1. Field of the Invention
The present invention relates to a decorative casing of an electronic device or the like, and specifically relates to a decorative casing using a multilayer film generating interference colors of which color tones change depending on a viewing angle of the decorative casing.
2. Description of the Related Art
As a factor that determines a salability of an electronic device or the like, obviously importance is attached to the actual function of the electronic device. However, the appearance of the product is also an important element. At present, functions of products in certain product groups have become similar, so it is difficult to differentiate the functions of the products. Consumers usually prefer a decorative product among many products which have substantially the same functions; however the appearance of the products is likely to have a greater influence on the sales of the products.
Based on the miniaturization of electronic devices in recent years, portable products are becoming more widespread, as represented by portable telephones and portable music players, compared with products used in a fixed state. As a result, users of portable products consider them to be accessories, and place importance on decoration, such as color, as well as shape and appearance. Therefore, manufacturers are differentiating their products from competitors' products through appearance. In addition to products that are always used in a mobile state, in products such as a mouse and a memory card of a personal computer, the outward appearance along with, surface decoration of the product is important to differentiate the product from those of competitors. Accordingly, manufacturers are actively studying techniques to enhance decoration of products.
One technique of applying decoration to a product is a decorative technique by which colors of the product appear different from different angles. This technique uses changing colors to give color depth. Various methods for achieving this kind of decoration are in use.
First, there is a method of coloring a casing of a product itself using a colored resin. However, it is difficult to obtain an impression of high quality from this method. Further, because a color of the surface does not change even if the surface shape is changed, the color of the surface becomes simple.
Second, there is a method of coating a casing of a product. However, a coating material which changes the color according to the viewing angle of the product is expensive, and this coating material gives an impression of low quality.
Third, there is a method of coating with a metal film and a dielectric film. However, the addition of a coating process increases tact time, which results in a decrease in productivity of the product.
Fourth, there is a method of attaching a seal to a casing. However, according to this method, a surface of the product is hidden under the seal, which results in the surface shape of the seal becoming flat, even when stereoscopic modeling is performed on the surface of the product under the seal.
Consequently, there is proposed a method of using a multilayer film utilizing interference colors of light. The multilayer film has a lamination of two or more layers having different refractive indexes which exhibits colors using interferential effect of light due to an optical path difference. The principle of coloring will be explained with reference to FIG. 1. When a multilayer film 101, having plural films of a thin film A and a thin film B are alternately laminated, and provided on a base material 100, light 102 incident to the multilayer film 101 is reflected at the interface between thin films A and B, and the reflected light interferes with each other. Following expression is established:
(nAdA cos θA+nBdB cos θB)=mλ (where m is an integer), where film thicknesses of the thin films A and B are expressed as dA and dB, light incident angles are expressed as θA and θB, refractive indexes are expressed as nA and nB, and a wavelength is expressed as λ.
(nAdA cos θA+nBdB cos θB)=mλ (where m is an integer). In this case, the lights of wavelength λ interfere with each other to increase the strength of the light. When an observer changes a viewing angle of the base material adhered with the multilayer film, incident angle θ of a reflected light reaching the observer relative to the multilayer film changes. Accordingly, wavelength λ of the interfering light changes. As a result, the coloring of the interference colors change. Therefore, when the multilayer film exhibiting interference colors is applied to a product, coloring that changes at different angles of viewing the product can be achieved, thereby providing decoration to the product.
Japanese Patent Application Laid-open No. 2005-219394 (Patent Document 1) discloses a decoration technique using a multilayer film exhibiting the interference colors. FIGS. 2A and 2B depict cross-sectional views of a decorative material described in above Patent Document 1. A multilayer film 201 is formed as a light-interference multilayer film, by laminating synthetic-resin thin films via adhesive layers. Synthetic resin films 202a and 202b for shape stabilization are laminated on multilayer film 201, to stereoscopically form a material film 203. Material film 203 is used as a decorative material 205. When multilayer film 201 is stereoscopically formed, the film is distorted, and is not totally uniform. As a result, a wavelength region for transmitting or reflecting the light at each part of decoration material 205 changes. Consequently, a rainbow-like pattern appears, or the pattern becomes substantially transparent due to the emphasis or reflection of a certain wavelength region, and various optical changes appear.
According to this conventional technique, coloring can be changed on curved parts, using a stereoscopic shape. However, it has been difficult to generate a change of color tone to a desired shape at a desired position. On a flat part, coloring is substantially uniform, because an angle formed between the flat surface and the observer becomes substantially constant. As a result, when this technique is applied to a product having many flat regions, the color tone becomes simple.