GB2135505A

GB2135505A - Low-pressure mercury vapour discharge lamp

Info

Publication number: GB2135505A
Application number: GB08400914A
Authority: GB
Inventors: Jan Willem Frederik Dorleijn; Ridder Adriaan Jan De; Theodorus Franciscus Lamboo
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1983-01-13
Filing date: 1984-01-13
Publication date: 1984-08-30
Also published as: GB2135505B; GB8400914D0; NL8303798A; SE8400097D0; FR2539555A1; SE8400097L; FR2539555B1; DE3400385A1

Abstract

A low-pressure mercury vapour lamp is provided with a luminescent layer 4 which emits long-wave ultraviolet radiation and a reflecting layer 5 comprising aluminium oxide preferably of size 0.1 to 20 mu m, on part of the inside of the vessel. The luminescent layer 4 comprises a material activated by bivalent europium and having a fundamental strontium tetraborate lattice and is preferably defined by the formula Sr1-x-pMxEapB4O7. Where M=Ba, Ca or Mg 0</=x</=0.25 and 0.03</=p</=0.20 e.g Sr0.97E0.03B4O7. The layer 4 may extend round all or part of the inside of the vessel 1. A further layer of luminescent material e.g. Y2O3:Eu may be used. The output of the lamp is extended by the combination of luminescent and reflecting layers. <IMAGE>

Description

SPECIFICATION Low-pressure mercury vapour discharge lamp The invention relates to a low-pressure mercury vapour discharge lamp which is capable of emitting long-wave ultraviolet radiation.

United States Patent Specification 3,987,331 describes a low-pressure mercury vapour discharge lamp comprising a tubular discharge vessel which is sealed in a vacuum-tight manner and on the inner wall of which a luminescent layer is disposed which emits long-wave ultraviolet radiation, a reflecting layer being present between this layer and the inner wall, which reflecting layer extends along the length of the discharge vessel round part of the circumference.

These lamps, which emit long-wave ultraviolet radiation, preferably in the range of from 320 to 400 nm (UV-A radiation), are mostly used for photochemical or photobiological purposes, for example in sun beds and solaria for direct pigmentation of the skin (tanning without formation of erythema), or in the phototherapy of skin diseases.

The lamp described in the aforementioned Patent Specification comprises a discharge vessel in which a luminescent layer is disposed which comprises a luminescent material, for example, barium mesosilicate activated by lead, emitting UV-A radiation. Further, a reflecting layer is present which preferably extends round more than 1 800 of the circumference and consists of zirconium dioxide. The advantages of the use of zirconium dioxide as compared with a number of other known reflecting materials, which are mostly used in conjunction with luminescent materials emitting visible light, are, according to the said Patent Specification, the favourable reflecting properties, especially for the said longwave ultraviolet radiation.

However, experiments have shown that the output of lamps emitting UV-A radiation in general and of the aforementioned lamps provided with a reflecting layer in particular strongly decreases already after a comparatively small number of operating hours. This is disadvantageous because the lamps have to be frequently replaced, but it may also be disadvantageous that the radiation dose per unit time during the operating time of the lamp decreases rapidly.

The invention has for its object to provide a low-pressure mercury vapour discharge lamp for emitting long-wave ultraviolet radiation, in which the aforementioned disadvantages are mitigated.

The invention provides a low-pressure mercury vapour discharge lamp comprising a tubular discharge vessel which is sealed in a vacuumtight manner, wherein a reflecting layer comprising aluminium oxide is disposed on the inside wall of the discharge vessel and extends along the length of the discharge vessel and round part of the circumference of the discharge vessel, and wherein a luminescent layer which emits long-wave ultraviolet radiation and comprises a luminescent material activated by bivalent europium and having a fundamental lattice of or derived from the strontium tetraborate lattice covers the reflecting layer.

Surprisingly, it has been found that the radiation output of the lamp according to the invention was remarkably high and constant during a long operating time with the aforementioned combination of a luminescent layer comprising strontium tetraborate and a reflecting layer comprising aluminium oxide.

Therefore, the use of these lamps is particularly advantageous because they need be replaced much less frequently than the known lamps, as a result of which a considerable saving in cost can be attained, and because during operation of the lamps a more constant value of the radiation dose per unit time is obtained.

In order to obtain the aforementioned advantages, a reflecting layer of aluminium oxide has to be used in combination with a luminescent material on the basis of strontium tetraborate activated by bivalent europium. The said luminescent material may have a fundamental lattice according to the formula SrB4Q, in which a part of the Sr, for example, of 0.3 to 20 mol.%, is replaced by bivalent europium. This strontium tetraborate is known, for example, from German Offenlegungsschrift 2,652,480. It is also possible to make modifications on the basis of the said tetraborate. For example, a part of the strontium (up to 25 mol.%) may be replaced by barium, magnesium and/or calcium (see the aforementioned Offenlegungsschrift and the Japanese published Patent Application 74/67892).Further, a fluorine-containing strontium tetraborate as known from USP 3,431,215 may be used. The said strontium tetraborates emit in a narrow band with a maximum at approximately 370 nm and a halfvalue width of approximately 20 nm.

