Abstract:
A flow cytometer comprises a flow cell configured to induce a flow of a sample containing object particles, a light source, an irradiating optical system configured to irradiate light from the light source on the flow of particles in the flow cell, a detecting part configured to detect light given off from the flow of particles which are irradiated by light. The irradiating optical system comprises a collective lens having an optical axis symmetric aspherical surface on one surface, and a cylindrical surface on the other surface.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from prior Japanese Patent Application No. 2014-149021, filed on Jul. 22, 2014, entitled “FLOW CYTOMETER, PARTICLE ANALYZER, AND FLOW CYTOMETRIC METHOD”. 
     FIELD OF THE INVENTION 
     The present invention relates to a flow cytometer, particle analyzer, and flow cytometric method. 
     BACKGROUND 
     Flow cytometers are used to analyze particles contained in a sample. In a flow cytometer, a sample containing particles is caused to flow through a flow cell where the particles in the sample flowing through the flow cell are irradiated by light from a light source. 
     In Japanese Laid-Open Patent Application No. 2008-32659, a collimator lens is used to convert the light from the light source into parallel light rays, a cylindrical lens is used to condense the light in a direction perpendicular to the flow in the flow cell, and a condenser lens is used to condense the light on the flow cell. 
     In Japanese Laid-Open Patent Application No. 2008-32659, a spherical lens is used as the condenser lens. However, large amount of distortion occurs when the rays are condensed by a single spherical lens, such that a condenser lens having a plurality of glued spherical lenses is usually used in order to reduce the distortion. A cylindrical lens also is usually arranged with spacing as a condenser lens. The thickness of the condenser lens, and the spacing of the condenser lens and the cylindrical lens are restricted by the compactness of the flow cytometer. 
     SUMMARY OF THE INVENTION 
     The scope of the present invention is defined solely by the appended claims, and is not affected to any degree by the statements within this summary. 
     The flow cytometer of a first aspect of the present invention includes a flow cell configured to induce a flow of a sample containing object particles; a light source; an irradiating optical system configured to irradiate light from the light source on the flow of particles in the flow cell; a detecting part configured to detect light given off from the flow of particles which are irradiated by light. In this case the irradiating optical system includes a collective lens having an optical axis symmetric aspheric surface on one surface, and a cylindrical surface on the other surface. 
     The particle analyzer of a second aspect of the present invention includes the flow cytometer of the first aspect, and an analyzing part configured to process the output from the detecting part, and analyze the object particles in the sample. 
     The flow cytometric method of a third aspect of the present invention includes flowing a sample containing an object particles through a flow cell, irradiating a flow of particles in the flow cell by condensing the light from a light source through a lens having an optical axis symmetric aspheric surface on one surface and a cylindrical surface on the other surface, and detecting the light given off from the flow in the flow cell. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of the structure of the flow cytometer of a first embodiment viewed in the Y axis negative direction; 
         FIG. 2A  is a schematic view of the structure of the flow cytometer of a first embodiment viewed in the X axis positive direction; 
         FIG. 2B  is a schematic view of the structure of the flow cytometer of the first embodiment; 
         FIG. 2C  is a schematic view of the collective lens exit surface and laser light convergence state in the first embodiment; 
         FIG. 3  is a block diagram showing the structure of the particle analyzer of the first embodiment; 
         FIG. 4A  is a schematic view of a conventional collective lens configured by a spherical lens, and the laser light convergence state; 
         FIG. 4B  shows the a spot diagram when a conventional collective lens is configured by a spherical lens; 
         FIG. 5A  is a schematic view of a collective lens configured by an aspheric lens, and the laser light convergence state; 
         FIG. 5B  shows the a spot diagram in the case of a collective lens configured by an aspheric lens; 
         FIG. 6A  illustrates the beam strength in the Y axis direction and X axis direction in the first embodiment; 
         FIG. 6B  illustrates the beam strength in the Y axis direction and X axis direction in the first embodiment; 
         FIG. 7A  is a schematic view of the collective lens exit surface and laser light convergence state in a second embodiment; 
         FIG. 7B  is a schematic view of the collective lens exit surface and laser light convergence state in a third embodiment; 
         FIG. 7C  is a schematic view of the collective lens exit surface and laser light convergence state in a fourth embodiment; 
