BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an imaging device for imaging an object using two-dimensional imaging elements. Furthermore, the present invention relates to a particle image capturing apparatus for imaging particles in an optical cell moving at high speed together with the medium.  
         [0003]     2. Description of the Related Art  
         [0004]     Conventional an imaging devices are known, such as digital cameras and the like, which capture the image of an object using an area sensor (two-dimensional imaging element such as a CCD or the like) to produce image data. U.S. Pat. No. 5,721,433 discloses a particle image analyzer capable of analyzing particle images obtained by sequentially imaging particles of a particle suspension fluid flowing within an optical gel and moving through an imaging region, and displaying a calculated distribution map of the shape parameters such as the degree of roundness and the like so as to analyze the shape and the like of micro particles.  
         [0005]     When interlace-type CCD area sensors for sequentially scanning odd pixels (ODD field) and even pixels (EVEN field) are used as the imaging elements of the particle image analyzer of U.S. Pat. No. 5,721,433, a striped pattern is introduced into the image when the area sensor is optically exposed during the EVEN field period. In general, the ODD field period of the area sensor is {fraction (1/60)} of a second, and the EVEN field period is {fraction (1/60)} of a second. Accordingly, when the particle image analyzer uses interlace-type CCD area sensors, it is difficult to image particles moving through the imaging region during the imaging intervals since the imaging interval must be approximately {fraction (1/30)} of a second.  
       SUMMARY  
       [0006]     The object of one embodiment of the present invention is to provide an imaging device which improves the probability of imaging each object even when imaging a plurality of objects moving at high speed.  
         [0007]     The first aspect of the present invention relates to an imaging device comprising: a first two-dimensional image sensing elements; a second two-dimensional image sensing element; an optical system for forming identical optical images on the first and second image sensing elements; a first shutter means for controlled exposure of the first image sensing element from the optical system; a second shutter means for controlled exposure of the second image sensing element from the optical system; and a control means for driving the first image sensing element based on field signals sequentially repeating ODD field period and EVEN field period, driving the second image sensing element based on field signals having a different phase than the first image sensing element, and controlling the operation of the first shutter means and second shutter means so as to expose with light from the optical system an image sensing element having the ODD field period among the first and second image sensing elements.  
         [0008]     The second aspect of the present invention relates to an imaging device comprising: a plurality of two-dimensional image sensing elements; an optical system for forming optical images on the respective image sensing elements; and a drive control means for driving the plurality of image sensing elements with respectively different timings, and controlling the operation of electronic shutters of the respective image sensing elements so as to expose one image sensing element among the plurality of image sensing elements.  
         [0009]     The third aspect of the present invention relates to a particle image capturing apparatus for imaging particles comprising: a flow cell for forming a flow of a particle suspension; a light source for irradiating the particle suspension flow with light; a first two-dimensional image sensing elements driven based on field signals sequentially repeating the ODD field period and EVEN field period; a second two-dimensional image sensing element driven based on field signals having a phase different than that of the first two-dimensional image sensing element; an optical system for forming identical optical images of the particle suspension flow on the first and second two-dimensional image sensing elements; a first shutter means for exposing the first two-dimensional image sensing element with light from the optical system when the first two-dimensional image sensing element has an ODD filed period; and a second shutter means for exposing the second two-dimensional image sensing element with light from the optical system when the second two-dimensional image sensing element has an ODD filed period.  
         [0010]     The fourth aspect of the present invention relates to a particle image capturing apparatus for imaging particles comprising: a flow cell for forming the flow of a particle suspension fluid; a light source for irradiating the particle suspension fluid; a first two-dimensional image sensing element; a second two-dimensional image sensing element; an optical system for forming identical optical images of particles in the particle suspension flow on the first and second particle image sensing elements; a first shutter means for controlled exposure of the first image sensing element from the optical system; a second shutter means for controlled exposure of the second image sensing element from the optical system; and a drive control means for driving the first image sensing element based on field signals sequentially repeating ODD field period and EVEN field period, driving the second image sensing element based on field signals having a different phase than the first image sensing element, and controlling the operation of the first shutter means and second shutter means so as to expose with light from the optical system an image sensing element having the ODD field period among the first and second image sensing elements.  
