Low-pressure gas discharge lamp having an improved efficiency

FIELD OF THE INVENTION

The invention relates to a low-pressure gas discharge lamp comprising a light- transmitting discharge vessel enclosing, in a gastight manner, a discharge space comprising a gas filling, the gas filling comprising a metal compound, the low-pressure gas discharge lamp further comprising discharge means for maintaining a discharge in the discharge space.

BACKGROUND OF THE INVENTION

Light generation in low-pressure gas discharge lamps is based on the principle that charge carriers, particularly electrons but also ions, are accelerated so strongly by an electric field applied to the discharge lamp that collisions with the gas atoms or molecules in the gas filling of the discharge lamp cause these gas atoms or molecules to be excited or ionized. When the atoms or molecules of the gas filling return to the ground state, a more or less substantial part of the excitation energy is converted into radiation.

Conventional low-pressure gas discharge lamps comprise mercury as a primary component for generating ultraviolet light (further also referred to as UV light). A luminescent layer comprising a luminescent material may be present on an inner wall of a discharge vessel so as to convert UV light into light having an increased wavelength, for example, UV-C for medical purposes, UV-B and UV-A for tanning purposes (sun-tanning lamps), or visible radiation for general illumination purposes. Such discharge lamps are therefore also referred to as fluorescent lamps. Fluorescent lamps for general illumination purposes usually comprise a mixture of luminescent materials, in which a combination of the luminescent materials determines a color of the light emitted by the fluorescent lamp. Examples of commonly used luminescent materials are a blue-luminescing europium- activated barium magnesium aluminate, BaMgAlioOi7:Eu ²⁺ (also referred to as BAM), a green- luminescing cerium-terbium co-activated lanthanum phosphate, LaPO ₄ ICe ₅ Tb (also referred to as LAP) and a red- luminescing europium-activated yttrium oxide, Y ₂ OsIEu (also referred to as YOX).

The discharge vessel of a low-pressure gas discharge lamp is usually constituted by a light-transmitting envelope enclosing a discharge space in a gastight manner.

The discharge vessel is generally circular and comprises both elongate and compact embodiments. Normally, the means for generating and maintaining a discharge in the discharge space are electrodes arranged near the discharge space. Alternatively, the low- pressure mercury vapor discharge lamp is an electrodeless low-pressure mercury vapor discharge lamp, for example, an induction lamp in which energy required to generate and/or maintain the discharge is transferred through the discharge vessel by means of an induced alternating electromagnetic field.

The known low-pressure mercury vapor discharge lamps have the drawback that they do not have an optimal luminescent conversion efficiency.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the invention to provide a low-pressure gas discharge lamp having an improved efficiency.

According to the invention, the object is achieved with a low-pressure gas discharge lamp comprising: a light-transmitting discharge vessel enclosing, in a gastight manner, a discharge space comprising a gas filling, the gas filling comprising a metal compound, the low-pressure gas discharge lamp further comprising discharge means for maintaining a discharge in the discharge space, - the low-pressure gas discharge lamp being substantially free of mercury, and the metal compound being selected from the group of chromium halide, vanadium halide and tantalum halide.

A molecular gas discharge takes place at a low pressure in the gas discharge lamp according to the invention, which gas discharge emits radiation in the visible and UV range of the electromagnetic spectrum comprising the characteristic lines of atomic chromium, atomic vanadium and atomic tantalum and the corresponding emission bands of the halides of these materials.

In combination with phosphors, the low-pressure gas discharge lamp according to the invention has a substantially higher luminous efficacy than conventional low-pressure mercury discharge lamps. The luminous efficacy, expressed in lumen/Watt, is the ratio between the total luminous flux emitted by the lamp and the total power input to the lamp. The relatively high luminous efficacy of the lamp according to the invention means that a specific quantity of light is obtained at a smaller power consumption.

The low-pressure gas discharge lamp according to the invention has the further advantage that the use of mercury is avoided. In this patent application, the term "substantially free of mercury" is understood to mean that the low-pressure gas discharge lamp contains a negligible amount of mercury, typically less than 10 micrograms of mercury per lamp. Mercury in the gas filling is regarded as an environmentally harmful and toxic substance, the use of which should be avoided as much as possible in present-day mass- products as its use, production and disposal pose a threat to the environment.

