Abstract:
In an optical lens used for a passing light having the maximum intensity on a wavelength (λ T ) of 780±10 nm, an optical is made such that a reflection preventing coating is provided on both or at least one of a light-incident surface (S 1 ) and a light-outgoing surface (S 2 ) and the following conditional formula is satisfied: R 2 (λ R )&gt;R 1 (λ R ), where R 1 (λ R ) and R 2 (λ R ) are a reflectance of said respective surfaces for light having a wavelength (λ R ) falling within a range from 500 to 700 nm.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to an optical lens for use in an optical head for reading an optical disk, and in particular, to a lens coating in relation to a high-precision mounting of an optical lens to an optical head with a use of a reflected light of a laser beam from a lens.  
           [0002]    In a conventional optical lens for use in an optical head for reading an optical disk, a reflection preventing coating (hereinafter referred to also as a coat) is provided at each of a light-incident surface into which light comes and a light-outgoing surface from which the incident light is emitted and a laser beam of 780 nm is used as a passing light for the lens. Further, the optical characteristic of the reflection preventing coat provided on each of the light-incident surface (S 1 ) and the light-outgoing surface (S 2 ) of an optical lens is such one as shown in FIG. 1.  
           [0003]    [0003]FIG. 1 is a drawing for explaining a conventional example of a coat (reflection preventing coat); FIG. 1( a ) is an illustration of the layer structure of the coat on the surface S 1  and the surface S 2 , and FIG. 1( b ) is a drawing showing the reflectance (spectral reflectance) vs. the wavelength of light.  
           [0004]    Moreover, the reflectance R (%) on the ordinate in FIG. 1 and in FIG. 2 to FIG. 7 which are described below is represented in logarithmic scale for the convenience of preparing the drawings. (Only in the last FIG. 8, the ordinate is represented with divisions of equal intervals.) The layer structure of the reflection preventing coat of the light-incident surface (S 1 ) and the light-outgoing surface (S 2 ) (for light having a wavelength of 780 nm) is made such one as stated below. Further, for a substrate material, a resin material such as an acrylic resin, “Arton” resin, “Zeonex” resin, or a polycarbonate resin is used.  
           [0005]    First layer: cerium oxide (refractive index n≈2.03)  
           [0006]    layer thickness d≈34 nm  
           [0007]    Second layer: silicon oxide (refractive index n≈1.45)  
           [0008]    layer thickness d≈177 nm  
           [0009]    With respect to the position adjustment in mounting an optical lens to an optical reading head, a lens which has been coated with reflection preventing coats is fitted in an optical reading head, and a He—Ne laser beam having a wavelength of 633 nm is irradiated through this lens, and the position adjustment is done by utilizing the reflected light.  
           [0010]    However, as shown in FIG. 1, the reflectance for the wavelength 633 nm of a He—Ne laser beam is as low as 4.3%, and there has been the problem that a high precision can not be obtained in the position adjustment of the lens.  
         SUMMARY OF THE INVENTION  
         [0011]    This invention has been made in order to solve the above-mentioned problem. That is, it is an object of the invention to provide means for improving the precision of the position adjustment of the lens, by preventing the lowering of the intensity of transmitting light having the wavelength (λ T ) and by raising the reflectance of the surface S 2  for the wavelength (λ R ) of the light for the position adjustment.  
           [0012]    The object of this invention can be accomplished by employing any one of the structures described below.  
           [0013]    That is, in an optical lens to be used for a passing light having the maximum intensity at the wavelength (λ T ) 780±10 nm, an optical component is made such that both or at least one of a light-incident surface (S 1 ) and a light-outgoing surface (S 2 ) is provided with a reflection preventing coating and the following inequality is satisfied:  
             R   2 (λ R )&gt; R   1 (λ R ),  
           [0014]    where R 1 (λ R ) and R 2 (λ R ) denote the reflectance of the respective surfaces for a light having a wavelength (λ R ) falling within a range from 500 to 700 nm.  
           [0015]    Further, in an optical lens to be used for a passing light having the maximum intensity at a wavelength (λ T ) falling within a range from 600 to 700 nm, an optical component is made such that both or at least one of the light-incident surface (S 1 ) and the light-outgoing surface (S 2 ) is provided with a reflection preventing coating, and the following inequality is satisfied:  
             R   2 (λ R )&gt; R   1 (λ R ),  
           [0016]    where R 1 (λ R ) and R 2 (λ R ) denote the reflectance of the respective surfaces for a light having a wavelength (λ R ) falling within a range from 750 to 850 nm.  
