The present invention relates to mercury-free metal halide lamps that do not contain mercury as a luminous material. In particular, the present invention relates to mercury-free metal halide lamps used for headlights of automobiles in combination with a reflecting mirror.
In recent years, metal halide lamps, which are one type of discharge lamps have been developed vigorously. The metal halide lamps enclose metal halide, in addition to mercury, in an arc tube (bulb) as luminous materials, and for example, the metal halide lamps are beginning to be used as head lamps of automobiles.
FIG. 7 shows a conventional metal halide lamp. The metal halide lamp shown in FIG. 7 includes the arc tube 1 made of quartz glass and sealing portions 2 for sealing the inside of the arc tube 1 that are positioned at both ends of the arc tube 1. A pair of electrodes 3 made of tungsten is arranged in the arc tube 1. A luminous material 17 including mercury and metal halide and a rare gas (not shown) are enclosed in the arc tube 1. The pair of electrodes 3 in the arc tube 1 is connected to first ends of molybdenum foils 4, and the molybdenum foils are sealed by the sealing portions 2. The other (second) ends of the molybdenum foils 4 are connected to lead wires 5. The lead wires 5 are electrically connected to an operating circuit (not shown).
The principle of the emission of this metal halide lamp will be described briefly. When the lamp is operated by applying a voltage to the lead wires 5 from the operating circuit, the metal halide (17) is partially or entirely evaporated, and then arc discharge generated between the pair of electrodes 3 causes dissociation between metal atoms and halogen atoms, and thus excitation and emission of the metal atoms occurs. In the vicinity of the tube wall of the arc tube 1, the dissociated metal atoms are recombined with halogen atoms to return to the metal halide. The lamp stays on stably by repeating this cycle phenomenon. In general, although the metal halide has a lower vapor pressure than that of mercury, the metal halide is readily excited and emits. Therefore, in the metal halide lamp, the emission of the added metal tends to be stronger than that of mercury. Therefore, the mercury mainly serves as a buffer gas for determining the voltage of the inside of the arc tube 1. The rare gas in the arc tube 1 serves as a start-up gas.
In these days, environmental issues are regarded as important issues to be tackled, so that a mercury-free metal halide lamp containing no mercury is desired in view of the global environment protection when it is disposed of. With this demand, development of mercury-free metal halide lamps has started, but mercury-free metal halide lamps having excellent characteristics have not been developed yet at present.
We made research to develop mercury-free metal halide lamps having excellent characteristics, and found during their pursuit for the development that the following phenomenon occurs. That is, when mercury-free metal halide lamps are operated horizontally, the arc significantly curves upward, compared with when metal halide lamps including mercury are horizontally operated. As a result, the upper portion of the arc tube is heated more than necessary, so that devitrification of the arc tube occurs.
Then, when a magnetic filed is applied to the mercury-free metal halide lamps to suppress the arc curving, the arc curving can be suppressed. However, the method for applying a magnetic field and the principle of curving suppression are different from those for the lamps containing mercury. Magnetic fields at certain intensities may cause the arc itself to become unstable and the arc to vibrate, although arc curving can be suppressed. This arc vibration is not preferable, because flickering is caused when the lamp is actually used.
In order to suppress arc vibration, we made various examinations and succeeded in suppressing arc curving and arc vibration by setting the parameters of the intensity (B) of a magnetic field applied to the center between the electrodes heads, the distance (d) between the heads of a pair of electrodes, the pressure (P0) inside the arc tube during steady-state operation (or the pressure (P) of an enclosed rare gas at 20xc2x0 C.), the power (w) consumed during steady-state operation, and the steady-state frequency (f) during steady-state operation so as to satisfy a certain relationship. The principle on which arc curving and arc vibration can be suppressed by applying a magnetic field is not clear at present. The relationship to be satisfied by the parameters to suppress arc curving and arc vibration is described in detail in Japanese Patent Application No. 2001-155385 (Applicant; Matsushita Electric Industrial Co., Ltd.), which is incorporated herein by reference.
Thus, a mercury-free metal halide lamp in which devitrification of the arc tube was prevented and flickering was suppressed was successfully realized by the success of suppression of arc curving while arc vibration was suppressed. However, it was observed in the experiments by the inventors of the present invention that in this mercury-free metal halide lamp, although devitrification can be prevented, blackening proceeds in the arc tube. Therefore, even if devitrification of the arc tube can be prevented by the above-described technique, if blackening proceeds, the lamp characteristics are degraded, and the lamp life is reduced. Therefore, practical use of the mercury-free metal halide lamps is still difficult.
Therefore, with the foregoing in mind, it is a main object of the present invention to provide a mercury-free metal halide lamp in which blackening occurring in the arc tube is effectively suppressed from proceeding.
A mercury-free metal halide lamp of the present invention includes an arc tube including a pair of electrodes inside the tube, wherein in the arc tube, a rare gas and a metal halide are contained, and no mercury is contained, and the mercury-free metal halide lamp is horizontally operated such that the pair of electrodes is substantially horizontal. The mercury-free metal halide lamp further includes magnetic field applying means for applying a magnetic field including a component substantially perpendicular to a straight line connecting the heads of the pair of electrodes in a substantially vertical direction, wherein the density of halogen atoms evaporated during steady-state operation with respect to unit inner volume of the arc tube is 20 xcexcmol/cc or more.
It is preferable that the density of the halogen atoms is 40 xcexcmol/cc or more.
It is preferable that the total amount of the metal halide enclosed in the arc tube with respect to unit inner volume of the arc tube is 20 xcexcmol/cc or more.
It is preferable that the total amount of the metal halide enclosed in the arc tube with respect to unit inner volume of the arc tube is 120 xcexcmol/cc or less.
The present invention is provided with magnetic field applying means for applying a magnetic field including a component substantially perpendicular to a straight line connecting the heads of the pair of electrodes in the substantially vertical direction, and the density of the halogen atoms evaporated during steady-state operation with respect to unit inner volume of the arc tube is 20 xcexcmol/cc or more. Therefore, blackening occurring in the arc tube effectively can be suppressed from proceeding. Furthermore, when the total amount of the metal halogen is 120 xcexcmol/cc or less, a lamp in which optical transmission loss by metal halide enclosed in the arc tube is suppressed can be realized.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.