         [0011]     The fifth aspect of the present invention relates to a particle image capturing apparatus comprising: a first two-dimensional image sensing element; a second two-dimensional image sensing element; an optical system for forming identical optical images of the particle on the first and second image sensing elements; and a drive control means for driving the first and second image sensing elements with different timings, and controlling the operation of electronic shutters of the respective image sensing elements so as to expose one or another of the first or second image sensing elements. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  shows the structure of an embodiment of an imaging device;  
         [0013]      FIG. 2  shows the field signals of a first CCD and a second CCD;  
         [0014]      FIG. 3  shows the structure of a particle image capturing apparatus using an embodiment of the imaging device;  
         [0015]      FIG. 4  is a control block diagram of the particle image capturing apparatus;  
         [0016]      FIG. 5  shows the particle image analyzer using the particle image capturing apparatus;  
         [0017]      FIG. 6  is a control block diagram of the particle image analyzer;  
         [0018]      FIG. 7  shows the flow of the analysis controls of the particle image analyzer;  
         [0019]      FIG. 8  illustrates the calculation of the surface area S and the circumference length L of the particle image;  
         [0020]      FIG. 9  shows a second embodiment of the measuring unit of the particle image capturing apparatus;  
         [0021]      FIG. 10  shows the optical cell of the measuring unit;  
         [0022]      FIG. 11  shows the structure light source unit for zonal light exposure; and  
         [0023]      FIG. 12  shows the A-A cross sectional view of  FIG. 11 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]     The structure of an embodiment of the imaging device  1  is shown in  FIG. 1 . The imaging device  1  is provided with an optical system  2 , first CCD  6 , second CCD  7 , a first CCD drive circuit  8  for driving the first CCD, a second CCD drive circuit  9  for driving the second CCD  7 , and a standard crystal oscillator  10 . The optical system  2  is provided with an objective lens  3  for collecting the image of an object, and a half-mirror  4  for dividing the light from the objective lens  3 . The first CCD  6  and the second CCD  7  are both interlace-type CCD area sensors provided with an electronic shutter. The exposure timing of the electronic shutter of the first CCD  6  is controlled by the first CCD drive circuit  8 . The exposure timing of the electronic shutter of the second CCD  7  is controlled by the second CCD drive circuit  9 .  
         [0025]     The first CCD drive circuit  8  for driving the first CCD  6  is provided with a synchronizing signal generator  8 a for generating synchronizing signals such as a vertical synchronizing signal VD and horizontal synchronizing signal HD based on the signal of the standard crystal generator  10 , timing generator  8   b  for receiving the input of the vertical synchronizing signal VD and horizontal synchronizing signal HD output from the synchronizing signal generator  8 a and generating various types of timing signals used for the first CCD  6 , and a driver  8   c  for receiving the timing signals output from the timing generator  8   b  and driving the first CCD  6  by providing a vertical transmission pulse, horizontal transmission pulse, and shutter pulses (electronic shutter starting pulse for the first CCD  6 ) for discharging accumulated signal loads and starting a new exposure. The signal processing system of the output signals of the first CCD  6  are omitted from the drawing. The second CCD drive circuit  9  for driving the second CCD  7  is similarly provided with a synchronizing signal generator  9   a  for generating synchronizing signals such as a vertical synchronizing signal VD and horizontal synchronizing signal HD, timing generator  9   b,  and driver  9   c.    
         [0026]     The first CCD  6  is driven based on the field signal which has the reverse phase of the second CCD  7 . In  FIG. 2 , FLD 1  is the field signal for driving the first CCD  6 , and FLD 2  is the field signal for driving the second CCD  7 . FLD 1  and FLD 2  are field signals which alternatingly repeat the ODD field and EVEN field. The field signal FLD  1  and the field signal FLD 2  have the same frequency, and their phase difference is π. The field signal FLD 1  has a phase which is the reverse that of the field signal FLD 2 . Therefore, when the field signal of the first CCD  6  is EVEN, the field signal of the second CCD  7  is ODD, and conversely, when the field signal of the first CCD  6  is ODD, the field signal of the second CCD  7  is EVEN. Either the first CCD  6  or the second CCD  7  are normally in a state capable of image sensing since they are driven by the field signals FLD 1  and FLD 2 .  