The measures according to the invention have the effect that the presence of the metal compound selected from the group of chromium halide, vanadium halide and tantalum halide results in an emission of light from the discharge space, in which part of the emitted light is in the visible range of the electromagnetic spectrum. A gas discharge takes place at a low pressure in the discharge space of the low-pressure gas discharge lamp according to the invention. Part of the generated spectrum is in the visible range. The main emission of light in the known low-pressure mercury vapor discharge lamp is in the ultraviolet range (the main ultraviolet light emission in the known low-pressure mercury vapor discharge lamp has a wavelength of approximately 254 nanometers). Luminescent materials are employed to produce visible light from these known low-pressure mercury vapor discharge lamps. The luminescent materials convert the emitted ultraviolet light into visible light. As a result of the conversion, energy is lost, which reduces the efficiency of the known low-pressure mercury vapor discharge lamps. The low-pressure gas discharge lamp according to the invention produces significant amounts of visible and near-UV light, which reduces the energy losses in the luminescent material and improves the efficiency. In some cases, the luminescent material may even be omitted.

The low-pressure gas discharge lamp according to the invention has the further advantage that part of the light emitted from the discharge space is in the near-ultraviolet range. The near-ultraviolet range comprises ultraviolet-B light (further also referred to as UV-B), having a wavelength between approximately 280 and 320 nanometers, and ultra violet- A light (further also referred to as UV-A), having a wavelength between approximately 320 and 400 nanometers. UV-A and UV-B light are generally used for medical purposes (for example, for treatment of psoriasis), and for germicidal, lacquer-curing and tanning purposes. Luminescent materials are used in the known low-pressure mercury vapor discharge lamps so as to convert the UV-radiation of the mercury into UV-A and UV-B. However, the known UV-generating low-pressure mercury vapor discharge lamp has the drawback that a relatively strong degradation of the luminescent materials used to

generate UV-A and UV-B leads to a limited lifetime. In addition to visible light, the low- pressure gas discharge lamp according to the invention emits also part of its emitted light in the near-ultraviolet range without the use of luminescent materials. Since no luminescent materials are used, the low-pressure gas discharge lamp according to the invention has an improved efficiency and an increased lifetime as compared to the known UV-generating low- pressure mercury vapor discharge lamps. To allow emission of light in the near-ultraviolet range, the discharge vessel is generally constituted by quartz or other UV-transmitting materials. Furthermore, instead of the blue emission of conventional UV-emitting lamps, the low-pressure gas discharge lamp according to the invention emits, in addition to the UV-A and UV-B light, light in the visible range of the electromagnetic spectrum.

The inventors have realized that the known low-pressure gas discharge lamps have a relatively low efficiency due to energy loss from a Stokes shift. The Stokes shift is an energy loss process due to conversion of one photon into another photon having an increased wavelength. In the known low-pressure mercury vapor discharge lamps, an ultraviolet photon from the mercury vapor discharge is generally converted by the luminescent material into a photon having an increased wavelength, for example, into UV-A, UV-B and/or visible light. The energy difference between the ultraviolet photon and the photon having an increased wavelength is typically lost and is known as the Stokes shift. The low-pressure gas discharge lamp according to the invention produces significant amounts of visible and near-UV light, which reduces the Stokes-shift losses in the luminescent material and improves the efficiency. In some cases, the luminescent material may even be omitted.

The use of a metal compound in a buffer gas of a low-pressure gas discharge lamp is disclosed in US patent US 6,972,521 in which the low-pressure gas discharge lamp has a gas discharge vessel containing a gas filling with an indium compound and a buffer gas. The gas filling with indium halides is particularly preferred. However, the use of an indium compound has the drawback that the temperature inside the discharge vessel of the known low-pressure gas discharge lamp comprising indium must be relatively high to maintain sufficient indium vapor in the discharge space. This requires special adaptations to the discharge vessel when these known low-pressure gas discharge lamps comprising indium are used for general illumination purposes, as these high temperatures reduce the efficiency due to the energy which is necessary to obtain the required high temperatures.

The efficiency of the low-pressure gas discharge lamp according to the invention can be further improved by adding a further halide or halogen as an additive to the gas filling. In an embodiment of the low-pressure gas discharge lamp, the gas filling further

comprises a halide-dispensing means. In an embodiment of the low-pressure gas discharge lamp, the halide-dispensing means comprises a halogen or a halide.

