Optical reflector and manufacturing method thereof

Abstract: In an optical reflector and a manufacturing method thereof, the reflector has superior durability and a high reflectivity to visible light, without color tones being apparent in the reflected light. The optical reflector comprises a glass substrate (1), on which are piled up a film (2) of refractive index n1, a film (3) of refractive index n2, a film (4) of refractive index n3, and a film (5) of refractive index n4, sequentially, wherein the thickness of the films are adjusted so that a relationship, n1 ≥ n4 > n3 > n2, is established among those refractive indices n1, n2, n3 and n4. The film (3) of refractive index n2 is from 30 nm to 60 nm in the film thickness, a reflectivity to visible light is equal to or greater than 75% upon the surface of the film (5) of refractive index n4, and the value is from 0 to 10 when representing the color tone of reflected light upon this film by a* and b* (psychometric chroma coordinates).

Description: The present invention relates to a reflector having a high reflectivity of visible light, in particular for use in glass products, such as a mirror, etc., and it relates to a manufacturing method thereof. Generally, upon a mirror is formed a metal film having a high reflectivity, such as a film of silver, aluminum, etc. With such a mirror using silver, a solution of silver salt is applied upon a glass plate, to thereby form a silver layer or film through a reductive reaction on the surface of the glass plate. However, the silver film itself is easily oxidized, and also is easily eroded, therefore being very weak or inferior in the durability thereof. Accordingly, it is impossible to obtain a mirror by this method alone that is practically useful or endurable, and there is a necessity of forming a protection layer upon the surface of the silver film. Further, because of encroachment or invasion of water and so on from the edge (i.e., an outer periphery edge portion of the silver film), there is also a necessity of protection treatment even on the edge thereof. Therefore a special protective coating treatment must be carried out, in particular, for the purpose of obtaining a mirror using silver which can be used on a washstand and/or in a bathroom. Also in many cases, an opaque material is applied as the protection layer or film. Where this is the case, the mirror can be formed only as a back-coated mirror, otherwise the mirror obtained may cause a dual-image due to the reflection of light upon a front surface and upon a reverse surface thereof. When a mirror uses an aluminum film in place of a film of silver, in general, the aluminum film is formed by a method such as a vacuum evaporation method and/or a sputtering method, necessitating the use of a vacuum apparatus or equipment as a facility thereof. Also, as it takes a period of time for forming the film by this method, there is a tendency that the cost for manufacturing thereof is increased due to a loss of efficiency in production or productivity. Further, the aluminum film formed through the above-mentioned method comes to be one which is low in durability, therefore a protection film is necessary for producing a practical mirror by this method as well. As a method not requiring the use of vacuum apparatus or equipment, a method for manufacturing a mirror having the reflectivity being equal to or higher than 70%, by forming one-by-one a reflection layer and a reflection enforcement layer, using a normal pressure CVD (Chemical Vapor Deposition) method, is disclosed in Japanese Laid-Open Patent Hei 6-183787 (1994). In more detail, it discloses a mirror in which a film having a high refractive index, such as a silicon film, is used as the reflection layer, on which is formed the reflection enforcement layer, including a layer having a low refractive index, such as a film of silicon oxide or the like, and a film of high refractive index, such as a silicon film, or a film of tin oxide, titanium oxide, etc., sequentially applied. In Japanese Laid-Open Patent Hei 6-183787 (1994), the silicon film is mainly used as a reflection layer, and a layer of silicon oxide is used as a low refractive index layer for forming the reflection enforcement layer, while as a high refractive index layer a film of silicon, titanium oxide, or tin oxide, etc., is used. In a case where the silicon film is used as the uppermost film, there is a necessity that a further protection layer is formed because of the low durability of the silicon layer. In a case where the film of titanium oxide or tin oxide is used, as described in US-A-5 745 291 though no such problem in the durability thereof arises, there is a necessity that the film of silicon oxide as the low refractive index layer must be equal to or greater than 70 nm in thickness, so as to make a reflection color inconspicuous, under the condition that the reflectivity to visible light is equal to or greater than 75%. However, a large amount of powder is generated when