         [0027]     Although the first CCD  6  and the second CCD  7  are driven based on field signals having reverse phases in the present embodiment, they also may be driven based on field signals having different phases.  
         [0028]     The structure of an embodiment of the particle image capturing apparatus  11  using the imaging device  1  is shown in  FIG. 3 . The particle image capturing apparatus  11  is provided with a measuring unit  12  provided with the imaging device  1 , input unit  23 , display unit  24 , and control unit  25 . The measuring unit  12  is provided with a particle suspension fluid  13  accommodated in a particle suspension bottle, suction pipette  14 , sample filter  15 , sample charging line  16 , sheath syringe  17 , flow cell  18 , sheath fluid bottle  19 , sheath fluid chamber  20 , waste fluid chamber  21 , light source (strobe)  22 , and imaging device  1 . The input unit  23  is an input device for performing various types of input operations and command operations, and is a keyboard, mouse and the like. The display unit  24  is a display unit such as a CRT display or the like. A touch panel type display may be used as the input device  23  and the display device  24 . Furthermore, since the structure of the imaging device  1  is shown in  FIG. 1 , the internal structure of the imaging device  1  is omitted from the drawing.  
         [0029]      FIG. 4  is a control block diagram of the particle image capturing apparatus  11 . The control unit  25  is provided with a central processing unit (CPU)  26 , memory  27 , measuring unit drive control circuit  28 , and signal processing circuit  29 . The memory  27  is provided with RAM, ROM, hard disk and the like. The memory  27  accommodates drive control programs for the measuring unit, signal processing programs for processing signals (particle image data) from the first CCD  6  and second CCD  7  and the like. The CPU  26  executes the drive control of the measuring unit through the measuring unit drive control circuit  28  based on the drive control programs accommodated in the memory  27 . The CPU  26  processes signals from the first CCD  6  and the second CCD  7  through the signal processing circuit  29  based on the signal processing program accommodated in the memory  27 . The particle image data from the first CCD  6  and the second CCD  7  are converted to digital data by an A/D converter of the signal processing circuit  29 , and thereafter stored in the memory  27 .  
         [0030]     The imaging of the particle image in the measuring unit  12  of  FIG. 3  is performed as described below. First, the particle suspension fluid  13  accommodated in the particle suspension bottle is suctioned by the suction pipette  14 , passed through the sample filter  15 , and delivered to the sample charging line  16  at the top part of the flow cell  18 . Coarse particles and debris in the suspension fluid are removed by the sample filter  15  so as to not clog the narrow flow cell  18  of the flow path. Furthermore, the sample filter  15  is also effective in unbinding coarse clumps. When the assayed particles are semitransparent, an appropriate staining of the particles may be performed.  
         [0031]     The suspension fluid  13  delivered to the charging line  16  is introduced to the flow cell  18  by the operation of the sheath syringe  17 , and the particle suspension fluid  13  is extracted a little at a time from the tip of the sample nozzle  18   a.  At the same time, the sheath fluid is also delivered to the flow cell  18  from the sheath fluid bottle  19  through the sheath fluid chamber  20 . As a result, the particle suspension fluid  13  is encapsulated in the sheath fluid, and the suspension fluid is constricted as it flows within the flow cell  18  via flow dynamics, and is discharged to the waste chamber  21 .  
         [0032]     The suspension flow in the flow cell  18  is periodically irradiated each {fraction (1/60)} second by a pulse of light from the light source (strobe)  22 . In this way a still image of a particle is introduced each {fraction (1/60)} second to the optical system  2  of the solid-state imaging device  1 . The still image is input to the first CCD  6  and second CCD  7  through the optical system  2 . The first CCD  6  is driven by a field signal having the reverse phase of the second CCD  7  as described previously. Therefore, the image of the particle input by the optical system  2  is sensed by the CCD which has the ODD field signal among the first CCD  6  and the second CCD  7 .  
