Abstract:
An optical member includes a synthetic resin substrate: and an antireflection film including at least a mixture film composed of tantalum oxide (Ta 2  O 5 ) and titanium oxide (TiO 2 ), and a film having a refractive index lower than that of the mixture film. The antireflection film is coated on the synthetic resin substrate.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is related to an optical member having an antireflection film, more specifically an optical member having at least one mixture film on a substrate composed of a synthetic resin. 
     Conventionally, in using a synthetic resin as a substrate, has been known the optical member having on the aforesaid substrate an antireflection film which decreases surface reflection. The optical member having the antireflection film has been applied to and practiced in cameras, video cameras and the like. Further, the synthetic resins applied to the substrate include acryl resins, polycarbonate resins, etc. It is particularly noted that in the optical member having the antireflection film of which substrate is composed of the synthetic resin, the synthetic resin has less heat-resistance than optical glass, and as a result, it is impossible to employ the means of vacuum evaporation. Accordingly, there have been problems such that no good adhesion between the synthetic resin substrate and the antireflection film is obtained and a crack is liable to be formed on the anitreflection film. 
     Generally, it has been known that as a reason that the crack is liable to be formed on the optical member on which the antireflection film is formed, stress remaining in the antireflection film is one of causes. In order to make the formation of the crack less liable, as described, for example, in Optical Thin Film (H. A. Macleod, Japanese version translated by Shigetaro Ogura et al., page 458), there is provided a method wherein a few percent of an impurity is intentionally put into the film for the decrease in the stress in the film which is used as an antirefelction film. Furthermore, in more detail, Pulker studied the relationship between the stress of an optical thin film and the fine structure of the film, and obtained good agreement of the measurement results with those obtained by the calculation based on a model. It is also described that the presence of a small amount of the impurity affects remarkably the stress; furthermore, the impurity is concentrated to the boundary of crystal grains in the film having a prismatic structure; as a result, pulling force between grain boundaries adjacent each other is decreased to reduce the stress, and furthermore, the stress of a magnesium fluoride film is reduced approximately to 50 percent by the presence of a small amount of calcium fluoride such as about four mole percent. 
     On the basis of the above-mentioned approach, is disclosed an optical member having an antireflection film composed of a mixture film. For example, is known an optical member having an antireflection film composed of a mixture film consisting of ZrO 2  and TiO 2  or ZrO 2  and TiO 2  (refer to Japanese Patent Publication Open to Public Inspection No. 181902/1992). 
     However, when the optical member having the antireflection film composed of the mixture film is kept at high temperatures (for example, 70° C.), the crack is formed on the antireflection film and no sufficient effect for preventing the formation of the crack has been obtained. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing problems, the present invention is accomplished. An object of the present invention is to provide an optical member having an antireflection film which is not liable to form the crack at high temperatures and has excellent antireflection properties. 
     The aforesaid object is accomplished by the following structures: 
     In structure (1), for an optical member having an antireflection film consisting of a substrate and the antireflection film, the object is accomplished by arranging on the synthetic resin substrate the antireflection film comprising at least one mixture film consisting of tantalum oxide (Ta 2  O 5 ) and titanium oxide (TiO 2 ). Here, the synthetic resin may be of optical use. For example, acryl resin (PMMA), polycarbonate resin (PC), etc. are available and also available in the following structures. 
     In structure (2), an optical member having the antireflection film described in the structure (1) is characterized by comprising the aforesaid mixture film consisting of tantalum oxide (Ta 2  O 5 ) and titanium oxide (TiO 2 ), which comprises from 0.5 to 30 weight percent of titanium oxide (TiO 2 ). Beyond these limits of the weight ratios, the crack will be liable to be formed. 
     In structure (3), an optical member having the antireflection film described in the structure (1) is characterized by comprising more preferably from 1 to 5 weight percent of titanium oxide (TiO 2 ). 
     In structure (4), an optical member having the antireflection films mentioned in one of structures (1) to (3) is characterized in that the aforesaid antireflection films are composed of two films which meet the following conditions, while the numbers of the aforesaid antireflection films are counted from the substrate side. 
     The first film: the aforesaid mixture film, 
     
         1.90≦n.sub.1 ≦2.10 
    
     
         0.03λ.sub.0 ≦n.sub.1 d.sub.1 ≦0.10λ.sub.0 
    
     The second film: having a refractive index lower than that of the mixture film, 
     
         1.36≦n.sub.2 ≦1.48 
    
     
         0.28λ.sub.0 ≦n.sub.2 d.sub.2 ≦0.4λ.sub.0 
    
     wherein 
     n i  is the refractive index of each of the films 
     d i  is the thickness of each of the films 
     λ 0  is the design wavelength 
     Beyond the limits, antireflection properties are deteriorated. 
     Further, the optical member of structure (4) satisfies the following conditions, 
     
         7 nm≦d.sub.1 ≦27 nm 
    
     
         96 nm≦d.sub.2 ≦150 nm. 
    
     Still further, in the optical member of structure (4), the second film contains silicon oxide (SiO 2 ) or magnesium fluoride (MgF 2 ). 
     In structure (5), an optical member having antireflection films described in one of structures (1) to (3) is characterized in that the aforesaid antireflection films are composed of three films which meet the following conditions, while the numbers of the aforesaid antireflection films are counted from the substrate side. 
     The first film: having a refractive index lower than that of the mixture film, 
     
         1.36≦n.sub.1 ≦1.48 
    
     
         0.10λ.sub.0 ≦n.sub.1 d.sub.1 ≦0.60λ.sub.0 
    
     The second film: the aforesaid mixture film, 
     
         1.90≦n.sub.2 ≦2.10 
    
     
         0.30λ.sub.0 ≦n.sub.2 d.sub.2 ≦0.10λ.sub.0 
    
     The third film: having a refractive index lower than that of the mixture film, 
     
         1.36≦n.sub.3 ≦1.48 
    
     
         0.28λ.sub.0 ≦n.sub.3 d.sub.3 ≦0.40λ.sub.0 
    
     Beyond the limits, antireflection properties are deteriorated. 
     Further, the optical member of structure (5) satisfies the following conditions, 
     
         34 nm≦d.sub.1 ≦225 nm 
    
     
         7 nm≦d.sub.2 ≦27 nm 
    
     
         96 nm≦d.sub.3 ≦150 nm. 
    
