Electric lamp and discharge devices – With gas or vapor – With particular gas or vapor
Patent
1997-03-17
1998-08-25
Patel, Nimeshkumar
Electric lamp and discharge devices
With gas or vapor
With particular gas or vapor
313638, 313639, H01J 6112
Patent
active
057986126
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a metal-halide discharge lamp for photo-optical purposes, and more particularly to a lamp of the type described in U.S. Pat. No. 5,689,154, Frey et al., to which Published International Application WO 95/05674 corresponds, and U.S. Pat. No. 5,691,601, Barthelmes et al., to which Published International Application WO 95/20822 corresponds, both assigned to the assignee of the present application.
BACKGROUND
Metal-halide discharge lamps of the type described in the above publications are installed predominantly in optical reflectors or other optical-projection systems. They are used for instance in projection or optical fiber waveguide technology, and among other purposes for overhead, slide, motion-picture and video projection, as well as endoscopy and boroscopy. Accordingly, very short arcs (of a few millimeters) and maximum luminance values (on average, a few tens of kcd/cm.sup.2) at color temperatures of more than 5000 K and with good to very good color reproduction (Ra>85) are required. Typical wattages are in the range of between 35 W and 600 W.
The referenced publications disclose on such lamp, with a fill that besides mercury and an inert gas also contains halogen compounds of the elements aluminum and indium. The lamps require a high starting voltages (typically about 12 kV), which is a disadvantage.
THE INVENTION
The object of the invention is to overcome the aforementioned disadvantage of high starting voltage and to create a metal-halide discharge lamp that has a color temperature of more than 5000 K--with very good color reproduction--and a relatively low starting voltage, and that accomplishes all this with the fewest possible fill components.
The discharge vessel of the metal-halide discharge lamp of the invention includes--besides the metals aluminum (Al) and indium (In)--only the element gallium (Ga) in addition, as a further metal for forming metal halides. The fill quantity of the element Ga per cm.sup.3 of the vessel volume is in the range between 0.02 mg and 1 mg, and in particular is in the range between 0.03 mg and 0.2 mg. In preliminary tests, it has unexpectedly been demonstrated that by the addition of Ga only, the starting voltage of the cold lamp drops from the typical value of 12 kV to below 8 kV. The the fill also contains the usual following further components: at least one inert gas, such as argon (Ar) or xenon (Xe) as a starting gas, with a typical fill pressure in the range between about 10 kPa and 40 kPa;
mercury, to adjust the desired arc voltage, which is typically in the range between 15 mg and 30 mg for arc voltages between 60 V and 90 V; and one or more halogens, preferably iodine (I) and/or bromine (Br), for forming metal halides.
Without intending to be limited to any particular theoretical explanation, it is currently thought that there are two primary reasons for the behavior observed. First, Ga with the halogen or halogens of the fill, and particularly with iodine (I), forms compounds with a lesser electron affinity than is the case with Al and In. Second, less formation of metal halide condensate and mercury condensate on the electrodes was observed. In previous fill systems for short-arc lamps--unlike long-arc lamps--condensate formation on the electrodes is thought to be primarily responsible for elevated starting voltages. Aside from the improved starting performance in both the cold and the hot lamp, an improved reproducibility of the arc onset can be observed at the electrode tips. Possibly, the more than 10 times higher vapor pressure of the GaI, as compared with InI, also contributes to the faster development of the arc. The starting performance can essentially be varied by means of a suitable stoichiometry of the fill components Al, In and Ga.
In East German Patent 254 270, a short-arc lamp is disclosed whose complex fill is composed substantially of the elements mercury (Hg), zinc (Zn), indium, sodium (Na), lithium (Li) and halogens. This patent does mention that In can be fully or partly substitu
REFERENCES:
patent: 3772557 (1973-11-01), Yoshida et al.
patent: 5220237 (1993-06-01), Maseki et al.
patent: 5689154 (1997-11-01), Barthelmes et al.
patent: 5691601 (1997-11-01), Frey et al.
Database WPI, Section Ch., Week 7716, Derwent Publications Ltd., London,GB; Class L03, AN 77-28139Y of JP,A,52 031 583 (Tokyo Shibaura Elec Ltd), Mar. 10, 1977.
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