In the light sources of projectors such as OHP (over head projectors) and liquid crystal projectors and in illuminators such as those used for spot illumination, reflectors are used for reflecting light that is emitted from light source lamps. The reflectors are members that are exposed to high temperatures resulting from the generation of heat by light source lamps such as ultra-high pressure mercury lamps, and are required to have a high heat resistance. Therefore, hitherto, reflectors have been used which comprise a reflector substrate composed of a heat-resisting glass having excellent heat resistance (e.g., PYREX (trademark) manufactured by Corning Glass Works) and having a reflective film formed on the substrate.
However, recently, the output and luminance of the light source lamps used for illuminators and projector light sources tend to be increased, and the level of heat resistance that is required is further enhanced. Under the circumstances, crystallized glasses that are superior to heat-resisting glasses in heat resistance are proposed as materials constituting reflector substrates.
For example, it has been proposed to use lithia-alumina-silica-based crystallized glasses (hereinafter sometimes referred to as “lithia-based crystallized glasses”) such as β-spodumene solid solution (β-spodumene: Li2O—Al2O3-4SiO2) and β-eucryptite solid solution (β-eucryptite: Li2O—Al2O3-2SiO2) (see, for example, JP-B-7-92527). Moreover, the inventors have also proposed using baria-alumina-silica-based crystallized glasses (hereinafter sometimes referred to as “baria-based crystallized glasses”) such as celsian (BaO—Al2O3-2SiO2) (see, for example, JP-A-2002-109923).
However, the lithia-based crystallized glass has such a problem in that it has a very small thermal expansion coefficient (JIS R1618: 40–400° C.) of about 0×10−7−20×10−7 (/° C.) and shows excellent heat resistance, but is inferior in meltability because of its high melting point of about 1500° C. or higher. Specifically, there are difficulties in that special equipment (such as a melting furnace) which can stand temperatures higher than 1500° C. are required for melting the glass and molding the glass to make the substrate for the reflector, which restricts the production conditions. On the other hand, the above-mentioned baria-based crystallized glass has a low melting point, namely, about 1450° C. and has no such high temperature production condition problems, but the thermal expansion coefficient (JIS R1618: 40—400° C.) is about 30×10−7−45×10−7 (/° C.), and further improvement is needed in the aspect of heat resistance.