Abstract:
In order to furnish an optical component and a phase contrast microscope which can indicate difference of phases of a specimen including information of frequency and color, at least two optical mediums are arranged side by side so that a constant difference of the phases is generated.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This Application is a Section 371 National Stage Application of International Application No. PCT/JP2009/054123, filed Mar. 5, 2009 and published as WO2009/110527 on Sep. 11, 2009, in English, the contents of which are hereby incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to a Phase contrast microscope and its optical component, in particular, relates to an optical component which changes difference of phases. 
     BACKGROUND ART 
     A phase contrast microscope has been introduced as a microscope which can observe clear and colorless living cells without killing them by dyeing or the like. This phase contrast microscope can observe clear and colorless specimens with contrast of light and shade, thereby achieving observation of living cells or the like with living condition.
     [Patent document 1] Tokukai-hei 9-230247   

     As mentioned above, the phase contrast microscope can observe clear and colorless living cells with contrast of light and shade. The phase contrast microscope needs to use a phase plate in order to observe items with contrast of light and shade. 
     The phase plate is formed of a certain thin membrane, and it can advance or delay a phase by one quarter of the wavelength of the light beam by transmitting the light beam outputted by a light source through the thin membrane. Since a material which composes the thin membrane of the phase plate has dispersion, it cannot make a phase lag of one quarter of the wavelength relating to any wavelengths, thereby reducing a contrast when observing specimens using a light beam with wide range of wavelengths. The ordinary phase contrast microscope has a configuration with a colored filter corresponding to the predetermined wavelength in order to observe items using only a light beam with the predetermined wavelength. According to such configuration, the ordinary phase contrast microscope can make a phase lag of one quarter of the wavelength of the light beam. 
     However, since the ordinary phase contrast microscope has a configuration with a filter, an image of specimens is expressed only by thickness of plain color. A user, therefore, has to observe specimens without having information relating to color of the specimens. 
     DISCLOSURE OF THE INVENTION 
     Means to Solve the Problem 
     Accordingly, the present invention is made considering the above mentioned problems, and it is an object of the present invention to provide an optical component and a phase contrast microscope, which can show difference of phases including a frequency information and color information. 
     To solve the problem, in the present invention, at least two optical mediums are arranged side by side so as to generate a constant difference of phases to at least two wavelengths. 
     Specifically, an optical component of the present invention is an optical component including an input surface which receives light beams of at least two wavelengths, and an output surface which outputs the light beams after changing a phase of the received light beam, comprising 
     an optical medium body which includes at least two optical mediums arranged side by side between said input surface and said output surface, 
     said optical medium body including 
     a first optical medium portion which outputs each of said light beams of at least two wavelengths after changing a phase thereof according to the wavelength, 
     a second optical medium portion which outputs each of said light beams of at least two wavelengths after changing a phase thereof according to the wavelength so that difference between a phase of the light beam outputted from said first optical medium portion and a phase of the light beam outputted from said second optical medium portion is approximately constant. 
     The optical component of the present invention includes the input surface and the output surface. The input surface receives light beams of at least two wavelengths. The optical component changes a phase of the received light beam. The light beam in which a phase is changed is outputted from the out put surface. 
     The optical medium body is arranged between the input surface and the output surface of the optical component. It is preferable that surfaces of the optical medium body are formed as the input surface and the output surface. The optical medium body includes at least two optical mediums. The at least two optical mediums are arranged side by side between the input surface and the out put surface. 
     Further, the optical medium body includes at least a first optical medium portion and a second optical medium portion. 
     The first optical medium portion receives the above mentioned light beams of at least two wavelengths. The first optical medium portion changes a phase of received light beam according to the wavelength due to its optical property such as a refractive index. The light beam in which a phase is changed is outputted from the first optical medium portion. 
     The second optical medium portion receives the above mentioned light beams of at least two wavelengths as well as the first optical medium portion. The second optical medium portion also changes a phase of received light beam according to the wavelength due to its optical property such as a refractive index. Further the second optical medium portion changes a phase of received light beam so that difference between a phase of the light beam outputted from said first optical medium portion and a phase of the light beam outputted from said second optical medium portion is approximately constant. The light beam in which a phase is changed in such a way is outputted from the second optical medium portion. 
     Accordingly, in the case of light beams of at least two wavelengths, difference between a phase of the light beam outputted from said first optical medium portion and a phase of the light beam outputted from said second optical medium portion is almost constant according to the first optical medium portion and the second optical medium portion. In this way, since it can make a phase lag to a plurality of wavelengths, it can adjust a phase of the light beam including information of frequency of the light beam. 
     In the optical component according to the present invention, said first optical medium portion is composed of at least one optical medium layer, said second optical medium portion is composed of at least one optical medium layer, the following three formulas are applied to said first optical medium portion and said second optical medium portion. 
     It is preferable to satisfy the following three formulas; 
                   ∑   i             ⁢           ⁢       t     1   ⁢   i       ×       n   1     ⁡     (     i   ,   1     )           -       ∑   j             ⁢           ⁢       t     2   ⁢   j       ×       n   2     ⁡     (     j   ,   1     )             =     C   ⁢           ⁢   1   ⁢     λ   1                         ∑   i             ⁢           ⁢       t     1   ⁢   i       ×       n   1     ⁡     (     i   ,   2     )           -       ∑   j             ⁢           ⁢       t     2   ⁢   j       ×       n   2     ⁡     (     j   ,   2     )             =     C   ⁢           ⁢   2   ⁢     λ   2                            C   ⁢           ⁢   1     -     C   ⁢           ⁢   2            &lt;   0.02         
where a thickness of i th  layer of said first optical medium portion is the thickness t 1i ,
 
a refractive index to the light beam of the wavelength λ 1  is the refractive index n 1 (i,1),
 
a refractive index to the light beam of the wavelength λ 2  is the refractive index n 1 (i,2),
 
a thickness of i th  layer of said second optical medium portion is the thickness t 1j ,
 
a refractive index to the light beam of the wavelength λ 1  is the refractive index n 2 (j,1),
 
a refractive index to the light beam of the wavelength λ 2  is the refractive index n 2 (j,2).
 