         FIG. 8A  illustrates the beam strength in the Y axis direction and X axis direction in the second embodiment; 
         FIG. 8B  illustrates the beam strength in the Y axis direction and X axis direction in the second embodiment; 
         FIG. 9A  is a schematic view of the collective lens and laser light convergence state in the fourth embodiment; 
         FIG. 9B  shows a spot diagram in the case of the collective lens of the fourth embodiment; 
         FIG. 10A  is a schematic view of the structure of the flow cytometer of a fifth embodiment viewed in the Y axis negative direction and the X axis positive direction; and 
         FIG. 10B  is a schematic view of the structure of the flow cytometer of the fifth embodiment viewed in the Y axis negative direction and the X axis positive direction. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention will be described hereinafter with reference to the drawings. 
     The first through fifth embodiments described below use the present invention in an apparatus which performs examination and analysis of blood by detecting the white blood cells, red blood cells, platelets and the like contained in a blood sample, and counting each blood cell. 
     First Embodiment 
     As shown in  FIG. 1 , a flow cytometer  100  is provided with a flow cell  110 , a light source  120 , an irradiating optical system  130 , and a detecting part  140 . For the sake of convenience, the mutual intersection of the XYZ axes is shown in  FIG. 1 . 
     As shown in  FIG. 2B , the flow cell  110  has a sheath fluid supply port  111 , a sample nozzle  112 , a pore part  113 , and a disposal port  114 . The sheath fluid supply port  111  supplies sheath fluid into the flow cell  110 . The sample nozzle  112  injects a measurement sample upward within the flow cell  110 . The measurement sample progresses through a flow path  115  formed in the pore part  113  while encapsulated in the sheath fluid, and toward the disposal port  114 . The flow path  115  extends in the Y axis direction. The measurement sample contains particles such as blood cells, and each particle passes through the flow path  115  while aligned in a row. 
     Returning to  FIG. 1 , the light source  120  emits laser light  201  in the Z axis positive direction. The laser light  201  has a wavelength of approximately 642 nm. The emission optical axis of the light source  120  matches the optical axis  202  of the irradiating optical system  130 . 
     The irradiating optical system  130  has a collimator lens  131  and a collective lens  132 . The collimator lens  131  converts the laser light emitted from the light source  120  to parallel rays. The collective lens  132  is made of glass and has an entrance surface  132   a  and an exit surface  132   b . The entrance surface  132   a  is an optical axis symmetric aspherical surface, and the exit surface  132   b  is a convex shape, that is, a cylindrical surface of fixed curvature. The generating line  132   c  of the cylindrical surface is parallel to the flow path  115 , as shown in  FIG. 2C . For the sake of convenience, the illustration of the entrance surface  132   a  of the collective lens  132  is omitted in  FIG. 2C . Although the shape of the collective lens  132  is illustrated as rectangular in the XY plane for convenience in  FIG. 2C  in order to schematically show the effect of the cylindrical surface, the actual external shape is circular. The same is true for  FIGS. 7A through 7C  below. 
     As shown in  FIGS. 2A and 2C , the laser light  201  is converged by the entrance surface  132   a  in the Y axis direction, and focused at position  203 . Position  203  matches the position of the flow path  115  of the flow cell  110 . However, the laser light  201  also is converged by the entrance surface  132   a  and the exit surface  132   b  in the X axis direction, and focused at position  204 , as shown in  FIG. 1  and  FIG. 2C  Position  204  is on the Z axis negative side from the position of the flow path  115  of the flow cell  110 . 
     As shown in  FIGS. 2B and 2C , the laser light  201  irradiates the flow path  115  by a shape which has a smaller width in the Y axis direction than the X axis direction, that is, by a shape which has a width in the direction parallel to the flow path  115  that is smaller than the width in a direction traversing the flow path  115 . When an optical axis symmetric aspheric surface is used to focus the laser light  201  at position  203 , the spherical aberration is effectively suppressed compared to when a spherical surface is used. The suppression of spherical aberration is described below with reference to  FIGS. 4A and 4B  and  FIGS. 5A and 5B . 
     In this case, in the plane of the intersection of the optical axis  202  and the entrance surface  132   a , the distance from the intersection point is designated r, and the distance from the tangent plane to the entrance surface  132   a  is designated z. The distance z shows the shape of the optical axis symmetric aspheric surface of the entrance surface  132   a . The curvature of the entrance surface  132   a  is designated c, the i th -order aspheric coefficient is designated ai, and the conic constant is designated k. The distance z is regulated by the following even order aspheric surface expression at this time. 
     
