample. 
         [0015]    Each pixel column of the image pickup element may be constituted by first and second column groups having respective readout directions different from each other. Such a configuration enables the field position of the objective lens to be bidirectionally scanned in a simple structure. 
         [0016]    The controller may control the second drive unit such that the focal position of the objective lens with respect to the sample reciprocates in ascending and descending directions during the synchronization of the movement of the predetermined part of the sample with the rolling readout of the image pickup element. This can acquire a greater amount of contrast information at the time when the focal position of the objective lens is changed, whereby the focus information can be calculated more accurately. 
         [0017]    The focus calculation unit may produce a focus map according to the calculated focus information. This can accurately produce the focus map. 
         [0018]    The focus calculation unit may calculate the focus information for each divisional region. This can accurately produce a focus map of the sample as a whole. 
         [0019]    The focusing method for an image acquisition device in accordance with one aspect of the present invention is a focusing method for an image acquisition device comprising a stage for mounting a sample; a light source for emitting light to the sample; a lightguide optical system including an objective lens arranged so as to oppose the sample on the stage; an image pickup element for capturing an optical image of the sample guided by the lightguide optical system; a focus calculation unit for calculating focus information of the sample according to image data from the image pickup element; a first drive unit for moving a field position of the objective lens with respect to the sample; a second drive unit for changing a focal position of the objective lens with respect to the sample; and a controller for controlling the image pickup element, first drive unit, and second drive unit; the method comprising using as the image pickup element a two-dimensional image pickup element, adapted to perform rolling readout, having a plurality of pixel columns; and causing the controller to synchronize movement of a predetermined part of the sample within a field of the objective lens caused by the first drive unit with the rolling readout of the image pickup element such that each pixel column of the image pickup element is exposed to an optical image of the predetermined part in the sample, while making the second drive unit change the focal position of the objective lens with respect to the sample. 
         [0020]    The method for acquiring focus information of a sample in accordance with one aspect of the present invention is a method for acquiring focus information of a sample by using a two-dimensional image pickup element, adapted to perform rolling readout, having a plurality of pixel columns; the method comprising synchronizing movement of a predetermined part of the sample within a field of an objective lens with the rolling readout of the image pickup element such that each pixel column of the image pickup element is exposed to an optical image of the predetermined part in the sample, while changing a focal position of the objective lens with respect to the sample; and acquiring the focus information of the sample according to image data from the image pickup element. 
         [0021]    The above-mentioned focusing method for an image acquisition device and method for acquiring focus information of a sample use as an image pickup element a two-dimensional image pickup element which is adapted to perform rolling readout while having a plurality of pixel columns. The rolling readout scheme, which varies image data readout timings among pixel columns and thus distorts images when used for moving objects, is typically employed for objects which stand still. In contrast, by utilizing a delay in image data readout timings among pixel columns in the rolling readout, the above-mentioned focusing method for an image acquisition device and method for acquiring focus information of a sample synchronize the movement of a predetermined part (the same part) of the sample within the field of the objective lens with the rolling readout such that each pixel column is exposed to an optical image of the predetermined part in the sample. As a consequence, image data from each pixel column includes contrast information obtained when the focal position of the objective lens is changed in the same part of the sample, whereby the focus information can be calculated rapidly and accurately according to the contrast information. 