     Further, it is preferable to satisfy the following three formulas;
 
 t ( n   1 (1,1)− n   2 (1,1))= C 1×λ1
 
 t ( n   1 (1,2)− n   2 (1,2))= C 1×λ2
 
| C 1 −C 2|&lt;0.02
 
     In this case, a thickness of said first optical medium portion and said second optical medium portion is the thickness t, 
     a refractive index of said first optical medium portion to the light beam of the first wavelength λ 1  is the refractive index n 1 (1,1), 
     a refractive index of said first optical medium portion to the light beam of the second wavelength λ 2  is the refractive index n 1 (1,2), 
     a refractive index of said second optical medium portion to the light beam of the first wavelength λ 2  is the refractive index n 2 (1,1), 
     a refractive index of said second optical medium portion to the light beam of the second wavelength λ 2  is the refractive index n 2 (1,2). 
     Further it is preferable to satisfy the following formula;
 
| n 1( i, 1)− n 2( i, 2)|&lt;0.3
 
     When presuming that the above mentioned  C 1 or  C 2 is C, C should be
 
¼≦| C|≦ ¾
 
     In this case, |C| means an absolute figure of C. 
     The above mentioned light beams of at least two wavelengths include the first wavelength λ 1  and the second wavelength λ 2 . In the above mentioned formulas, “t” is a thickness of the first optical medium portion and a thickness of the second optical medium portion. “C” is a constant not related to wavelengths. 
     Further, n(1,1) is a refractive index of the first optical medium portion to the first wavelength λ 1 , n(2,1) is a refractive index of the first optical medium portion to the second wavelength λ 2 , n(2,1) is a refractive index of the second optical medium portion to the first wavelength λ 1 , and n(2,2) is a refractive index of the second optical medium portion to the second wavelength λ 2 . 
     With using the optical mediums which satisfy the above mentioned two formulas, a phase of the light beam outputted from the first optical medium portion can be advanced or delayed to the light beam outputted from the second optical medium portion by C both in the case of the light beam of wavelength λ 1  and the light beam of wavelength λ 2 . 
     In the optical component of the present invention, it is preferable that said first wavelength λ 1  and said second wavelength λ 2  are wavelengths of visual light beams. 
     Accordingly, it can adjust a phase of the light beam including information of frequency of a visual light, that is, information of color. 
     Further, an phase contrast microscope of the present invention is an phase contrast microscope including an illumination optical system, and an image optical system, comprising 
     an opening arranged at a pupil position of said illumination optical system, 
     a phase plate arranged at a position having a conjugated relationship with said opening, said phase plate including 
     an input surface which receives light beams of at least two wavelengths, 
     an output surface which outputs the light beams after changing a phase of the received light beam, and 
     an optical medium body which includes at least two optical mediums arranged side by side between said input surface and said output surface, 
     said optical medium body including 
     a first optical medium portion which outputs each of said light beams of at least two wavelengths after changing a phase thereof according to the wavelength, 
     a second optical medium portion which outputs each of said light beams of at least two wavelengths after changing a phase thereof according to the wavelength so that difference between a phase of the light beam outputted from said first optical medium portion and a phase of the light beam outputted from said second optical medium portion is approximately constant. 
     The phase contrast microscope of the present invention includes the illumination optical system, the image optical system, the opening and the phase plate. The illumination optical system is used for illuminating a specimen. The image optical system is used for focusing the image of the specimen. The opening is arranged at a pupil position of the illumination optical system, and used for limiting a light beam which illuminates the specimen. 
     The phase plate is arranged at a position having a conjugated relationship with the opening. The phase plate includes the input surface and the output surface. The input surface receives the light beam of at last two wavelengths. The phase plate changes a phase of the received light beam. The light beam in which a phase is changed is outputted from the output surface. 
     The optical medium body is arranged between the input surface and the output surface of the optical component. It is preferable that surfaces of the optical medium body are formed as the input surface and the output surface. The optical medium body includes at least two optical mediums. The at least two optical mediums are arranged side by side between the input surface and the output surface. 
     Further, the optical medium body includes at least a first optical medium portion and a second optical medium portion. 
     The first optical medium portion receives the above mentioned light beams of at least two wavelengths. The first optical medium portion changes a phase of received light beam according to the wavelength due to its optical property such as a refractive index. The light beam in which a phase is changed is outputted from the first optical medium portion. 
     The second optical medium portion receives the above mentioned light beams of at least two wavelengths as well as the first optical medium portion. The second optical medium portion also changes a phase of received light beam according to the wavelength due to its optical property such as a refractive index. Further the second optical medium portion changes a phase of received light beams so that difference between a phase of the light beam outputted from said first optical medium portion and a phase of the light beam outputted from said second optical medium portion is approximately constant. The light beam in which a phase is changed in such a way is outputted from the second optical medium portion. 
     Accordingly, in the case of light beams of at least two wavelengths, difference between a phase of the light beam outputted from the first optical medium portion and a phase of the light beam outputted from the second optical medium portion is almost constant according to the first optical medium portion and the second optical medium portion. In this way, since it can make a phase lag to a plurality of wavelengths, it can adjust a phase of the light beam including information of frequency of the light beam. Especially, it can observe a specimen which is near to transparent including its color information. Since it can observe a specimen without using a filter, configuration of the phase contrast microscope can be simplified. 
     Effect of the Invention 
     It can show difference of phase of a specimen including information of frequency and color. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic drawing of a phase contrast microscope according to a first embodiment of the present invention. 
         FIG. 2  is a plane view (a), a sectional view (b) and a theoretical explanatory drawing of a phase plate  100  according to a first embodiment of the present invention. 
         FIG. 3  is a sectional view of a phase plate according to second to fifth embodiments of the present invention. 
         FIG. 4  is a sectional view to show a general concept of a phase plate according to first to fifth embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments according to the present invention are explained based on the drawings. 
     First Embodiment 
     &lt;&lt;Configuration of Phase Contrast Microscope&gt;&gt; 
       FIG. 1  illustrates a configuration of a phase contrast microscope  10 . 
     The phase contrast microscope  10  comprises a light source (not shown in the drawing), an aperture ring  12 , a condenser lens  14 , a field lens  14 , a phase plate  100 . These components are supported at predetermined positions by a supporting body (not shown in the drawing). 
