(51) Int.Cl.4

Specification

The invention relates to a high pressure mercury vapor discharge lamp with an envelope, which is made of a material that is stable to high temperature. Said envelope contains two tungsten electrodes and a filling, which comprises in essence mercury, a rare gas and halogen, which is free in the operating state. In operation the wall load of the envelope is higher than 1 W/mm². The envelope, which is stable to high temperature, may be made, for example, of quartz glass or aluminum oxide.

This type of super high pressure mercury vapor discharge lamp, which is known from the DE-AS 14 89 417 has an elongated quartz glass envelope having a volume of 55 mm³. This envelope is filled with rare gas and 6.5 mg of mercury; this is equivalent to a quantity of mercury of 0.12 mg/mm³. The mercury vapor pressure may be about 120 bar. The lamp has a wall load of about 14.5 W/mm³. In order to increase the lifespan, not only the wall of the envelope is cooled, for example, by means of flowing water, but also 5 x 10^-4 to 5 x 10^-2 g. atoms of at least one of the halogens per cubic millimeter are fed into the envelope.

Although such lamps at mercury vapor pressures of more than 100 bar produce a high luminance and a relatively continuous spectrum, they yield essentially a typical mercury spectrum with a low content of red.

The GB PS 11 09 135 discloses a super high pressure mercury vapor discharge lamp comprising a capillary tube, which is made of quartz glass and which is filled with mercury up to a quantity of 0.15 mg per cubic millimeter of volume; this is equivalent to a mercury vapor pressure of about 150 bar. In order to improve the color rendering, this lamp is, moreover, filled with at least one metal iodide. The high electrode load of these lamps results in the evaporation of the tungsten from the electrodes and the deposition of said tungsten on the wall of the envelope. This leads to a blackening of the envelope, as a result of which the latter is heated to an extremely high temperature, which in turn may give rise to an explosion of the envelope, especially at high mercury vapor pressures.

Therefore, the object of the invention is to provide a high pressure mercury vapor discharge lamp, which has not only a high luminance but also a good color rendering and a long lifespan.

This object is achieved in a high pressure mercury vapor discharge lamp of the kind mentioned in the introductory part in that the quantity of mercury is larger than 0.2 mg/mm³ and that the mercury vapor pressure during operation is higher than 200 bar and that at least one of the halogens - chlorine, bromine, or iodine - is present in a quantity between 10^-6 and 10^-4 mmol/mm³.

Up to a mercury vapor pressure of about 150 bar, the light output and the color rendering properties of mercury high pressure lamps are practically constant because in essence a line radiation of the mercury is emitted and an amount of continuous radiation, which originates from the recombination of electrons and mercury atoms. It was a surprise to find that at higher mercury vapor pressures there is a significant increase in the light output and the color rendering index. This increase is due to a drastic increase in the amount of continuous radiation. It is presumed that at high pressures of more than 200 bar, not only a continuous emission from quasi molecular states but also the band emission of real, bonded molecular states also make a significant contribution. At an operating pressure of about 300 bar, the continuum part of the visible radiation lies well above 50%. As a result, the red content of the emitted light spectrum is also increased.

In order to maintain this high mercury vapor pressure, the wall temperature of the envelope has to be high (about 1,000 deg. C). Moreover, the lamp envelope has to withstand this high pressure. Therefore, the envelop has to be kept as small as possible.

The upper limit of the mercury vapor pressure depends upon the strength of the material of the envelope, but in practical cases may be about 400 bar. Preferably the quantity of mercury ranges from 0.2 to 0.35 mg/mm³; and the mercury vapor pressure ranges from 200 to 350 bar.

The very small dimensions of the envelope could lead to an increased blackening of the wall by the tungsten that has evaporated from the electrodes. However, such a blackening of the wall must be absolutely avoided, because otherwise the wall temperature increases during the lifetime due to increased absorption of thermal radiation, which in turn would lead to the destruction of the envelope of the lamp. As a measure to avoid such a blackening of the wall by the tungsten transport, the high pressure mercury vapor discharge lamp of the invention contains a small quantity of at least one of the halogens - chlorine, bromine or iodine. These halogens bring about a tungsten transport cycle, by means of which the tungsten, which is deposited on the envelope of the lamp, is transported back to the electrodes.

