Abstract:
A measuring plate for use in a sensor utilizing the phenomenon of attenuation in total internal reflection provides a dielectric block and a film layer and includes a dielectric plate provided with a plurality of recesses each provided with a film layer and holding a sample in contact with the film layer, and a reflecting optical system including a reflecting surface which is formed on the dielectric plate for each of the recesses to cause the light beam emitted from the light source to impinge upon the interface between the film layer of the recess and the dielectric plate and/or to cause the light beam reflected at the interface between the film layer of the recess and the dielectric plate to travel toward a predetermined position.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a measuring plate for use in a sensor using attenuation in total internal reflection such as a surface plasmon sensor for quantitatively analyzing a material in a sample on the basis of generation of surface plasmon.  
           [0003]    2. Description of the Related Art  
           [0004]    In metal, free electrons vibrate in a group to generate compression waves called plasma waves. The compression waves generated in a metal surface are quantized into surface plasmon.  
           [0005]    There have been proposed various surface plasmon sensors for quantitatively analyzing a material in a sample utilizing a phenomenon that such surface plasmon is excited by light waves.  
           [0006]    Among those, one employing a system called “Kretschmann configuration” is best known. See, for instance, Japanese Unexamined Patent Publication No. 6(1994)-167443.  
           [0007]    The plasmon resonance sensor using the Kretschmann configuration basically comprises a dielectric block shaped, for instance, like a prism, a metal film which is formed on one face of the dielectric block and is brought into contact with a sample, a light source emitting a light beam, an optical system which causes the light beam to enter the dielectric block to impinge upon the interface of the dielectric block and the metal film at various angles of incidence so that total internal reflection conditions are satisfied at the interface, and a photodetector means which detects the intensity of the light beam reflected in total internal reflection at the interface and detects a state of surface plasmon resonance, i.e., a state of attenuation intotal internal reflection.  
           [0008]    In order to obtain various angles of incidence of the light beam to the interface, a relatively thin incident light beam may be caused to impinge upon the interface while deflecting the incident light beam so that the angle of incidence changes or a relatively thick incident light beam may be caused to impinge upon the interface in the form of convergent light or divergent light so that components of the incident light beam impinge upon the interface at various angles. In the former case, the light beam which is reflected from the interface at an angle which varies as the incident light beam is deflected may be detected by a photodetector which is moved in synchronization with deflection of the incident light beam or by an area sensor extending in the direction in which reflected light beam is moved as a result of deflection. In the latter case, an area sensor which extends in directions so that all the components of light reflected from the interface at various angles can be detected by the area sensor may be used.  
           [0009]    In such a plasmon resonance sensor, when a light beam impinges upon the interface at a particular angle of incidence θsp not smaller than the angle of total internal reflection, evanescent waves having an electric field distribution in the sample in contact with the metal film are generated and surface plasmon is excited in the interface between the metal film and the sample. When the wave number vector of the evanescent waves is equal to the wave number of the surface plasmon and wave number matching is established, the evanescent waves and the surface plasmon resonate and light energy is transferred to the surface plasmon, whereby the intensity of light reflected in total internal reflection at the interface of the dielectric block and the metal film sharply drops. The sharp intensity drop is generally detected as a dark line by the photodetector.  
           [0010]    The aforesaid resonance occurs only when the incident light beam is p-polarized. Accordingly, it is necessary to set the light beam to impinge upon the interface in the form of p-polarized light.  
           [0011]    When the wave number of the surface plasmon can be known from the angle of incidence θsp at which the phenomenon of attenuation in total internal reflection (ATR) takes place, the dielectric constant of the sample can be obtained. That is,  
           K   sp          (   ω   )       =       ω   c                  ɛ   m          (   ω   )            ɛ   s             ɛ   m          (   ω   )       +     ɛ   s                                   
 
           [0012]    wherein K sp  represents the wave number of the surface plasmon, ω represents the angular frequency of the surface plasmon, c represents the speed of light in a vacuum, and ε m  and ε s  respectively represent the dielectric constants of the metal and the sample.  
