1. Technical Field
The present invention relates to an arc tube and a projector.
2. Related Art
A related-art arc tube included in a light source of a projector has a tube spherical portion containing a pair of electrodes, and a pair of seal portions extending from both sides of the tube spherical portion (for example, see JP-A-2005-5183). Each electrode provided on this type of arc tube has a shaft portion made of metal having high melting point such as tungsten, and a coil winding portion formed by winding coil such as tungsten around the shaft portion (for example, see JP-A-8-273599).
Recently, there is an increasing demand for an arc tube capable of emitting light having high luminance with the development of a high-luminance projector.
A possible method for increasing luminance of the arc tube in the related art is to raise the temperature of the coolest portion in the discharge space within the tube spherical portion (such as root of the electrode (around a position indicated by A in FIG. 2A which will be described later)). When the temperature of the coolest portion is low, the evaporation capability of metal substance sealed within the tube spherical portion deteriorates. In this case, the lowered efficiency of halogen cycle and blackened tube wall thus caused lowers transmissivity of the arc tube.
For raising the temperature of the coolest portion in the related-art arc tube, it is possible to increase the length of the coil winding portion and wind the coil up to an area close to the root of the electrode. In this case, however, the arc path is guided from the area of the coil winding portion closest to the seal portion toward the tube spherical portion, and the high-temperature arc contacts and collides with the inner wall of the tube spherical portion. As a result, quartz glass (SiO2) as base material of the arc tube evaporates. Then, the evaporated SiO2 is separated into Si and O by discharge plasma, and oxidizes the tungsten constituting the electrode. Thus, the oxide of the tungsten evaporates and easily adheres to the inner surface of the tube spherical portion in the vicinity of the root of the electrode. In this case, the transmissivity of the arc tube easily lowers due to the deposit thus produced. Accordingly, increase in the luminance of the arc tube becomes difficult.
Therefore, in case of the related-art arc tube which increases the length of the coil winding portion and winds the coil up to the vicinity of the root of the electrode, the temperature of the coolest portion in the discharge space within the tube spherical portion can be raised, but the luminance of light from the arc tube cannot be easily increased.