     The aperture ring  12  has a ring shape opening and is arranged at a pupil position of a illumination optical system  1 , i.e., a front focal surface of the field lens  18  as mentioned later. A light beam outputted by the light source (not shown in the drawing) is converted to a parallel light beam in advance by an optical element such as a collimator lens before reaching the aperture ring  12 . A light beam converted to a parallel one is narrowed down by the aperture ring  12  so as to pass only through the ring shape opening, then converted to a ring shape light beam. 
     The condenser lens  14  converts the light beam which is converted to the ring shape light beam by the aperture ring  12  so as to be focused at a specimen  16  as described later. The light beam which is converted to the focused light beam by the condenser lens  14  forms an image at the specimen  16  as an example. The light beam which incidents the specimen  16  is divided into a direct light beam which transmits strait through the specimen  16  (shown by a solid line of  FIG. 1 ) and a diffractive light beam which is diffracted by a phase object of the specimen  16  and travels on the skew (shown in a dotted line of  FIG. 1 ). The diffractive light beam is diffracted by the phase object as well as a phase of the diffractive light beam is delayed by around one quarter of the wavelength. 
     The illumination optical system  1  is composed of the above mentioned light source, the aperture ring  12 , and the condenser lens  14 . 
     The field lens  18  is positioned in the traveling direction of the direct light beam and the diffractive light beam which are divided by the specimen  16 . Further, the phase plate  100  is arranged at a pupil position of an image optical system  2 , i.e., a rear focal surface of the field lens  18 , which means that the phase plate  100  is positioned at location having a conjugated relationship with the aperture ring  12 . The phase plate  100 , as mentioned later, is composed of a first optical medium portion  110  and a second optical medium portion  120 . The first optical medium portion  110  is formed in a ring shape (Refer to  FIG. 2 ). Construction and function of the phase plate  100  is described in detail later. 
     The direct light beam which passes through the specimen  16  is converted to a parallel light beam and injected to the phase plate  100 . The first optical medium portion  110  of the phase plate  100  which is formed in a ring shape is positioned so as to overlap the direct light beam which is a light beam with a ring shape. Therefore it is said that the inner diameter d 1  and the outer diameter d 2  of the first optical medium portion  110  which is formed in a ring shape on the phase plate  100  (Refer to (a) and (b) of  FIG. 2 ) correspond to the value calculated by multiplying an inner diameter and an outer diameter of the aperture ring  12  by magnifications of the condenser lens  14  and the field lens  18  respectively. 
     On the contrary, the diffractive light beam which is diffracted by the phase object of the specimen  16  passes through the outer second optical medium portion  120   b  as mentioned later, 
     The direct light beam and the diffractive light beam which pass through the phase plate  100  are focused, interfered by each other and form an image on an imaging surface  20 . 
     &lt;Configuration of Phase Plate  100 &gt; 
       FIG. 2  shows a plane view (a), a sectional view (b) and a theoretical explanatory drawing of the phase plate  100  according to the first embodiment. 
     The phase plate  100  has an approximately circular plate as shown in  FIG. 2(   a ). The phase plate  100  comprises the first optical medium portion  110  composed of a first optical medium area and the second optical medium portion  120  composed of a second optical medium area. Optical property of the first optical medium portion  110  and the second optical medium portion  120  will be described later. The phase plate  100  has an input surface  140  (a bottom surface shown in  FIG. 2  ( b )) and an output surface  150  (a top surface shown in  FIG. 2  ( b )). As mentioned later, a light beam outputted by a light source incidents the input surface  140 , passes through the first optical medium portion  110  and the second optical medium portion  120 , then is outputted from the output surface  150 . Further, a base portion  130  is formed at a side of the input surface  140  of the phase plate  100 . The base portion  130  is composed of the second optical medium portion  120  and acts as a base of the phase plate  100 . 
     The first optical portion  100  is positioned at a side of the output surface  150  of the phase plate  100  as shown in  FIG. 2(   a ). The first optical medium portion  110  has a ring shape on the output surface  150 . An inner diameter of the first optical medium portion  110  is d 1  and an outer diameter of the first optical medium portion  110  is d 2 . The inner diameter d 1  and the outer diameter d 2  can be determined according to the magnifications of the condenser lens  14  and the field lens  18 . Thickness of the first optical medium portion  110  is t 1 . The thickness t 1  of the first optical medium portion  110  can be determined so as to satisfy Formula 1 and Formula 2 as mentioned later. 
     On the contrary, the second optical medium portion  120  is positioned at an area of the phase plate  100  excluding the first optical medium portion  110 . For easily understanding, the second optical medium portion  120  is separated into an inner second optical medium portion  120   a , an outer second optical medium portion  120   b , and the base portion  130 . The base portion  130  is, as mentioned above, formed at the side of the input surface  140  of the phase plate  100  and acts as the base of the phase plate  100 . The above mentioned first optical medium portion  110 , inner second optical medium portion  120   a  and second optical medium portion  120   b  are formed concentrically. 
     The inner second optical medium portion  120   a  is positioned inside of the first optical medium portion  110 . The inner second optical medium portion  120   a  has an approximately circular shape on the input surface  140  and the output surface  150 . 
     A diameter of the inner second optical medium portion  120   a  is determined according to the above mentioned inner diameter d 1  of the first optical medium portion  110 . The outer second optical medium portion  120   b  is positioned outside of the first optical medium portion  110 . The outer second optical medium portion  120   b  has an approximately ring shape on the input surface  140  and the output surface  150 . An inner diameter of the outer second optical medium portion  120   b  is determined according to the outer diameter d 2  of the first optical medium portion  110 , and an outer diameter of the outer second optical medium portion  120   b  is determined according to a diameter of the phase plate  100 . Thickness of the second optical medium portion  120  is also t 1  and it can be determined according to Formula 1 and Formula 2 as mentioned later. 
     As just described, the first optical medium portion  110  and the second optical medium portion  120  (the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b ) is arranged side by side between the input surface  140  and the output surface  150 . 
     The first optical medium portion  110  shows a refractive index n 1  (1,1) to a light beam of the wavelength λ 1 , and shows the refractive index n 1  (1,2) to a light beam of the wavelength λ2. The first optical medium portion  110  preferably made of a plastic, a resin or the like. 
     On the contrary, the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b  show the refractive index n 2  (1,1) to a light beam of the wavelength λ 1 , and shows the refractive index n 2 (1,2) to a light beam of the wavelength λ2. 
     The inner second optical medium portion  120   a  and the outer second optical medium portion  120   b  is preferably a medium having an approximately circular plate shape. The base portion  130  shows the same refractive index because it is also made of the material of the second optical medium portion. 
     These refractive indexes n 1  (1,1), n 1 (1,2), n 2 (1,1) and n 2 (1,2) satisfy the following formulas.
 