It has been found that an excess of free halide is advantageous with respect to the coldest spot temperature to achieve a sufficient vapor pressure. In the case of chromium halides, for the pure CrX ₃ phase (X = F, Cl, Br), a vapor pressure of 10 ^~3 Pa is obtained in the discharge phase at temperatures above T = 750K. For example, the presence of an excess of free halogen allows the formation of a CrX ₆ vapor phase in the discharge phase with pressures above 10 ^~3 Pa, even at T = 300K. In addition, the vapor pressure of Cr-halides over the pure CrX ₃ phase can be significantly increased by adding some halogen loosely bound on another material. For chlorine, AuCl ₃ and TeCl ₄ are suitable examples. AuCl ₃ and TeCl ₄ can be dosed as a solid-state phase. These halide-dispensing means stimulate CrX ₆ to go into the vapor phase in a low-pressure gas discharge lamp during operation.

In an embodiment of the low-pressure gas discharge lamp, the gas filling further comprises a buffer gas. The buffer gas is generally constituted by an inert gas, for example, helium, neon, argon, krypton and/or xenon. The inert gas serves as a buffer gas enabling the gas discharge in the low-pressure gas discharge lamp to be more readily ignited. The efficiency of the low-pressure gas discharge lamp can be further improved by optimizing the internal pressure of the lamp during operation. Generally, the buffer gas has a cold filling pressure of below 10 kPa (=100 mbar). In an embodiment of the Io w- pressure gas discharge lamp, the buffer gas pressure ranges between 10 and 1000 Pa.

In a preferred embodiment of the low-pressure gas discharge lamp, the metal compound is selected from the group of chromium chloride, vanadium chloride and tantalum chloride. Other suitable halides are, for example, vanadium bromide, tantalum bromide and tantalum iodide. Also mixtures of the mentioned halides can be used. In another embodiment of the low-pressure gas discharge lamp, in operation, the concentration of the density of the chromium halide, vanadium halide or tantalum halide is between 10 ^"11 and 10 ^"7 mole/cm ³ .

In an embodiment of the low-pressure gas discharge lamp, the low-pressure gas discharge lamp comprises an outer vessel enclosing the discharge vessel. The presence of the additional outer vessel has the advantage that it provides an additional thermal insulation which further reduces the energy loss from loss of heat. Furthermore, a luminescent layer may conveniently be applied at the inner side of the outer vessel, preventing the luminescent material from reacting with the gas filling inside the discharge vessel.

In an embodiment of the low-pressure gas discharge lamp, the low-pressure gas discharge lamp comprises elements which maintain the discharge via inductive operation, further also referred to as inductive coupler. The elements may also maintain the discharge via capacitive operation, microwave operation, or via electrodes. Such a lamp, referred to as electrodeless low-pressure gas discharge lamp, has the advantage that its average lifetime is considerably longer than that of conventional low-pressure gas discharge lamps which have electric contacts through the discharge vessel to transfer power into the discharge space. Generally, the electric contacts, also referred to as electrodes, limit the life span of conventional low-pressure gas discharge lamps. For example, the electrodes may become contaminated with residue or may get damaged by the discharge and cannot transfer sufficient power into the discharge space so as to guarantee operation of the conventional low-pressure gas discharge lamp. Providing the low-pressure gas discharge lamp with an inductive coupler according to the invention considerably increases the lifetime of such a low-pressure gas discharge lamp. An example of such an inductive coupler is a coil which is arranged, for example, around the light-transmitting discharge vessel, or in a glass protrusion protruding into the discharge vessel. Besides maintaining the discharge, the inductive coupler may also be used to generate the discharge in the discharge vessel of the low-pressure gas discharge lamp according to the invention.

