WO2016193694A2

WO2016193694A2 - Mercury-free gas discharge lamp

Info

Publication number: WO2016193694A2
Application number: PCT/GB2016/051573
Authority: WO
Inventors: Christoph Kaiser; Celal Mohan Ögün; Rainer Kling; Robert GUST; Ian MAYOR-SMITH; Mark Aston
Original assignee: Hanovia Limited
Priority date: 2015-05-29
Filing date: 2016-05-27
Publication date: 2016-12-08
Also published as: WO2016193694A3; WO2016193694A8

Abstract

A radiation source for a gas-discharge lamp, the radiation source comprising: a metal halide, comprising a metal and a halogen; and an additional source of the metal; wherein the metal halide dissociates, in use, whereby in gaseous form the ratio of metal to halogen exceeds the stoichiometric ratio of the metal halide.

Description

Mercury-free gas-discharge lamp

The invention relates to gas-discharge lamps that produce electromagnetic radiation in the ultra-violet region of the electromagnetic spectrum. Such lamps may find use in various applications relating to disinfection, such as for the purification of water or treatment of food and beverages, in the manufacture of pharmaceuticals and also for curing and drying. More specifically, the invention relates to a mercury-free gas-discharge lamp and, in particular, a mercury-free radiation source for a gas-discharge lamp. In a typical gas-discharge lamp, UV light is generated by passing an electrical discharge through an ionised gas (or "plasma"), as a consequence of the resulting transitions of electrons between energy states emitting photons of particular energies.

The use of ultra-violet (UV) electromagnetic radiation or light for disinfection and purification purposes is known. The most desirable wavelengths of UV radiation for disinfection purposes are generally understood to be in the 180nm to 320nm range, more preferably 200nm to 300nm (often referred to as UV-C), and optimally around 265nm. UV radiation of such wavelengths has both a biological effect, inactivating (if only temporarily) microorganisms primarily by genomic damage preventing replication, and a chemical effect, breaking chemical bonds (including those of micro-pollutants) by a process called photodissociation or photolysis.

UV electromagnetic radiation, typically of slightly higher wavelengths (up to approximately 400nm), is also used for curing and drying.

Conventional UV gas-discharge lamps comprise an elongate tube of quartz or silica with electrodes at either end. The lamps are filled with a starting gas, typically a noble gas such as argon or xenon, and also a small quantity of radiating working material, typically mercury. At room temperature most of the mercury inside the lamp is in liquid form. The lamp is ignited by passing an electrical current across the electrodes of the lamp, which ionises the starting gas, the resulting atomic/electron collisions causing the mercury to evaporate. Once the lamp has reached operating condition, the mercury partial pressure is much higher than that of the starting gas, and mercury therefore dominates the electrical and radiating behaviour of the lamp. Two different operating conditions are possible:

1 . Unsaturated - In the unsaturated condition, all of the mercury is vaporized and in the gas phase. The partial pressure of mercury then is determined by the filling amount of mercury.

2. Saturated - By adding more mercury, some will stay liquid and condense at the coldest point of the lamp, referred to as the 'cold spot'. The partial pressure of mercury is therefore limited by the vapour pressure at the cold spot temperature. Mercury is used because it fortuitously has a relatively low vapour pressure at room temperature (aiding the starting of the lamp), a suitably high vapour pressure at lamp running temperatures (enabling a local thermal equilibrium (LTE) and benefitting from increase in lamp efficiency) and radiates strongly at the desired UV wavelengths. Typical mercury discharge lamps for UV applications have an efficiency of -15%.

However, mercury is known to be toxic, and there are increasing numbers of regulations restricting mercury use. Although various forms of mercury-free UV lamps exist (for example, xenon automotive headlights, excimer lamps and LEDs), these do not offer equivalent running characteristics (both electrically and as a plasma) to those of a high-pressure mercury gas-discharge lamp.

For mercury-free 'high'-pressure gas-discharge lamps (defined here as operating under LTE), many of the same general principles apply as for mercury gas-discharge lamps. The main problem is the vapour pressure of the materials used for the UV radiation source in the lamp. Most of the materials which could be used for the UV radiation source are solid under room temperature. All of them show low vapour pressures at cold spot temperatures suitable for use with silica, the material of which the lamps are constructed.