 t ( n   1 (1,1)− n   2 (1,1))= C 1×λ1  Formula 1
 
 t ( n   1 (1,2)− n   2 (1,2))= C 1×λ2
 
| C 1− C 2|&lt;0.02  Formula 2
 
     “t” of Formula 1 and Formula 2 is thickness of the first optical medium portion  110 , the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b . In the embodiment shown in  FIG. 2(   b ), it is thickness t 1 , in other words, it is a thickness calculated by deducting the thickness of the base portion  130  from the total thickness t 2  of the phase plate  100 . The thickness t 1  contributes to change of a phase of the first optical medium portion  110  and change of a phase of the second optical medium portion  120 . The thickness t 1  is one of the elements to make difference between a phase of the light beam passing through the first optical medium portion  110  and a phase of the light beam passing through the second optical medium portion  120 . 
     &lt;Manufacture of Phase plate  100 &gt; 
     The above mentioned phase plate  100  is formed by machining a glass plate having an approximately circular shape. This glass plate becomes the second optical medium portion  120  (the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b ) and the base portion  130 . In machining process of the glass plate, at first, a groove having a ring shape is formed as concentrically to the phase plate  100  by machining a surface of a side of the output surface  150  of the glass plate. The machining is performed to make depth of the groove U. Then an adhesive agent is poured into the groove and solidified. The solidified adhesive agent becomes the first optical medium portion  110 . Since the groove is formed and the first optical medium portion  110  is formed with adhesive agent, the phase plate  100  is made easily and with low costs. Since the groove is formed by machining, thickness of the first optical medium portion  110  can be adjusted according to the thickness of the groove. It is easy to make thickness of the first optical medium portion  110  thin or thick. If it is desired to make thickness of the first optical medium portion  110  thick, it may be formed by a thin film forming process such as a deposition or a sputtering. 
     A resin material such as a clear plastic may be utilized as the second optical medium portion  120  instead of a glass plate if the above mentioned Formula 1 and Formula 2 are satisfied. Accordingly, the phase plate  100  can be made easily with low costs. 
     &lt;Function of Phase plate  100 &gt; 
     As mentioned above, the light beam outputted by the light source is divided into the direct light beam and the diffractive light beam by the phase object of the specimen  16 . A phase of the diffractive light beam is delayed by approximately one quarter of the wavelength when it is diffracted by the phase object of the specimen  16 . On the contrary, the direct light beam is not diffracted by the phase object of the specimen  16  and its phase is not changed. 
     As shown in the dotted line in  FIG. 1 , traveling direction of the diffractive light beam is adjusted by the field lens  18  so as to pass through the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b  of the phase plate  100 . Specifically, the diffractive light beam which reaches the input surface  140  of the phase plate  100  passes through the base portion  130 . Then, the diffractive light beam which passes through the base portion  130  reaches the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b . Further, the light beam passes through the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b , then is outputted from the output surface  150  of the phase plate  100 , and travels toward the image surface  20 . As mentioned above, while traveling direction of the diffractive light beam is explained using the inner second optical medium portion  120   a , the outer second optical medium portion  120   b  and the base portion  130 , which are divided for easily understanding, there is no actual border line and no border surface. 
     On the contrary, as shown in the solid line in  FIG. 1 , traveling direction of the direct light beam is adjusted by the field lens  18  so as to pass through the first optical medium portion  110  of the phase plate  100 . Specifically, the direct light beam which reaches the input surface  140  of the phase plate  100  passes through the base portion  130 . Then, the direct light beam which passes through the base portion  130  reaches the first optical medium portion  110 . Further, the light beam passes through the first optical medium portion  110 , then is outputted from the output surface  150  of the phase plate  100 , and travels toward the image surface  20 . 
     As mentioned above, both the direct light beam and the diffractive light beam pass through the base portion  130  after they enter the phase plate from the input surface  140 . Therefore, phases of both the direct light beam and the diffractive light beam are changed in the same way by the refractive index of the second optical medium portion  120  of which the base portion  130  is composed and by the thickness of the base portion  130 . Therefore, difference between a phase of the direct light beam and a phase of the diffractive light beam before passing through the base portion  130  is the same as difference of the phases after passing through the base portion  130 . 
     After passing through the base portion  130 , the direct light beam passes through the first optical medium portion  110 , and the diffractive light beam passes through the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b . Accordingly, a phase of the direct light beam is changed by the refractive index and thickness of the first optical medium portion  110 , and a phase of the diffractive light beam is changed by the refractive index and thickness of the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b . Therefore, it makes difference between difference of phases of the direct light beam and the diffractive light beam just after passing through the base portion  130  and difference of phases of the direct light beam and the diffractive light beam when outputted from the output surface  150  of the phase plate  100 . Difference between a phase of the direct light beam and a phase of the diffractive light beam after passing through the base portion  130  is approximately one quarter of the wavelength, which is caused by the phase object of the specimen  16 . 