forming the film of silicon oxide through a pyrolytic method, and this may be taken into the film as a contaminating foreign body or may cause pinholes therein, thereby reducing a yield rate. Further, the powder is accumulated or piled up in a periphery of the apparatus and/or in exhaust pipes thereof, thus presenting a cause of reduction in the stability of the film-forming process. For avoiding the drawbacks mentioned above, there can be considered a countermeasure wherein the supplied amount of the raw material is reduced so as to slow down the speed of forming films, however it is difficult to obtain the film thickness being equal to or greater than 70 nm, by this method. Also, in Japanese Laid-Open Patent Hei 8-508708 (1996), there is proposed a mirror, wherein the low refractive film is made to be equal to or greater than 10 nm and be equal to or less than 40 nm in the thickness thereof, and a film of tin oxide is formed upon the surface thereof. However, for this mirror, the reflectivity to visible light is equal to or greater than 35% and is less than 70%, and further the color tone of the reflected light has a bluish tinge. According to the present invention, there is provided an optical reflector having a glass substrate according to claim 1. Also, according to the present invention, it is preferable that said film having the refractive index n2 is one which contains silicon oxide (SiO 2 ) as a main ingredient thereof. Preferably said film having the refractive index n3 is one which contains tin oxide (SnO) as a main ingredient thereof. Preferably said film having the refractive index n4 is one which contains titanium oxide (TiO 2 ) as a main ingredient thereof. Preferably said film having the refractive index n1 is one which contains silicon (Si) as a main ingredient thereof. Further, according to the present invention, it is preferable that an undercoating film is formed between said film having the refractive index n1 and said glass substrate, and it is further preferable that said undercoating film is a film of tin oxide. Also, according to the present invention, it is preferable that film thickness is equal to or greater than 15 nm and is equal to or less than 45 nm for the film having the refractive index n1. Preferably the film thickness is equal to or greater than 10 nm and is equal to or less than 80 nm for the film having the refractive index n3. Preferably the film thickness is equal to or greater than 10 nm and is equal to or less than 80 nm for the film having the refractive index n4. An opaque layer may be formed on an opposed side of said glass substrate to that on which said films are formed. Also, according to the present invention, there is provided a method for manufacturing the optical reflector having a glass substrate according to claim 10. Further, according to the present invention, it is also preferable that an undercoating film is formed between said film having the refractive index n1 and said glass substrate, and that said undercoating film, said film having the refractive index n1, said film having the refractive index n2, said film having the refractive index n3 and said film having the refractive index n4 are formed, sequentially, by means of a pyrolytic method upon a surface of said glass substrate at high temperature. Namely, according to the present invention, forming the film having the refractive index n1 at the wavelength of 550 nm; the film having the refractive index n2 at the said wavelength; the film having the refractive index n3 at the said wavelength; and the film having the refractive index n4 at the said wavelength, sequentially, upon at least one of main surfaces of the glass substrate, and making the film thickness of the film of the refractive index n2 equal or greater than 30 nm and is equal or less than 60 nm, it is possible to improve the efficacy of the optical reflector by employing film formation using a pyrolytic method, thereby providing the optical reflector of high performance and good durability, having a value of {(a*) 2 + (b*) 2 } 1/2 being equal to or greater than 0 and being equal to or less than 10, when representing a reflective color tone upon surface of said film of refractive indices n4 by a* and b* (psychometric chroma coordinates). Certain preferred embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figs. 1 (a) to 1(c) are enlarged cross-sectional views showing embodiments of a preferred optical reflector according to the present invention, respectively; and Fig. 2 shows an outline view of an apparatus or equipment for forming films, in a float glass plate production line for manufacturing the optical reflector according to a preferred embodiment of the present invention. According to one aspect of the present invention, there can be obtained an optical reflector having a glass substrate, a film having a refractive index n1 at a wavelength of 550 nm; a film having a refractive index n2 at the wavelength of 550 nm; a film having a refractive index n3 at