       
         
           
             
               
                 
                   z 
                   = 
                   
                     
                       
                         cr 
                         2 
                       
                       
                         1 
                         + 
                         
                           
                             1 
                             - 
                             
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   k 
                                 
                                 ) 
                               
                               ⁢ 
                               
                                 c 
                                 2 
                               
                               ⁢ 
                               
                                 r 
                                 2 
                               
                             
                           
                         
                       
                     
                     + 
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         8 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           α 
                           i 
                         
                         ⁢ 
                         
                           r 
                           
                             2 
                             ⁢ 
                             i 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     For example, the entrance surface  132   a  can be designed using the r of the fourth order item or the r of the sixth order item. Thus, the entrance surface  132   a  can be regulated and designed for a desired optical axis symmetric aspheric surface. In the first embodiment, the optical axis symmetric aspheric surface of the entrance surface  132   a  is regulated using the r to the fourth order item, and the r of the second order coefficient and the r of the sixth order and subsequent r are designated 0. Each value in the first embodiment, for example, may be set as shown in Table 1 below. 
     
       
         
               
             
               
               
               
               
             
               
               
               
             
               
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Parameters of Collective Lens 132 in the First embodiment 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1) 
                 Entrance surface 132a radius of curvature 
                 7.665 
                 mm 
               
             
          
           
               
                 2) 
                 Entrance surface 132a curvature c 
                 0.13046 
               
               
                 3) 
                 Conic constant k 
                 −0.62809 
               
               
                 4) 
                 4th-order aspheric surface coefficient α4 
                 0.00001688 
               
             
          
           
               
                 5) 
                 Entrance surface 132a focal length 
                 15.0 
                 mm 
               
               
                 6) 
                 Exit surface 132b radius of curvature 
                 −65. 
                 mm 
               
             
          
           
               
                 7) 
                 Collective lens 132 refractive index 
                 1.5110 
               
               
                   
               
             
          
         
       
     