     Still further, in the optical member of structure (5), the first or third film contains silicon oxide (SiO 2 ) or magnesium fluoride (MgF 2 ). 
     In structure (6), an optical member having antireflection films mentioned in one of structures (1) to (3) is characterized in that the aforesaid antireflection films are composed of four films which meet the following conditions, while the numbers of the aforesaid antireflection films are counted from the substrate side. 
     The first film: the aforesaid mixture film, 
     
         1.90≦n.sub.1 ≦2.10 
    
     
         0.02λ.sub.0 ≦n.sub.1 d.sub.1 ≦0.15λ.sub.0 
    
     The second film: having a refractive index lower than that of the mixture film, 
     
         1.36≦n.sub.2 ≦1.48 
    
     
         0.03λ.sub.0 ≦n.sub.2 d.sub.2 ≦0.18λ.sub.0 
    
     The third film: the aforesaid mixture film, 
     
         1.90≦n.sub.3 ≦2.10 
    
     
         0.09λ.sub.0 ≦n.sub.3 d.sub.3 ≦0.32λ.sub.0 
    
     The fourth film: having a refractive index lower than that of the mixture film, 
     
         1.36≦n.sub.4 ≦1.48 
    
     
         0.22λ.sub.0 ≦n.sub.4 d.sub.4 ≦0.40λ.sub.0 
    
     Beyond the limits, antireflection properties are deteriorated. 
     Further, the optical member of structure (6) satisfies the following conditions, 
     
         4 nm≦d.sub.1 ≦41 nm 
    
     
         11 nm≦d.sub.2 ≦68 nm 
    
     
         22 nm≦d.sub.3 ≦86 nm 
    
     
         75 nm≦d.sub.4 ÷150 nm 
    
     Still further, in the optical member of structure (6), the second or fourth film contains silicon oxide (SiO 2 ) or magnesium fluoride (MgF 2 ). 
     In structure (7), is an optical member having antireflection films mentioned in one of structures (1) to (3) is characterized in that the aforesaid antireflection films are composed of five films which meet the following conditions, while the numbers of the aforesaid antireflection films are counted from the substrate side. 
     The first film: having a refractive index lower than that of the mixture film, 
     
         1.36≦n.sub.1 ≦1.48 
    
     
         0.10λ.sub.0 ≦n.sub.1 d.sub.1 ≦0.60λ.sub.0 
    
     The second film: the aforesaid mixture film, 
     
         1.90≦n.sub.2 ≦2.10 
    
     
         0.02λ.sub.0 ≦n.sub.2 d.sub.2 ≦0.15λ.sub.0 
    
     The third film: having a refractive index lower than that of the mixture film, 
     
         1.36≦n.sub.3 ≦1.48 
    
     
         0.03λ.sub.0 ≦n.sub.3 d.sub.d ≦0.18λ.sub.0 
    
     The fourth film: the aforesaid mixture film 
     
         1.90≦n.sub.4 ≦2.10 
    
     
         0.09λ.sub.0 ≦n.sub.4 d.sub.4 ÷0.32λ.sub.0 
    
     The fifth film: having a refractive index lower than that of the mixture film, 
     
         1.36≦n.sub.5 ≦1.48 
    
     
         0.22λ.sub.0 ≦n.sub.5 d.sub.5 ≦0.40λ.sub.0 
    
     Beyond the limits, antireflection properties are deteriorated. 
     Further, the optical member of structure (7) satisfies the following conditions, 
     
         34 nm≦d.sub.1 ≦225 nm 
    
     
         4 nm≦d.sub.2 ≦41 nm 
    
     
         11 nm≦d.sub.3 ≦68 nm 
    
     
         22 nm≦d.sub.4 ≦86 nm 
    
     
         75 nm≦d.sub.5 ≦150 nm 
    
     Still further, in the optical member of structure (7), the first, third or fifth film contains silicon oxide (SiO 2 ) or magnesium fluoride (MgF 2 ). 
     In structure (8), an optical member having antireflection films described in one of structures (1) to (3), comprises at least each one of mixture films consisting of tantalum oxide (Ta 2  O 3 ) and titanium oxide (TiO 2 ), aforesaid film having the refractive index lower than that of the mixture film and an intermediate refractive index film having a refractive index of 1.58 to 1.62. 
     Further, aforesaid film having the refractive index lower than that of the mixture film contains silicon oxide (SiO 2 ) or magnesium fluoride (MgF 2 ). 
     Still further, the intermediate refractive index film contains aluminum oxide (Al 2  O 3 ). 
     In structure (9), an optical member having antireflection films described in one of structures (1), (2), (3) and (8), is characterized in that the aforesaid antireflection films are composed of by six films which meet the following conditions, while the numbers of the aforesaid antireflection films are counted from the substrate side. 
     The first film: the aforesaid intermediate refractive index film, 
     