the wavelength of 550 nm; and a film having a refractive index n4 at the wavelength of 550 nm, sequentially formed upon at least one of main surfaces thereof, wherein there is established a relationship n1 ≥ n4 > n3 > n2 among the refractive indices n1, n2, n3 and n4, said optical reflector thereby having a reflectivity to visible light being equal or greater than 75% and having a value of {(a*) 2 + (b * ) 2 } 1/2 being equal to or greater than 0 and being equal to or less than 10, when representing a reflective color tone by a* and b* (psychometric chroma coordinates), even when the film thickness of said film of the refractive index n2 is equal or greater than 30 nm and is equal or less than 60 nm. An optical reflector having a high reflectivity and an inconspicuous color tone of reflection can be obtained by applying the construction of multi-layered films, wherein they are formed upon the film of refractive index n1 to function as the reflection layer, gradually changing the refractive index thereof from a low refractive index to a high one as the number of layers increase. If the thickness of the above-mentioned film of refractive index n2 is less than 30 nm, it is impossible to satisfy both a predetermined reflectivity and a predetermined color, while if it exceeds 60 nm, the productivity of manufacture is impaired, thereby causing increase in manufacturing costs, and it is impossible to achieve the advantages offered by the present invention. The larger the difference between the refractive indices n1 and n2 (n1 > n2), the larger the reflectivity to visible light, therefore it is preferable that this difference is as large as possible. The film of refractive index n1 at a wavelength of 550 nm, according to the present invention, is preferably a film that contains silicon as the main ingredient thereof. The reason for this is that, in particular, the silicon has a high refractive index and further that such a film can be formed easily through pyrolytic methods. It is also possible to include carbon, nitrogen, oxygen, etc., in the silicon film. In this case, the refractive index n1, though varying depending upon the included substances and the amount of impurities contained therein, lies within a range, being equal to or greater than 3.0 and being equal to or less than 5.5. The film of refractive index n2 at a wavelength of 550 nm, according to the present invention, is preferably a film that contains silicon oxide as the main ingredient thereof. The reason for this is that, in particular, the silicon oxide has a low refractive index and further that such a film can be formed easily through pyrolytic methods. It is also possible to include nitrogen, carbon, fluorine, titanium, tin, aluminum, boron, phosphorus, etc., in the silicon oxide film. In this case, the refractive index n2, though varying depending upon the included substances and the amount of impurities contained therein, lies within a range, being equal to or greater than 1.4 and being equal to cr less than 1.6. The film of refractive index n4 at a wavelength of 550 nm, according to the present invention, is preferably made of titanium oxide. The reason for this is that, in particular, the titanium oxide has a relatively high refractive index, that such a film can be formed easily through pyrolytic methods, and further that it has good durability thus introducing no problems if it is exposed at the outermost surface. It is also possible to include tin, fluorine, chlorine, carbon, nitrogen, silicon, etc., in the titanium oxide film. In this case, the refractive index n4, though varying depending upon the included substances and the amount of impurities contained therein, lies within a range, being equal to or greater than 2.2 and being equal to or less than 2.8. The film of refractive index n3 at a wavelength of 550 nm, according to the present invention, is preferably made of tin oxide. The reason for this is that the refractive index thereof is of a value between those of above-mentioned refractive indices n2 and n4 and further that such a film can be formed easily through pyrolytic methods. It is also possible to include indium, fluorine, antimony, chlorine, carbon, silicon, etc., in the tin oxide film. In this case, the refractive index n3, though varying depending upon the included substances and the amount of impurities contained therein, lies within a range, being equal to or greater than 1.8 and being equal to or less than 2.2. According to the present invention, it may be possible to form an undercoating film between the film having a refractive index of n1 and the glass substrate. When using soda lime glass as the glass substrate, it is possible to prevent alkali from being diffused into the film of refractive index n1 by forming the undercoating film, thereby preventing any decrease in the refractive index thereof. A film comprised of tin oxide is one example of an undercoating film which could be used. Also, it may be possible to include