           [0013]    When the dielectric constant ε s  of the sample is known, the concentration of the specific material in the sample can be calculated and accordingly a property related to the dielectric constant ε s  (refractive index) of the sample can be detected by detecting the angle of incidence θsp at which the intensity of light reflected in total internal reflection from the interface of the prism and the metal film sharply drops (this angel θsp will be referred to as “the attenuation angle θsp”, hereinbelow).  
           [0014]    As a similar apparatus utilizing the phenomenon of attenuation in total internal reflection (ATR), there has been known a leaky mode sensor described in, for instance, “Spectral Research” Vol. 47, No. 1 (1998), pp 21 to 23 &amp; pp 26 and 27. The leaky mode sensor basically comprises a dielectric block shaped, for instance, like a prism, a clad layer which is formed on one face of the dielectric block, an optical waveguide layer which is formed on the clad layer and is brought into contact with a sample, a light source emitting a light beam, an optical system which causes the light beam to enter the dielectric block to impinge upon the interface of the dielectric block and the metal film at various angles of incidence so that total internal reflection conditions are satisfied at the interface and attenuation in total internal reflection takes place due to excitation of a waveguide mode in the optical waveguide layer, and a photodetector means which detects the intensity of the light beam reflected in total internal reflection at the interface and detects a state of waveguide mode excitation, i.e., a state of attenuation in total internal reflection.  
           [0015]    In the leaky mode sensor with this arrangement, when the light beam is caused to impinge upon the clad layer through the dielectric block at an angle not smaller than an angle of total internal reflection, only light having a particular wave number and impinging upon the optical waveguide layer at a particular angle of incidence comes to propagate through the optical waveguide layer in a waveguide mode after passing through the clad layer. When the waveguide mode is thus excited, almost all the incident light is taken in the optical waveguide layer and accordingly, the intensity of light reflected in total internal reflection at the interface of the dielectric block and the clad layer sharply drops. That is, attenuation in total internal reflection occurs. Since the wave number of light to be propagated through the optical waveguide layer in a waveguide mode depends upon the refractive index of the sample on the optical waveguide layer, the refractive index and/or the properties of the sample related to the refractive index can be detected on the basis of the angle of incidence at which the attenuation in total internal reflection occurs.  
           [0016]    In the conventional surface plasmon resonance sensors or leaky mode sensors, there has been proposed a system in which a plurality of measuring chips are arranged on a plate in order to increase the measuring speed or to automate the measurement.  
           [0017]    However, this system is disadvantageous in that it is necessary to transfer the measuring chips from the plate to the sensor one by one. When measurement is performed with the measuring chips held on the plate, the light beam can be eclipsed, for instance, by the bottom portion of an adjacent measuring chip, which deteriorates the accuracy of measurement.  
         SUMMARY OF THE INVENTION  
         [0018]    In view of the foregoing observations and description, the primary object of the present invention is to provide a measuring plate which allows to perform accurate measurement on a plurality of samples held thereon with the samples kept thereon.  
           [0019]    In accordance with a first aspect of the present invention, there is provided a measuring plate for use in a sensor utilizing the phenomenon of attenuation in total internal reflection comprising a dielectric block provided with a film layer to be brought into contact with a sample, a light source which emits a light beam, an incident optical system which causes the light beam to enter the dielectric block so that total internal reflection conditions are satisfied at the interface of the dielectric block and the film layer and various angles of incidence of the light beam to the interface can be obtained, and a photodetector means which detects the intensity of the light beam reflected in total internal reflection at the interface and detects a state of attenuation in total internal reflection, the measuring plate being for providing the dielectric block and the film layer and comprising a dielectric plate provided with a plurality of recesses each provided with a film layer and holding a sample in contact with the film layer, and a reflecting optical system including a reflecting surface which is formed on the dielectric plate for each of the recesses to cause the light beam emitted from the light source to impinge upon the interface between the film layer of the recess and the dielectric plate and/or to cause the light beam reflected at the interface between the film layer of the recess and the dielectric plate to travel toward a predetermined position.  