The present invention therefore seeks to provide a mercury-free radiation source for a gas- discharge lamp that has comparable radiation properties to conventional mercury-containing lamps, such as similar starting behaviour, power density per arc length and efficiencies. The absence of mercury in such gas-discharge lamps, preferably while offering comparable performance to conventional lamps, offers considerable safety and environmental advantages. Ideally, all of the substances that constitute the radiation source should be non-toxic and non-carcinogenic in the used quantities.

According to an aspect of the invention there is provided a radiation source for a gas- discharge lamp, the radiation source comprising: a metal halide, comprising a metal and a halogen; and an additional source of the metal; wherein the metal halide dissociates, in use, whereby in gaseous form the ratio of metal to halogen exceeds the stoichiometric ratio of the metal halide. By altering the stoichiometric ratio in this way detrimental effects due to the presence of the halogen, for example discharge instability, may be reduced.

In use, the metal halide may dissociate into metal and halogen, the halogen also reacting with the excess metal to form the metal halide. This may assist in the transformation of the metal into the gaseous state. Only a small amount of halide may suffice for this.

Preferably, the radiation source emits electromagnetic radiation in the ultraviolet spectrum. The metal is preferably zinc, magnesium, tin or antimony. The halogen may be iodine or bromine. The metal halide may comprise zinc iodine and the ratio by mass in gaseous form of zinc to iodine is in excess of 0.2576, preferably in excess of 1 .5, more preferably approximately 2.0.

Preferably, the radiation source further comprises an additive, wherein in use the additive emits electromagnetic radiation in substantially the same spectral range as the metal (which may be complementary to that of the metal). Preferably, the additive comprises one or more of: a metal (eg. tellurium), a halide (preferably, the halogen of the halide additive is the same as that of the radiating gas) and an alloy (eg. gallium-indium).

The additive may comprise an elemental metal, preferably tellurium, gallium or iron or an alkali metal, preferably caesium or rubidium.

The additive may comprise a halide, preferably the halogen of the halide additive being the same as that of the metal halide, more preferably an iodide of magnesium, germanium, iron, gallium, lead, bismuth, antimony, thulium or thallium or a bromide of lead or bismuth.

The additive may comprise an alloy, preferably gallium-indium. Preferably, the radiation source further comprises a combination of additives.

Preferably, the radiation source further comprises a gas mixture comprising a noble gas, preferably argon, xenon or neon. The gas mixture may be a Penning mixture.

Preferably, the radiation source is adapted to emit electromagnetic radiation in the ultraviolet UV spectrum. The UV radiation may have a wavelength that is germicidal, preferably having a wavelength of between 200 and 300 nanometres, preferably between 250 and 280 nanometres, more preferably 265 nanometres. The UV radiation may have a wavelength suitable for curing or drying, preferably having a wavelength of between 200 and 400 nanometres.

Preferably, the radiation source does not contain any mercury. According to another aspect of the invention there is provided a gas discharge lamp comprising a radiation source as described herein.

Such a gas-discharge lamp may be particularly suitable for disinfection applications in the 200-300nm wavelength range. The particular efficiencies of the lamp design at wavelengths above 300nm may be advantageous for curing.

The gas discharge lamp may comprise a tube arranged to contain the radiation source, preferably wherein the tube comprises a ceramic material configured to be transparent to UV radiation.

The gas discharge lamp may comprise two electrodes disposed within the tube in a spaced arrangement, preferably with an electrode positioned at either end of the tube, more preferably wherein the lamp has an inner diameter of around 9mm and the electrode gap is around 250mm.

The gas discharge lamp may further comprise an infra-red reflective coating, preferably gold or platinum, at each end of the tube.

According to another aspect of the invention there is provided a fluid treatment apparatus incorporating a gas-discharge lamp as described herein. According to another aspect of the invention there is provided curing apparatus incorporating a gas-discharge lamp as described herein.

Also provided is a radiation source and/or a gas discharge lamp substantially as herein described with reference to the accompanying drawings.

According to another aspect of the invention there is provided a method of operating a gas- discharge lamp substantially as herein described with reference to the accompanying drawings.

According to another aspect of the invention there is provided a radiation source for a gas- discharge lamp, comprising: a first metal, a halogen, and a further substance comprising a second metal, whereby the first metal reacts with the halogen such that at least some ions of the first metal are transported to a vapour phase by the halogen, thereby causing the further substance to evaporate and emit a desired spectral emission, wherein there is a non- stoichiometric excess of first metal relative to the halogen.