indium, fluorine, antimony, chlorine, carbon, silicon, etc., in this tin oxide film. The film thickness of this undercoating film is preferably equal to or greater than 1 nm and is equal to or less than 100 nm, and ideally, it lies within a range being equal to or greater than 10 nm and being equal to or less than 50 nm. When the film thickness of the undercoating film is too thick, a haze factor or ratio becomes too large, bringing about diffused or irregular reflection in the reflection light, therefore it is not preferable. In an optical reflector having such the films as mentioned above stacked thereon, it is preferable that the film thickness of the films of refractive indices n1, n3 and n4 are within a range indicated below, that the reflectivity to visible light is equal to or greater than 75%, and that the value of {(a*) 2 + (b*) 2 } 1/2 is equal to or greater than 0 and is equal to or less than 10, when representing a reflective color tone by a* and b* (psychometric chroma coordinates). When the film thickness of any among the films falls out of the range indicated below, the reflectivity to visible light may become lower than 75%, or the value of {(a*) 2 + (b*) 2 } 1/2 may become greater than 10, when representing the reflective color tone by a* and b* (psychometric chroma coordinates). The thickness of the film having the refractive index n1 is equal to or greater than 15 nm, and is equal to or less than 45 nm; the thickness of the film having the refractive index n3 is equal to or greater than 10 nm, and is equal to or less than 80 nm; and/or the thickness of the film having the refractive index n4 is equal to or greater than 10 nm, and is equal to or less than 80 nm. The optical reflector obtains the transmittance to visible light being equal to or less than 25%, thereby enabling that it may be used as a half mirror. Also, in a case of the reflector being used as a mirror, an opaque layer may be formed on the glass surface on the reverse side with respect to the above-described films, so that the image on the reverse side surface cannot be seen through the reflector. It is also possible to adjust the color of the reflection upon the film surface by changing the color of the opaque layer. Also, by forming another opaque layer outside (relative to the substrate) of the film of refractive index n4 as the most outer layer, it is possible to use the reflector as a back-coated mirror. In this case, for increasing the reflectivity to visible light at the back surface, it is preferable to form the films mentioned above on the substrate, i.e., the film of refractive index n4, the film of refractive index n3 and the film of refractive index n1, on the glass substrate, sequentially. On the outermost surface of the films mentioned above, i.e., on an outer side of the film of refractive index n4 as the most outer layer, there can be formed concave/convex patterning by utilizing the crystal structure of tin oxide and/or titanium oxide. Further, by forming on this an overcoating film, such as of silicon oxide, etc., it is possible to obtain a hydrophilic property thereof, and further it is possible to obtain a surface which exhibits such hydrophilic properties durably and lastingly. As the method for forming each layer, any one can be applied, such as a vacuum evaporation method, a sputtering method, a sol-gel method, a liquid-phase epitaxy method, an enameling method, a spray method, a CVD (Chemical Vapor Deposition) method, etc., however, in particular the spray pyrolytic method and the CVD (Chemical Vapor Deposition) are effective, in which the ingredients are supplied upon the glass plate of high temperature so as to form the films thereon utilizing the heat of the glass plate. Any kind of glass plate upon which the above-mentioned films can be formed may be used as the glass substrate. However, a glass substrate, such as a soda lime glass, etc., is preferably which can be produced through a float glass process having a high level of productivity. The above-mentioned glass plate or substrate of high temperature may be obtained by re-heating the glass which was formed in advance in a plate-like form. However it is preferable to apply such a method, wherein the ingredients are supplied upon the glass surface within a tin bath or when the glass plate comes out therefrom in the manufacturing of the float glass, because there is no necessity of a heating apparatus nor equipment when utilizing what is sometimes referred to as an on-line CVD method, and it is possible to deposit the layers on a large and/or wide area upon a large glass surface. It is also possible to apply a method combining the above-mentioned pyrolytic method with other manufacturing method(s), for example, manufacturing the films of refractive indices n1, n2 and n3 through the pyrolytic method, whereafter the film of refractive index n4 may be manufactured thereupon by the sol-gel method or the sputtering method. For the ingredients to be used when forming a film or layer of silicon through the pyrolytic