In a preferred embodiment of the low-pressure gas discharge lamp, the discharge vessel is provided with a luminescent layer comprising a luminescent material. The luminescent material, for example, absorbs part of the ultraviolet light emitted by the chromium halide, vanadium halide or tantalum halide and converts the absorbed ultraviolet light into visible light. When the low-pressure gas discharge lamp according to the invention is used for general illumination purposes, it should produce substantially white light at the required color temperature. The light emitted by the low-pressure gas discharge lamp is in the visible range of the electromagnetic spectrum. Altering a gas pressure and/or operating temperature inside the discharge vessel changes the spectrum of the emitted light and as such changes the color of the light emitted by the low-pressure gas discharge lamp according to the invention. However, the required color temperature of the low-pressure gas discharge lamp may not be achieved by only altering the gas pressure and/or operating temperature. Addition of a luminescent layer comprising luminescent materials enables the light emitted by the luminescent material to be mixed with the light emitted from the discharge space so as to produce the required color temperature. Although in this preferred embodiment luminescent materials are used to adapt the color of the emitted light from the low-pressure

gas discharge lamp, this lamp has an efficiency which is still higher as compared to conventional low-pressure mercury vapor discharge lamps. The ultraviolet part of the light emitted by the low-pressure gas discharge lamp according to the invention is in the near- ultraviolet range which has a substantially longer average wavelength than the main ultraviolet emission of the mercury vapor (which is around 254 nanometers). This shift of the average wavelength of the ultraviolet part of the emitted light in the low-pressure gas discharge lamp according to the invention to longer wavelengths results in a reduced Stokes shift, because the difference between the average energy of the ultraviolet photon which is absorbed by the luminescent material and the average energy of the emitted photon in the visible range is reduced, resulting in a reduction of losses. Part of the light emitted from the discharge space of the low-pressure gas discharge lamp according to the invention is in the visual range of the electromagnetic spectrum, which already improves the efficiency of the low-pressure gas discharge lamp according to the invention with respect to conventional low- pressure mercury vapor discharge lamps. The luminescent layer comprising the luminescent material may be applied to the inner or the outer side of the discharge vessel. Applying the luminescent layer on the outer side of the discharge vessel prevents the luminescent material from reacting with the gas filling inside the discharge vessel.

In an embodiment of the low-pressure gas discharge lamp, the discharge vessel comprises a coating for thermal insulation. Generally, the low-pressure gas discharge lamp according to the invention has a higher operating temperature than conventional low- pressure mercury vapor discharge lamps so as to ensure that enough atoms and/or molecules are in the gas filling. The additional coating for thermal insulation may be an infrared radiation-reflecting coating reflecting the emitted infrared radiation from the discharge space back into the discharge space. The result of the added infrared radiation-reflecting coating is an increased temperature inside the discharge vessel. Alternatively, the coating for thermal insulation may shield the increased temperature inside the discharge vessel from the outer side of the discharge vessel and as such shield this increased temperature from a user handling the low-pressure gas discharge lamps.

The invention also relates to the use of the low-pressure gas discharge lamp according to the invention for diagnostic or therapeutic applications, such as, for example, medical imaging and radiotherapy for treatment of psoriasis, and for germicidal and cosmetic applications such as, for example, tanning. Another application is use as a lacquer-curing lamp.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

In the drawings: Figures IA and IB are cross-sectional views of two embodiments of a low- pressure gas discharge lamp according to the invention, and

Figures 2A, 2B, 2C and 2D show emission spectra of low-pressure gas discharge lamps comprising chromium chloride, vanadium chloride, tantalum iodide and tantalum chloride, respectively. Figures IA and IB are purely diagrammatic and not drawn to scale.

Particularly for clarity, some dimensions are exaggerated strongly. Similar components in the Figures are denoted by the same reference numerals as much as possible.

DESCRIPTION OF EMBODIMENTS Figures IA and IB show very schematically two embodiments of a low- pressure gas discharge lamp 10, 20 according to the invention in cross-sectional views. The low-pressure gas discharge lamp 10, 20 according to the invention comprises a light- transmitting discharge vessel 12, 22 which encloses a discharge space 14, 24 in a gastight manner. The discharge space 14, 24 comprises a gas filling of, for example, a metal compound and a buffer gas. The low-pressure gas discharge lamp 10, 20 further comprises discharge means 18, 28 for maintaining a discharge in the discharge space 14, 24. The discharge means 18, 28 couple energy into the discharge space 14, 24, for example, via capacitive coupling, inductive coupling, microwave coupling, or via electrodes.