It is efficacious for the high pressure discharge lamp of the invention to use as the halogen bromine, which is introduced into the lamp in the form of CH₂Br₂ at a filling pressure of about 0.1 mbar.

The mercury vapor discharge lamp, according to the invention, does not contain a metal halide, because such a high metal halide concentration would be required for a substantial increase of the continuum part of the radiation that a very rapid corrosion of the electrodes would occur as a consequence of the high tungsten transport rates. Therefore, heavily loaded metal halide lamps, as described, for example, in the GB‑PS 11 09 135, typically reach only lifespans of a few hundred hours, whereas in the lamps, according to the invention, lifespans of more than 5,000 hours could be reached with a substantially constant light output (Dh < 2%) and substantially constant color rendering properties (Dx, Dy < 0.005 during 5,000 hours). In this case h stands for the efficiency; and x and y stand for the color coordinates.

The lamps, according to the invention, have a color temperature of more than 8,000 K. The drawback with this high color temperature and the color rendering, which is worse as compared to metal halogenide lamps, can be improved in a discharge lamp according to the invention in that the lamp is surrounded by a filter to block blue radiation content.

In this context it should be pointed out that it is known from the GB‑PS 15 39 429 to reduce the blue content of the radiation in high pressure mercury vapor discharge lamps with the addition of halide by the use of a filter and, thus, to achieve an improvement in the color of the emitted radiation. In mercury vapor discharge lamps at a mercury vapor pressure of up to about 150 bar, such a filter would be practically ineffective, because the emitted light exhibits almost no red content. However, the spectrum of the inventive lamp contains such a high content of the continuous red radiation that with the use of a filter for the blue radiation content, with a loss of light of only 15%, it is possible to achieve an emission of white light having a color temperature of 5,500 K and a color rendering index of 70.

At this point a few embodiments of the invention are described with reference to the drawings. In the drawing:

Figure 1 depicts a high pressure mercury vapor discharge lamp having an elliptical lamp envelope.

Figure 2 depicts a high pressure mercury vapor discharge lamp having a cylindrical lamp envelope that is surrounded by an outer envelope, which is coated with a filter.

Figure 3 depicts the emitted light spectrum of a high pressure mercury vapor discharge lamp at a mercury vapor pressure of more than 200 bar; and

Figure 4 depicts the transmission spectrum of a filter that is used in the lamp, shown in Figure 2.

The high pressure mercury vapor discharge lamp 1, depicted in Figure 1, has an elliptical lamp envelope 2 that is made of quartz glass. The envelope ends are adjoined by cylindrical quartz parts 3 and 4, into which molybdenum foils 5 and 6 are molten in a vacuum tight manner. The inner ends of the molybdenum foils 5 and 6 are connected to the pole leads 7 and 8, which carry the electrodes 9 and 10 that are made of tungsten.

The outer ends of the molybdenum foils 5 and 6 are adjoined by current supply wires 11 and 12 that are made of molybdenum and that run to the exterior.

The high pressure mercury vapor discharge lamp 13, depicted in Figure 2, is constructed in a manner similar to the lamp, depicted in Figure 1. However, the envelope 14 of the lamp exhibits a cylindrical shape. The lamp 13 is surrounded by an outer envelope 15, which is made of quartz glass and which is coated on the inner side with an interference filter 16. This filter 16 serves to reduce the blue radiation that is emitted by the lamp 13.