method, may be included a silane gas, such as of monosilane, disilane, or dichlorosilane, silicon tetrachloride, etc. For the ingredients to be used when forming a film or layer of silicon oxide through the pyrolytic method, included may be a silicon ingredient such as a silane gas such as monosilane, disilane, or dichlorosilane, an inorganic compound such as silicon tetrachloride, etc., or an organic compound such as tetraethoxysilane, tetramethoxysilane, dibutoxy-diacetoxysilane, etc. Also, as an oxygen source, included may be oxygen, ozone, acetone, carbon dioxide, etc. In the case of using the silane gas, it is possible to form the film with improved stability by adding ethylene or ethane, etc., thereto. As the ingredients to be used when forming a film of tin oxide through the pyrolytic method, included may be an inorganic compound, such as tin tetrachloride, etc., or monomethyl tin trichloride, monobutyl tin trichloride, dimethyl tin dichloride, dibutyl tin dichloride, tetramethyl tin, tetrabutyl tin, dibutyl tin diacetate, dioctyl tin diacetate, etc. As the ingredients to be used when forming a film of titanium oxide through the pyrolytic method, included may be an inorganic compound, such as titanium tetrachloride, titanium isopropoxide, acetyl acetonate titanium, etc. The films or layers which are formed by the pyrolytic method are not caused to deteriorate during heating processes. Therefore it is possible to apply a tempering process or a bending process to the optical reflector, after the manufacturing thereof. (Embodiment) Figs. 1 (a) to (c) are enlarged cross-sectional views showing examples of optical reflectors according to the present invention. In the embodiment shown in the cross-sectional view of Fig. 1 (a), upon the surface of the glass substrate 1 are formed a silicon film 2, a silicon oxide film 3, a tin oxide film 4 and a titanium oxide film 5, sequentially. In measurement of the distance between the concave and the convex portions, observed using an atomic force microscope (AFM) or an electron microscope, an average value (Sm) thereof is calculated from the measured cross-section curve. However, the average distance (Sm) between the concave and the convex portions is preferably between 4 nm and 300 nm. Also, a roughness (Ra) of the surface is preferably between 1.5 nm and 50 nm. In the embodiment shown in the cross-sectional view of Fig. 1 (b), as an undercoating film 6 is formed a film of tin oxide between the glass substrate 1 and the silicon film 2. And, in the embodiment shown in the cross-sectional view of Fig. 1 (c), an opaque layer 7 is formed upon the reverse surface of the glass substrate. Fig. 2 shows an outline view of an example of the apparatus or equipment for forming films, in a float glass plate production line for manufacturing the optical reflector according to a preferred embodiment of the present invention. In the present embodiment, the above-mentioned films or layers are formed through the pyrolytic CVD (Chemical Vapor Deposition) method within a tin bath 13 when producing the float glass plate as shown in Fig. 2. The forming of the films or layers is performed, after the glass plate is formed at a predetermined thickness, in a time period when the glass has a high temperature, i.e., within a range from 600°C to 750°C. For forming the films or layers, the film forming apparatuses 15 to 19 are inserted within the tin bath 13, individually, wherein the silicon film is formed by supplying the monosilane being diluted with nitrogen, the silicon oxide film by supplying the monosilane, ethylene and oxygen being diluted with nitrogen, the tin oxide film by supplying the dimethyl tin dichloride, oxygen and water being diluted with nitrogen, and the titanium oxide film by supplying the titanium isopropoxide and oxide being diluted with nitrogen, to the respective film forming apparatuses. (Examples 1 to 5 and Comparisons 1 to 4) Examples 1 to 5 and comparisons 1 to 4 are manufactured by washing and cutting the glass plate into plates being 100mm × 100mm, after forming the silicon film, the silicon oxide film, the tin oxide film and the titanium oxide film on the glass surface, at the respective predetermined thickness thereof, sequentially, using the film forming apparatuses 15 to 19, and then cooling the glass plate using a lehr. (Examples 6 to 12 and Comparison 5) Examples 6 to 12 and comparison 5 are manufactured by washing and cutting the glass plate into plates being 100mm × 100mm, after forming the tin oxide film (the undercoating film), the silicon film, the silicon oxide film, the tin oxide film and the titanium oxide film on the glass surface, at the respective predetermined thickness thereof, sequentially, using the film forming apparatuses 15 to 19, and then cooling the glass plate using a lehr. Though all the films or layers are formed within the tin bath 13 using the pyrolytic CVD (Chemical Vapor Deposition) method in the present embodiments, however the films or layers of oxides, such