High Pressure Mercury Vapor Discharge Lamp With Metal Halide Additive

Abstract

High pressure mercury vapor discharge lamp without outer jacket and with a all loading of between 10 and 100 W/cm.sup.2 preferably 20-60 W/cm.sup.2 and a medium arc length corresponding to a load per cm of arc length of between 100 and 1000 W/cm, preferably 150-850 W/cm. Additives of thallium halide and halides of rare-earth metals, preferably of thulium and holmium, in a ratio of weight of halogen to metal between 2 and 10. Color temperature is 6,000.degree. K, index of color rendition 92.

Description

The invention relates to a high pressure mercury vapor discharge lamp with metal halide additive, the discharge vessel being preferably of ellipsoidal shape, and made of heat-resisting, light-transmissive material.

Two different types of high pressure discharge lamps with metal halide filling are generally known:

1. The type of lamp which is chiefly used for general lighting purposes such as, for instance, street lighting (see for example, German Patent specification No. 1,184,008) is a development of the high pressure mercury vapor discharge lamps. It comprises a tubular discharge vessel of quartz glass with one lead-in wire each sealed or pinch-sealed into each end. The spacing of the electrodes is approximately 50 mm. The wall loading of such lamps is about 10 W/cm.sup.2. The vessel is filled, besides mercury and a starting gas, mostly with the halides, preferably the iodides, of sodium and thallium; rare-earth metals like dysprosium, holmium, erbium or thulium (see U.S. Pat. No. 3,536,947; U.S. Pat. No. 3,452,238) are used. The discharge vessel, the so-called arc tube, is enclosed in a single-ended outer envelope, which is exhausted or filled with gas in order to reduce heat losses. The lamps have a luminous efficiency of over 50 lm/W, and mostly below 80 lm/W; and a life of several 1,000 hours.

2. The lamps of the other type are used for projection purposes and in signaling and they are, with regard to their field of application, a further development of the high pressure xenon discharge lamps used for these purposes. They provide high luminance and have, therefore, short arcs. The lamps are operated with extremely high wall loading of far more than 100 W/cm.sup.2, for instance with 400 to 1,000 W/cm.sup.2. The wall thickness of the discharge vessel is from 3 to 5 mm; the spacing of the electrodes is in most cases less than 10 mm. The lamps are so-called one-component lamps and contain, besides a basic gas as the filling excitable to luminance, the halides, for instance, of indium or gallium or aluminum; in addition, mercury is often used as a buffer gas but not as an element to be excited. An outer envelope is not necessary with these lamps (see U.S. Pat. No. 3,259,777, Dutch published Patent application 67 10 944). A projection lamp which contains scandium halide as the excitable element (see German Patent 1,177,248) is also known. The luminous efficiency of these one-component lamps is more than 90 lm/W, the useful life is several 100 hours.

It is an object of the invention to provide a lamp which represents a further development of lamps of the first-mentioned type, and which at the same time, shows characteristics of the second type, for instance, with regard to luminous efficiency. Another object is the optimum dimensioning of such a lamp.

SUBJECT MATTER OF THE INVENTION:

The lamp is a high pressure mercury vapor discharge lamp with metal halide additive; it has a single wall, preferably ellipsoidal discharge vessel of heat-resisting, light-transmissive material. According to the invention, the lamp has a wall loading of between 10 and 100 W/cm.sup.2, preferably 20-60 W/cm.sup.2 with a load per cm of arc length between 100 and 1,000 W/cm, preferably from 150 to 850 W/cm; and the additive is composed of halides of rare-earth metals, preferably of thulium and/or holmium, and/or halide of thallium, and wherein the ratio of weight of halogen to metal is between 2 and 10. No outer envelope is used.

The invention will be described by way of example with reference to the accompanying drawing wherein the single figure is a schematic longitudinal view of the lamp.