           [0017]    Further, in an optical lens to be used for a passing light having the maximum intensity at a wavelength falling within a range from 350 to 500 nm, an optical component is made such that both or at least one of the light-incident surface (S 1 ) and the light-outgoing surface (S 2 ) is provided with a reflection preventing coating, and the following inequality is satisfied:  
             R   2 (λ R )&gt; R   1 (λ R ),  
           [0018]    where R 1 (λ R ) and R 2 (λ R ) denote the reflectance of the respective surfaces for a light having a wavelength (λ R ) within a range from 500 to 800 nm.  
           [0019]    Further, the reflectance R 2 (λ R ) of the light-outgoing surface (S 2 ) of the optical component is made not smaller than 5% for the wavelength (λ R ).  
           [0020]    For example, these are as follows.  
           [0021]    (1) When the transmittance T(λ T ) for a laser beam having a peak intensity at the wavelength (λ T ) 780 nm is made 96% or more and the wavelength (λ R ) falls within a range from 500 to 700 nm, desirably is the wavelength of 633 nm of a He—Ne laser beam, the following conditional formula is satisfied:  
             R   1 (λ R )&lt; R   2 (λ R ),  
           [0022]    where R 1 (λ R ) and R 2 (λ R ) denote the reflectance of the light-incident surface and the light-outgoing surface respectively.  
           [0023]    (2) When a lens is fitted in a pickup for an optical disk player, light is irradiated to the surface S 2  of the lens and the position adjustment in the fitting is done by detecting the reflected light. Assuming that the wavelength of the reflected light is λ R , the yield of assembly in the fitting greatly depend upon the reflectance R 2 (λ R ) of the surface S 2  for light having the wavelength λ R . In the case where R 2 (λ R ) ≧5%, the yield of 88% or more can be obtained, and in the case where R 2 (λ R )≧7%, the yield of 95% or more can be obtained. In order to make the expense of assembly smaller as far as possible, it is necessary to raise the yield as much as possible; it is required at least that R 2 (λ R )≧5%, and it is desirably required that R 2 (λ R )≧7%.  
           [0024]    This invention is capable of solving these requirements. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    [0025]FIG. 1( a ) and FIG. 1( b ) are drawings for explaining an example of a conventional coat.  
         [0026]    [0026]FIG. 2( a ), FIG. 2( b ), FIG. 2( c ), and FIG. 2( d ) are drawings for explaining the coat of Embodiment 1.  
         [0027]    [0027]FIG. 3( a ), FIG. 3( b ), and FIG. 3( c ) are drawings for explaining the coat of Embodiment 2.  
         [0028]    [0028]FIG. 4( a ), FIG. 4( b ), and FIG. 4( c ) are drawings for explaining the coat of Embodiment 3.  
         [0029]    [0029]FIG. 5( a ), FIG. 5( b ), and FIG. 5( c ) are drawings for explaining the coat of Embodiment 4.  
         [0030]    [0030]FIG. 6( a ), FIG. 6( b ), and FIG. 6( c ) are drawings for explaining the coat of Embodiment 5.  
         [0031]    [0031]FIG. 7( a ), FIG. 7( b ), and FIG. 7( c ) are drawings for explaining the coat of Embodiment 6.  
         [0032]    [0032]FIG. 8 is a drawing for explaining the coat of Embodiment 7. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0033]    In the following, examples of practice will be shown.  
         [0034]    &lt;Embodiment 1&gt; 
         [0035]    [0035]FIG. 2 are drawings for explaining the coat of Embodiment 1; FIG. 2( a ) is an illustration of the structure of the coat layer of the surface S 1 , FIG. 2( b ) is an illustration of the structure of the coat layer of the surface S 2 , FIG. 2( c ) is a drawing showing a typical reflectance of the surface S 1  vs. light wavelength, and FIG. 2( d ) is a drawing showing a typical reflectance of the surface S 2  vs. light wavelength.  
         [0036]    For the base material in Embodiment 1, any one out of an acrylic resin, “Arton” resin, “Zeonex” resin, and a polycarbonate resin is used.  