         1.58≦n.sub.1 ≦1.62 
    
     
         0.05λ.sub.0 ≦n.sub.1 d.sub.1 ≦0.30λ.sub.0 
    
     The second film: having a refractive index lower than that of the mixture film, 
     
         1.36≦n.sub.2 ≦1.48 
    
     
         0.10λ.sub.0 ≦n.sub.2 d.sub.2 ≦0.60λ.sub.0 
    
     The third film: the aforesaid mixture film 
     
         1.90≦n.sub.3 ≦2.10 
    
     
         0.02λ.sub.0 ≦n.sub.3 d.sub.3 ≦0.08λ.sub.0 
    
     The fourth film: having a refractive index lower than that of the mixture film, 
     
         1.36≦n.sub.4 ≦1.48 
    
     
         0.10λ.sub.0 ≦n.sub.4 d.sub.4 ≦0.18λ.sub.0 
    
     The fifth film: the aforesaid mixture film, 
     
         1.90≦n.sub.5 ≦2.10 
    
     
         0.09λ.sub.0 ≦n.sub.5 d.sub.5 ≦0.35λ.sub.0 
    
     The sixth film: having a refractive index lower than that of the mixture film, 
     
         1.36≦n.sub.6 ≦1.48 
    
     
         0.22λ.sub.0 ≦n.sub.6 d.sub.6 ≦0.40λ.sub.0 
    
     Beyond the limits, antireflection properties are deteriorated. 
     Further, the optical member of structure (9) satisfies the following conditions, 
     
         15 nm≦d.sub.1 ≦97 nm 
    
     
         34 nm≦d.sub.2 ≦225 nm 
    
     
         4 nm≦d.sub.3 ≦22 nm 
    
     
         34 nm≦d.sub.4 ≦68 nm 
    
     
         22 nm≦d.sub.5 ≦94 nm 
    
     
         75 nm≦d.sub.6 ≦150 nm 
    
     Still further, in the optical member of structure (9), the second film contains silicon oxide (SiO 2 ) or magnesium fluoride (MgF 2 ). 
     Still further, in the optical member of structure (9), the first film contains aluminum oxide (Al 2  O 3 ). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a film structure of Example. 
     FIG. 2 shows the relationship between weight ratio of a mixture film and refractive index. 
     FIG. 3 shows the spectral reflectance of the optical member having the antireflection film of Example A. 
     FIG. 4 shows the spectral reflectance of the optical member having the antireflection film of Example B. 
     FIG. 5 shows the spectral reflectance of the optical member having the antireflection film of Example C. 
     FIG. 6 shows the spectral reflectance of the optical member having the antireflection film of Example D. 
     FIG. 7 shows the spectral reflectance of the optical member having the antireflection film of Example E. 
     FIG. 8 shows the spectral reflectance of the optical member having the antireflection film of Example F. 
     FIG. 9 shows the spectral reflectance of the optical member having the antireflection film of Example G. 
     FIG. 10 shows the spectral reflectance of the optical member having the antireflection film of Example H. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Example 
     In the following, Examples of the present invention are explained. Further, the weight ratio of titanium oxide (TiO 2 ) to tantalum oxide (Ta 2  O 5 ) is converted using the correlation diagram of weight ratio versus refractive index in FIG. 2. 
     Experimental Apparatus 
     For the preparation of the antireflection film, a vacuum evaporation method was employed. A vacuum vessel was evacuated up to 2.0×10 -2  Pa, and materials were heated by an electron gun and evaporated to prepare a film. Further, during the preparation of the film, as reactive gas, oxygen was introduced up to 1.5×10 -2  Pa at the total pressure. In addition, the evaporation was performed without heating particularly a substrate. For the preparation of a mixture film consisting of tantalum oxide (Ta 2  O 3 ) and titanium oxide (TiO 2 ), in the present invention, individual materials were mixed according to the predetermined weight ratio, followed by melting, cooling and crashing and the resulting mixture was employed as a material for evaporation. The optical member obtained was left alone in ambient conditions of 60° C. and 90% relative humidity for 240 hours, and 70° C. (drying) for eight hours. Further, individual materials may be evaporated by another evaporation source (2-element simultaneous evaporation) and an intended mixture film may be prepared while regulating evaporation rate. 
     Evaluation Method 
     An evaluation item is of crack. The crack was visually inspected under a reflection-type optical microscope (40 times). 
     Cracks in Examples and Comparative Examples were evaluated in accordance with the following criteria. 
     a: no formation of crack 
     b: formation of pale streak-like crack 
     c: formation of tortoiseshell-like crack 
     Methods for measuring refractive index and film thickness 
     The film thickness is measured as follows. A sample (material to be measured) is cut perpendicularly against an antireflection film and the cross section is enlarged by an SEM (scanning-type electron microscopy) and the film thickness is measured. 
     The refractive index is obtained as follows. Materials composing each antireflection film are analyzed by an X-ray electron spectroscopy and the refractive index can be retrieved from the known references (for example, Kogaku Hakumaku (Optical Thin Film) Kyoritsu Shuppan). 
     The refractive index of a mixture film is obtained as follows. 
     (1) Analysis of composition ratio of materials in the mixture film by the X-ray electron spectroscopy 
     (2) Retrieval of refractive index of each material composing the mixture film from the known references 
     (3) The refractive index of the mixture film is obtained as the total of products of refractive index of each material by composition ratio 
     Example A 
     Examples of the present invention are explained. FIG. 1 is a cross section of an optical member 1 having an antireflection film of the present invention. In FIG. 1, the optical member 1 having the antireflection film is composed of a synthetic resin substrate 2 and the antireflection film 3, which is composed of a first antireflection film 31 and a second antireflection film 32. 
     Preparation of Samples 
     Samples of Example A are of Example 1 and of Comparative Examples 1, 2 and 3. Table 1 illustrates materials for the substrates and antireflection films, optical film thickness and film structures. 
     