The luminous efficiency of lamps increases with the specific loading in Watt per centimeter of arc length. With the same power input and a reduction of the electrode spacing an increase in luminous efficiency is then to be expected. Surprisingly, however, the luminous efficiency obtainable with lamps of medium electrode spacing, according to the invention, is much greater than could be expected from theoretical considerations and amounts to 90 to 130 lm/W depending on the respective power input of the lamp, without, however, approaching the wall loading of the second type of lamp. A greater power input with lamp dimensions maintained constant implies, however, that the wall loading of the lamp rises and hence lamp life decreases. Unexpectedly, however, it has been found that --compared with the above described short-arc lamps-- a halogen cycle starts already with low wall loadings of the lamps according to the invention. This is in contrast to the known high pressure mercury vapor discharge lamps with metal halide additives in which the halogen cycle is hardly still effective. The invention shows that by choosing the specific load per cm of arc length and wall loading, it is possible to obtain a maximum of luminous efficiency far beyond that which would be expected from a mere change of dimensions. In order to obtain the halogen cycle an excess of halogen must be present in the lamp; yet, this excess must be chosen so low that the luminous efficiency is not impaired. From the high wall loading, it is obvious that the heat-accumulating outer envelope has to be omitted. This facilitates at the same time lamp manufacture and renders it less expensive. Besides, providing the lamp with bases at both ends allows the application of higher starting voltage and enables restarting of the lamp when still hot.

As an addition to mercury, halogen compounds, preferably the iodides of thulium or holmium in a quantity of from 0.25 to 11 mg/cm.sup.3, preferably from 0.5 to 4 mg/cm.sup.3 when filled into the discharge vessel are advantageous. An excess of iodine of up to 50 percent of the quantity of iodine equivalent to the rare-earth metals is necessary to ensure the halogen-cycle process and thus long lamp life. When using a thulium iodide additive, a high color rendition index of about 92 is obtained with a color temperature of 6,000.degree. K which renders the lamp particularly suitable for color television and color photography.

The discharge tube 1 is of quartz glass which contains only little hydrogen and can be coated inside with a protective layer 2, preferably of boron oxide. The maximum inner diameter of the discharge vessel is 15 mm, the volume 2.3 cm.sup.3. Each end of the discharge vessel is provided with a rod-shaped non-activated tungsten electrode 3, 4, having a diameter of 0.9 mm. Electrode spacing is 12 mm. The ends of the discharge vessel are provided with a reflecting coating 5. The bases 6 and 7 are connected to a voltage source; operating voltage is approximately 80 V, current approximately 7.5 amps.

The filling of the discharge vessel 1 consists of a basic gas as starting aid, for instance, argon at a pressure of 50 torr; and 1.5 mg of thulium, 4 mg of mercury and 7 mg of HgJ.sub.2, the ratio of weight of iodine to thulium being 2.3. The mercury iodide reacts in the discharge vessel with thulium according to the formula

2 Tm+3 HgJ.sub.2 .uparw. 2 TmJ.sub.3 +3 Hg

The structural datas and the fill quantities are applicable to a lamp of 500 W power input. The luminous flux is 45 klm, the luminous efficiency 90 lm/W, the color temperature 6,000.degree. K, the chromaticity coordinates x/y = 0.32/0.32, the color rendition index 92. Lamps having a power input of 600 W, had a luminous efficiency of 110 lm/W. Lamps having a power input of 5 kW had a luminous efficiency of 130 lm/W.

Claims

We claim:

1. High pressure mercury vapor discharge lamp comprising

a single, unenclosed envelope of heat-resisting light-transmissive material forming a discharge vessel;

a fill of rare gas forming a starting gas in said vessel;

said vessel, when the lamp is operating, having a wall loading of between 10 and 100 W/cm.sup.2, and a loading per cm of arc length of between 100 and 1,000 W/cm;

the discharge vessel further containing

from 0.25 to 11 mg/cm.sup.3 of a halide of the rare-earth metals,

from 0 to 0.2 mg/cm.sup.3 of thallium halide,

from 2 to 10 mg/cm.sup.3 of mercury,

and an excess of halogen of up to 50 percent of the quantity of halogen equivalent to the rare-earth metal.

2. Lamp according to claim 1 wherein the wall loading is from 20 to 60 W/cm.sup.2.

3. Lamp according to claim 1 wherein the load per cm of arc length is between 150 to 850 W/cm.

4. Lamp according to claim 1 wherein the additive of rare-earth metal comprises a halide of thulium or holmium.

5. Lamp according to claim 1 wherein the additive is thallium iodide.

6. Lamp according to claim 1, wherein the lamp envelope is ellipsoidal in shape.