     While the base portion  130  affects a phase of the direct light beam and a phase of the diffractive light beam, it does not affect difference between a phase of the direct light beam and a phase of the diffractive light beam when outputted from the output surface  150  of the phase plate  100 . On the contrary, the first optical medium portion  110 , the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b  which satisfy Formula 1 and Formula 2 affect difference between phases of the direct light beam and a phase of the diffractive light beam when outputted from the output surface  150  of the phase plate  100 . 
     Hereinafter, light beams of the wavelength λ, 1  and the wavelength λ 2  are explained in detail using  FIG. 2(   c ) which is a principle drawing. The base portion  130  of the phase plate  100  shown in  FIG. 2(   b ) is omitted in  FIG. 2(   c ) 
     A light beam having the wavelength λ 1  is divided into a direct light beam LR 1  and a diffractive light beam LD 1  by a specimen  16 . 
     The direct light beam LR 1  incidents a first optical medium portion  110  shown in  FIG. 2(   c ) by a field lens  18 . Since the direct light beam LR 1  which incidents the first optical medium portion  110  has wavelength it is outputted from the first optical medium portion  110  having a certain phase lag according to the refractive index n 1  ( 1 , 1 ) and the thickness t 1  of the first optical medium portion  110 . On the contrary, the diffractive light beam LD 1  incidents the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b  shown in  FIG. 2(   c ) by the field lens  18 . Since the diffractive light beam LD 1  which incidents the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b  also has the wavelength λ 1 , it is outputted from the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b  having a certain phase lag according to the refractive index n1 (1,2) and the thickness t 1  of the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b.    
     Accordingly, the direct light beam LR 1  outputted from the first optical medium portion  110  has a phase lag of (λ 1 )/4 to the diffractive light beam LD 1  outputted from the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b  due to the relationship shown in Formula 1. 
     Correspondingly, the direct light beam LR 2  incidents a first optical medium portion  110  shown in  FIG. 2(   c ) by a field lens  18 . Since the direct light beam LR 2  which incidents the first optical medium portion  110  has the wavelength λ 2 , it is outputted from the first optical medium portion  110  having a certain phase lag according to the refractive index n1 (2,1) and the thickness t 1  of the first optical medium portion  110 . On the contrary, the diffractive light beam LD 2  incidents the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b  shown in  FIG. 2(   c ) by a field lens  18 . Since the diffractive light beam LD 2  which incidents the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b  also has the wavelength λ 2 , it is outputted from the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b  having a certain phase lag according to the refractive index n1 (2,2) and the thickness t 1  of the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b.    
     Accordingly, the direct light beam LR 2  outputted from the first optical medium portion  110  has a phase lag of (λ 2 )/4 to the diffractive light beam LD 2  outputted from the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b  due to the relationship shown in Formula 2. 
     With using the first optical medium portion  110 , the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b  which satisfy the above mentioned Formula 1 and Formula 2, a phase of the light beam outputted from the first optical medium portion can be advanced or delayed to the light beam outputted from the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b  by one quarter of the wavelength both in the case of the light beam of wavelength λ 1  and the light beam of wavelength λ 2 . 
     A phase plate used in the ordinary phase contrast microscope uses air as a medium corresponding to the second optical medium portion, and it can advance or delay a phase of the direct light beam to the diffractive light beam by one quarter of the wavelength due to the refractive index of the first optical medium portion and a refractive index of air. In the case of light beams having a plurality of wavelengths, however, a phase of the direct light beam cannot have a phase lag of one quarter of the wavelength to the diffractive light beam due to dispersion of the first optical medium portion. 
     On the contrary, the present invention can utilize not only dispersion of the first optical medium but also dispersion of the second optical medium, and satisfy the relationship shown in Formula 1 and Formula 2. Therefore, even in the case of light beams having a plurality of wavelengths, a phase of the direct light beam can have a phase lag of one quarter of the wavelength to the phase of the diffractive light beam. 
     It is preferable that both the wavelength λ 1  and the wavelength λ 2  are within a rage of visible light beams, and difference between the wavelength λ 1  and the wavelength λ 2  is maximized. Accordingly, a phase of the direct light beam can have a phase lag of one quarter of the wavelength to a phase of the diffractive light beam in a wide range of visual light beams. 
     Example 1 
         C 1= t 1( n   1 (1,1)− n   2 (1,1))/λ1=0.24990
 