     The focal length of the entrance surface  132   a , that is, the focal length of the collective lens  132  in the Y axis direction, is set at 5 mm or greater but not more than 100 mm. The focal length of the entrance surface  132   a  is preferably 5 mm or greater but not more than 70 mm, and ideally 5 mm or greater but not more than 35 mm. 
     Returning to  FIG. 1 , a forward scattered light  211 , side scattered light  212 , and fluorescent light  213  are produced when the particle in the flow path  115  is irradiated by the laser light  201 . The forward scattered light  211  issues mainly from the flow cell  110  in the Z axis positive direction. The side scattered light  212  issues mainly from the flow cell  110  in the X axis positive direction and the X axis negative direction. The fluorescent light  213  issues around the periphery of the flow cell  110 . 
     The detecting part  140  is provided with a collective lens  141 , beam stopper  142 , pinhole  143 , light sensor  144 , collective lens  145 , dichroic mirror  146 , light sensor  147 , spectral filter  148 , and light sensor  149 . 
     The collective lens  141  converges the forward scattered light at the position of the pinhole  143 . The collective lens  141  converges the laser light  201 , which has passed through the flow cell  110  without irradiating a particle, at the position of the beam stopper  142 . The beam stopper  142  transmits the majority of the forward scattered light  211  and blocks the laser light  201  that has passed through the flow cell  110 . The light sensor  144  is a photodiode. The light sensor  144  receives the forward scattered light  211  transmitted through the pinhole  143 , and outputs signals based on the forward scattered light  211 . 
     The collective lens  145  converges the side scattered light  212  and the fluorescent light  213 . The dichroic mirror  146  reflects the side scattered light  212  and transmits the fluorescent light  213 . The light sensor  147  is a photodiode. The light sensor  147  receives the side scattered light  212  that was reflected by the dichroic mirror  146 , and outputs signals based on the side scattered light  212 . The spectral filter  148  transmits only the fluorescent light  213 . The light sensor  149  is an avalanche photodiode. The light sensor  149  receives the fluorescent light  213 , and outputs signals based on the fluorescent light  213 . 
     As shown in  FIG. 3 , the particle analyzer  10  is provided with a measuring part  20 , analyzing part  30 , and memory part  40 . The measuring part  20  incorporates a flow cytometer  100 . 
     The measuring part  20  prepares a measurement sample to be used for measurements by mixing reagent and the like with a blood sample of peripheral blood collected from a patient. The measuring part  20  outputs the signals from the light sensors  144 ,  147 , and  149  of the flow cytometer to the analyzing part  30 . The analyzing part  30  performs processing to calculate a plurality of characteristics parameters from the waveforms of the received signals. The analyzing part  30  performs analyses based on the calculated characteristics parameters using a computer program stored in the memory part  40 , and subsequently stores the analysis results in the memory part  40 . 
     Spherical aberration simulation results are described below referring to  FIGS. 4A and 4B , and  FIGS. 5A and 5B . The following simulation evaluated the performance of the entrance surface  132   a  of the collective lens  132 . In the simulation, a collective lens  134  configured by an aspheric lens was used instead of the collective lens  132  for purposes of this evaluation. The performance of a collective lens  133  configured by a glued spherical lens also was evaluated as a conventional example for comparison. In the simulation, the sizes of the spot diagrams were obtained and the magnitudes of the spherical aberrations were evaluated when using the collective lens  133  of the conventional example, and when using the collective lens  134 . 
     As shown in  FIG. 4A , the collective lens  133  of the conventional example is configured by two glued glass spherical lenses. The entrance surface  133   a  and the exit surface  133   b  are both optical axis symmetric spherical surfaces, and the surface  133   c  formed by gluing the two lenses also is an optical axis symmetric spherical surface. In the simulation, parallel rays impinged the entrance surface  133   a , and a spot diagram was obtained at the position of most convergence by the collective lens  133  of the conventional example. The conditions of the simulation are described in Table 2 below. 
     
       
         
               
             
               
               
               
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Simulation conditions of collective 
               
               
                 lens 133 (conventional example) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1) 
                 Entrance surface 133a radius of curvature 
                 11.8 
                 mm 
               
               
                 2) 
                 Entrance surface 133c curvature 
                 −5.9 
                 mm 
               
               
                 3) 
                 Exit surface 133b radius of curvature 
                 −11.7 
                 mm 
               
               
                 4) 
                 Collective lens 133 focal length 
                 15.7 
                 mm 
               
             
          
           
               
                 5) 
                 Collective lens 133 refractive index 
                 1.5110 
               
             
          
           
               
                 6) 
                 Parallel light wavelength 
                 642 
                 nm 
               
               
                 7) 
                 Parallel light radius 
                 3.24 
                 mm 
               
               
                   
               
             
          
         
       
     