           [0020]    When the measuring plate in accordance with the present invention is to be used in a surface plasmon resonance sensor, the film layer of each of the recesses comprises a metal film, whereas when the measuring plate in accordance with the present invention is to be used in a leaky mode sensor, the film layer of each of the recesses comprises a clad layer and an optical waveguide layer formed on the clad layer.  
           [0021]    In the measuring plate of the present invention, the reflecting optical system provided for each recess is for confining the light beam for measuring the sample in the recess within a predetermined area not to be interfered with recesses adjacent to the recess or elements for the recesses adjacent to the recess, and at the same time, it is preferred that the recesses be formed at substantially regular intervals.  
           [0022]    In one embodiment, the dielectric plate is provided with a plurality of recesses, each having a flat and smooth bottom, on the upper side thereof, the film layer is formed on the bottom of each recess, and the reflecting optical system for each recess comprises a reflecting surface formed on the lower side of the dielectric plate to reflect a light beam impinging thereupon from below toward the interface between the film layer and the dielectric plate.  
           [0023]    In another embodiment, the dielectric plate is provided with a plurality of recesses, each having a flat and smooth bottom, on the upper side thereof, the film layer is formed on the bottom of each recess, and the reflecting optical system for each recess comprises a reflecting surface formed on the lower side of the dielectric plate to reflect downward a light beam reflected at the interface between the film layer and the dielectric plate.  
           [0024]    In still another embodiment, the dielectric plate is provided with a plurality of recesses, each having a flat and smooth bottom, on the upper side thereof, the film layer is formed on the bottom of each recess, and the reflecting optical system for each recess comprises a reflecting surface formed on the lower side of the dielectric plate to reflect a light beam impinging thereupon from above toward the interface between the film layer and the dielectric plate.  
           [0025]    In still another embodiment, the dielectric plate is provided with a plurality of recesses, each having a flat and smooth bottom, on the upper side thereof, the film layer is formed on the bottom of each recess, and the reflecting optical system for each recess comprises a reflecting surface formed on the lower side of the dielectric plate to reflect upward a light beam reflected at the interface between the film layer and the dielectric plate.  
           [0026]    In still another embodiment, the dielectric plate is provided with a plurality of recesses, each having a flat and smooth side surface, on the upper side thereof, the film layer is formed on the side surface of each recess, and the reflecting optical system for each recess comprises a reflecting surface formed on the upper side of the dielectric plate to reflect a light beam impinging thereupon from below toward the interface between the film layer and the dielectric plate.  
           [0027]    In still another embodiment, the dielectric plate is provided with a plurality of recesses, each having a flat and smooth side surface, on the upper side thereof, the film layer is formed on the side surface of each recess, and the reflecting optical system for each recess comprises a reflecting surface formed on the upper side of the dielectric plate to reflect downward a light beam impinging upon the interface between the film layer and the dielectric plate from below and reflected at the interface.  
           [0028]    In still another embodiment, the dielectric plate is provided with a plurality of recesses, each having a flat and smooth side surface, on the upper side thereof, the film layer is formed on the side surface of each recess, and the reflecting optical system for each recess comprises a reflecting surface formed on the lower side of the dielectric plate to reflect a light beam impinging thereupon from above toward the interface between the film layer and the dielectric plate.  
           [0029]    In still another embodiment, the dielectric plate is provided with a plurality of recesses, each having a flat and smooth side surface, on the upper side thereof, the film layer is formed on the side surface of each recess, and the reflecting optical system for each recess comprises a reflecting surface formed on the lower side of the dielectric plate to reflect upward a light beam impinging upon the interface between the film layer and the dielectric plate from below and reflected at the interface.  
           [0030]    It is preferred that the dielectric plate be formed of glass or transparent resin. It is further preferred that the dielectric plate be formed by one-piece injection molding.  
           [0031]    It is further preferred that each of the recesses flares upward.  
           [0032]    In the measuring plate of the present invention, since the reflecting optical system provided for each recess confines the light beam for measuring the sample in the recess within a predetermined area not to be interfered with recesses adjacent to the recess or elements for the recesses adjacent to the recess, the light beam for each recess cannot be eclipsed, for instance, by the bottom portion of recesses adjacent to the recess, and accordingly, the sample in each recess can be accurately analyzed. 