                                           TABLE 1__________________________________________________________________________               Comparative    Comparative   ComparativeExample 1           Example 1      Example 2     Example 3PMMA                PMMA           PMMA          PMMAMate-    Optical          Refractive               Mate-                   Optical                         Refractive                              Mate-                                 Optical                                       Refractive                                            Mate-                                               Optical                                                     Refractiverial     Thickness          Index               rial                   Thickness                         Index                              rial                                 Thickness                                       Index                                            rial                                               Thickness                                                     Index__________________________________________________________________________First    Ta.sub.2 O.sub.5    0.05λ.sub.0          2.02 Ta.sub.2 O.sub.5                   0.50λ.sub.0                         2.00 TiO.sub.2                                 0.05λ.sub.0                                       2.15 ZrO.sub.2                                               0.06λ.sub.0                                                     1.82Film    (97 wt %)    TiO.sub.2    (3 wt %)Second    SiO.sub.2    0.35λ.sub.0          1.46 SiO.sub.2                   0.35λ.sub.0                         1.46 SiO.sub.2                                 0.35λ.sub.0                                       1.46 SiO.sub.2                                               0.34λ.sub.0                                                     1.46Film__________________________________________________________________________ Note: Design wavelength λ.sub.0 = 510 nm 
    
     Test Results 
     Table 2 shows test results on the aforesaid samples. Furthermore, FIG. 3 illustrates antireflection characteristics (spectral reflectance) measured by a spectrophotometer. 
     
                       TABLE 2______________________________________          Comparative                    Comparative                              Comparative   Example 1          Example 1 Example 2 Example 3______________________________________60° C. 90% RH     a        a         a       a240 H70° C. Drying     a        b         c       b______________________________________ 
    
     Table 2 shows that no significant difference in the formation of the crack is found between the sample of Example 1 and samples of Comparative Examples 1, 2 and 3, when the samples are stored at 60° C. and 90% relative humidity for 240 hours but during the storage at 70° C. (drying), the sample of Example 1 of the present invention has only formed no crack. 
     Example B 
     Samples of Example B are of Example 1 and of Comparative Examples 4, 5 and 6. Table 3 shows materials for substrates and antireflection films, film thickness and film structures. 
     
                                           TABLE 3__________________________________________________________________________            Comparative Comparative ComparativeExample 2        Example 4   Example 5   Example 6PMMA             PMMA        PMMA        PMMA    Optical         Refrac-                Optical                       Refrac-                            Optical                                   Refrac-                                        Optical                                               Refrac-Mater-   Thick-        tive            Mater-                Thick-                    tive                        Mater-                            Thick-                                tive                                    Mater-                                        Thick-                                            tiveial      ness         Index            ial ness                     Index                        ial ness                                 Index                                    ial ness                                             Index__________________________________________________________________________First    SiO.sub.2    0.15λ.sub.0        1.46            SiO.sub.2                0.15λ.sub.0                    1.46                        SiO.sub.2                            0.15λ.sub.0                                1.46                                    SiO.sub.2                                        0.15λhd                                            1.46FilmSecond    Ta.sub.2 O.sub.5    0.05λ.sub.0        2.02            Ta.sub.2 O.sub.5                0.05λ.sub.0                    2.00                        TiO.sub.2                            0.05λ.sub.0                                2.15                                    ZrO.sub.2                                        0.07λ.sub.0                                            1.82Film    (97 wt %)    TiO.sub.2    (3 wt %)Third    SiO.sub.2    0.36λ.sub.0        1.46            SiO.sub.2                0.36λ.sub.0                    1.46                        SiO.sub.2                            0.36λ.sub.0                                1.46                                    SiO.sub.2                                        0.35λ.sub.0                                            1.46Film__________________________________________________________________________ Note: Design wavelength λ.sub.0 = 510 nm 
    
     Test Results 
     Table 4 shows test results on the aforementioned samples. Further, FIG. 4 illustrates the antireflection characteristics (spectral reflectance) measured by a spectrophotometer. 
     
                       TABLE 4______________________________________          Comparative                    Comparative                              Comparative   Example 2          Example 4 Example 5 Example 6______________________________________60° C. 90% RH     a        a         a       a240 H70° C. Drying     a        b         c       b8 H______________________________________ 
    
     Table 4 shows that no significant difference in the formation of the crack is found between the sample of Example 2 and samples of Comparative Examples 4, 5 and 6, when the samples are stored at high temperature conditions such as 60° C. and 90% relative humidity for 240 hours but during the storage at 70° C. (drying), the sample of Example 2 of the present invention has only formed no crack. 
     Example C 
     Samples of Example C are of Example 3 and of Comparative Examples 7, 8 and 9. Table 5 shows materials for substrates and antireflection films, film thickness and film structures. 
     