     As shown in  FIG. 4B , the spot diagram radius was 0.001161 mm when parallel rays were converged by the collective lens  133  of the conventional example. 
     As shown in  FIG. 5A , the collective lens  134  was configured by a single glass aspheric lens. The entrance surface  134   a  is an optical axis symmetric aspheric surface identical to the entrance surface  132   a  of the first embodiment, and was configured as shown in Table 1 (1) through (5) and (7). The exit surface  134   b  is parallel to the X-Y plane. In the simulation, parallel rays impinged the entrance surface  134   a , and a spot diagram was obtained at the position of most convergence by the collective lens  134 . The simulation conditions in this case were identical to Table 2 (6) and (7). 
     As shown in  FIG. 5B , the spot diagram radius was 4.561×10 −7  mm when parallel rays were converged by the collective lens  134 . 
     According to the results of the simulation, it can be understood that the spherical aberration can be suppressed and laser light converged with a smaller spot diagram in the case of the collective lens  134  configured by an aspheric lens compared to the case of the aspheric lens  133  configured by the spherical lens of the conventional example. In the flow cytometer  100  shown in  FIG. 1 , the use of the entrance surface  132   a  which has an optical axis symmetric aspheric surface can be said to more effectively suppress spherical aberration of the laser light  201  because laser light  201  is converged on the flow path  115  of the flow cell  110 , compared to the use of the conventional collective lens configured by a spherical surface. 
     The simulation results of beam intensity at the position of the flow path  115  of the flow cell  110  using the collective lens  132  is described below. 
     In the simulation, parallel light rays impinged the entrance surface  132   a , and beam intensities in the Y axis direction and X axis direction were obtained at the position of the flow path  115  of the flow cell  110 . The conditions of this simulation were identical to Table 1 (1) through (7) and Table 2 (6) and (7). The diameter of the flow path  115  was 25 μm. 
     In  FIGS. 6A and 6B , the horizontal axis represents the distance from the intersection of the optical axis  202  and the flow path  115 , and the vertical axis represents the beam intensity normalized by the maximum value. The shape of the beam intensity in the X axis direction shown in  FIG. 6B  has a moderate slope in a direction separated from the optical axis compared to the shape of the beam intensity in the Y axis direction shown in  FIG. 6A . The beam intensity had a maximum value of 97.4% at the position on the X axis direction 12.5 μm and −12.5 μm, as shown in  FIG. 6B . Accordingly, a substantially uniform light irradiated the flow path  115 . In this way the shape of the beam intensity in the Y axis direction and the X axis direction in the flow path  115  can be appropriately set for the collective lens  132  so as to detect particles in the measurement sample flowing through the flow path  115 . 
     According to the first embodiment, light from the light source irradiates the flow cell  110  with low aberration and scant distortion without using a plurality of spherical lenses. This permits a reduction in the thickness of the lens for converging the laser light  201 . The surface on the opposite side from the cylindrical surface, that is, the entrance surface  132   a , can be an optical axis symmetric aspheric surface since a plurality of spherical lenses become unnecessary by using the optical axis symmetric aspheric lens. Since both the shaping and focusing of laser light  201  can be realized by a single collective lens  132 , the number of lenses in the irradiation optical system  130  can be reduced and the irradiation optical system  130  can be made more compact. The members and structures for installing and holding lenses, such as holders and the like, can be similarly reduced by the reduction of the number of lenses in the irradiating optical system  130 . Therefore, the space for arranging the irradiating optical system  130  can be greatly reduced, and as a result the flow cytometer  100  can be made effectively more compact. 
     According to the first embodiment, the entrance surface  132   a  and the exit surface  132   b  of the collective lens  132  are configured by an optical axis symmetric aspheric surface and a cylindrical surface, respectively. Therefore, the lens surfaces of the collective lens  132  can be designed so that the laser light  201  emitted by the light source  120  irradiates the flow path  115  of the flow cell  110  in a low aberration state with scant distortion. 