       Advantageous Effects of Invention 
       [0022]    The present invention can acquire focus information of samples rapidly and accurately. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0023]      FIG. 1  is a diagram illustrating an embodiment of the image acquisition device in accordance with the present invention; 
           [0024]      FIG. 2  is a diagram illustrating an example of an image pickup element, in which (a) and (b) represent a light-receiving surface of the image pickup element and rolling readout in the image pickup element, respectively; 
           [0025]      FIG. 3  is a diagram illustrating an example of scanning an image acquisition region with respect to a sample; 
           [0026]      FIG. 4  is a diagram illustrating how movement of a predetermined part of the sample within a field of an objective lens is synchronized with the rolling readout of an image pickup element  6 , in which (a) represents the positional relationship between the field of the objective lens and divisional regions, while (b) exhibits the predetermined part of the sample with respect to each pixel column and timings at which the image pickup element is exposed and read out; 
           [0027]      FIG. 5  is a diagram illustrating a state subsequent to  FIG. 4 ; 
           [0028]      FIG. 6  is a diagram illustrating a state subsequent to  FIG. 5 ; 
           [0029]      FIG. 7  is a diagram illustrating an example of pixel column configurations in the image pickup element; 
           [0030]      FIG. 8  is a diagram illustrating an example of contrast information processed by an image processing unit; 
           [0031]      FIG. 9  is a flowchart illustrating operations of the image acquisition device represented in  FIG. 1 ; 
           [0032]      FIG. 10  is a diagram illustrating the relationship between divisional regions and the predetermined part of the sample when producing a focus map; and 
           [0033]      FIG. 11  is a flowchart illustrating operations of the image acquisition device when producing the focus map. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0034]    In the following, preferred embodiments of the image acquisition device and method and system for acquiring focus information of a sample in accordance with the present invention will be explained in detail with reference to the drawings. 
         [0035]      FIG. 1  is a diagram illustrating an embodiment of the image acquisition device in accordance with the present invention. As illustrated in the diagram, an image acquisition device  1  comprises a stage for mounting a sample S, a light source  3  for emitting light to the sample, a lightguide optical system  5  including an objective lens  25  arranged so as to oppose the sample S on the stage  2 , and an image pickup element  6  for capturing an optical image of the sample S guided by the lightguide optical system  5 . 
         [0036]    The image acquisition device  1  also comprises a stage drive unit (first drive unit)  11  for moving a field position of the objective lens  25  with respect to the sample S, an objective lens drive unit (second drive unit)  12  for changing the focal position of the objective lens  25  with respect to the sample S, a controller  13  for controlling the image pickup element  6 , stage drive unit  11 , and objective lens drive unit  12 , and an image processing unit  14 . 
         [0037]    The sample S to be observed by the image acquisition device  1 , an example of which is a living sample such as a tissue cell, is mounted on the stage  2  while being sealed with a glass slide. The light source  3  is arranged on the bottom face side of the stage  2 . For example, any of laser diodes (LD), light-emitting diodes (LED), superluminescent diodes (SLD), and light sources of lamp type such as halogen lamps is used as the light source  3 . 
         [0038]    The lightguide optical system  5  is constituted by an illumination optical system  21  arranged between the light source  3  and stage  2  and a microscope optical system  22  arranged between the stage  2  and image pickup element  6 . The illumination optical system  21  has a Köhler illumination optical system constituted by a condensing lens  23  and a projection lens  24 , for example, and guides the light from the light source  3  so as to irradiate the sample S with uniform light. On the other hand, the microscope optical system  22  has an objective lens  25  and an imaging lens  26  arranged on the downstream side (image pickup element  6  side) of the objective lens  25  and guides an optical image of the sample S to the image pickup element  6 . The optical image of the sample S is an image formed by transmitted light in the case of bright field illumination, scattered light in the case of dark field illumination, and emission (fluorescence) in the case of emission measurement. It may also be an image formed by reflected light from the sample S. 
         [0039]    The image pickup element  6  is a two-dimensional image pickup element which is adapted to perform rolling readout while having a plurality of pixel columns. An example of such an image pickup element  6  is a CMOS image sensor. As illustrated in  FIG. 2(   a ), a plurality of pixel columns  31 , each of which is constructed by arranging a plurality of pixels in a direction perpendicular to a readout direction, align in the readout direction on a light-receiving surface  6   a  of the image pickup element  6 . 
