The present invention relates to a high-pressure discharge lamp that has a discharge vessel made of light-transmitting ceramic, and a lighting apparatus that uses the lamp.
There is a demand for a high-pressure discharge lamp having a smaller lamp power of 20 W or less, which is made of light-transmitting ceramic and which has a long lifetime and a high efficiency.
It was found that a leak occurs at the seal shortly after the lamp is turned on even if a small high-pressure discharge lamp is manufactured by proportionally reducing directly the specifications of the discharge vessel and electrodes of a conventional, relatively large, high-pressure lamp to meet the demand. This is because the modes of conveying heat to the seal from a heat source such as discharge plasma, i.e., heat conduction, convection and radiation, are unbalanced.
To realize small high-pressure discharge lamps, the prior art technology of high-pressure discharge lamps should therefore be reviewed fundamentally to create new specifications that are suitable for small, high-pressure discharge lamps.
In contrast, the present inventors have already invented a high-pressure discharge lamp comprising light-transmitting ceramic that has a desirable long lifetime and a preferable light-emission efficiency even with a small size. That invention has already been filed as Japanese Patent Application No. 10-196322. The invention according to this application discloses a light-transmitting ceramic discharge vessel comprising a bulging section having both ends narrowed by continuous curved surfaces, and small-diameter cylindrical sections communicating with the ends of the bulging section and having an inner diameter smaller than the bulging section. It is very advantageous for a small high-pressure discharge lamp to use this discharge vessel because the discharge vessel can be formed integral with ease and has no optically and thermally discontinuous portion.
It was found, however, a problem occurs depending on the size of the space in the bulging section around the electrodes in the light-transmitting ceramic discharge vessel having the shaped as described above. That is, when a high-pressure discharge lamp which has been lightened is turned off, vapor of fillings such as a halide and mercury which has been dispersed in the discharging space moves toward narrow gaps in the small-diameter cylindrical sections of the light-transmitting ceramic discharge vessel having a low temperature. At this time, a turbulent flow is generated around the electrodes. If a turbulent flow is generated, the fillings such as halide and mercury easily adhere to surfaces of distal ends of the electrodes. Once the fillings have adhered to the distal ends of the electrodes, the electric discharging power attenuates so operation errors are caused at starting or transition from a glow discharge to an arc discharge becomes difficult. As a result, sputtering is excited and blackening occurs in the light-transmitting ceramic discharge vessel due to the sputtering.