         [0033]     Although a first CCD drive circuit  8  and a second CCD drive circuit  9  are used as exposure control means in the above embodiment, the exposure timing of the first CCD  6  and second CCD  7  also may be controlled by the control unit  25 . Furthermore, although an electronic shutter is used as a shutter means for controlling the exposure of the first CCD  6  and second CCD  7 , a mechanical shutter also may be used.  
         [0034]     Although the first CCD  6  and second CCD  7  are driven based on field signals having reverse phases, they also may be driven based on field signals having different phases.  
         [0035]     The structure of a particle image analyzer  30  provided with the particle image capturing apparatus  11  is shown in  FIG. 5 . The particle image analyzer  30  is provided with the particle image capturing apparatus  11 , image processing device (personal computer)  31 , operation input unit  32  for inputting various types of operations and the like, and display unit  33 . The operation input unit  32  is a keyboard (or mouse), and the display unit  33  is a display.  
         [0036]      FIG. 6  is a block diagram of the image processing system in the particle image analyzer; the particle image data from the particle image capturing apparatus  11  is processed in the image processing device (personal computer)  31 , and displayed on the display  33  (display unit) functioning as a display device. The image processing device  31  is provided with a CPU  34 , memory unit  35 , and signal processing circuit  36 . The memory unit  35  is provided with a RAM, ROM, hard disk and the like, and stores analysis programs for executing the image processes described below.  
         [0037]     The image processing sequence of particle image data of each {fraction (1/60)} second is shown in  FIG. 7 . The image processing device  31  executes the processes of steps S 1  through S 12  shown in  FIG. 7 .  
         [0038]     The particle image signals from the first CCD  6  and second CCD  7  are subjected to A/D conversion by the signal processing circuit  36  of the image processing device  31 , to obtain particle image data (step S 1 ). First, the obtained image data are subjected to background correction to correct unevenness in the intensity of light (shading) irradiating the suspension fluid flow (step S 2 ).  
         [0039]     Specifically, image data obtained by light exposure when particles are not moving through the flow cell  18  are collected prior to the measuring, and these image data and the image data of the actual particle image screen are compared. Then, a contour enhancement process is executed to accurately extract the contour of the particle image (step S 3 ). Specifically, the generally well-known Laplacean enhancement process is executed.  
         [0040]     Next, the image data are binarized at an appropriate threshold level (step S 4 ). Then, a determination is made as to whether or not the binarized particle image has an edge point, and information on a possible edge point adjacent to the observed edge point. That is, a chain code, is generated (step S 5 ). Thereafter, the particle image is subjected to edge tracing while referring to the chain code, and the total number of pixels, total number of edges, and number of inclined edges of each particle image are determined (step S 6 ).  
         [0041]     If an image processing device capable of high-performance pipeline processing is used, the aforesaid image processing of a screen imaged every {fraction (1/60)} second can be accomplished in real time. Furthermore, the particle image can be extracted from the imaged frame, and the extracted particle image can be stored in the image memory of the memory unit  35  of the image processing device  31  (step S 7 ).  
         [0042]     When the imaging ends (step S 8 ), particle characteristics parameters such as circular equivalent diameter (granularity) and roundness and the like are calculated as described below (step S 9 ). First, the projection surface area S and circumferential length L of each particle image are determined from the total number of pixels, total number of edges, and number of inclined edges of each particle image using the equations below.  
         [0043]     As shown in  FIG. 8 , the surface area S within the frame and the length of the frame (period length L) which can be connected to the center of the edges of the circumferences of binary images can be expressed by equations (1) and (2) below when the surface area per unit pixel is “1”. 
 
Surface area S=total number of pixels−(total edges×0.5)−1   (1) 
 
Circumferential length L=(total number of edges−number of inclined edges)+(number of inclined edges×2 1/2 )   (2) 
 
         [0044]     Then, the circular equivalent diameter is determined using the surface area S and circumferential length L. The circular equivalent diameter is the diameter of a circle having the same surface area as the projection image of the particle, and is expressed by equation (3). The roundness is a value defined by equation (4); the roundness is “1” when the particle image is circular, and the roundness value becomes smaller the larger the irregularities of the exterior edge of the particle image. 