       
    
    
     BREIF DESCRIPTION OF THE DRWAINGS  
       [0033]    [0033]FIG. 1 is a perspective view of a measuring plate in accordance with a first embodiment of the present invention,  
         [0034]    [0034]FIG. 2 is a cross-sectional view taken along line A-A in FIG. 1,  
         [0035]    [0035]FIG. 3 is a fragmentary side view showing a part of a surface plasmon resonance sensor employing the measuring plate in accordance with the first embodiment of the present invention,  
         [0036]    [0036]FIGS. 4A and 4B are graphs showing the relation between the angle of incidence of light to the interface between the metal film and the dielectric plate and the intensity of the reflected light beam detected by the photodetector in the surface plasmon resonance sensor,  
         [0037]    [0037]FIG. 5 is a fragmentary cross-sectional view showing the measuring plate in accordance with a second embodiment of the present invention,  
         [0038]    [0038]FIG. 6 is a fragmentary cross-sectional view showing the measuring plate in accordance with a third embodiment of the present invention,  
         [0039]    [0039]FIG. 7 is a fragmentary cross-sectional view showing the measuring plate in accordance with a fourth embodiment of the present invention,  
         [0040]    [0040]FIG. 8 is a fragmentary cross-sectional view showing the measuring plate in accordance with a fifth embodiment of the present invention,  
         [0041]    [0041]FIG. 9 is a fragmentary cross-sectional view showing the measuring plate in accordance with a sixth embodiment of the present invention,  
         [0042]    [0042]FIG. 10 is a fragmentary cross-sectional view showing the measuring plate in accordance with a seventh embodiment of the present invention, and  
         [0043]    [0043]FIG. 11 is a fragmentary cross-sectional view showing the measuring plate in accordance with an eighth embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0044]    [0044]FIG. 1 shows a measuring plate in accordance with a first embodiment of the present invention. The measuring plate of this embodiment is for a surface plasmon resonance sensor utilizing surface plasmon resonance.  
         [0045]    In FIG. 1, the measuring plate  10   a  comprises a dielectric plate la of a dielectric material such as glass. A plurality of recesses  2   a  each functioning as a sample holder for storing therein a sample liquid are formed on the upper surface of the dielectric plate  1   a . For example, each of the recesses  2   a  may be flared upward. The dielectric plate la may be of any size and the recesses  2   a  formed on the upper surface thereof may be any in number (e.g., 96, 384 or 1536) and may be arranged in any way.  
         [0046]    The dielectric plate la may be easily produced by one-piece injection molding of, for instance, glass or transparent resin. As the transparent resin, PMMA, polycarbonate, amorphous polyolefin, or cycloolefin may be preferably employed. Generally it is preferred that the dielectric plate la be formed of a material which is 1.45 to 2.5 in refractive index since the surface plasmon resonance angle (the attenuation angle θsp) is practically obtained in the refractive index range.  
         [0047]    As shown in FIG. 2, which is a cross-sectional view taken along line A-A in FIG. 1, the bottom surface of each recess  2   a  is flat and a metal film  3   a , for instance, of gold, silver, copper, or aluminum, is provided on the flat bottom surface of each recess  2   a . A reflecting optical system, comprising a mirror  4   a  which reflects a light beam, impinging thereupon from below, toward the interface  5   a  between the metal film  3   a  and the dielectric plate la and a mirror  4   b  which reflects downward the light beam reflected at the interface  5   a , is provided on the lower side of the dielectric plate  1   a  for each of the recesses  2   a . The metal films  3   a  and mirrors  4   a  and  4   b  can be formed by depositing metal in predetermined positions of the dielectric plate  1   a.    
         [0048]    The measuring plate  10   a  can be modified to a measuring plate for a leaky mode sensor by replacing the metal films  3   a  with a clad layer and an optical waveguide layer.  
         [0049]    A surface plasmon resonance sensor employing the measuring plate  10   a  of this embodiment will be described, hereinbelow.  
         [0050]    [0050]FIG. 3 shows a surface plasmon detecting portion of the surface plasmon resonance sensor.  