     According to the first embodiment, the optical axis symmetric aspheric surface of the entrance surface  132   a  has a shape configured to converge the entering laser light  201  at the position  203 . The cylindrical surface of the exit surface  132   b  has a shape configured to shape the laser light  201  irradiating the flow path  115  of the flow cell  110  so that the width in a direction parallel to the flow path  115  is less than the width on a direction intersecting the flow path  115 . Therefore, when the functions are divided for each surface of the collective lens  132 , the design and fabrication of each surface of the collective lens  132  is facilitated. 
     According to the first embodiment, the cylindrical surface of the exit surface  132   b  has a convex shape. Therefore, the focus position of the laser light  201  in the direction intersecting the flow path  115  becomes the position  203  on the light source  120  side of the flow path  115 . Hence, the irradiating optical system  130  can be made more compact. 
     According to the first embodiment, there is no position shift around the optical axis between the entrance surface  132   a  and the exit surface  132   b  due to the optical axis symmetric shape of the entrance surface  132   a . The expected performance is secured in the collective lens  132  without fabricating the collective lens  132  by strictly managing the rotational positions of the entrance surface and the exit surface. 
     Second Embodiment 
     The flow cytometer of the second embodiment has a structure identical to the flow cytometer of the first embodiment with the exception that the shape of the exit surface of the collective lens  132  is different. As shown in  FIG. 7A , in the second embodiment the exit surface  132   b  of the collective lens  132  is a cylindrical surface of which has a concave shape and constant curvature compared to the first embodiment. 
     The laser light  201  in this case is converged by the entrance surface  132   a  in the Y axis direction, and focused at the position  203 . Position  203  matches the position of the flow path  115  of the flow cell  110 . On the other hand, the laser light  201  is converged by the entrance surface  132   a  in the X axis direction, diffused by the exit surface  132   b , and focused at position  204 . Position  204  is on the Z axis positive side from the position of the flow path  115  of the flow cell  110 . In the second embodiment, therefore, the laser light  201  irradiates the flow path  115  so that the width of the laser light  201  in a direction parallel to the flow path  115  is less than the width in a direction intersecting the flow path  115  similar to the first embodiment. 
     In the second embodiment, the entrance surface  132   a  is designed to converge the light on the flow path  115  similar to the first embodiment. Accordingly, the entrance surface  132   a  markedly suppresses spherical aberration similar identically to  FIGS. 5A and 5B . In the second embodiment, the lens surfaces of the collective lens  132  can be designed so that the laser light  201  irradiates the flow path  115  of the flow cell  110  in a low aberration state with scant distortion. 
     The simulation results of beam intensity at the position of the flow path  115  of the flow cell  110  using the collective lens  132  of the second embodiment is described below. 
     In the simulation, parallel light rays impinged the entrance surface  132   a , and beam intensities in the Y axis direction and X axis direction were obtained at the position of the flow path  115  of the flow cell  110 . The conditions of this simulation were identical to Table 1 (1) through (5) and Table 2 (6) and (7), and the radius of curvature of the exit surface  132   b  was 65 mm. The diameter of the flow path  115  was 25 μm. 
     The shape of the beam intensity in the X axis direction shown in  FIG. 8B  has a moderate slope in a direction separated from the optical axis compared to the shape of the beam intensity in the Y axis direction shown in FIG. SA. The beam intensity had a maximum value of 97.8% at the positions on the X axis direction 12.5 μm and −12.5 μm, as shown in  FIG. 8B . Accordingly, a substantially uniform light irradiated the flow path  115 . In this way the shape of the beam intensity in the Y axis direction and the X axis direction in the flow path  115  can be appropriately set for the collective lens  132  of the second embodiment so as to detect particles in the measurement sample flowing through the flow path  115 . 
     Third Embodiment 
     The flow cytometer of the third embodiment has a structure identical to the flow cytometer of the first embodiment with the exception that the shape of the exit surface of the collective lens  132  is different. As shown in  FIG. 7B , in the third embodiment the generating line  132   c  of the cylindrical surface of the exit surface  132   b  is in the X axis direction, that is, in a direction intersecting the flow path  115  compared with the first embodiment. 