as the silicon oxide film, the tin oxide film, the titanium oxide film, etc., may be formed within the cooling portion 14. In this case, it is possible to employ not only the CVD (Chemical Vapor Deposition) method, but also to use a spray method using the ingredients in a liquid and/or powder form. Further, they may be manufactured by using the pyrolytic method and another film forming method(s). For example, the silicon film, the silicon oxide film and the tin oxide film may be formed through the pyrolytic method, while the titanium oxide film through the sol-gel method or the sputtering method. In each of those embodiments being formed in the methods mentioned above, the silicon film 2, the silicon oxide film 3, the tin oxide film 4, the titanium oxide film 5 and the undercoating film are as described below in the film thickness thereof, while a reflectivity to visible light which is incident upon the front surface of the films is equal to or greater than 75%, and the value of {(a*) 2 + (b*) 2 } 1/2 is equal to or greater than 0 and is equal to or less than 10, when representing the reflective color tone by a* and b* (psychometric chroma coordinates). Film thickness: the undercoating film of tin oxide: being equal to or greater than 0 nm and equal to or less than 100 nm; the silicon film: being equal to or greater than 15 nm and equal to or less than 45 nm; the silicon oxide film: being equal to or greater than 30 nm and equal to or less than 60 nm; the tin oxide film: being equal to or greater than 10 nm and equal to or less than 80 nm; and the titanium oxide film: being equal to or greater than 10 nm and equal to or less than 80 nm. In this instance, the refractive indices of the silicon film, the silicon oxide film, the tin oxide film and the titanium oxide film at the wavelength of 500 nm lie within the following ranges: the silicon film: from 4.1 up to 4.7; the silicon oxide film: from 1.44 to 1.46; the tin oxide film: from 1.9 to 2.1; and the titanium oxide film: from 2.3 to 2.5. In Table 1, there are shown the film thickness of each layer in the respective embodiments and the comparisons, and the value of {(a*) 2 + (b*) 2 } 1/2 when representing the reflective color tone by a* and b* (psychometric chroma coordinates). Further, the reflectivity to visible light is measured in accordance with JIS R3106-1998, the color tone of reflection light with JIS Z 8722-1982, by means of a spectrophotometer of type UV-3100, Shimazu-Seisakusho (Manufacturer) Co., Ltd. (Japan), and L* and a* and b* of the psychometric chroma coordinates of a* and b* color system, defined in JIS Z 8729-1980, are calculated therefrom. See also JIS (Japanese Industrial Standard) Z 8105-1982 2075. Film Thickness of Respective Layers (nm) Reflectivity to Visible Light (%) √(a* 2 +b* 2 ) Titanium Oxide Tin Oxide Silicon Oxide Silicon Tin Oxide Example 1 50 20 60 30 83.3 7.6 Example 2 50 30 40 30 79.7 8.3 Example 3 50 10 60 45 77.2 5.6 Example 4 10 80 50 25 76.3 9.5 Example 5 80 10 40 25 78.3 9.6 Example 6 40 20 60 30 20 81.3 5.3 Example 7 50 10 60 40 60 75.0 2.8 Example 8 50 30 60 15 30 76.0 8.7 Example 9 50 30 30 30 20 76.4 7.9 Example 10 80 10 50 20 10 77.6 8.6 Example 11 60 10 60 25 50 78.3 5.2 Example 12 50 20 60 30 100 79.2 7.9 Comparison 60 40 20 30 75.9 18.5 Comparison 1 60 40 20 30 75.9 18.5 Comparison 2 10 90 50 30 72.6 32.5 Comparison 3 90 10 40 30 71.9 42.5 Comparison 4 50 10 60 50 72.1 4.6 Comparison 5 60 20 60 10 40 69.0 10.7 As is indicated in Table 1, the reflectivity to visible light is equal to or greater than 75% with the film construction of each of the embodiments 1 to 12, and the values of {(a*) 2 + (b*) 2 } 1/2 are equal to or less than 10. Alternately, with the film construction of each of the comparisons 1 to 5, the reflectivity to visible light is less than 75% and the values of {(a*) 2 + (b*) 2 } 1/2 are greater than 10. As is fully explained in the above, according to the present invention, the glass substrate formed directly or indirectly via the undercoating film, with the film of refractive index n1, the film of refractive index n2, the film of refractive index n3, and the film of refractive index n4, wherein the film of low refractive index n2 comprised of such material as silicon oxide is set to be equal to or greater than 30 nm and be equal to or less than 60 nm, it is possible to produce an optical reflector, having the reflectivity to visible light being equal to or greater than 75% upon the surface and having the value of {(a*) 2 + (b*) 2 } 1/2 being equal to or greater than 0 and equal to or less than 10, when representing the reflective color tone by a* and b* (psychometric chroma coordinates), by using pyrolysis to take advantage of its good yield rate and to achieve better efficiency, thereby enabling to reduce the manufacturing cost thereof. Further, by covering the outermost surface with the film having the good durability, overcome is a necessity for a special process of treating, such as with a protection layer or the like.