         [0051]    As shown in FIG. 3, the surface plasmon resonance sensor comprises the measuring plate  10   a , and the surface plasmon detecting portion. The surface plasmon detecting portion comprises a laser (e.g., a semiconductor laser)  14  which emits a light beam  13 , an incident optical system  15  which causes the light beam  13  by way of the mirror  4   a  to impinge upon the interface  5   a  between the metal film  3   a  and the dielectric plate la at various angles of incidence, first and second photodetectors  16  and  17  which detect the amount of light beam  13  reflected at the interface  5   a , and a comparator  18  connected to the first and second photodetectors  16  and  17 .  
         [0052]    In this particular embodiment, the laser  14 , the incident optical system  15  and the photodetectors  16  and  17  are disposed below the measuring plate  10   a  and the light beam  13  reflected at the interface  5   a  to impinge upon the first and second photodetectors  16  and  17  by way of the mirror  4   b  is detected by the first and second photodetectors  16  and  17 .  
         [0053]    The incident optical system  15  comprises a collimator lens  15   a  which collimates the light beam  13  emitted from the laser  14  as a divergent light beam and a condenser lens  15   b  which converges the collimated light beam  13  on the interface  5   a.    
         [0054]    Since converged by the condenser lens  15   b  as described above, the laser beam  13  includes components impinging upon the interface at various angles of incidence θ. The laser  14  and the incident optical system  15  are arranged so that the angles of incidence θ are all not smaller than the angle of total internal reflection. Accordingly, the laser beam  13  is reflected in total internal reflection at the interface  5   a  and the reflected laser beam  13  includes components reflected at the interface  5   a  at various angles of reflection. The incident optical system  15  may be arranged to cause the laser beam  13  to impinge upon the interface  5   a  in a defocused state. This arrangement averages errors in detecting states of surface plasmon resonance and improves measuring accuracy.  
         [0055]    The laser beam  13  is caused to impinge upon the interface  5   a  in a p-polarized state. This can be realized by positioning the laser  14  so that the laser beam  13  impinges upon the interface  5   a  in a p-polarized state. Otherwise, the direction of polarization of the laser beam  13  may be controlled by a wavelength plate.  
         [0056]    The first and second photodetectors  16  and  17  may comprise, for instance, a split photodiode. The first photodetector  16  is disposed to detect the amount of components of the light beam  13  in a first predetermined range (relatively small angle range) of angle of reflection and the second photodetector  17  is disposed to detect the amount of components of the light beam  13  in a second predetermined range (relatively large angle range) of angle of reflection.  
         [0057]    Analysis of the sample by the surface plasmon resonance sensor will be described, hereinbelow. A samples is put in each of the recesses  2   a  of the measuring plate  10   a  to be held in contact with the metal film  3   a  in the recess  2   a . A light beam  13  is converged on the interface  5   a  between the metal film  3   a  and the dielectric plate  1   a  and the light beam  13  reflected in total internal reflection at the interface  5   a  is detected by the first and second photodetectors  16  and  17 . A first detecting signal S 1  output from the first photodetector  16  representing the amount of light beam  13  impinging upon the first photodetector  16  is input into the comparator  18  and a second detecting signal S 2  output from the second photodetector  17  representing the amount of light beam  13  impinging upon the second photodetector  17  is input into the comparator  18 . The comparator  18  outputs a differential signal S representing the difference between the first and second detecting signals S 1  and S 2 .  
         [0058]    The component impinging upon the interface at a particular angle of incidence θsp excites the surface plasmon and the intensity I of light reflected in total internal reflection at the interface  5   a  sharply drops for this component. That is, the relation between the intensity I of the light beam  13  reflected in total internal reflection at the interface  5   a  and the angle of incidence θ is substantially as shown by curve a in FIG. 4A and by curve b in FIG. 4B. When the attenuation angle θsp and the curves representing the relation between the intensity I of the light beam  13  reflected in total internal reflection at the interface  5   a  and the angle of incidence θ are known, the specific material in the sample can be quantitatively analyzed. The reason for this will be described in detail, hereinbelow.  