Below are the data of a few practical embodiments:

Lamp 1

Elliptical lamp envelope, according to Figure 1, having a wall thickness of 1.8 mm. The inside dimensions and operating data are:

length 7 mm

diameter 2.5 mm

envelope volume 23 mm³

electrode gap 1.2 mm

filling:

mercury 5 mg Hg (0.218 mg/mm³)

halogen 5 x 10^-6 mmol/mm³ CH₂Br₂

operating pressure approximately 170 bar

power 50 W

operating voltage 70 V

light output 56 lm/W

wall load 1.30 W/mm²

Lamp 2

Elliptical lamp envelope, according to Figure 1, having a wall thickness of 1.7 mm. The inside dimensions and operating data are:

length 5 mm

diameter 2.5 mm

envelope volume 16.5 mm³

electrode gap 1.0 mm

filling:

mercury 4 mg Hg (0.243 mg/mm³)

halogen 5 x 10^-6 mmol/mm³ CH₂Br₂

operating pressure approximately 220 bar

power 40 W

operating voltage 80 V

light output 56 lm/W

wall load 1.30 W/mm²

Lamp 3

Cylindrical lamp envelope, according to Figure 2, having a wall thickness of 1.3 mm without an outer envelop. The inside dimensions and operating data are:

length 4 mm

diameter 1.5 mm

envelope volume 7 mm³

electrode gap 1.0 mm

filling:

mercury 2.5 mg Hg (0.357 mg/mm³)

halogen 5 x 10^-6 mmol/mm³ CH₂Br₂

operating pressure 300 bar

power 30 W

operating voltage 92 V

light output 60 lm/W

wall load 1.36 W/mm²

The described lamps have a color temperature of more than 8,000 K; however, the color rendering is significantly improved in comparison with lamps having a low operating pressure. For example, the color rendering index R_a is for the three lamps, just described, 51.5, 55.2 and 61.6, whereas with a lamp, corresponding to example 1, at an operating pressure of 100 bar only a color rendering index of 32.7 was attained.

In Figure 3, the light spectrum that is emitted by a lamp, according to Example 2, is plotted as the intensity I over the wavelength l. It shows that the continuum portion of the visible radiation is about 50%.

In the lamp, according to Figure 2, the interference filter 16 consists, for example, of an alternating sequence of layers of chemically modified titanium dioxide and amorphous silicon dioxide. Such interference filters are the subject mater of the German patent application P 37 24 216.4. In a practical embodiment, the filter that is used had a degree of transmission Tr, as shown in Figure 4, as a function of the wavelength l. The results are reflected in the following lighting technical data:

Without a filter:

color temperature: 8,580 K

color rendering index: 55.2

light output: 56 lm/W

With a filter:

color temperature: 5,500 K

color rendering index: 69.7

light output: 48 lm/W

This shows that the interference filter causes not only a significant reduction in the color temperature, but also at the same time a significant improvement in the color rendering index.

Although in comparison with comparable, heavily loaded metal halide lamps, the inventive lamps exhibit a somewhat lower light output and, moreover, during operation without a filter also exhibit poorer color rendering properties, the inventive lamps exhibit an extremely high constancy of the lighting technical data, a light output that does not change during the service life, and a very long life. While lifespans of a few hundred hours are attained with heavily loaded metal halide lamps, the inventive lamps do not exhibit any changes even after a service life of more than 5,000 hours.

Patent Claims

1. High pressure mercury vapor discharge lamp with an envelope, which is made of a material that is stable to high temperature and which contains two tungsten electrodes and a filling, which comprises in essence mercury, rare gas, and halogen, which is free in the operating state, and the load on the envelope wall during operation is higher than 1 W/mm², characterized in that the amount of mercury exceeds 0.2 mg/mm³ and the mercury vapor pressure exceeds 200 bar and that at least one of the halogens - chlorine, bromine, or iodine - is present in an amount ranging from 10^-6 to 10^-4 mmol/mm³.

2. Discharge lamp, as claimed in claim 1, characterized in that the quantity of mercury ranges from 0.2 to 0.35 mg/mm³; and the mercury vapor pressure during operation ranges from 200 to 350 bar.

3. Discharge lamp, as claimed in any one of the preceding claims, characterized in that the lamp is surrounded by a filter for the blue radiation content.