         [0037]    Reflection preventing coat of the surface S 1   
         [0038]    (herer, n ij : the refractive index of the j-th layer of the surface S i , d ij : the layer thickness (mm) of the j-th layer of the surface S i , i: 1 or 2, j: an integer)  
         [0039]    First layer: cerium oxide (refractive index n 11 ≈2.03)  
         [0040]    layer thickness d 11 =340 Å±30 Å 
         [0041]    Second layer: silicon oxide (refractive index n 12 ≈1.45)  
         [0042]    layer thickness d 12 =1770 Å±150 Å 
         [0043]    Reflection preventing coat of the surface S 2   
         [0044]    First layer: cerium oxide (refractive index n 21 ≈2.03)  
         [0045]    layer thickness d 21 =395 Å±15 Å 
         [0046]    Second layer: silicon oxide (refractive index n 22 ≈1.45)  
         [0047]    layer thickness d 22 =2075 Å±75 Å 
         [0048]    In the vacuum deposition method, heating by an electronic gun is employed. For the evaporation source, a pellet of cerium oxide or particles of silicon oxide are placed. Oxide gas is introduced with its pressure made to be 1.5×10 −2  pas to carry out the vacuum deposition.  
         [0049]    &lt;EFFECT&gt; 
         [0050]    In this way, the following result was obtained.  
         [0051]    As shown in FIG. 2( c ) and FIG. 2( d ), for the reflectance of 4.3% of the surface S 1  for the wavelength 633 nm of the laser beam for the position adjustment, the reflectance of the surface S 2  was 9.8%, which is larger than the conventional one, thereby making it possible to improve the precision of the position adjustment. Further, the transmittance for the passing laser beam having the wavelength of 780 nm was kept at 96% or more.  
         [0052]    The summary of the results is as follows:  
         [0053]    Transmittance T(λ T )≧96%  
         [0054]    (λ T : wavelength of a laser beam having the maximum intensity at 780 nm)  
         [0055]    R 1 (λ R )=1.5% to 7.0%  
         [0056]    (λ R : wavelength of a light having the maximum intensity at 633 nm (He—Ne laser beam))  
         [0057]    R 2 (λ R )=9.7% to 13.0%  
         [0058]    (λ R : the same as the above).  
         [0059]    In this way, as the transmittance for the passing laser beam for use in an optical head having the above-mentioned wavelength, 96% or more could be secured. Further, the reflectance R 2 (λ R ) of the light-outgoing surface of the lens (S 2 ) for the laser beam for the position adjustment of the lens was 9.7% to 13.0%, which is larger than 4.3% of the conventional example; thus, it has become possible that a reflectance of at least 5% or more is secured and the reflectance R 2 (λ R ) is made larger than the reflectance R 1 (λ R ) of the light-incident surface of the lens (S 1 ) which is 1.5% to 7.0%, thereby improving the precision and the easiness of operation of the position adjustment of the lens as will be explained later in Embodiment 7.  
         [0060]    &lt;Embodiment 2&gt; 
         [0061]    [0061]FIG. 3 are drawings for explaining the coat of Embodiment 2; FIG. 3( a ) is an illustration of the structure of the coat layer of the surface S 1 , FIG. 3( b ) is an illustration of the structure of the coat layer of the surface S 2 , and FIG. 3( c ) is a drawing showing a typical reflectance of the surface S 2  vs. light wavelength.  
         [0062]    The base material of Embodiment 2 is the same as Embodiment 1.  
         [0063]    Further, for the coat of the surface S 1 , the same one as Embodiment 1 is used.  
         [0064]    Reflection preventing coat of the surface S 2   
         [0065]    First layer: silicon oxide (refractive index n 21 ≈1.45)  
         [0066]    layer thickness d 21 =1480 Å±80 Å 
         [0067]    Second layer: cerium oxide (refractive index n 22 ≈2.03)  
         [0068]    layer thickness d 22 =530 Å±30 Å 
         [0069]    hird layer: silicon oxide (refractive index n 23 ≈1.45)  
         [0070]    layer thickness d 23 =1840 Å±90 Å 
         [0071]    The method of vapor deposition is the same as Embodiment 1.  