     The laser light  201  in this case is converged by the entrance surface  132   a  in the X axis direction, and focused at the position  204 . Position  204  is on the Z axis positive side from the position of the flow path  115  of the flow cell  110 . On the other hand, the laser light  201  is converged by the entrance surface  132   a  and the exit surface  132   b  in the Y axis direction, and focused at position  203 . Position  203  matches the position of the flow path  115  of the flow cell  110 . In the third embodiment, therefore, the laser light  201  irradiates the flow path  115  so that the width of the laser light  201  in a direction parallel to the flow path  115  is less than the width in a direction intersecting the flow path  115  similar to the first embodiment. 
     In the third embodiment, the entrance surface  132   a  is designed to converge the light at a position farther than the flow path  115  unlike the first embodiment. The converging of light in the Y axis direction is realized by using the convergence effect of the entrance surface  132   a  and the convergence effect of the exit surface  132   b  which has a spherical surface. In the third embodiment, therefore, the entrance surface  132   a  is designed to suppress aberration and distortion while considering the effect of the exit surface  132   b . In the third embodiment, light irradiates the flow path  115  of the flow cell  110  in a state of low aberration and scant distortion by designing the entrance surface  132   a  while considering the effect of the exit surface  132   b.    
     Fourth Embodiment 
     The flow cytometer of the fourth embodiment has a structure identical to the flow cytometer of the first embodiment with the exception that the shape of the exit surface of the collective lens  132  is different. As shown in  FIG. 7C , in the fourth embodiment the exit surface  132   b  is a cylindrical surface of which has a concave shape and constant curvature compared to the first embodiment. The generating line  132   c  of the cylindrical surface of the exit surface  132   b  is in the X axis direction, that is, in a direction intersecting the flow path  115 . 
     The laser light  201  in this case is converged by the entrance surface  132   a  in the X axis direction, and focused at the position  204 . Position  204  is on the Z axis negative side from the position of the flow path  115  of the flow cell  110 . On the other hand, the laser light  201  is converged by the entrance surface  132   a  in the Y axis direction, diffused by the exit surface  132   b , and focused at position  203 . Position  203  matches the position of the flow path  115  of the flow cell  110 . In the fourth embodiment, therefore, the laser light  201  irradiates the flow path  115  so that the width of the laser light  201  in a direction parallel to the flow path  115  is less than the width in a direction intersecting the flow path  115  similar to the first embodiment. 
     In the fourth embodiment, the entrance surface  132   a  is designed to converge the light at a position closer than the flow path  115  unlike the first embodiment. The converging of light in the Y axis direction is realized by using the convergence effect of the entrance surface  132   a  and the diffusion effect of the exit surface  132   b  which has a spherical surface. In the fourth embodiment, therefore, the entrance surface  132   a  is designed to suppress aberration and distortion while considering the effect of the exit surface  132   b . In the fourth embodiment, light irradiates the flow path  115  of the flow cell  110  in a state of low aberration and scant distortion by designing the entrance surface  132   a  while considering the effect of the exit surface  132   b.    
     The spherical aberration simulation results of the fourth embodiment are described below referring to  FIGS. 9A and 9B . This simulation evaluated the performance of the entrance surface  132   a  of the collective lens  132  of the fourth embodiment. The collective lens  132  described above converges the light in the Y axis direction through the convergence effect of the entrance surface  132   a  and the diffusion effect of the exit surface  132   b . In the simulation, a collective lens  135  was used which had an optical axis symmetric aspheric surface as the entrance surface and an optical axis symmetric spherical surface as the exit surface instead of the collective lens  132  so as to evaluate spherical aberration by obtaining a spot diagram with the light in a circular convergence state. 
     The entrance surface  135   a  of the collective lens  135  was an optical axis symmetric aspheric surface designed according to Table 3 (1) through (4). The exit surface  135   b  of the collective lens  135  was an optical axis symmetric spherical surface which had the radius cf curvature shown in Table 3 (5). The refractive index of the collective lens  135  was as shown in Table 3 (6). In the simulation, the size of a spot diagram was obtained when parallel rays were converged by the convergence lens  135  designed as described above, and the magnitude of the spherical aberration was evaluated. The radius and wavelength of the parallel light were as shown in Table 3 (7) and (8). 
     