 C 2= t 1( n   1 (1,2)− n   2 (1,2))/λ2=0.25046
 
Where
 
λ1=0.248613 μm, λ2=0.65627 μm
         for wavelength,
 
 n   1 (1,1)=1.60940
 
 n   1 (1,2)=1.60019
 
 n   2 (1,1)=1.58835
 
 n   2 (1,2)=1.57171
   for refractive index,
 
 t 1= t 2=5.7714 μm
   for thickness.       

     Example 2 
         C 1= t 1( n   1 (1,1)− n   2 (1,1))/λ1=0.25352
 
 C 2= t 1( n   1 (1,2)− n   2 (1,2))/λ2=0.24656
 
Where
 
λ1=0.48613 μm, λ2=0.65627 μm
         for wavelength,
 
 n   1 (1,1)=1.59230
 
 n   1 (1,2)=1.58323
 
 n   2 (1,1)=1.57616
 
 n   2 (1,2)=1.56204
   for refractive index,
 
 t 1= t 2=7.636 μm
   for thickness.       

     Second Embodiment 
       FIG. 3  ( a ) illustrates a phase plate  200  according to the second embodiment of the present invention. The same numeral is specified to the items similar to those of the phase plate  100  according to the first embodiment. 
     The phase plate  200  of the second embodiment is formed by adding a protection glass cover  210  to the phase plate  100  of the first embodiment. While an output surface of the phase plate  200  becomes a top surface of the protection glass cover  210 , both the direct light beam and the diffractive light beam can pass through the protection glass cover. Accordingly, with using the first optical medium portion  110 , the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b  which satisfy the above mentioned Formula 1 and Formula 2, a phase of the light beam outputted from the first optical medium portion can be advanced or delayed to the light beam outputted from the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b  by one quarter of the wavelength both in the case of the light beam of wavelength λ 1  and the light beam of wavelength λ 2 . 
     Further, since the first optical medium portion  110 , the inner second optical medium portion  120   a  and the outer second optical medium portion  120   b  are protected by the protection glass cover in the phase plate  200  of the second embodiment, it can increase strength, achieve ease for handling, and protect against damages such as scratches, thereby maintaining an optical property of the phase plate  200 . 
     Third Embodiment 
       FIG. 3  ( b ) illustrates a phase plate  300  according to the third embodiment of the present invention. The phase plate  300  of the third embodiment is formed by forming a first optical medium portion  310  and a second optical medium portion  320  on a base portion  330 . 
     As a shape, a size or a material of the first optical medium portion  310  except thickness, those similar to the first optical medium portion  110  of the first embodiment can be utilized. The second optical medium portion  320  is composed of an inner second optical medium portion  320   a  and an outer second optical medium portion  320   b  as well as the first embodiment. A bottom surface of the phase plate  300  is an input surface  340  and a top surface of the phase plate  300  is an output surface  350 . 
     As a base portion  330 , a glass or a plastic can be used if a light beam can pass through it. Accordingly, since it can omit machine process or mold process or the like, the phase plate  300  can be made easily. 
     A thickness of the first optical medium portion  310  is different from a thickness of the second optical medium portion  320  in the phase plate  300  of the third embodiment as shown in  FIG. 3(   b ). In this case, a phase can be adjusted using the following formula. 
     
       
         
           
             
               
                 
                   
                     
                       t 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       3 
                       × 
                       
                         
                           n 
                           1 
                         
                         ⁡ 
                         
                           ( 
                           
                             1 
                             , 
                             1 
                           
                           ) 
                         
                       
                     
                     - 
                     
                       [ 
                       
                         
                           t 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           4 
                           × 
                           
                             
                               n 
                               2 
                             
                             ⁡ 
                             
                               ( 
                               
                                 1 
                                 , 
                                 1 
                               
                               ) 
                             
                           
                         
                         + 
                         
                           ( 
                           
                             
                               t 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               3 
                             
                             - 
                             
                               t 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               4 
                             
                           
                           ) 
                         
                       
                       ] 
                     
                   
                   = 
                   
                     
                       1 
                       4 
                     
                     × 
                     λ1 
                   
                 
               
               
                 
                   Formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   3 
                 
               
             