         [0072]    &lt;EFFECT&gt; 
         [0073]    In this way, the following result was obtained.  
         [0074]    As shown in FIG. 3, by making up the coat of the surface S 2  of three layers, the reflectance of the surface S 2  for the wavelength 633 nm of the laser beam for the position adjustment became higher to 13%.  
         [0075]    Further, it was accomplished to make the transmittance T(780 nm) 96% or more, which is practically of no problem, and the precision of the position adjustment was improved more than Embodiment 1.  
         [0076]    The summary of the results is as follows:  
         [0077]    Transmittance T(λ T )≧96%  
         [0078]    (λ T : wavelength of a laser beam having the maximum intensity at 780 nm)  
         [0079]    R 2 (λ R )=9.0% to 16.0%  
         [0080]    (λ R : wavelength of a laser beam having the maximum intensity at 633 nm).  
         [0081]    In this way, as the transmittance of the optical head for the passing laser beam for use as an optical head having the above-mentioned wavelength, 96% or more could be secured. Further, the reflectance R 2 (λ R ) of the light-outgoing surface (S 2 ) of the lens for the laser beam for the position adjustment of the lens was 9.0% to 16.0%, which is larger than 4.3% of the conventional example; thus, it has become possible that a reflectance of at least 5% or more is secured and the reflectance R 2 (λ R ) is made larger than Embodiment 1, thereby improving the precision and the easiness of operation of the position adjustment of the lens as will be explained later in Embodiment 7.  
         [0082]    &lt;Embodiment 3&gt; 
         [0083]    [0083]FIG. 4 are drawings for explaining the coat of Embodiment 3; FIG. 4( a ) is an illustration of the structure of the coat layer of the surface S 1 , FIG. 4( b ) is an illustration of the structure of the coat layer of the surface S 2 , and the broken line and the solid line in FIG. 4( c ) are curves showing a typical reflectance of the surface S 1  and S 2  vs. light wavelength respectively.  
         [0084]    The base material of Embodiment 3 is the same as Embodiment 1.  
         [0085]    Reflection preventing coat of the surface S 1   
         [0086]    First layer: cerium oxide (refractive index n 11 ≈2.03)  
         [0087]    layer thickness d 11 =283 Å±28 Å 
         [0088]    Second layer: silicon oxide (refractive index n 12 ≈1.45)  
         [0089]    layer thickness d 12 =1470 Å±150 Å 
         [0090]    Reflection preventing coat of the surface S 2   
         [0091]    First layer: silicon oxide (refractive index n 21 ≈1.45)  
         [0092]    layer thickness d 21 =920 Å±70 Å 
         [0093]    Second layer: cerium oxide (refractive index n 22 ≈2.03)  
         [0094]    layer thickness d 22 =328 Å±28 Å 
         [0095]    Third layer: silicon oxide (refractive index n 23 ≈1.45)  
         [0096]    layer thickness d 23 =1140 Å±90 Å 
         [0097]    The method of vapor deposition is the same as Embodiment 1.  
         [0098]    &lt;EFFECT&gt; 
         [0099]    In this way, the following result was obtained:  
         [0100]    Transmittance T(λ T )≧96%  
         [0101]    (λ T : wavelength of a laser beam having the maximum intensity at 650 nm)  
         [0102]    R 1 (λ R )=0.5% to 2.8%  
         [0103]    (λ R : wavelength of a laser beam having the maximum intensity at 780 nm)  
         [0104]    R 2 (λ R )=5.1% to 6.8%  
         [0105]    (λ R : the same as the above).  
         [0106]    In this way, as the transmittance of the optical head for the passing laser beam for use as an optical head having the above-mentioned wavelength, 96% or more could be secured. Further, the reflectance R 2 (λ R ) of the light-outgoing surface (S 2 ) of the lens for the laser beam for the position adjustment of the lens was 5.1% to 6.8%; thus, it has become possible that a reflectance of at least 5% or more is secured and the reflectance R 2 (λ R ) is made larger than the reflectance R 1 (λ R ) of the light-incident surface (S 1 ) of the lens which is 0.5% to 2.8%, thereby improving the precision and the easiness of operation of the position adjustment of the lens as will be explained later in Embodiment 7.  