         [0040]    In the image pickup element  6 , as illustrated in  FIG. 2(   b ), a reset signal, a readout start signal, and a readout end signal are outputted according to a drive period of a drive clock, whereby exposure and readout are controlled for each pixel column  31 . An exposure period of one pixel column  31  is a duration from discharge of electric charges triggered by the reset signal to readout of the electric charges triggered by the readout start signal. A readout period of one pixel column  31  is a duration from the start of readout of electric charges triggered by the readout start signal to an end of readout of electric charges triggered by the readout end signal. The readout start signal for the next pixel column can also be used as the readout end signal. 
         [0041]    In the rolling readout, readout start signals to be outputted for the respective pixel columns  31  are sequentially outputted with a predetermined time difference. Therefore, unlike global readout in which all the pixel columns are read out at the same time, respective readout operations for the pixel columns  31  are sequentially performed with the predetermined time difference. The readout speed in the rolling readout is controlled by a time interval of the readout start signals for reading the respective pixel columns  31 . The readout speed becomes faster and slower as the time interval of readout start signals is shorter and longer, respectively. The readout interval between the pixel columns  31 ,  31  adjacent to each other can be adjusted by techniques such as adjustment of the frequency of the drive clock, setting of a delay period in the readout period, and change of a clock number specifying the readout start signal, for example. 
         [0042]    The stage drive unit  11  is constituted by a motor or actuator such as a stepping motor (pulse motor) or piezoelectric actuator, for example. Under the control of the controller  13 , the stage drive unit  11  moves the stage  2  in the XY directions about a plane having a predetermined angle (e.g., 90°) with respect to a plane orthogonal to the optical axis of the objective lens  25 . As a consequence, the sample S secured to the stage  2  moves relative to the optical axis of the objective lens, thereby shifting the field position of the objective lens  25  with respect to the sample S. 
         [0043]    The image acquisition device  1  performs imaging of the sample at a high magnification of 20× to 40×, for example. Therefore, the objective lens  25  has a field V which is small with respect to the sample S, whereby a region in which an image can be captured in one imaging operation also becomes small with respect to the sample S as illustrated in  FIG. 3 . This makes it necessary for the field V of the objective lens  25  to be scanned with respect to the sample S in order to capture an image of the sample S as a whole. 
         [0044]    Therefore, the image acquisition device  1  sets an image acquisition region  32  so as to include the sample S with respect to a sample container (e.g., a glass slide) holding the sample S and configures positions of a plurality of divisional regions  33  according to the image acquisition region  32  and the field V on the sample S of the objective lens  25 . Then, an image of a part of the sample S corresponding to the divisional region  33  is captured, so as to acquire partial image data corresponding to the divisional region  33 , and thereafter the stage drive unit  11  is driven such that the field V of the objective lens  25  is located at the next divisional region  33  to be subjected to imaging, where an image is captured again, so as to acquire partial image data. At this time, the controller  13  drives the stage drive unit  11 , so as to accelerate/decelerate the stage  2  when moving the field V on the sample S of the objective lens  25  from the divisional region  33  to the next divisional region  33  and stop the stage  2  at such a position that the field V on the sample S is at the next divisional region  33 . Subsequently, the image acquisition device  1  repeatedly executes this operation, whereupon the image processing unit  14  combines thus obtained partial image data, so as to produce an image of the sample S as a whole. 
         [0045]    For capturing an image of the sample S as a whole, in the image acquisition device  1 , the controller  13  controls the objective lens  25  so as to move its field position with respect to the sample S along imaging lines Ln (where n is a natural number) constituted by a plurality of divisional regions  33 . At this time, the controller  13  moves the stage  2  along the scan direction such that the field V on the sample S of the objective lens  25  is located at the next divisional region  33  to be subjected to imaging. For moving the field position of the objective lens  25  with respect to the sample S between the imaging lines Ln adjacent to each other, bidirectional scanning in which scan directions are reversed between the imaging lines Ln adjacent to each other, for example, is employed as illustrated in  FIG. 3 . Unidirectional scanning in which the same scan direction is used for all the imaging lines Ln may be employed as well. Also employable is random scanning in which the field position of the objective lens  25  moves randomly among the divisional regions  33 . 