                                           TABLE 5__________________________________________________________________________               Comparative    Comparative   ComparativeExample 3           Example 7      Example 8     Example 9PMMA                PMMA           PMMA          PMMAMate-    Optical          Refractive               Mate-                   Optical                         Refractive                              Mate-                                 Optical                                       Refractive                                            Mate-                                               Optical                                                     Refractiverial     Thickness          Index               rial                   Thickness                         Index                              rial                                 Thickness                                       Index                                            rial                                               Thickness                                                     Index__________________________________________________________________________First    Ta.sub.2 O.sub.5    0.055λ.sub.0          2.02 Ta.sub.2 O.sub.5                   0.055λ.sub.0                         2.00 TiO.sub.2                                 0.055λ.sub.0                                       2.15 ZrO.sub.2                                               0.055λ.sub.0                                                     1.82Film    (97 wt %)    TiO.sub.2    (3 wt %)Second    SiO.sub.2    0.145λ.sub.0          1.46 SiO.sub.2                   0.145λ.sub.0                         1.46 SiO.sub.2                                 0.145λ.sub.0                                       1.46 SiO.sub.2                                               0.135λ.sub.0                                                     1.46FilmThird    Ta.sub.2 O.sub.5    0.125λ.sub.0          2.02 Ta.sub.2 O.sub.5                   0.125λ.sub.0                         2.00 TiO.sub.2                                 0.125λ.sub.0                                       2.15 ZrO.sub.2                                               0.155λ.sub.0                                                     1.82Film    (97 wt %)    TiO.sub.2    (3 wt %)Fourth    SiO.sub.2    0.320λ.sub.0          1.46 SiO.sub.2                   0.320λ.sub.0                         1.46 SiO.sub.2                                 0.320λ.sub.0                                       1.46 SiO.sub.2                                               0.320λ.sub.0                                                     1.46Film__________________________________________________________________________ Note: Design wavelength λ.sub.0 = 510 nm 
    
     Test 
     Table 6 shows results on the above-mentioned samples. Furthermore, FIG. 5 illustrates antireflection characteristics (spectral reflectance) measured by a spectrophotometer. 
     
                       TABLE 6______________________________________          Comparative                    Comparative                              Comparative   Example 3          Example 7 Example 8 Example 9______________________________________60° C. 90% RH     a        a         c       a240 H70° C. Drying     a        c         c       c8 H______________________________________ 
    
     Table 6 shows that no significant difference in the formation of the crack is found between the sample of Example 3 and samples of Comparative Examples 7, 8 and 9, when the samples are stored at high temperature conditions such as 60° C. and 90% relative humidity for 240 hours but during the storage at 70° C. (drying), the sample of Example 3 of the present invention has only formed no crack. 
     Example D 
     Samples of Example D are of Example 4 and of Comparative Examples 10, 11 and 12. Table 7 shows materials for substrates and antireflection films, film thickness and film structures. 
     
                                           TABLE 7__________________________________________________________________________               Comparative    Comparative   ComparativeExample 4           Example 10     Example 11    Example 12PMMA                PMMA           PMMA          PMMAMate-    Optical          Refractive               Mate-                   Optical                         Refractive                              Mate-                                 Optical                                       Refractive                                            Mate-                                               Optical                                                     Refractiverial     Thickness          Index               rial                   Thickness                         Index                              rial                                 Thickness                                       Index                                            rial                                               Thickness                                                     Index__________________________________________________________________________First    TaO.sub.2 O.sub.5    0.135λ.sub.0          2.02 Ta.sub.2 O.sub.5                   0.135λ.sub.0                         2.00 TiO.sub.2                                 0.135λ.sub.0                                       2.15 ZrO.sub.2                                               0.135λ.sub.0                                                     1.82Film    (97 wt %)    TiO.sub.2    (3 wt %)Second    SiO.sub.2    0.06λ.sub.0          1.46 SiO.sub.2                   0.06λ.sub.0                         1.46 SiO.sub.2                                 0.06λ.sub.0                                       1.46 SiO.sub.2                                               0.06λ.sub.0                                                     1.46FilmThird    Ta.sub.2 O.sub.5    0.265λ.sub.0          2.02 Ta.sub.2 O.sub.5                   0.265λ.sub.0                         2.00 TiO.sub.2                                 0.265λ.sub.0                                       2.15 ZrO.sub.2                                               0.265λ.sub.0                                                     1.82Film    (97 wt %)    TiO.sub.2    (3 wt %)Fourth    MgF.sub.2    0.265λ.sub.0          1.38 MgF.sub.2                   0.265λ.sub.0                         1.38 MgF.sub.2                                 0.265λ.sub.0                                       1.38 MgF.sub.2                                               0.265λ.sub.0                                                     1.38Film__________________________________________________________________________ Note: Design wavelength λ.sub.0 = 510 nm 
    
     Test 
     Table 8 shows results on the above-mentioned samples. Furthermore, FIG. 6 illustrates antireflection characteristics (spectral reflectance) measured by a spectrophotometer. 
     
                       TABLE 8______________________________________          Comparative                    Comparative                              Comparative   Example 4          Example 10                    Example 11                              Example 12______________________________________60° C. 90% RH     a        a         c       a240 H70° C. Drying     a        c         c       c8 H______________________________________ 
    
     Table 8 shows that no significant difference in the formation of the crack is found between the sample of Example 4 and samples of Comparative Examples 10, 11 and 12, when the samples are stored at high temperature conditions such as 60° C. and 90% relative humidity for 240 hours but during the storage at 70° C. (drying), the sample of Example 3 of the present invention has only formed no crack. 
     Samples of Example E are of Example 5 and of Comparative Examples 13, 14 and 15. Table 9 shows materials for substrates and antireflection films, film thickness and film structures. 
     