         [0107]    &lt;Embodiment 4&gt; 
         [0108]    [0108]FIG. 5 are drawings for explaining the coat of Embodiment 4; FIG. 5( a ) is an illustration of the structure of the coat layer of the surface S 1 , FIG. 5( b ) is an illustration of the structure of the coat layer of the surface S 2 , and FIG. 5( c ) is a drawing showing a typical reflectance of the surface S 2  vs. light wavelength.  
         [0109]    The base material of Embodiment 4 is the same as Embodiment 1.  
         [0110]    Reflection preventing coat of the surface S 1   
         [0111]    First layer: cerium oxide (refractive index n 11 ≈2.03)  
         [0112]    layer thickness d 11 =283 Å±28 Å 
         [0113]    Second layer: silicon oxide (refractive index n 12 ≈1.45)  
         [0114]    layer thickness d 12 =1470 Å±150 Å 
         [0115]    (the reflection preventing coat of the surface S 1  is the same as Embodiment 3)  
         [0116]    Reflection preventing coat of the surface S 2   
         [0117]    First layer: cerium oxide (refractive index n 21 ≈2.03)  
         [0118]    layer thickness d 21 =1370 Å±95 Å 
         [0119]    Second layer: silicon oxide (refractive index n 22 ≈1.45)  
         [0120]    layer thickness d 22 =1490 Å±104 Å 
         [0121]    Third layer: cerium oxide (refractive index n 23 ≈2.03)  
         [0122]    layer thickness d 23 =1010 Å±70 Å 
         [0123]    Fourth layer: silicon oxide (refractive index n 24 ≈1.45)  
         [0124]    layer thickness d 24 =834 Å±58 Å 
         [0125]    The method of vapor deposition is the same as Embodiment 1.  
         [0126]    &lt;EFFECT&gt; 
         [0127]    In this way, the following result was obtained:  
         [0128]    Transmittance T(λ T )≧96%  
         [0129]    (λ T : wavelength of a laser beam having the maximum intensity at 650 nm)  
         [0130]    R 1 (λ R )=0.5% to 2.8%  
         [0131]    (λ R : wavelength of a laser beam having the maximum intensity at 780 nm)  
         [0132]    R 2 (λ R )=6.0% to 25.0%  
         [0133]    (λ R : the same as the above).  
         [0134]    In this way, as the transmittance of the optical head for the passing laser beam for use as an optical head having the above-mentioned wavelength, 96% or more could be secured. Further, the reflectance R 2 (λ R ) of the light-outgoing surface (S 2 ) of the lens for the laser beam for the position adjustment of the lens was 6.0% to 25.0%, which is larger than 4.3% for the example of conventional one; thus, it has become possible that a reflectance of at least 5% or more is secured, the reflectance R 2 (λ R ) is made larger than the reflectance R 1 (λ R ) of the light-incident surface (S 1 ) of the lens which is 0.5% to 2.8%, and it is made greatly higher than the example of the practice 3, thereby improving the precision and the easiness of operation of the position adjustment of the lens as will be explained later in Embodiment 7.  
         [0135]    &lt;Embodiment 5&gt; 
         [0136]    [0136]FIG. 6 are drawings for explaining the coat of Embodiment 5; FIG. 6( a ) is an illustration of the structure of the coat layer of the surface S 1 , FIG. 6( b ) is an illustration of the structure of the coat layer of the surface S 2 , and the broken line and the solid line in FIG. 6( c ) are curves showing a typical reflectance of the surface S 1  and S 2  vs. light wavelength respectively.  
         [0137]    The base material of Embodiment 5 is the same as Embodiment 1.  
         [0138]    Reflection preventing coat of the surface S 1   
         [0139]    First layer: cerium oxide (refractive index n 11 ≈2.03)  
         [0140]    layer thickness d 11 =174 Å±21 Å 
         [0141]    Second layer: silicon oxide (refractive index n 12 ≈1.45)  
         [0142]    layer thickness d 12 =898 Å±110 Å 
         [0143]    Reflection preventing coat of the surface S 2   
         [0144]    First layer: silicon oxide (refractive index n 21 ≈1.45)  
         [0145]    layer thickness d 21 =680 Å±65 Å 
         [0146]    Second layer: cerium oxide (refractive index n 22 ≈2.03)  
         [0147]    layer thickness d 22 =258 Å±25 Å 
         [0148]    Third layer: silicon oxide (refractive index n 23 ≈1.45)  
         [0149]    layer thickness d 23 =849 Å±84 Å 
         [0150]    The method of vapor deposition is the same as Embodiment 1.  