         [0046]    As with the stage drive unit  11 , the objective lens drive unit  12  is constituted by a motor or actuator such as a stepping motor (pulse motor) or piezoelectric actuator, for example. Under the control of the controller  13 , the objective lens drive unit  12  moves the objective lens  25  in the Z direction along the optical axis of the objective lens  25 . This shifts the focal position of the objective lens  25  with respect to the sample S. 
         [0047]    The controller  13  is a part which controls respective operations of the image pickup element  6 , stage drive unit  11 , and objective lens drive unit  12 . Specifically, the controller  13  causes the objective lens drive unit  12  to change the focal position (focal plane) of the objective lens  25  with respect to the sample S. At this time, the focal position of the objective lens  25  with respect to the sample S moves along one direction. Specifically, the controller  13  causes the objective lens drive unit  12  to change the position in the Z direction of the objective lens  25  with respect to the stage  2 , thereby varying the distance between the stage  2  and objective lens  25 . 
         [0048]    When the stage drive unit  11  can move the stage  2  along the Z direction aligning with the optical axis of the objective lens  25 , the controller  13  may cause the stage drive unit  11  to change the position in the Z direction of the stage  2  with respect to the objective lens  25 , thereby varying the distance between the stage  2  and objective lens  25 . In this case, the stage drive unit  11  serves as a drive unit for moving the focal position of the objective lens  25  with respect to the sample S, thereby fulfilling functions equivalent to those of this embodiment. 
         [0049]    The controller  13  also synchronizes movement of a predetermined part of the sample within the field V of the objective lens  25  caused by the stage drive unit  11  with the rolling readout of the image pickup element  6  such that each pixel column  31  of the image pickup element  6  is exposed (receives) an optical image of the predetermined part in the sample S. For example, the movement of the stage  2  caused by the stage drive unit  11  is synchronized with the rolling readout of the image pickup element  6 . When the objective lens drive unit  12  can move the lightguide optical system  5  including the objective lens  25  in the XY directions, the controller  13  may synchronize the movement of the predetermined part of the sample within the field V of the objective lens  25  caused by the objective lens drive unit  12  with the rolling readout of the image pickup element  6  such that each pixel column  31  of the image pickup element  6  is exposed to (receives) the optical image of the predetermined part in the sample S. In this case, the objective lens drive unit  12  serves as a drive unit for moving the field position of the objective lens  25  with respect to the sample S, thereby fulfilling functions equivalent to those of this embodiment. 
         [0050]    This embodiment employs a dynamic prefocus scheme in which focus information of the sample S in the next divisional region  33  to be subjected to imaging is acquired immediately before imaging. In this embodiment, the focus information can be acquired while moving the field position of the objective lens  25  to the next divisional region to be subjected to imaging, whereby the sample S can be placed at an in-focus position at the time when the field V of the objective lens  25  is located at the next divisional region  33  to be subjected to imaging. Therefore, the imaging of the sample S can be executed rapidly. 
         [0051]    As illustrated in  FIG. 4(   a ), the controller  13  controls the stage drive unit  11  such that the sample S moves at a fixed speed within the field V of the objective lens  25  when the field V of the objective lens  25  shifts from one divisional region  33   a  to the next divisional region  33   b . As illustrated in  FIG. 4(   b ), the controller  13  also controls the stage drive unit  11  and image pickup element  6  such that the moving direction of a focused image Sb of the optical image of the sample S on the light-receiving surface  6   a  of the image pickup element  6  and the readout direction of each pixel column  31  of the image pickup element  6  coincide with each other. When an image pickup element which can variably set the readout speed for the rolling readout is used, the controller  13  may change the readout speed for the rolling readout according to the moving speed of the sample S within the field V of the objective lens  25 . 