Claims: An optical reflector having a glass substrate (1), upon at least one of main surfaces thereof are formed, sequentially, a film (2) having a refractive index n1 at a wavelength of 550 nm; a film (3) having a refractive index n2 at said wavelength; a film (4) having a refractive index n3 at said wavelength; and a film (5) having a refractive index n4 at said wavelength, wherein there is established a relationship as indicated below between refractive indices n1, n2, n3 and n4; n1 ≥ n4 > n3 > n2 characterised in that film thickness of said film of the refractive index n2 is equal to or greater than 30 nm and is equal to or less than 60 nm; a reflectivity to visible light of said film of refractive index n4 is equal to or greater than 75%; and a value of {(a*) 2 + ( b*) 2 } 1/2 is equal to or greater than 0 and is equal or less than 10, when representing a color tone of reflected light upon a surface of said film of refractive indices n4 by the psychometric chroma coordinates a* and b * . An optical reflector as defined in Claim 1, wherein said film (3) having the refractive index n2 contains silicon oxide as a main ingredient thereof. An optical reflector as defined in Claim 1 or 2, wherein said film (4) having the refractive index n3 contains tin oxide as a main ingredient thereof. An optical reflector as defined in Claim 1, 2 or 3, wherein said film (5) having the refractive index n4 contains titanium oxide as a main ingredient thereof. An optical reflector as defined in any one of Claims 1 to 4, wherein said film (2) having the refractive index n1 contains silicon as a main ingredient thereof. An optical reflector as defined in any one of Claims 1 to 5, wherein an undercoating film (6) is formed between said film (2) having the refractive index n1 and said glass substrate (1). An optical reflector as defined in Claim 6, wherein said undercoating film (6) is a film of tin oxide. An optical reflector as defined in any one of Claims 1 to 7, wherein film thickness of said film (2) having the refractive index n1, of said film (4) having the refractive index n3 and of said film (5) having the refractive index n4 are in a region as indicated below, respectively: the film (2) having the refractive index n1 is equal to or greater than 15 nm and is equal to or less than 45 nm; the film (4) having the refractive index n3 is equal to or greater than 10 nm and is equal to or less than 80 nm; and the film (5) having the refractive index n4 is equal to or greater than 10 nm and is equal to or less than 80 nm. An optical reflector as defined in any one of Claims 1 to 8, wherein an opaque layer (7) is formed on a side of said glass substrate (1), being reverse to that on which said films are formed. A method for manufacturing the optical reflector having a glass substrate (1), as defined in any one of Claims 1 to 9, comprising the steps of: forming a film (2) having a refractive index n1 at a wavelength of 550 nm; forming a film (3) having a refractive index n2 at said wavelength; forming a film (4) having a refractive index n3 at said wavelength; and forming a film (5) having a refractive index n4 at the said wavelength wherein n1 ≥ n4 > n3 > n2, sequentially, upon at least one of main surfaces thereof. A method for manufacturing the optical reflector as defined in Claim 10, wherein an undercoating film (6) is formed between said film (2) having the refractive index n1 and said glass substrate (1). A method for manufacturing the optical reflector as defined in Claim 10 or 11, wherein said undercoating film (6), said film (2) having the refractive index n1, said film (3) having the refractive index n2, said film (4) having the refractive index n3 and said film (5) having the refractive index n4 are formed, sequentially, through a pyrolytic method upon a surface of said glass (1) at high temperature.

Inventors: HIRATA MASAHIRO, SUEYOSHI YUKIO, YAMAMOTO TORU, HIRATA, MASAHIRO, SUEYOSHI, YUKIO, YAMAMOTO, TORU, Hirata, Masahiro, c/o Nippon Sheet Glass Co., Ltd., Sueyoshi, Yukio, c/o Nippon Sheet Glass Co., Ltd., Yamamoto, Toru, c/o Nippon Sheet Glass Co., Ltd.
Applicants: NIPPON SHEET GLASS CO LTD, NIPPON SHEET GLASS CO., LTD.
Application year: 2000
Main classification: G02B5
IPC codes: G02B5, C03C17