 
Circular equivalent diameter=(particle projection image area/π)½×2   (3) 
 
Roundness=(circumferential length of a circle having a projection surface area value identical to the particle image)/(circumferential length of the particle image)   (4) 
 
         [0045]     When the circular equivalent diameter (granularity) and roundness of each particle image is calculated in this way, then a required scattergram and histogram are created based on commands from the keyboard  32  and displayed on the display  33  (step S 10 ).  
         [0046]     When analysis items and analysis regions are specified from the keyboard  32 , these items and regions of the displayed scattergram and histogram are analyzed, that is, various analysis data, such as average value, standard deviation, variable coefficient, median value, mode value, 10% cumulative value, 50% cumulative value, 90% cumulative value and the like are calculated and the calculation results are displayed (steps S 11 , S 12 ).  
         [0047]      FIG. 9  shows the structure of a second embodiment of the particle image capturing apparatus.  FIG. 10  shows details of the optical cell and the particle suspension fluid discharge nozzle of  FIG. 9 . The first CCD drive circuit for driving the first CCD  6  and the second CCD drive circuit fro driving the second CCD  7  are omitted from  FIG. 9  since they are identical to the first CCD drive circuit  8  and second CCD drive circuit  9  of  FIG. 1 .  
         [0048]     The measuring unit  40  is provided with a first light source unit  41  having a red semiconductor laser light source with a wavelength of 660 nm, conical exterior surface reflective mirror  42 , conical interior surface reflective mirror  43 , ring mirror  44 , conical interior surface reflective mirror  45 , optical cell  46 , objective lens  49 , dichroic mirror  50 , lens  51 , mirror  52 , pinhole plate  53 , collimator lens  54 , bandpass filter  55 , photosensor element (photomultiplier tube)  56 , imaging control unit  57 , second light source unit  58  having a pulse semiconductor light source with a wavelength of 870 nm, half-mirror  59 , focusing lens  60 , half-mirror  61 , mirror  62 , first CCD  63 , and second CCD  64 .  
         [0049]     First, when a laser beam of 600 nm wavelength is emitted from the first light source unit  41 , the laser light is converted to zonal light by the conical exterior surface reflective mirror  42  and the conical interior surface reflective mirror  43 . The zonal light is guided to the conical interior surface reflective mirror  45  by the ring mirror  44 , and converges at the detection region  48  of  FIG. 10 . In  FIG. 10 , when the particle in the suspension fluid discharged from the nozzle  47  in the optical cell  46  reaches the detection region  48 , the particle is excessively irradiated by the 600 nm zonal light. The scattered light (600 nm) from the excessively irradiated particle is reflected by the dichroic mirror  50  through the objective lens  49 , and enters the photosensor element (photomultiplier tube)  56  through the lens  5   1 , mirror  52 , pinhole plate  53 , collimator lens  54 , and bandpass filter  55 . In this way the photosensor element  56  measures the intensity of the scattered light from the detection region  48 . When the scattered light from the detection region  48  is detected by the photosensor element  56 , the imaging control unit  57  determines the imaging object particle when the scattered light intensity is in a predetermined range, and the pulse semiconductor laser light source (wavelength: 870 nm) of the second light source unit  58  generates a pulse. The pulse semiconductor laser light having a wavelength of 870 nm is reflected by the half-mirror  59 . The light reflected by the half-mirror  59  passes through the dichroic mirror  50 , and converges at the detection region  48  via the objective lens  49 . The dichroic mirror  50  transmits light having a wavelength of 870 nm, and reflects light having a wavelength of 600 nm.  
         [0050]     The scattered light from the irradiated particle enters the first CCD  63  through the objective lens  49 , dichroic mirror  50 , half-mirror  59 , objective lens  60 , and half-mirror  61 . The light reflected by the half-mirror  61  enters the second CCD  64  through the mirror  62 . This assay unit  40  is capable of high efficiency imaging of particles since it detects and images particles moving in the imaging region. Although the detection region  48  shown in  FIG. 10  is set so as to closely match the imaging region, the imaging region also may be set to the left side of the detection region  48  in  FIG. 10  (downstream in the medium discharge direction from the nozzle  47 ).  