         [0151]    &lt;EFFECT&gt; 
         [0152]    In this way, the following result was obtained:  
         [0153]    Transmittance T(λ T )≧96%  
         [0154]    (λ T : wavelength of a laser beam having the maximum intensity at 408.3 nm)  
         [0155]    R 1 (λ R )=4. 0% to 5.5%  
         [0156]    (λ R : wavelength of a laser beam having the maximum intensity at 633 nm)  
         [0157]    R 2 (λ R )=6.0% to 7.5%  
         [0158]    (λ R : the same as the above).  
         [0159]    In this way, as the transmittance of the optical head for the passing laser beam for use as an optical head having the above-mentioned wavelength, 96% or more could be secured. Further, the reflectance R 2 (λ R ) of the light-outgoing surface (S 2 ) of the lens for the laser beam for the position adjustment of the lens was 6.0% to 7.5%, which is larger than 5.0% for the example of conventional one; thus, it has become possible that the reflectance R 2 (λ R ) is made larger than the reflectance R 1 (λ R ) of the light-incident surface (S 1 ) of the lens which is 4.0% to 5.5%, thereby improving the precision and the easiness of operation of the position adjustment of the lens as will be explained later in Embodiment 7.  
         [0160]    &lt;Embodiment 6&gt; 
         [0161]    [0161]FIG. 7 are drawings for explaining the coat of Embodiment 6; FIG. 7( a ) is an illustration of the structure of the coat layer of the surface S 1 , FIG. 7( b ) is an illustration of the structure of the coat layer of the surface S 2 , and the broken line and the solid line in FIG. 7( c ) are curves showing a typical reflectance of the surface S 1  and S 2  vs. light wavelength respectively.  
         [0162]    The base material of Embodiment 6 is the same as Embodiment 1.  
         [0163]    Reflection preventing coat of the surface S 1   
         [0164]    First layer: cerium oxide (refractive index n 11 ≈2.03)  
         [0165]    layer thickness d 11 =174 Å±17 Å 
         [0166]    Second layer: silicon oxide (refractive index n 12 ≈1.45)  
         [0167]    layer thickness d 12 =898 Å±89 Å 
         [0168]    Reflection preventing coat of the surface S 2   
         [0169]    First layer: zirconium oxide (refractive index n 21 ≈2.03)  
         [0170]    layer thickness d 21 =910 Å±90 Å 
         [0171]    Second layer: silicon oxide (refractive index n 22 ≈1.45)  
         [0172]    layer thickness d 22 =982 Å±95 Å 
         [0173]    Third layer: zirconium oxide (refractive index n 23 ≈2.03)  
         [0174]    layer thickness d 23 =645 Å±64 Å 
         [0175]    Fourth layer: silicon oxide (refractive index n 24 ≈1.45)  
         [0176]    layer thickness d 24 =548 Å±54 Å 
         [0177]    The method of vapor deposition is the same as Embodiment 1.  
         [0178]    &lt;EFFECT&gt; 
         [0179]    In this way, the following result was obtained:  
         [0180]    Transmittance T(λ T )≧96%  
         [0181]    (λ T : wavelength of a laser beam having the maximum intensity at 408.3 nm)  
         [0182]    R 1 (λ R )=4.0% to 5.5%  
         [0183]    (λ R : wavelength of a laser beam having the maximum intensity at 633 nm)  
         [0184]    R 2 (λ R )=30.0% to 36.5%  
         [0185]    (λ R : the same as the above).  
         [0186]    In this way, as the transmittance of the optical head for the passing laser beam for use as an optical head having the above-mentioned wavelength, 96% or more could be secured. Further, the reflectance R 2 (λ R ) of the light-outgoing surface (S 2 ) of the lens for the laser beam for the position adjustment of the lens was 30.0% to 36.5%, which is larger than 4.3% for the example of conventional one; thus, it has become possible that the reflectance is made larger than the reflectance R 1 (λ R ) of the light-incident surface (S 1 ) of the lens which is 4.0% to 5.5%, and it is made greatly higher than the example of the practice 5, thereby improving the precision and the easiness of operation of the position adjustment of the lens as will be explained later in Embodiment 7.  