             
               
                 
                   
                     
                       t 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       3 
                       × 
                       
                         
                           n 
                           1 
                         
                         ⁡ 
                         
                           ( 
                           
                             1 
                             , 
                             2 
                           
                           ) 
                         
                       
                     
                     - 
                     
                       [ 
                       
                         
                           t 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           4 
                           × 
                           
                             
                               n 
                               2 
                             
                             ⁡ 
                             
                               ( 
                               
                                 1 
                                 , 
                                 2 
                               
                               ) 
                             
                           
                         
                         + 
                         
                           ( 
                           
                             
                               t 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               3 
                             
                             - 
                             
                               t 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               4 
                             
                           
                           ) 
                         
                       
                       ] 
                     
                   
                   = 
                   
                     
                       1 
                       4 
                     
                     × 
                     λ2 
                   
                 
               
               
                 
                   Formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   4 
                 
               
             
           
         
       
     
     The wavelength λ 1 , wavelength λ 2 , refractive index n 1 (1,1), n 1 (1,2), n 2 (1,1) and n 2 (1,2) are the same as those of the first embodiment, “t 3 ” is a thickness of the first optical medium portion  310 , and “t 4 ” is a thickness of the second optical medium portion  320 , as shown in  FIG. 3(   b ). A refractive index of air is 1. 
     With using the first optical medium portion  310  and the second optical medium portion  320  which satisfy the above mentioned Formula 3 and Formula 4, a phase of the light beam outputted from the first optical medium portion  310  can be advanced or delayed to the light beam outputted from the second optical medium portion  320  by one quarter of the wavelength both in the case of the light beam of wavelength λ 1  and the light beam of the wavelength λ 2 . 
     In the above mentioned case, Formula 3 and Formula 4 are conditions which should be applied to two wavelengths λ 1  and λ 2 . If Formula 3 and Formula 4 are satisfied, there is a case to apply the following formula to the wavelength λ 3  which is close to the wavelength λ 1  and the wavelength λ 2 .
 
 t 3× n   1 (1,3)−[ t 4× n   2 (1,3)+( t 3− t 4)]=δ×λ3  Formula 5
 
Where
 
|δ−¼|&lt;0.02
         for 5.       

     The refractive index n 1 (1,3) is a refractive index of the first optical medium portion  310  to the wavelength λ 3 , and the refractive index n 2 (1,3) is a refractive index of the first optical medium portion  320  to the wavelength λ 3 . If “δ” is fully close to ¼, it shows that an adequate phase plate can be obtained in a wide range of λ 1 , λ 2  and λ 3 . 
     Fourth Embodiment 
       FIG. 3(   c ) illustrates a phase plate  400  according to the fourth embodiment of the present invention. The same numeral is specified to the items similar to those of the phase plate  300  according to the third embodiment. 
     The phase plate  400  of the fourth embodiment is formed by adding a protection glass cover  410  to the phase plate  300  of the third embodiment. While an output surface of the phase plate  400  becomes a top surface of the protection glass cover  410 , both the direct light beam and the diffractive light beam can pass through the protection glass cover. Accordingly, with using the first optical medium portion  310 , the inner second optical medium portion  320   a  and the outer second optical medium portion  320   b  which satisfy the above mentioned Formula 3 and Formula 4, a phase of the light beam outputted from the first optical medium portion  310  can be advanced or delayed to the light beam outputted from the inner second optical medium portion  320   a  and the outer second optical medium portion  320   b  by one quarter of the wavelength both in the case of the light beam of the wavelength λ 1  and the light beam of the wavelength λ 2 . 
     Further, since the first optical medium portion  310 , the inner second optical medium portion  320   a  and the outer second optical medium portion  320   b  are protected by the protection glass cover in the phase plate  400  of the fourth embodiment, it can increase strength, achieve ease in handling, and protect against damages such as scratches, thereby maintaining an optical property of the phase plate  400 . 
     Fifth Embodiment 
       FIG. 3(   d ) illustrates a phase plate  500  according to the fifth embodiment of the present invention. The same numeral is specified to the items similar to those of the phase plate  300  according to the third embodiment. 
     The phase plate  500  of the fifth embodiment is configured such that the first optical medium portion  310  is covered with a second optical medium portion  520 . Further, the second optical medium portion  520  is covered with a protection layer  510 . A bottom surface of the phase plate  510  is an input surface  540  and a top surface of the phase plate  510  is an output surface  550 . 
     For the second optical medium portion  520 , those similar to the second optical medium portion  120  according to the first embodiment can be utilized. The second optical medium portion  520  of the phase plate  500  is composed of an inner second optical medium portion  520   a  and an outer second optical medium portion  520   b  as well as the first embodiment. Accordingly, the first optical medium portion  310  can be adequately protected by covering the first optical medium portion  310  with the second optical medium portion  520 . Further, since entire surface of the second optical medium portion  520  is covered with the protection layer  510 , both the first optical medium layer  310  and the second optical medium portion  520  can be protected. It can also achieve ease in handling, and protect against damages, thereby maintaining an optical property of the phase plate for a long period. 
     &lt;&lt;&lt;Common Description of Embodiments 1 to 5&gt;&gt;&gt; 
       FIG. 4  illustrates a phase plate  600  which generalizes phase plates according to the above mentioned embodiments 1 to 5. An item shown at the left side of the phase plate  600  in  FIG. 4  is a first optical medium portion  600   a  to which the first optical medium portion is generalized, and an item shown at the right side of the phase plate  600  is a second optical medium portion  600   b  to which the second optical medium portion is generalized. The first optical medium portion  600   a  and the second optical medium portion  600   b  are formed of at least one layer. 
     In the embodiment shown in  FIG. 4 , a first layer  602 - 1  of a first optical medium portion  600   a  shows the refractive index n 1 (1,1) to a light beam of the wavelength λ 1  with the thickness t 11 , and shows the refractive index n 1 (1,2) to a light beam of the wavelength λ 2 . A second layer  602 - 2  of the first optical medium portion  600   a  shows the refractive index n 1 (2,1) to a light beam of the wavelength λ 1  with the thickness t 12 , and shows the refractive index n 1 (2,2) to the light beam of the wavelength λ 2 . Further, an “i th ” layer  602 - i  of the first optical medium portion  600   a  shows the refractive index n 1 (i,1) to the light beam of the wavelength λ 1  with the thickness t 1   i , and shows the refractive index n 1 (i,2) to the light beam of the wavelength λ 2 . 
     A first layer  604 - 1  of a second optical medium portion  600   b  shows the refractive index n 2 (1,1) to the light beam of the wavelength λ 1  with the thickness t 21 , and shows the refractive index n 2 (1,2) to the light beam of the wavelength λ 2 . A second layer  604 - 2  of the second optical medium portion  600   b  shows the refractive index n 2 (2,1) to the light beam of the wavelength λ 1  with the thickness t 22 , and shows the refractive index n 2 (2,2) to the light beam of the wavelength λ 2 . Further, an “i th ” layer  604 - i  of the second optical medium portion  600   b  shows the refractive index n 2 (i,1) to the light beam of the wavelength λ 1  with the thickness t 2i , and shows the refractive index n 2 (i,2) to the light beam of the wavelength λ 2 . 
     Both light beams of the wavelength λ 1  and the wavelength λ 2  can pass through all the layers of the above mentioned first optical medium portion  600   a , and both light beams of the wavelength λ 1  and the wavelength λ 2  can pass through all the layers of the above mentioned second optical medium portion  600   b.    
     In the case of the phase plate  600  composed of the first optical medium portion  600   a  and the second optical medium portion  600   b , a phase can be adjusted with using the following three formulas. 
     
