Electric lamp and discharge devices: systems – Special application – Vehicle
Reexamination Certificate
2001-08-24
2003-12-30
Clinger, James (Department: 2821)
Electric lamp and discharge devices: systems
Special application
Vehicle
C313S641000
Reexamination Certificate
active
06670765
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a high-intensity discharge lamp, also known as a metal halide lamp, for use in a vehicle headlamp, fog lamp etc. and other illumination devices. The invention more particularly relates to a mercury-free high intensity discharge lamp with high lumen output efficiency in the visible light wavelength, appropriate color rendering property and excellent discharge stability, enabling practical dimming of a headlight incorporating the mercury-free high intensity discharge lamp.
2. Description of the Related Art
In a conventional high-intensity discharge lamp such as a metal halide lamp, mercury has been used not only as a light emitting material, but also as a buffer gas in order to promote vaporization of other light emitting materials by increasing the temperature of a light emitting tube (arc tube) and to adjust lamp voltage of the light emitting tube. The lamp voltage can be understood as a voltage of the light emitting tube during steady lighting of the high intensity discharge lamp comprising the light emitting tube. Steady lighting is a state of lighting after a start-up or initial lighting period has finished. However, mercury is a toxic substance which has the potential to cause damage to the environment. Therefore, development of a light emitting tube which does not contain mercury is a long-felt need for manufacturers of high-intensity discharge lamps.
In another conventional metal halide lamp, a light emitting tube which comprises no mercury (referred hereinafter as “a mercury-free light emitting tube”) can be made by sealing a starter gas such as xenon (Xe) gas in the light emitting tube. The amount of sealed Xe gas corresponds to a few atmospheres or more at room temperature. Room temperature means substantially a normal, comfortable temperature. Thus, metal halides on a wall of a discharge chamber of the light emitting tube are vaporized by heat transmission from a xenon arc that has a high temperature and extends towards the wall of the chamber.
In the conventional mercury-free light emitting tube, major light emitting materials are metal halides which have similar thermodynamic properties to mercury. However, the conventional mercury-free metal halide lamp has different light emitting characteristics from the conventional mercury metal halide lamp. For example, in the conventional mercury metal halide lamp, if a dimming function is operated by decreasing input electric power to the metal halide lamp, the color of light emitted from the light emitting tube greatly changes because intensity of light emitted from mercury (having relatively high vapor pressure) is maintained while emission of light from other metals (metal halides) greatly decreases. On the other hand, in the conventional mercury-free metal halide lamp, if input electric power to the metal halide lamp is decreased, the color of light emitted from the light emitting tube changes in a smaller range, because light emission from each metal decreases keeping substantially the same ratio to all metals in the discharge chamber, and light emitted from each metal collectively constitutes the light emitted from the light emitting tube. However, the conventional mercury-free metal halide lamps have problems, some of which are described later in detail with reference to Japanese Patent Publications.
In yet another conventional light emitting tube capable of instant lighting, a starter gas including Xe gas is sealed in the discharge chamber in an amount of more than a few atmospheres at room temperature. A few times the rated current is supplied in an initial lighting period just after start-up of the light emitting tube. When the light emitting tube is started up from room temperature (referred hereinafter as “cold start”), electrodes disposed in the light emitting tube are heated to temporarily reach a high temperature, which expedites deterioration of the electrodes. Further, in a light emitting tube made of silica glass, electrodes which are made of tungsten are embedded in sealed portions of the light emitting tube located adjacent the discharge chamber. In this structure, mercury and metal halides creep and stay in a gap between the electrodes and the sealed portion when the light emitting tube was cooled by turning off the light emitting tube. Such mercury and metal halides located in this gap are instantly vaporized by a steep temperature rise on cold start of the light emitting tube, which may destroy the sealed portions of the light emitting tube where the electrodes are embedded. The lifetime of this kind of light emitting tube is substantially determined by the number of times cold starts that occur rather than the lighting hours. In cases where the metal halide lamp is used in devices which are frequently and repeatedly turned on and off, the lifetime of the light emitting tube can be greatly improved if the turn-off mechanism includes a dimming mechanism, i.e., number of times turnoff is decreased by replacing it with a certain dimming operations.
Japanese Patent Publication No. 6-84496 discloses a mercury-free high pressure metal halide discharge lamp capable of dimming. According to an embodiment of the patent publication, the high pressure metal halide discharge lamp comprises NaI 20 mg, ScI
3
4 mg, and Xe gas which is sealed into a discharge chamber in an amount of approximately 8 atmospheres at room temperature. Rated electric power of the high pressure metal halide discharge lamp is 150W. If the rated electric power is decreased to 75W, the light color of the lamp is maintained, and a certain level of dimming without accompanying strangeness to a viewer is achieved. Further, the lamp voltage of approximately 90V is achieved by setting the multiplication factor of Xe gas pressure (atm.) and distance between the electrodes (mm) to be greater than or equal to 40.
According to results of the inventors trial and experiments, combination of NaI and ScI
3
provides relatively good color rendering property and color reproducibility, i.e., color maintenance property before and after dimming, and high lumen output efficiency. However, the color of light obtained by the combination is rather greenish, and not pure white. According to testing and experiments, the light obtained did not fall within the scope of tolerance for white automobile light in the chromaticity diagram. Accordingly, usage of the high pressure metal halide lamp as a light source for illumination devices is limited depending on the required color rendering property for the illumination devices.
Lamp voltage is determined by the sum of voltage drop caused by electrodes and impedance produced by, for example, the electron scattering effect by metal atoms and produced by attachment of free halogens and electrons. Mercury greatly commits itself to areas of voltage because it has especially large collision cross section with an electron. According to the embodiment of the patent publication, no mercury is contained in the chamber of the light emitting tube. However, the light emitting tube achieved the same voltage as that of mercury-containing light emitting tubes. It is understood that vapor pressure of the metal halide was increased by operating the light emitting tube at a very high temperature. Since vapor pressure of metal halides is very high, it causes devitrification of the wall of the chamber and deterioration of electrodes due to reaction of the silica glass light emitting tube and the metal halides.
Japanese Patent Publication No. 11-238488 discloses a substantially mercury-free metal halide discharge lamp that includes a first halide with at least one metal selected from the group consisting of sodium, scandium, and a rare earth metal capable of predetermined light emission. The substantially mercury-free metal halide includes a second halide having relatively high vapor pressure and tendency of declination to emit visible light. The second halide includes at least one metal selected from the group consisting of aluminum (Al), iron (Fe), cadmiu
Clinger James
Morgan & Lewis & Bockius, LLP
Stanley Electric Co. Ltd.
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