         [0059]    Assuming that the first and second predetermined ranges of angle of reflection are contiguous to each other on opposite sides of angle of reflection θ M , and the first photodetector  16  detects the components of the light beam  13  which impinges upon the interface Sa at an angle of incidence smaller than M, whereas the second photodetector  17  detects the components of the light beam  13  which impinges upon the interface Sa at an angle of incidence larger than θ M , the first photodetector  16  detects the components of the light beam  13  in the range represented by the hatched portion in FIGS. 4A and 4B and the amount of light detected by the first photodetector  16  is larger in the case shown by FIG. 4B than in the case shown by FIG. 4A. To the contrast, the amount of light detected by the second photodetector  17  is smaller in the case shown by FIG. 4B than in the case shown by FIG. 4A. Thus, the outputs of the first and second photodetectors  16  and  17  exhibit a specific difference according to the relation between the intensity I of the light beam  13  reflected in total internal reflection at the interface  5   a  and the angle of incidence θ.  
         [0060]    Accordingly, the attenuation angle θsp, the curves representing the relation between the intensity I of the light beam  13  reflected in total internal reflection at the interface  5   a  and the angle of incidence θ and the like can be estimated on the basis of the output S of the comparator  18  (representing the difference between the first and second detecting signals S 1  and S 2 ) by referring to a calibration curve which has been prepared for each sample, whereby the specific material in the sample can be quantitatively analyzed.  
         [0061]    Even if the first and second predetermined ranges of angle of reflection are not contiguous to each other, the outputs of the first and second photodetectors  16  and  17  exhibit a specific difference according to the relation between the intensity I of the light beam  13  reflected in total internal reflection at the interface  5   a  and the angle of incidence θ and accordingly, the specific material in the sample can be quantitatively analyzed in the same manner.  
         [0062]    By linearly or two-dimensionally arranging a plurality of the surface plasmon detecting portions with each surface plasmon detecting portion opposed to one of the recesses  2   a  of the measuring plate  10   a , samples in a plurality of recesses  2   a  can be simultaneously analyzed. Since the optical path of the light beam  13  for measuring the sample in each recess  2   a  is confined within the space between adjacent recesses  2   a , the light beam  13  for each recess  2   a  cannot be eclipsed, for instance, by the bottom portion of recesses  2   a  adjacent to the recess  2   a , and accordingly, the sample in each recess  2   a  can be accurately analyzed.  
         [0063]    A measuring plate in accordance with a second embodiment of the present invention will be described with reference to FIG. 5, hereinbelow. In FIG. 5, elements analogous to those shown in FIGS. 1 and 2 are given the same reference numerals and will not be described here unless necessary.  
         [0064]    In FIG. 5, the measuring plate  10   b  in accordance with the second embodiment comprises a dielectric plate  1   b  and a plurality of recesses  2   a  are formed on the upper surface of the dielectric plate  1   b . The bottom surface of each recess  2   a  is flat and a metal film  3   a , for instance, of gold, silver, copper, or aluminum, is provided on the flat bottom surface of each recess  2   a . A reflecting optical system, comprising a mirror  4   c  which reflects a light beam, impinging thereupon from above, toward the interface Sa between the metal film  3   a  and the dielectric plate  1   b  and a mirror  4   d  which reflects upward the light beam reflected at the interface  5   a , is provided on the lower side of the dielectric plate  1   b  for each of the recesses  2   a.    
         [0065]    In this particular embodiment, the laser  14 , the incident optical system  15  and the photodetectors  16  and  17  are disposed above the measuring plate  10   b  and the light beam  13  reflected at the interface Sa to impinge upon the first and second photodetectors  16  and  17  by way of the mirror  4   d  is detected by the first and second photodetectors  16  and  17 .  
         [0066]    With this embodiment, result similar to that obtained with the first embodiment can be obtained.  
         [0067]    A measuring plate in accordance with a third embodiment of the present invention will be described with reference to FIG. 6, hereinbelow. In FIG. 6, elements analogous to those shown in FIGS. 1 and 2 are given the same reference numerals and will not be described here unless necessary.  