United States Patent Specification 3,225,241 discloses lamps having a reflecting layer which may consist of suitable materials, such as titanium dioxide or aluminium oxide. However, these layers are only used in combination with a luminescent layer emitting visible light. This specification also discloses that aluminium oxide reflects resonance radiation of mercury having a wavelength of 254 nm. However, nothing is stated about problems of decrease of the output of long-wave UV radiation in the range in the vicinity of 370 nm.

Preferably the luminescent layer comprises a luminescent material which has a composition defined by the formula Sr1####M#Eux B407, wherein M presents at least one of the metals Ba, Ca and Mg, 0 < #x < #0.25, and 0.003#p < #0.20. These iast-mentioned tetraborates yield the highest radiation intensities and can be obtained more readily than the fluorine-containing tetraborates.

In a particular embodiment of a lamp according to the invention, the luminescent layer comprises a further luminescent material (for example up to 10% by weight of the total quantity of luminescent material) which emits light in the wavelength range of from 580 to 650 nm. Such a material is preferably yttrium oxide activated by trivalent europium. The lamp then has a warmer colour.

Very favourable results were obtained with a lamp according to the invention in which the size of the major part of the particles in the reflecting layer of aluminium oxide was between 0.1 and 20 Mm.

The reflecting layer may extend, for example, round from 1 80 to 2400 of the circumference of the inside wall of the discharge vessel.

An embodiment of the invention will now be described with reference to the drawing, in which: Figure 1 is a diagrammatic longitudinal section of a low-pressure mercury vapour discharge lamp according to the invention, and Figure 2 is a cross-section of the discharge vessel of the lamp shown in Figure 1.

The lamp shown in Figure 1 comprises a tubular discharge vessel 1 which is sealed in a vacuum-tight manner and in which two electrodes 2, 3 are arranged, between which a discharge is maintained during operation of the lamp. On the inner wall of the discharge vessel 1 is disposed a luminescent layer 4 which emits long-wave ultraviolet radiation (UV-A) radiation.

The layer comprises a luminescent material on the basis of strontium tetraborate activated by bivalent europium. Between part of the luminescent layer and the glass wall of the discharge vessel is disposed a reflecting layer 5 (see Fig. 2) which extends along the length of the discharge vessel and round approximately 2000 of the circumference. This reflecting layer comprises aluminium oxide.

In a practical embodiment, the tubular discharge vessel of the aforementioned lamp has a length of approximately 1.80 m and an outer diameter of approximately 38 mm. The power consumption was about 100 W. The luminescent layer 4 comprises strontium tetraborate according to the formula SrO 97EuO o3B407 This material emits in a narrow band at 370 nm and at a halfvalue width of 20 nm.

The layer of Al203 (up to approximately 5 mg/cm2) extends over approximately 2000 of the circumference. The average particle size of the Al2O3 was approximately 5 ym. The radiation output of this lamp was 23 UV-A Watt after 20 operating hours. After 2000 operating hours, the radiation output was 20 UV-A Watt.

In the lamp described above, the luminescent layer extends round the whole inside circumference of the discharge vessel. In an alternative embodiment, the luminescent layer is present only on the reflecting layer so that a window is formed which is free from both luminescent material and reflecting material.

Claims

1. A low-pressure mercury vapour discharge lamp comprising a tubular discharge vessel which is sealed in a vacuum-tight manner, wherein a reflecting layer comprising aluminium oxide is disposed on the inside wall of the discharge vessel and extends along the length of the discharge vessel and round part of the circumference of the discharge vessel, and wherein a luminescent layer which emits longwave ultraviolet radiation and comprises a luminescent material activated by bivalent europium and having a fundamental lattice of or derived from the strontium tetraborate lattice covers the reflecting layer.

2. A low-pressure mercury vapour discharge lamp as claimed in Claim 1, wherein the luminescent material has a composition defined by the formula Srt x pMxEupB407, wherein M represents at least one of the elements Ba, Ca and Mg, 0axe0.25 and 0.003#p#0.20.

3. A low-pressure mercury vapour discharge lamp as claimed in Claim 2, wherein the particle size of the aluminium oxide is between 0.1 and 20yam.

4. A low-pressure mercury vapour lamp as claimed in any preceding Claim, wherein the luminescent layer also covers all or part of the inside surface of the discharge vessel which is not covered by the reflecting layer.

5. A low-pressure mercury vapour lamp, substantially as herein described with reference to Figures 1 and 2 of the drawing.