In the embodiment shown in Figure IA, the discharge elements 18 are a set of electrodes 18, only one of which is shown in Figure IA. The electrodes 18 are electric connections through the discharge vessel 12 of the low-pressure gas discharge lamp 10. A discharge is initiated between the two electrodes 18 by applying an electric potential difference between them. This discharge is generally located between the two electrodes 18 and is indicated in Figure IA as the discharge space 14. A molecular gas discharge takes place at a low pressure in the gas discharge lamp according to the invention, which gas discharge emits radiation in the visible and UV range of the electromagnetic spectrum comprising the characteristic lines of atomic chromium, atomic vanadium and atomic tantalum and the corresponding emission bands of the halides of these materials.

In general, light generation in a low-pressure gas discharge lamp is based on the principle that charge carriers, particularly electrons but also ions, are accelerated by an electric field applied between the electrodes in the discharge vessel. Collisions of these accelerated electrons and ions with the gas atoms or molecules in the gas filling in the discharge vessel cause these gas atoms or molecules to be dissociated, excited or ionized. When the atoms or molecules of the gas filling return to the ground state, a more or less substantial part of the excitation energy is converted into radiation. In the known low- pressure mercury vapor discharge lamps, the light emitted by the excited mercury atoms is mainly ultraviolet light at a wavelength of approximately 254 nanometers. This ultraviolet light is subsequently absorbed by a luminescent layer comprising a luminescent material which converts the absorbed ultraviolet light, for example, into visible light of a predetermined color.

In the low-pressure gas discharge lamp 10 according to the invention, the discharge space 14 comprises a metal compound and a buffer gas. The metal compound in the low-pressure gas discharge lamp 10 according to the invention is selected from the group of chromium halide, vanadium halide and tantalum halide. The buffer gas is generally constituted by an inert gas, for example, helium, neon, argon, krypton and/or xenon, preferably at a pressure between 1 Pa and 10 kPa (typically at room temperature, not in operation). The metal compound added to the gas filling of the low-pressure gas discharge lamp 10 according to the invention and its fragments in the discharge are excited by the accelerated electrons and ions and subsequently emit light. The emission spectrum of the low-pressure gas discharge lamp 10 is determined by the type of metal compound in the gas filling, together with, for example, the pressure and temperature inside the discharge vessel 12. The gas filling comprises different metal compounds such as metal atoms and molecules, all of which contribute to the emission spectrum of the low-pressure gas discharge lamp 10 according to the invention. Figures 2A, 2B, 2C and 2D show emission spectra of low- pressure gas discharge lamps comprising chromium chloride, vanadium chloride, tantalum iodide and tantalum chloride, respectively.

In the embodiment of the low-pressure gas discharge lamp 10 shown in Figure IA, a luminescent layer 16 is applied to the inner side of the discharge vessel 12.

Alternatively, the luminescent layer is applied to the outer side of the discharge vessel or on an additional outer bulb. The luminescent layer 16, for example, absorbs part of the near- ultraviolet light emitted from the discharge space 14 and converts the absorbed ultraviolet light into visible light of a predetermined color. A few commonly used examples of

luminescent materials among the vast range of possible luminescent materials are europium- activated barium magnesium aluminate, BaMgAlioOi7:Eu ²⁺ (also referred to as BAM) which emits substantially blue light, and europium-activated yttrium oxysulfϊde, Y ₂ O ₂ SiEu (also referred to as YOS) which emits substantially red light. By choosing a specific luminescent material or a mixture of luminescent materials, the color of the low-pressure gas discharge lamp 10 can be determined by mixing the visible light emitted from the discharge space 14 with the light emitted by the luminescent layer 16.

The low-pressure gas discharge lamp 10 also comprises, for example, a thermal insulation coating 19. This thermal insulation coating can be applied to the inner or outer side of the discharge vessel or on an additional outer bulb. Generally, the low-pressure discharge lamp 10 according to the invention has a higher operating temperature than conventional low-pressure mercury vapor discharge lamps so as to ensure that enough metal vapor is in the gas filling. The additional thermal insulation coating 19 is, for example, an infrared radiation-reflecting coating of indium-tin oxide (also known as ITO or tin-doped indium oxide) or fluorine-doped tin oxide (SnO ₂ :F; also known as FTO), reflecting the infrared radiation emitted from the discharge space 14 back towards the discharge space 14. The result of the added infrared radiation-reflecting coating is that the temperature inside the discharge vessel 12 increases.