                                           TABLE 9__________________________________________________________________________               Comparative    Comparative   ComparativeExample 5           Example 13     Example 14    Example 15PMMA                PMMA           PMMA          PMMAMate-    Optical          Refractive               Mate-                   Optical                         Refractive                              Mate-                                 Optical                                       Refractive                                            Mate-                                               Optical                                                     Refractiverial     Thickness          Index               rial                   Thickness                         Index                              rial                                 Thickness                                       Index                                            rial                                               Thickness                                                     Index__________________________________________________________________________First    SiO.sub.2    0.50λ.sub.0          1.46 SiO.sub.2                   0.50λ.sub.0                         1.46 SiO.sub.2                                 0.50λ.sub.0                                       1.46 SiO.sub.2                                               0.50λ.sub.0                                                     1.46FilmSecond    Ta.sub.2 O.sub.5    0.055λ.sub.0          2.02 Ta.sub.2 O.sub.5                   0.055λ.sub.0                         2.00 TiO.sub.2                                 0.055λ.sub.0                                       2.15 ZrO.sub.2                                               0.075λ.sub.0                                                     1.82Film    (97 wt %)    TiO.sub.2    (3 wt %)Third    SiO.sub.2    0.145λ.sub.0          1.46 SiO.sub.2                   0.145λ.sub.0                         1.46 SiO.sub.2                                 0.145λ.sub.0                                       1.46 SiO.sub.2                                               0.106λ.sub.0                                                     1.46FilmFourth    Ta.sub.2 O.sub.5    0.125λ.sub.0          2.02 Ta.sub.2 O.sub.5                   0.125λ.sub.0                         2.00 TiO.sub.2                                 0.125λ.sub.0                                       2.15 ZrO.sub.2                                               0.207λ.sub.0                                                     1.82Film    (97 wt %)    TiO.sub.2    (3 wt %)Fifth    SiO.sub.2    0.320λ.sub.0          1.46 SiO.sub.2                   0.320λ.sub.0                         1.46 SiO.sub.2                                 0.320λ.sub.0                                       1.46 SiO.sub.2                                               0.293λ.sub.0                                                     1.46Film__________________________________________________________________________ Note: Design wavelength λ.sub.0 = 510 nm 
    
     Test 
     Table 10 shows results on the aforesaid samples. Furthermore, FIG. 7 illustrates antireflection characteristics (spectral reflectance) measured by a spectrophotometer. 
     
                       TABLE 10______________________________________          Comparative                    Comparative                              Comparative   Example 5          Example 13                    Example 14                              Example 15______________________________________60° C. 90% RH     a        a         c       a240 H70° C. Drying     a        c         c       c8 H______________________________________ 
    
     Table 10 shows that no significant difference in the formation of the crack is found between the sample of Example 4 and samples of Comparative Examples 13, 14 and 15, when the samples are stored at high temperature conditions such as 60° C. and 90% relative humidity for 240 hours but during the storage at 70° C. (drying), the sample of Example 3 of the present invention has only formed no crack. 
     Example F 
     Samples of Example F are of Example 6 and of Comparative Examples 16, 17 and 18. Table 11 shows materials for substrates and antireflection films, film thickness and film structures. 
     
                                           TABLE 11__________________________________________________________________________               Comparative    Comparative   ComparativeExample 6           Example 16     Example 17    Example 18PMMA                PMMA           PMMA          PMMAMate-    Optical          Refractive               Mate-                   Optical                         Refractive                              Mate-                                 Optical                                       Refractive                                            Mate-                                               Optical                                                     Refractiverial     Thickness          Index               rial                   Thickness                         Index                              rial                                 Thickness                                       Index                                            rial                                               Thickness                                                     Index__________________________________________________________________________First    SiO.sub.2    0.500λ.sub.0          1.46 SiO.sub.2                   0.500λ.sub.0                         1.46 SiO.sub.2                                 0.500λ.sub.0                                       1.46 SiO.sub.2                                               0.500λ.sub.0                                                     1.46FilmSecond    Ta.sub.2 O.sub.5    0.120λ.sub.0          2.02 Ta.sub.2 O.sub.5                   0.120λ.sub.0                         2.00 TiO.sub.2                                 0.120λ.sub.0                                       2.15 ZrO.sub.2                                               0.120λ.sub.0                                                     1.82Film    (97 wt %)    TiO.sub.2    (3 wt %)Third    SiO.sub.2    0.065λ.sub.0          1.46 SiO.sub.2                   0.065λ.sub.0                         1.46 SiO.sub.2                                 0.065λ.sub.0                                       1.46 SiO.sub.2                                               0.065λ.sub.0                                                     1.46FilmFourth    Ta.sub.2 O.sub.5    0.290λ.sub.0          2.02 Ta.sub.2 O.sub.5                   0.290λ.sub.0                         2.00 TiO.sub.2                                 0.290λ.sub.0                                       2.15 ZrO.sub.2                                               0.290λ.sub.0                                                     1.82Film    (97 wt %)    TiO.sub.2    (3 wt %)Fifth    MgF.sub.2    0.270λ.sub.0          1.38 MgF.sub.2                   0.270λ.sub.0                         1.38 MgF.sub.2                                 0.270λ.sub.0                                       1.38 MgF.sub.2                                               0.270λ.sub.0                                                     1.38Film__________________________________________________________________________ Note: Design wavelength λ.sub.0 = 510 nm 
    
     Test 
     Table 12 shows results on the aforesaid samples. Furthermore, FIG. 8 illustrates antireflection characteristics (spectral reflectance) measured by a spectrophotometer. 
     
                       TABLE 12______________________________________          Comparative                    Comparative                              Comparative   Example 6          Example 16                    Example 17                              Example 18______________________________________60° C. 90% RH     a        a         c       a240 H70° C. Drying     a        c         c       c8 H______________________________________ 
    
     Table 12 shows that no significant difference in the formation of the crack is found between the sample of Example 7 and samples of Comparative Examples 16, 17 and 18, when the samples are stored at high temperature conditions such as 60° C. and 90% relative humidity for 240 hours but during the storage at 70° C. (drying), the sample of Example 3 of the present invention has only formed no crack. 
     Example G 
     Samples of Example G are of Example 7 and of Comparative Examples 19, 20 and 21. Table 13 shows materials for substrates and antireflection films, film thickness and film structures. 
     