       
         
           
             
               
                 
                   
                     
                       
                         ∑ 
                         i 
                         
                             
                         
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           t 
                           
                             1 
                             ⁢ 
                             i 
                           
                         
                         × 
                         
                           
                             n 
                             1 
                           
                           ⁡ 
                           
                             ( 
                             
                               i 
                               , 
                               1 
                             
                             ) 
                           
                         
                       
                     
                     - 
                     
                       
                         ∑ 
                         j 
                         
                             
                         
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           t 
                           
                             2 
                             ⁢ 
                             j 
                           
                         
                         × 
                         
                           
                             n 
                             2 
                           
                           ⁡ 
                           
                             ( 
                             
                               j 
                               , 
                               1 
                             
                             ) 
                           
                         
                       
                     
                   
                   = 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                     ⁢ 
                     
                       λ 
                       1 
                     
                   
                 
               
               
                 
                   Formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   6 
                 
               
             
             
               
                 
                   
                     
                       
                         ∑ 
                         i 
                         
                             
                         
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           t 
                           
                             1 
                             ⁢ 
                             i 
                           
                         
                         × 
                         
                           
                             n 
                             1 
                           
                           ⁡ 
                           
                             ( 
                             
                               i 
                               , 
                               2 
                             
                             ) 
                           
                         
                       
                     
                     - 
                     
                       
                         ∑ 
                         j 
                         
                             
                         
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           t 
                           
                             2 
                             ⁢ 
                             j 
                           
                         
                         × 
                         
                           
                             n 
                             2 
                           
                           ⁡ 
                           
                             ( 
                             
                               j 
                               , 
                               2 
                             
                             ) 
                           
                         
                       
                     
                   
                   = 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                     ⁢ 
                     
                       λ 
                       2 
                     
                   
                 
               
               
                 
                   Formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   7 
                 
               
             
             
               
                 
                   
                      
                     
                       
                         C 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                       - 
                       
                         C 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         2 
                       
                     
                      
                   
                   &lt; 
                   0.02 
                 
               
               
                 
                   Formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   8 
                 
               
             
           
         
       
     
     With using the first optical medium portion  600  and the second optical medium portion  600   b  which satisfy the above mentioned Formula 6, Formula 7 and Formula 8, a phase of the light beam outputted from the first optical medium portion  600   a  can be advanced or delayed to the light beam outputted from the second optical medium portion  600   b  by C1 (to C2) both in the case of the light beam of the wavelength λ 1  and the light beam of the wavelength λ 2 . 
     DESCRIPTION OF REFERENCE NUMERALS 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 10 
                 Phase contrast microscope 
               
               
                 100, 200, 300, 
                 Phase plate (Optical component) 
               
               
                 400, 500, 600 
                   
               
               
                 110, 310 
                 First optical medium portion (First optical medium area) 
               
               
                 120, 320, 520 
                 Second optical medium portion (Second optical medium  
               
               
                   
                 area) 
               
               
                 140, 340, 540 
                 Input surface 
               
               
                 150, 350, 550 
                 Output surface 
               
               
                 600a 
                 First optical medium portion (First optical medium area) 
               
               
                 600b 
                 Second optical medium portion (Second optical medium  
               
               
                   
                 area)