The low-pressure gas discharge lamp 10 also comprises, for example, an outer vessel 11 enclosing the discharge vessel 12. The additional outer vessel 11 provides additional thermal insulation which further reduces the energy loss from loss of heat. Furthermore, a luminescent layer 16 may be applied, for example, at the inner side of the outer vessel 11, preventing the luminescent material in the luminescent layer 16 from reacting with the gas filling inside the discharge vessel 12. Figure IB shows an embodiment of a low-pressure gas discharge lamp 20 comprising an inductive coupler 28 for inductively maintaining the discharge in the low- pressure gas discharge lamp 20. Alternatively, the inductive coupler 28 may also be used for generating the discharge. The inductive coupler 28, also referred to as power coupler, generally comprises a coil wound on a ferrite core, for example, nickel-zinc ferrite or manganese-zinc ferrite. The inductive coupler 28 is arranged in a protrusion 23 in the discharge vessel 22 and generates a varying electromagnetic field inside the discharge vessel 22 at the discharge space 24. Electrons and ions in the gas filling of the discharge space 24 are accelerated by the electromagnetic field and collide with the metal compounds added to the gas filling. Due to the collision, the metal compounds are excited and subsequently emit

light. Inductively generating and/or maintaining the discharge in the low-pressure gas discharge lamp 20 has the advantage that the electrodes 18, which generally limit the lifetime of low-pressure gas discharge lamps, can be omitted. Alternatively, the inductive coupler 28 may be arranged outside (not shown) the discharge vessel 22, resulting in a simplification of the manufacturing process for the discharge vessel 22. In the embodiment shown in

Figure IB, the luminescent layer 26 is applied to the outer side of the discharge vessel 22.

EXAMPLE 1

A cylindrical discharge vessel of glass, which is transparent to UV-radiation, having a length of 25 cm and a diameter of 2.5 cm was provided with outer electrodes of conductive material. The discharge vessel was evacuated and a dose of 0.25 mg chromium (III) chloride was added. In this example, some further chlorine was added; the halide-dispensing means comprises 0.26 mg of tellurium (IV) chloride. Also argon as a buffer gas was introduced into the discharge vessel at a cold pressure of 500 Pa. A high- frequency field having a frequency of 13.56 MHz originating from an external source was applied.

Figure 2A shows an emission spectrum of the low-pressure gas discharge lamp comprising chromium (III) chloride at an operating temperature of 115°C. In operation, atomic chromium generates 70 to 80% of the emission of the low-pressure gas discharge lamp, whereas ionized chromium generates the remainder of the light emission.

EXAMPLE 2

A cylindrical discharge vessel of glass, which is transparent to UV-radiation, having a length of 9 cm and a diameter of 6.5 cm was provided with a coil of conductive material around the vessel. The discharge vessel was evacuated and a dose of 1.5 mg of vanadium (III) chloride was added. Also xenon as a buffer gas was introduced into the discharge vessel at a cold pressure of 100 Pa. A high-frequency field having a frequency of 2.65 MHz originating from an external source was applied.

Figure 2B shows an emission spectrum of the low-pressure gas discharge lamp comprising vanadium (III) chloride at an operating temperature of 200°C.

EXAMPLE 3

A cylindrical discharge vessel of glass, which is transparent to UV-radiation, having a length of 9 cm and a diameter of 6.5 cm was provided with a coil of conductive

material around the vessel. The discharge vessel was evacuated and a dose of 5 mg of tantalum (III) iodide was added. Xenon was introduced as a buffer gas into the discharge vessel at a cold pressure of 100 Pa. A high-frequency field having a frequency of 2.65 MHz originating from an external source was applied. Figure 2C shows an emission spectrum of the low-pressure gas discharge lamp comprising tantalum (V) iodide.

EXAMPLE 4

A cylindrical discharge vessel of glass, which is transparent to UV-radiation, having a length of 9 cm and a diameter of 6.5 cm was provided with a coil of conductive material around the vessel. The discharge vessel was evacuated and a dose of 5 mg of tantalum (III) chloride was added. Xenon was introduced as a buffer gas into the discharge vessel at a cold pressure of 100 Pa. A high-frequency field having a frequency of 2.65 MHz originating from an external source was applied. Figure 2D shows an emission spectrum of the low-pressure gas discharge lamp comprising tantalum (V) chloride.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. Use of the article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.