         [0052]    The position of a predetermined part Sa of the sample S used for acquiring focus information can be set according to a time from the start of movement of the field V of the objective lens  25  with respect to the sample S to the start of exposure of the pixel column  31 . When the predetermined part Sa is located at any position in the divisional region  33 , the controller  13  outputs a reset signal for starting exposure to the image pickup element  6  such as to start the exposure of the pixel column  31  after a lapse of a predetermined time since the field V of the objective lens  25  is moved. When the predetermined part Sa is set at a specific position of the divisional region  33 , the controller  13  calculates the time from the start of movement of the field V of the objective lens  25  with respect to the sample S to the start of exposure of the pixel column  31  according to the position of the predetermined part Sa in the divisional region  33  and the moving speed (or acceleration of movement) of the sample S within the field V of the objective lens  25 . Then, the controller  13  starts the exposure of the pixel column  31  after a lapse of the calculated time from the start of movement of the field V. 
         [0053]    For example, there is a case where it is preferable for the predetermined part Sa of the sample S to be set in a region other than the end parts of the divisional regions  33  (regions abutting on boundaries of the divisional regions  33 ). In this case, the predetermined part Sa of the sample S is set so as to keep away from the boundaries of the divisional regions  33 , whereby the start of exposure of the pixel column  31  is controlled according to the position of a region other than the end parts of the divisional region (regions abutting on the boundaries of the divisional regions  33 ) and the moving speed (or acceleration of movement) of the sample S within the field V of the objective lens  25 . 
         [0054]    The exposure time in each pixel column  31  is set according to at least the width of the predetermined part Sa of the sample S in the scan direction and the moving speed of the predetermined part Sa of the sample S within the field V of the objective lens  25 . More preferably, the magnification of the lightguide optical system  5  is also taken into consideration. This enables each pixel column  31  to be exposed to the optical image of the predetermined part Sa of the sample S. 
         [0055]    Here, when the focused image Sb of light from the predetermined part Sa of the sample S on the light-receiving surface  6   a  of the image pickup element  6  reaches the first pixel column  31  at time T 1  as illustrated in  FIG. 4(   b ), the exposure of the first pixel column  31  is started. 
         [0056]    At time T 2 , the position of the sample S within the field V of the objective lens  25  shifts as illustrated in  FIG. 5(   a ). The focal position of the objective lens  25  also changes as compared to that at the time T 1  in such a direction that the gap between the sample S and objective lens  25  becomes narrower, for example. The focal position of the objective lens  25  with respect to the sample S moves along one direction. At this time, as illustrated in  FIG. 5(   b ), the focused image Sb of light from the predetermined part Sa of the sample S reaches the second pixel column  31 , thereby starting exposure of the second pixel column  31 . The readout of the first pixel column  31  is started at the timing when the focused image Sb of light from the predetermined part Sa of the sample S passes through the first pixel column  31 . 
         [0057]    At time T 3 , the position of the sample S within the field V of the objective lens  25  further shifts in the scan direction as illustrated in  FIG. 6(   a ). The focal position of the objective lens  25  also changes as compared to that at the time T 1  in such a direction that the gap between the sample S and objective lens  25  becomes narrower. At this time, as illustrated in  FIG. 6(   b ), the focused image Sb of light from the predetermined part Sa of the sample S reaches the third pixel column  31 , thereby starting exposure of the third pixel column  31 . The readout of the second pixel column  31  is started at the timing when the focused image Sb of light from the predetermined part Sa of the sample S passes through the second pixel column  31 . The readout of the first pixel column  31  ends at the same time when the readout of the second pixel column  31  is started. 
         [0058]    Subsequently, the movement of the predetermined part Sa of the sample S within the field V of the objective lens  25  and the rolling readout at the pixel column  31  are performed in the same procedure until a predetermined number of pixel columns is reached. Respective image data read out from the pixel columns  31  are sequentially outputted to the image processing unit  14 . It is preferable for the image pickup element  6  to be able to switch readout directions of the rolling readout. This makes it easy for the moving direction of the focused image Sb of light from the sample S and the readout direction of each pixel column  31  of the image pickup element  6  to coincide with each other even when the scan direction of the field position of the objective lens  25  with respect to the sample S changes as in bidirectional scanning and random scanning. 