                                           TABLE 13__________________________________________________________________________               Comparative   Comparative    ComparativeExample 7           Example 19    Example 20     Example 21PMMA                PMMA          PMMA           PMMAMate-    Optical          Refractive               Mate-                   Optical                        Refractive                             Mate-                                 Optical                                       Refractive                                            Mate-                                                Optical                                                     Refractiverial     Thickness          Index               rial                   Thickness                        Index                             rial                                 Thickness                                       Index                                            rial                                                Thickness                                                     Index__________________________________________________________________________First    Al.sub.2 O.sub.3    0.10λ.sub.0          1.60 Al.sub.2 O.sub.3                   0.10λ.sub.0                        1.60 Al.sub.2 O.sub.3                                 0.10λ.sub.0                                       1.60 Al.sub.2 O.sub.3                                                0.10λ.sub.0                                                2    1.60FilmSecond    SiO.sub.2    0.50λ.sub.0          1.46 SiO.sub.2                   0.50λ.sub.0                        1.40 SiO.sub.2                                 0.50λ.sub.0                                       1.46 SiO.sub.2                                                0.50λ.sub.0                                                     1.46FilmThird    Ta.sub.2 O.sub.5    0.135λ.sub.0          2.02 Ta.sub.2 O.sub.5                   0.135λ.sub.0                        2.00 TiO.sub.2                                 0.135λ.sub.0                                       2.15 ZrO.sub.2                                                0.135λ.sub.0                                                     1.82Film    (97 wt %)    TiO.sub.2    (3 wt %)Fourth    SiO.sub.2    0.065λ.sub.0          1.46 SiO.sub.2                   0.05λ.sub.0                        1.46 SiO.sub.2                                 0.05λ.sub.0                                       1.46 SiO.sub.2                                                0.05λ.sub.0                                                     1.46FilmFifth    Ta.sub.2 O.sub.5    0.265λ.sub.0          2.02 Ta.sub.2 O.sub.5                   0.265λ.sub.0                        2.00 TiO.sub.2                                 0.265λ.sub.0                                       2.15 ZrO.sub.2                                                0.265λ.sub.0                                                     1.82Film    (97 wt %)    TiO.sub.2    (3 wt %)Sixth    MgF.sub.2    0.27λ.sub.0          1.38 MgF.sub.2                   0.27λ.sub.0                        1.38 MgF.sub.2                                 0.27λ.sub.0                                       1.38 MgF.sub.2                                                0.27λ.sub.0                                                     1.38Film__________________________________________________________________________ Note: Design wavelength λ.sub.0 = 510 nm 
    
     Test 
     Table 14 shows results on the aforesaid samples. Furthermore, FIG. 9 illustrates antireflection characteristics (spectral reflectance) measured by a spectrophotometer. 
     
                       TABLE 14______________________________________          Comparative                    Comparative                              Comparative   Example 7          Example 19                    Example 20                              Example 21______________________________________60° C. 90% RH     a        a         c       a240 H70° C. Drying     a        c         c       c8 H______________________________________ 
    
     Table 14 shows that no significant difference in the formation of the crack is found between the sample of Example 7 and samples of Comparative Examples 19, 20 and 21, but during the storage at 70° C. (drying), the sample of Example 7 of the present invention has only formed no crack. 
     Example H 
     Samples of Example H are of Examples 7, 8 and 9, and of Comparative Examples 22. Tables 15 and 16 show materials for substrates and antireflection films, film thickness and film structures. 
     
                                           TABLE 15__________________________________________________________________________Example 7             Example 8PMMA                  PMMAMate-     Optical           Refractive Optical                            Refractiverial      Thickness           Index Material                      Thickness                            Index__________________________________________________________________________FirstAl.sub.2 O.sub.3     0.10λ.sub.0           1.60  Al.sub.2 O.sub.3                      0.10λ.sub.0                            1.60FilmSecondSiO.sub.2     0.50λ.sub.0           1.46  SiO.sub.2                      0.50λ.sub.0                            1.46FilmThirdTa.sub.2 O.sub.5     0.135λ.sub.0           2.02  Ta.sub.2 O.sub.5                      0.135λ.sub.0                            2.00Film (97 wt %)        (99.5 wtTiO.sub.2        %)(3 wt %)         TiO.sub.2                 (0.5 wt %)FourthSiO.sub.2     0.065λ.sub.0           1.46  SiO.sub.2                      0.05λ.sub.0                            1.46FilmFifthTa.sub.2 O.sub.5     0.265λ.sub.0           2.02  Ta.sub.2 O.sub.5                      0.265λ.sub.0                            2.00Film (97 wt %)TiO.sub.2(3 wt %)SixthMgF.sub.2     0.27λ.sub.0           1.38  MgF.sub.2                      0.27λ.sub.0                            1.38Film__________________________________________________________________________ Note: Design wavelength λ.sub.0 = 510 nm 
    
     
                                           TABLE 16__________________________________________________________________________                 ComparativeExample 9             Example 22PMMA                  PMMAMate-     Optical           Refractive Optical                            Refractiverial      Thickness           Index Mate-rial                      Thickness                            Index__________________________________________________________________________FirstAl.sub.2 O.sub.3     0.10λ.sub.0           1.60  Al.sub.2 O.sub.3                      0.10λ.sub.0                            1.60FilmSecondSiO.sub.2     0.50λ.sub.0           1.46  SiO.sub.2                      0.50λ.sub.0                            1.46FilmThirdTa.sub.2 O.sub.5     0.135λ.sub.0           2.07  Ta.sub.2 O.sub.5                      0.135λ.sub.0                            2.09Film (70 wt %)        (60 wt %)TiO.sub.2        TiO.sub.2(30 wt %)        (40 wt %)FourthSiO.sub.2     0.05λ.sub.0           1.46  SiO.sub.2                      0.05λ.sub.0                            1.46FilmFifthTa.sub.2 O.sub.5     0.265λ.sub.0           2.07  Ta.sub.2 O.sub.5                      0.265λ.sub.0                            2.09Film (70 wt %)        (60 wt %)TiO.sub.2        TiO.sub.2(30 wt %)        (40 wt %)SixthMgF.sub.2     0.27λ.sub.0           1.38  MgF.sub.2                      0.27λ.sub.0                            1.38Film__________________________________________________________________________ Note: Design wavelength λ.sub.0 = 510 nm 
    