         [0187]    &lt;Embodiment 7&gt; 
         [0188]    [0188]FIG. 8 is a drawing for explaining the coat of Embodiment 7; it is a drawing showing curves of the reflectance of the surface S 2  of a lens, of which the reflection preventing coat of the surface S 1  and the surface S 2  in Embodiment 1 is made up in such a manner as shown in Table 1 below, vs. wavelength of light.  
         [0189]    In addition, the ordinate of this FIG. 8, the reflectance R(%) is represented in equal-interval scale, which is different from FIG. 1 to FIG. 7 as described in the above.  
                                                                                                                                                                                               TABLE 1                                       Reflection               preventing   Kind of reflection preventing           coat of   coat of surface S2                surface S1   1   2   3   4   5                        1st layer   Coat   d 11     d 21         cerium   No.            oxide   (Å)    340    318    326   344   357    387            2nd layer   Coat   d 12     d 22         silicon   No.            oxide   (Å)   1770   1656   1700   1793    1862    2023           λ R                 790   820            Reflectance in   a   b   c   d   e   f                          
 
         [0190]    Further, in Table 2 described below, the reflectance, the transmittance, the yield in assembly, and the judgment whether it is good or bad for practical use are shown for the case where the reflection preventing coat of the surfaces S 1  and S 2  is formed in each of the combinations shown in the above-mentioned Table 1. Besides, the wavelength λ R  of the laser beam for the position adjustment of the lens is 633 nm, and the wavelength λ T  of the passing laser beam for use in an optical head is 780 nm.  
                                                                           TABLE 2                                       Kind of reflection preventing coat of surface S2                1   2   3   4   5                        R 1  (λ R ) (%)   4.3   4.3   4.3   4.3   4.3       R 2  (λ R ) (%)   2.3   3.0   5.0   7.0   11.0       T (λ T ) (%)   97.8   98.0   98.2   97.2   96.8       Yield (%)   70   75   88   95   98       Evaluation   C   C   B   A   A                  
 
         [0191]    As shown in Table 2, when the reflectance R 2 (λ R ) of the kind 1 and the kind 2 of the reflection preventing coat of the surface S 2  is 2.3% and 3.0% respectively which are smaller than the reflectance R 1 (λ R ) 4.3% of the surface S 1 , the assembly yield becomes 70% and 75% respectively, which do not make the practical quality come up to the standard, because the position adjustment using the reflected light by the surface S 2  becomes hard to observe owing to insufficient illumination.  
         [0192]    When the reflectance R 2 (λ R ) of the kind 3 of the coat of the surface S 2  is 5.0% which is larger than the reflectance R 1 (λ R ) 4.3% of the surface S 1 , resulting in the assembly yield of 88%, the practical quality comes up to the standard, because the position adjustment using the reflected light by the surface S 2  becomes easy to observe owing to sufficient illumination which improves the precision of position adjustment and the easiness of operation.  
         [0193]    Further, when the reflectance R 2 (λ R ) of the kind 4 and kind 5 of the coat of the surface S 2  are 7.0% and 11.0% respectively which are larger than the reflectance R 1 (λ R ) 4.3% of the surface S 1 , and when the value of the reflectance R 2 (λ R ) becomes larger 7.0% to 11.0%, the assembly yield becomes 95% to 98% which come near to almost 100% and the practical quality comes sufficiently up to the standard, because the position adjustment using the reflected light by the surface S 2  becomes easy to observe owing to sufficient illumination which improves the precision of position adjustment and the easiness of operation. Accordingly, if the reflectance R 2 (λ R ) of the surface S 2  is larger than the reflectance R 1 (λ R ) of the surface S 1 , and in particular, if the reflectance R 2 (λ R ) of the surface S 2  is so large as to get a value described in the above, the assembly yield becomes nearly 100% owing to the improvement of the precision of the position adjustment and the easiness of operation as described in the above; that is very desirable.  
         [0194]    According to this invention, it has become possible to provide means for preventing the lowering of the intensity of the transmitted light for the wavelength (λ T ), for raising the reflectance of the surface S 2  for the wavelength (λ R ) of the light for the position adjustment, and for improving the precision of the position adjustment of the lens.