         [0068]    In FIG. 6, the measuring plate  10   c  in accordance with the third embodiment comprises a dielectric plate  1   c  and a plurality of recesses  2   a  are formed on the upper surface of the dielectric plate  1   c . The bottom surface of each recess  2   a  is flat and a metal film  3   a , for instance, of gold, silver, copper, or aluminum, is provided on the flat bottom surface of each recess  2   a . A reflecting optical system, comprising a mirror  4   a  which reflects a light beam, impinging thereupon from below, toward the interface  5   a  between the metal film  3   a  and the dielectric plate  1   c  and a mirror  4   d  which reflects upward the light beam reflected at the interface  5   a , is provided on the lower side of the dielectric plate  1   b  for each of the recesses  2   a.    
         [0069]    In this particular embodiment, the laser  14  and the incident optical system  15  are disposed below the measuring plate  10   c  with the first and second photodetectors  16  and  17  disposed above the measuring plate  10   c  and the light beam  13  reflected at the interface  5   a  to impinge upon the first and second photodetectors  16  and  17  by way of the mirror  4   d  is detected by the first and second photodetectors  16  and  17 .  
         [0070]    With this embodiment, result similar to that obtained with the first embodiment can be obtained.  
         [0071]    A measuring plate in accordance with a fourth embodiment of the present invention will be described with reference to FIG. 7, hereinbelow. In FIG. 7, elements analogous to those shown in FIGS. 1 and 2 are given the same reference numerals and will not be described here unless necessary.  
         [0072]    In FIG. 7, the measuring plate  10   d  in accordance with the fourth embodiment comprises a dielectric plate  1   d  and a plurality of recesses  2   a  are formed on the upper surface of the dielectric plate id. The bottom surface of each recess  2   a  is flat and a metal film  3   a , for instance, of gold, silver, copper, or aluminum, is provided on the flat bottom surface of each recess  2   a . A reflecting optical system, comprising a mirror  4   c  which reflects a light beam, impinging thereupon from above, toward the interface  5   a  between the metal film  3   a  and the dielectric plate  1   d  and a mirror  4   b  which reflects downward the light beam reflected at the interface  5   a , is provided on the lower side of the dielectric plate  1   d  for each of the recesses  2   a.    
         [0073]    In this particular embodiment, the laser  14  and the incident optical system  15  are disposed above the measuring plate  10   d  with the first and second photodetectors  16  and  17  disposed below the measuring plate  10   d  and the light beam  13  reflected at the interface  5   a  to impinge upon the first and second photodetectors  16  and  17  by way of the mirror  4   b  is detected by the first and second photodetectors  16  and  17 .  
         [0074]    With this embodiment, result similar to that obtained with the first embodiment can be obtained.  
         [0075]    A measuring plate in accordance with a fifth embodiment of the present invention will be described with reference to FIG. 8, hereinbelow. In FIG. 8, elements analogous to those shown in FIGS. 1 and 2 are given the same reference numerals and will not be described here unless necessary.  
         [0076]    In FIG. 8, the measuring plate  10   e  in accordance with the fifth embodiment comprises a dielectric plate le and a plurality of recesses  2   b  are formed on the upper surface of the dielectric plate le. A side surface of each recess  2   b  is flat and a metal film  3   b , for instance, of gold, silver, copper, or aluminum, is provided on the flat side surface of each recess  2   b . A reflecting optical system, comprising a mirror  4   e  which reflects downward a light beam, impinging upon the interface  5   b  between the metal film  3   b  and the dielectric plate le from below and reflected at the interface  5   b , is provided on the upper side of the dielectric plate  1   e  for each of the recesses  2   b.    
         [0077]    In this particular embodiment, the laser  14 , the incident optical system  15  and the first and second photodetectors  16  and  17  are disposed below the measuring plate  10   e  and the light beam  13  reflected at the interface  5   b  to impinge upon the first and second photodetectors  16  and  17  by way of the mirror  4   e  is detected by the first and second photodetectors  16  and  17 .  
         [0078]    With this embodiment, result similar to that obtained with the first embodiment can be obtained.  