     Test 
     Table 17 shows results on the aforesaid samples. Furthermore, FIG. 10 illustrates antireflection characteristics (spectral reflectance) measured by a spectrophotometer. 
     
                       TABLE 17______________________________________                              Comparative   Example 7          Example 7 Example 9 Example 22______________________________________60° C. 90% RH     a        a         a       a240 H70° C. Drying     a        a         a       c8 H______________________________________ 
    
     Table 17 shows that no significant difference in the formation of the crack is found between the sample of Examples 7, 8 and 9 and the sample of Comparative Example 22, but during the storage at 70° C. (drying), the samples of Examples 7, 8 and 9 of the present invention have only formed no crack. According to the foregoing, it has been found that in order to prevent the formation of the crack, mixing ratio of titanium oxide (TiO 2 ) will be in the range of 0.5 to 30 percent by weight. 
     Furthermore, test results show that when stored at 80° C. (drying), cracks are formed on Examples 8 and 9, while no crack is formed on Example 7 under this condition. Accordingly, on the use at higher temperatures, the mixing ratio of titanium oxide (TiO 2 ) is preferably from 1 to 5 percent by weight. 
     Further, as synthetic resins for the substrate, any of synthetic resins are available which have been used in this industry without particular limitation. Though polymethylmethacrylate (PMMA) and polycarbonates (PC) have been illustrated as representative synthetic resins, other resins are employed. 
     In addition, the vacuum evaporation was applied to Examples of the present invention. However, an ion plating, sputtering, etc. may be employed. 
     As composed above, the following advantages are effected, that is: 
     According to structure (1), in an optical member composed of a substrate and antireflection films, is provided the optical member which has excellent antireflection properties and is available for a high precision optical system, and is not liable to form the crack, because on the synthetic resin substrate, is arranged antireflection films having at least one mixture film consisting of tantalum oxide (Ta 2  O 3 ) and titanium oxide (TiO 2 ). 
     According to structure (2), in an optical member having the antireflection films described in the structure (1), the crack is not liable to be formed, because in the aforesaid antireflection films, a mixture film consisting of tantalum oxide (Ta 2  O 5 ) and titanium oxide (TiO 2 ) comprises from 0.5 to 30 weight percent of titanium oxide (TiO 2 ). 
     According to structure (3), in an optical member having the antireflection films described in the structure (1), the crack is not liable to be formed, because in the aforesaid antireflection films, a mixture film consisting of tantalum oxide (Ta 2  O 5 ) and titanium oxide (TiO 2 ) comprises from 1 to 5 weight percent of titanium oxide (TiO 2 ). 
     According to structure (4), in an optical member having the antireflection films described in one of structures (1) to (3), the crack is not liable to be formed; antireflection properties are excellent; the cost is low and the productivity is high, because the aforesaid antireflection films are composed of two films which meet conditions described in the structure (4), while the number of the antireflection film of the optical member having the antireflection films is counted from the substrate side. 
     According to structure (5), in an optical member having the antireflection films described in one of the structures (1) to (3), the crack is not liable to be formed; antireflection properties are excellent; the cost is low and the productivity is high, because the aforesaid antireflection films are composed of three films described in structure (5), while the number of the antireflection film is counted from the substrate. 
     According to structure (6), in an optical member having the antireflection films described in one of the structures (1) to (3), the crack is-not liable to be formed; antireflection properties are excellent; the cost is low and the productivity is high because the aforesaid antireflection films are composed of four films described in structure (6), while the number of the antireflection film is counted from the substrate. 
     According to structure (7), in an optical member having the antireflection films described in one of the structures (1) to (3), the crack is not liable to be formed; antireflection properties are excellent; the cost is low and the productivity is high, because the aforesaid antireflection films are composed of five films described in structure (7), while the number of the antireflection film is counted from the substrate. 
     According to structure (8), in an optical member having the antireflection films described in one of the structures (1) to (3), the crack is not liable to be formed; the cost are low and the productivity is high, because the aforesaid antireflection films comprise at least one of mixture films consisting of tantalum oxide (Ta 2  O 5 ) and titanium oxide (TiO 2 ), one of the low refractive index films and one of the intermediate refractive index films. 
     According to structure (9), in an optical member having the antireflection films described in one of the structures (1), (2), (3) and (8), the crack is not liable to be formed; the cost is low and the productivity is high, because the aforesaid antireflection films are composed of six films described in structure (9), while the number of the antireflection film of the optical member having the antireflection films is counted from the substrate side. 
     According to structure (10), in an optical member having the antireflection films described in one of the structures (4) to (9), the crack is not liable to be formed; the cost is low and the productivity is high, because the low refractive index film is composed of silicon oxide (SiO 2 ) or magnesium fluoride (MgF 2 ). 
     According to structure (11), in an optical member having the antireflection films described in one of the structures (8) and (9), the crack is not liable to be formed; the cost is low and the productivity is high, because the intermediate refractive index film is composed of aluminum oxide (Al 2  O 3 ).