         [0059]    As illustrated in  FIG. 7 , a plurality of pixel columns  31  constructing the light-receiving surface  6   a  of the image pickup element  6  may be separated into first and second pixel column groups  31   a ,  31   b , each constituted by a plurality of pixel columns, the first and second pixel column groups  31   a ,  31   b  being read out separately from each other. In this case, the readout direction of the first pixel column group  31   a  and the readout direction of the second pixel column group  31   b  may be set opposite to each other, and the pixel column group used for acquiring focus information may be selected according to the scan direction. Specifically, the first pixel column group  31   a  is controlled such that pixel groups are sequentially read out from those at an end to those at the center, and the second pixel column group  31   b  is also controlled such that pixel groups are sequentially read out from those at an end to those at the center. Of course, the first pixel column group  31   a  may be controlled such that pixel groups are sequentially read out from those at the center to those at an end, and the first pixel column group  31   a  may also be controlled such that pixel groups are sequentially read out from those at the center to those at an end. This can adapt to bidirectional scanning. The first pixel column group  31   a  may be controlled such that pixel groups are sequentially read out from those at an end to those at the center, while the second pixel column group  31   b  may be controlled such that pixel groups are sequentially read out from those at the center to those at an end. In this case, the first and second pixel groups  31   a ,  31   b  are exposed to (receive) optical images of different sample positions, whereby focus information can be obtained at two positions at the same time. In bidirectional scanning, it is preferable for the first and second pixel groups  31   a ,  31   b  to have readout directions opposite to each other. 
         [0060]    The image processing unit  14  is a part which combines partial image data captured by the image pickup element  6 , so as to generate an observation image of the sample S. The image processing unit  14  sequentially receives the respective partial image data of the divisional regions  33  outputted from the image pickup element  6  and combines them, so as to generate an observation image of the sample S as a whole. 
         [0061]    The image processing unit  14  also functions as a focus calculation unit which calculates focus information of the sample S according to image data from the image pickup element  6 . Specifically, the image processing unit  14  calculates the focus information of the sample S according to the respective image data from the pixel columns  31  of the image pickup element  6 . An example of the focus information is such positional information in the Z direction of the objective lens  25  or stage  2  that the sample S is located at the focal position of the objective lens  25 . Examples of such information include the position in the Z direction of the objective lens  25 , the height (distance) of the objective lens  25  with respect to the sample S (stage  2 ), the position in the Z direction of the stage  2 , and the height (distance) of the sample S (stage  2 ) with respect to the objective lens  25 . 
         [0062]    As mentioned above, the controller  13  synchronizes the movement of the predetermined part Sa of the sample S within the field V of the objective lens  25  caused by the stage drive unit  11  with the rolling readout of the image pickup element  6  such that each pixel column  31  of the image pickup element  6  is exposed to an optical image of the predetermined part Sa in the sample S, while causing the objective lens drive unit  12  to change the focal position of the objective lens  25 . Therefore, in order for each pixel column  31  to be exposed to the optical image of the predetermined part Sa of the sample S, the image data from the image pickup element  6  obtained when the focal position is acquired includes contrast information at the time when the focal position of the objective lens  25  is changed at the predetermined part Sa (the same part) of the sample S. 
         [0063]      FIG. 8  is a diagram illustrating an example of contrast information processed by the image processing unit. The example illustrated in the diagram represents contrast values of image data from the first pixel column  31  to the n th  pixel column  31  in the imaging region, in which the contrast value of the image data in the i th  pixel column  31  is a peak value. In this case, assuming that the focal position of the objective lens  25  is an in-focus position when exposing the i th  pixel column to the predetermined part Sa of the sample S, the image processing unit  14  generates focus information. As the contrast value, the contrast value in a specific pixel in the pixels included in each pixel column  31  or an average value of contrast values in part or whole of the pixels included in each pixel column  31  may be used. 