       
         
               
             
               
               
               
               
             
               
               
               
             
               
               
               
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Spherical aberration simulation conditions of fourth embodiment 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1) 
                 Entrance surface 135a radius of curvature 
                 7.115 
                 mm 
               
             
          
           
               
                 2) 
                 Entrance surface 135a curvature c 
                 0.14053 
               
               
                 3) 
                 Conic constant k 
                 −0.58578 
               
               
                 4) 
                 4th-order aspheric surface coefficient α4 
                 0.00002995 
               
             
          
           
               
                 5) 
                 Exit surface 135b radius of curvature 
                 90 
                 mm 
               
             
          
           
               
                 6) 
                 Collective lens 135 refractive index 
                 1.5110 
               
             
          
           
               
                 7) 
                 Parallel light wavelength 
                 642 
                 nm 
               
               
                 8) 
                 Parallel light radius 
                 3.24 
                 mm 
               
               
                   
               
             
          
         
       
     
     As shown in  FIG. 9B , the spot diagram radius was 3.548×10 −5  mm when parallel rays were converged by the collective lens  135 . 
     In the simulation results, the size of the spot diagram somewhat increased compared to the simulation results of the first embodiment described referring to  FIGS. 5A and 5B . However, the size of the spot diagram in the simulation results was several levels smaller in value compared to the simulation results of the collective lens  133  of the conventional example configured by a spherical lens described referring to  FIGS. 4A and 4B . Thus, spherical aberration is effectively suppressed even in the collective lens  132  of the fourth embodiment compared to the collective lens of the conventional example configured by a spherical lens. 
     In the third embodiment, light was converged in the Y axis direction by the optical effects of both the entrance surface  132   a  and the exit surface  132   b  of the collective lens  132  similar to the fourth embodiment. Accordingly, the effect of suppressing spherical aberration identical to the fourth embodiment can be obtained by adjusting the design of the entrance surface  132   a.    
     Fifth Embodiment 
     The flow cytometer of the fifth embodiment has a structure identical to the flow cytometer of the first embodiment with the exception that the shape of the exit surface of the collective lens  132  is different. As shown in  FIGS. 10A and 10B , in the fifth embodiment the entrance surface  132   a  of the collective lens  132  is a cylindrical surface which has a convex shape and constant curvature, and the exit surface  132   b  is an optical axis symmetric aspheric surface compared to the first embodiment. The generating line of the cylindrical surface is parallel to the flow path  115 . In the fifth embodiment, the functions of the entrance surface and the exit surface of the collective lens  132  are switched compared to the first embodiment. In  FIGS. 10A and 10B , the structures other than the flow cell  110  are omitted for the sake of convenience. 
     The spherical aberration simulation results of the fifth embodiment are described below. In the simulation below, the size of the spot diagram was obtained when using the collective lens  132  of the fifth embodiment, and the magnitude of the spherical aberration was evaluated. 
     In the simulation, parallel rays impinged the entrance surface  132   a , and a spot diagram was obtained at position  203  shown in  FIG. 10B . The conditions of the simulation are described in Table 4 below. Note that in the simulation the entrance surface  132   a  is parallel to the X-Y axis in order to evaluate the magnitude of spherical aberration at position  203 , that is, at the position of the flow path  115  of the flow cell  110 . 
     
       
         
               
             
               
               
               
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 Spherical aberration simulation conditions of fifth embodiment 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1) 
                 Exit surface 132b radius of curvature 
                 −7.6983 
                 mm 
               
             
          
           
               
                 2) 
                 Exit surface 132b curvature c 
                 −0.12990 
               
               
                 3) 
                 Conic constant k 
                 0.11835 
               
               
                 4) 
                 4th-order aspheric surface coefficient α4 
                 6.598 × 10 −4   
               
               
                 5) 
                 Collective lens 132 refractive index 
                 1.5110 
               
             
          
           
               
                 6) 
                 Parallel light wavelength 
                 642 
                 nm 
               
               
                 7) 
                 Parallel light radius 
                 3.24 
                 mm 
               
               
                   
               
             
          
         
       
     
     In the simulation results, the radius of the spot diagram was 5.955×10 −5  mm when parallel rays were converged by the collective lens  132  of the fifth embodiment. 
     In the simulation results, the size of the spot diagram somewhat increased compared to the simulation results of the first embodiment described referring to  FIGS. 5A and 5B . However, the size of the spot diagram in the simulation results was several levels smaller in value compared to the simulation results of the collective lens  133  of the conventional example configured by a spherical lens described referring to  FIGS. 4A and 4B . Thus, spherical aberration is effectively suppressed even in the collective lens  132  of the fifth embodiment compared to the collective lens of the conventional example configured by a spherical lens. 
     Modifications 
     The collimating lens  131  also may be omitted in the first through fifth embodiments. In this case, the setting values of the collective lens  132  may be modified so that the laser light  201  irradiates the flow path  115  of the flow cell  110  in a shape identical to the first through fifth embodiments. 
     Although the curvature of the cylindrical surface in the collective lens  132  is constant in the first through fifth embodiments, the present invention is not limited to this configuration inasmuch as the curvature of the cylindrical surface need not be constant. In this case, the shape of the intensity distribution of the laser light  201  on the flow path  115  of the flow cell  110  may be a so-called top hat shape that is flatter close to the top. In this way the intensity of the light in a direction intersecting the flow path  115  can be more uniform so that a uniform light irradiates the flow path  115 . 
     The flow cytometer  100  also may be configured so that the laser light  201  and a laser light of a different wavelength than the laser light  201  irradiate the flow path  115  of the flow cell  110  in the first through fifth embodiments. In this case, another light source is arranged in the flow cytometer  100 , and emits laser light of a different wavelength than the laser light  201 . 
     Although blood is the object of measurement in the first through fifth embodiments, urine also may be an object of measurement. The present invention is applicable to apparatuses for measuring particles in biological samples such as blood and urine.