         [0079]    A measuring plate in accordance with a sixth embodiment of the present invention will be described with reference to FIG. 9, hereinbelow. In FIG. 9, elements analogous to those shown in FIGS. 1 and 2 are given the same reference numerals and will not be described here unless necessary.  
         [0080]    In FIG. 9, the measuring plate  10   f  in accordance with the sixth embodiment comprises a dielectric plate  1   f  and a plurality of recesses  2   b  are formed on the upper surface of the dielectric plate  1   f . A side surface of each recess  2   b  is flat and a metal film  3   b , for instance, of gold, silver, copper, or aluminum, is provided on the flat side surface of each recess  2   b . A reflecting optical system, comprising a mirror  4   e  which reflects a light beam, impinging thereupon from below, toward the interface  5   b  between the metal film  3   b  and the dielectric plate if, is provided on the upper side of the dielectric plate if for each of the recesses  2   b.    
         [0081]    In this embodiment, the laser  14 , the incident optical system  15  and the first and second photodetectors  16  and  17  are disposed below the measuring plate  10   f  and the light beam  13  reflected downward at the interface  5   b  to impinge upon the first and second photodetectors  16  and  17  is detected by the first and second photodetectors  16  and  17 .  
         [0082]    With this embodiment, result similar to that obtained with the first embodiment can be obtained.  
         [0083]    A measuring plate in accordance with a seventh embodiment of the present invention will be described with reference to FIG. 10, hereinbelow. In FIG. 10, elements analogous to those shown in FIGS. 1 and 2 are given the same reference numerals and will not be described here unless necessary.  
         [0084]    In FIG. 10, the measuring plate 10 g in accordance with the seventh embodiment comprises a dielectric plate  1   g  and a plurality of recesses  2   c  are formed on the upper surface of the dielectric plate  1   g . A side surface of each recess  2   c  is flat and a metal film  3   c , for instance, of gold, silver, copper, or aluminum, is provided on the flat side surface of each recess  2   c . A reflecting optical system, comprising a mirror  4   f  which reflects upward a light beam, impinging upon the interface  5   c  between the metal film  3   c  and the dielectric plate  1   g  from below and reflected at the interface  5   b , is provided on the lower side of the dielectric plate  1   g  for each of the recesses  2   c.    
         [0085]    In this embodiment, the laser  14  and the incident optical system  15  are disposed below the measuring plate 10 g with the first and second photodetectors  16  and  17  disposed above the measuring plate 10 g and the light beam  13  reflected at the interface  5   c  to impinge upon the first and second photodetectors  16  and  17  by way of the mirror  4   f  is detected by the first and second photodetectors  16  and  17 .  
         [0086]    With this embodiment, result similar to that obtained with the first embodiment can be obtained.  
         [0087]    A measuring plate in accordance with an eighth embodiment of the present invention will be described with reference to FIG. 11, hereinbelow. In FIG. 11, elements analogous to those shown in FIGS. 1 and 2 are given the same reference numerals and will not be described here unless necessary.  
         [0088]    In FIG. 11, the measuring plate  10   h  in accordance with the eighth embodiment comprises a dielectric plate  1   h  and a plurality of recesses  2   c  are formed on the upper surface of the dielectric plate  1   h . A side surface of each recess  2   c  is flat and a metal film  3   c , for instance, of gold, silver, copper, or aluminum, is provided on the flat side surface of each recess  2   c . A reflecting optical system, comprising a mirror  4   f  which reflects a light beam, impinging thereupon from above, toward the interface  5   c  between the metal film  3   c  and the dielectric plate  1   h , is provided on the lower side of the dielectric plate  1   h  for each of the recesses  2   c.    
         [0089]    In this particular embodiment, the laser  14  and the incident optical system  15  are disposed above the measuring plate  10   h  with the first and second photodetectors  16  and  17  disposed below the measuring plate  10   h  and the light beam  13  reflected at the interface  5   c  to impinge upon the first and second photodetectors  16  and  17  by way of the mirror  4   f  is detected by the first and second photodetectors  16  and  17 .  
         [0090]    With this embodiment, result similar to that obtained with the first embodiment can be obtained.