         [0064]    Operations of the above-mentioned image acquisition device  1  will now be explained.  FIG. 9  is a flowchart illustrating the operations of the image acquisition device. 
         [0065]    In the image acquisition device  1  employing the dynamic prefocus scheme, an image of the sample S is captured in one divisional region  33  (step S 01 ), whereupon the field V of the objective lens  25  moves toward the next divisional region  33  (step S 02 ). Next, while the focal position of the objective lens  25  is changed during the movement of the field V of the objective lens  25 , the movement of the predetermined part Sa of the sample S within the field V of the objective lens  25  is synchronized with the rolling readout of the image pickup element  6  such that each pixel column  31  of the image pickup element  6  is exposed to an optical image of the predetermined part Sa in the sample S after a lapse of a predetermined time (step S 03 ). 
         [0066]    After completing the readout of image data at a predetermined pixel column  31 , focus information is calculated according to respective contrast values of image data in the pixel columns (step S 04 ). Then, the focal position of the objective lens  25  with respect to the sample S is adjusted according to the calculated focus information, and an image of the sample S is captured in the next divisional region  33  after the field V of the objective lens  25  is located there (step S 05 ). The processing of steps S 01  to S 05  is repeated until the imaging of the sample S is completed in all the divisional regions  33 , whereupon the respective images of the divisional regions  33  are combined, so as to generate an observation image of the sample S as a whole. 
         [0067]    As explained in the foregoing, the image acquisition device  1  uses as the image pickup element  6  a two-dimensional image pickup element which is adapted to perform rolling readout while having a plurality of pixel columns  31 . The rolling readout scheme, which varies image data readout timings among the pixel columns  31  and thus may distort images when used for movable objects, is typically employed for objects which stand still. In contrast, by utilizing a delay in image data readout timings among the pixel columns  31  in the rolling readout, the image acquisition device  1  synchronizes the movement of the predetermined part Sa (the same part) of the sample S within the field V of the objective lens  25  with the rolling readout such that each pixel column  31  of the image pickup element  6  is exposed to an optical image of the predetermined part Sa in the sample S, while changing the focal position of the objective lens  25 . As a consequence, image data from each pixel column  31  includes contrast information obtained when the focal position of the objective lens  25  is changed in the same part of the sample S, whereby the focus information can be calculated rapidly and accurately according to the contrast information. 
         [0068]    The image acquisition device  1  also controls the stage drive unit  11  such that the predetermined part Sa of the sample S is moved at a fixed speed within the field V of the objective lens  25 . This can easily control the synchronization of the movement of the predetermined part Sa of the sample S within the field V of the objective lens  25  with the rolling readout. 
         [0069]    The predetermined part Sa of the sample S is set so as to be located in a region other than end parts of the divisional regions  33  in the image acquisition region  32  of the image pickup element  6 . When set in an end part of the divisional regions  32 , the predetermined part Sa of the sample S is more susceptible to acceleration at the time of being moved by the stage drive unit  11 . Therefore, setting the predetermined part Sa of the sample S in a region other than end parts of the divisional regions  33  makes it possible to control the synchronization of the movement of the predetermined part Sa of the sample S within the field V of the objective lens  25  with the rolling readout more easily. 
         [0070]    The present invention is not limited to the above-mentioned embodiment. For example, while the above-mentioned embodiment assumes to control the objective lens drive unit  12  so as to move the focal position of the objective lens  25  in one of ascending and descending directions during the synchronization of the predetermined part Sa of the sample S with the rolling readout of the image pickup element  6 , the objective lens drive unit  12  may be controlled so as to reciprocate the focal position of the objective lens in ascending and descending directions. In this case, the distance (gap) in the Z direction between the objective lens  25  and stage  2  is controlled so as to expand and contract repeatedly. 