         [0051]     Furthermore, a zonal irradiating light source unit having the structure shown in  FIGS. 11 and 12  may be used as the second light source unit  58 .  FIG. 11  is a cross sectional view of the structure of a zonal irradiation light source unit, and  FIG. 12  is an A-A cross sectional view of  FIG. 11 .  
         [0052]     In  FIGS. 11 and 12 , a multimode optical fiber  72  is inserted into a through-hole provided on the same axis as the center axis of a cylindrical body  71 . The multimode optical fiber  72  has a core  73  and clad  74 . The body  71  is provided with six through-holes parallel to the through-hole disposed on the same axis as the center axis of the body  71  on the circular circumference centered on the center axis of the body  71 , and provided at the end of these respective through-holes are laser light sources  76   a,    76   b,    76   c,    76   d,    76   e,    76   f,  and collimator lenses  77   a,    77   b,    77   c,    77   d,    77   e,  and  77   f  (refer to  FIG. 12 ). inside these through-holes are provided light source drive circuit boards  75   a,    75   b,    75   c ,  75   d,    75   e,  and  75   f  (boards  75   b,    75   c,    75   d,    75   e,  and  75   f  are not shown).  
         [0053]     The light-emitting sides of the through-holes provided on the same axis as the center axis of the body  71  are provided with three collimator lenses  79   a,    79   b,  and  79   c.  A concave mirror  78  is provided at the left endface of the body  71  shown in  FIG. 11 . The optical axis of the multimode optical fiber  72  matches the optical axis of the concave mirror  78 , that is, the light receiving opening is arranged at the focus point of the concave mirror  78 .  
         [0054]     A multimode optical fiber having a core diameter of 800 □m is used as the multimode optical fiber  72 . Furthermore, Pulse semiconductor lasers are used as the laser light sources  76   a  through  76   f.    
         [0055]     In the aforesaid structure, the plurality of light fluxes emitted from the laser light sources  76   a  through  76   f  are converted parallel light which is parallel to the optical axis of the mirror  78  by the collimator lenses  77   a  through  77   f.  The parallel light is condensed by the concave mirror  78  and enters the light receiving end of the multimode optical fiber  72  from different directions at predetermined identical entrance angles. Since the length of the optical paths are mutually identical from the laser light sources  76   a  through  76   f  to the multimode optical fiber  72 , all of the light flux enters the light receiving opening having the same spot diameter.  
         [0056]     The multimode optical fiber  72  mixes the plurality of entering light fluxes and reduces the coherence and smoothes the light intensity distribution and emits the radiant zonal light fluxes from the emission opening to the three collimator lenses  79   a,    79   b,  and  79   c.  The collimator lenses  79   a,    79   b,  and  79   c  convert the radiant zonal light fluxes from the optical fiber  72  to parallel light flux having a single optical axis.  
         [0057]     From the perspective of good zonal light formation, the plurality of laser light sources are arranged on the circumference centered on the optical axis of the multimode optical fiber  72  such that the spacing of the adjacent laser light sources are equidistant. The number of zonal light forming light sources, that is, the laser light sources emitting light of the zonal light wavelength, is desirably four to eight, and preferably 5 to eight.  
         [0058]     According to this structure, coherence can be reduced and zonal light effectiveness improved by the multimode optical fiber using a plurality of laser light sources which emit light flux of a predetermined wavelength. That is, when a particle imaged by zonal light is irradiated, optical resolution is improved since only the light flux entering at an angle to the particle is used. Furthermore, the detection signal to noise ratio is improved by using laser light to reduce coherence.
The present invention relates to an imaging device comprising a plurality of two-dimensional image sensing elements, optical system for forming optical images on the respective image sensing elements and drive control means for driving the plurality of image sensing elements with respectively different timings, and controlling the operation of shutters of the respective image sensing elements so as to expose one image sensing element among the plurality of image sensing elements.