         [0071]    When the sample S is a tissue cell, its thickness is about 10 μm, for example. Therefore, when the moving distance of the focal position of the objective lens  25  for each pixel column  31  is set to about 0.1 μm, contrast information can be acquired for the total thickness of the sample S by about 100 pixel columns. In contrast, a two-dimensional image pickup element such as a CMOS image sensor has about several thousands of pixel columns, for example, whereby contrast information can be acquired a plurality of times during one frame. Consequently, by reciprocating the objective lens  25  in the height direction, focus information can be calculated for a plurality of predetermined regions of the sample S, which makes it possible to calculate focus information more accurately. 
         [0072]    Though the above-mentioned embodiment uses the dynamic prefocus scheme, a focus map scheme can also be employed. While the dynamic prefocus scheme acquires focus information between imaging of one divisional region  33  and imaging of the next divisional region  33 , the focus map scheme acquires focus information in each divisional region  33  for the image acquisition region  32  or imaging line Ln before capturing an image of the sample S. 
         [0073]    In this case, the controller  13  controls the stage drive unit  11  such that the field V of the objective lens  25  moves at a fixed speed over a plurality of divisional regions  33 . At this time, the moving direction of the focused image Sb of light from a predetermined position of the sample S and the readout direction of the image pickup element  6  are made to coincide with each other on the light-receiving surface  6   a  of the image pickup element  6 . At the timing when the readout of one frame ends, the readout of the next frame is started, which enables the predetermined part Sa of the sample S used for calculating the focus information to appear at fixed intervals, whereby at least one piece of focus information can be acquired in each divisional region  33 . A focus map of the imaging line Ln or the sample S as a whole can be produced accurately by applying the method of least squares or the like to the focus information in each divisional region  33 . 
         [0074]      FIG. 11  is a flowchart illustrating operations of the image acquisition device when producing a focus map. In the image acquisition device  1  employing the focus map scheme, as illustrated the chart, the stage drive unit  11  starts moving the stage  2 , whereupon the field V of the objective lens  25  moves over a plurality of divisional regions  33  (step S 11 ). It also synchronizes the movement of the predetermined part Sa of the sample S with the rolling readout of the image pickup element  6  such that each pixel column  31  of the image pickup element  6  is exposed to the focused image Sb of light of the predetermined part Sa in the sample S, while changing the focal position of the objective lens  25  so as to reciprocate it along the Z direction (step S 12 ), and calculates focus information in each divisional region  33  (step S 13 ). 
         [0075]    Thereafter, it is determined whether or not the calculation of focus information is completely acquired for a desirable imaging line Ln (step S 14 ); when the calculation of focus information is not completely acquired, the field V of the objective lens  25  is moved to the next imaging line Ln (step S 15 ), and the processing of steps S 01  to S 03  is repeatedly executed. When the calculation of focus information is completely acquired, a focus map of the sample S is produced according to the focus information (step S 16 ). Then, the image data of each divisional region  33  is acquired while locating the focal position of the objective lens  25  at the sample S according to the produced focus map. 
         [0076]    After focus information for one imaging line Ln is acquired, a focus map for this imaging line Ln may be produced, and the respective image data of the divisional regions  33  constituting the imaging line Ln may be acquired while locating the focal position of the objective lens  25  at the sample S according to the produced focus map. The focus map may be constituted by the focus information in each divisional region  33  itself instead of being produced by applying the method of least squares or the like to the focus information in each divisional region  33 . 
       REFERENCE SIGNS LIST 
       [0077]      1 : image acquisition device;  2 : stage;  3 : light source;  5 : lightguide optical system;  6 : image pickup element;  11 : stage drive unit (first drive unit);  12 : objective lens drive unit (second drive unit);  13 : controller;  14 : image processing unit (focus calculation unit);  25 : objective lens;  31 : pixel column;  32 : image acquisition region;  33 : divisional region; S: sample; Sa: predetermined part of the sample; V: objective lens field.