For convenience and understanding, the following is an explanation of some terms used herein:

• "radiation source" refers to a chemical and physical composition comprising one or more substances, which may be present in various states, used to produce a desired spectral emission.

• "emitting substance" may refer to a substance that only emits radiation in a desired spectral range when in use.

• "ignition" refers to the process of causing ionisation and an initial electric discharge in a starting gas. This discharge may then produce heat to cause other substances of the radiation source to evaporate.

• "discharge" or "electrical discharge" refers to a continuous flow of electricity through an ionised gas or plasma.

• "vapour pressure" refers to the pressure of a vapour in contact with its liquid or solid form, at a given temperature in a closed system.

• "high pressure" (referred by some skilled in the art as "medium pressure") lamps refers to gas-discharge lamps operating under defined here as operating under local thermal equilibrium (LTE) and having an internal pressure during operation of above 0.1 bar, typically 1 -2 bar (10⁵ Pa), potentially higher, say 50 bar (referred to by some as "super high pressure").

The following terms may be used synonymously:

· "radiation source" and "filling" or "filling gas"

• "working gas", "radiating gas" and "illuminating gas"

• "auxiliary gas", "starting gas" and "start gas"

• "emitting substance", "additive" and "dopant" It should be appreciated that the working gas and/or emitting substances may be in solid or liquid phase prior to ignition of the radiation source. Where reference is made to these components as gases it should also be understood to refer to these components in other states. As referred to herein, the term "working material" may therefore refer to a "working gas" in a solid or liquid state.

Any apparatus feature as described herein may also be provided as a method feature, and vice versa.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa. Furthermore, any, some and/or all features in one aspect can be applied to any, some and/or all features in any other aspect, in any appropriate combination.

It should also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and/or supplied and/or used independently.

These and other aspects of the present invention will become apparent from the following exemplary embodiments that are described with reference to the following figures in which:

Figure 1 shows a graph of relative germicidal efficiency of electromagnetic radiation in the 200nm to 300 nm spectral region;

Figure2 shows a gas-discharge lamp;

Figure 3 shows a gas-discharge lamp having respectively A) a mercury filling and B) a metal halide, mercury-free filling; Figure 4 shows an electrical discharge of a radiation source within a lamp exhibiting turbulent behaviour and emissions in the visual spectrum;

Figure 5 shows a chart plotting the proportion of zinc to iodine used in a UV radiation source against efficacy and maximum power in a gas-discharge lamp;

Figure 6 is a chart showing spectral irradiances of different emitting substances; and

Figure 7 is a chart showing a comparison of lamp (germicidal) efficacy of different emitting substances.

Figure 1 shows a graph of the relative germicidal efficiency of electromagnetic radiation in the region of 200nm to 300nm based on a study by Meulemans, "The Basic Principle of UV- Disinfection of Water", Ozone, volume 9, issue 4, 1987, which is hereby incorporated by reference in its entirety. Further discussion of the germicidal effects of UV may be found in Meulemans. A radiation source for a gas-discharge lamp arranged to produce radiation in the UV range close to the optimum wavelength (~265nm) of the Meulemans work function may therefore be used in disinfection applications.

Figure 2 shows a gas-discharge lamp 10, comprising an elongate sealed tube 20, preferably of fused quartz or fused silica, filled with a starting or auxiliary gas and, in operation, a gaseous quantity of radiating working material 30. Two spaced electrodes 40,42 are disposed in the lamp, which are used to ignite the starting gas. These electrodes are typically made from tungsten doped with thorium, and are preferably sealed into opposite ends of the lamp. In a preferred embodiment, a lamp may be 1 m-2m in length, and have an outer diameter that is less than 29mm such that it can replace a pre-existing mercury lamp without further modification required.

Lamps are typically filled in an inert atmosphere in controlled conditions, so as to minimise the presence of impurities within the lamp.

A working gas is provided as part of the radiation source in solid or liquid form, which evaporates in use. For UV lamps a working gas is chosen which exhibits line radiation in the UV region of the emission spectrum. Ideally, for water purification applications, at least one spectral line is positioned close to the maximum of the Meulemans work function.

Figure 3A shows a conventional gas-discharge lamp with a filling comprising mercury as the working gas and argon as the starting gas. Use of metal halides

A working gas having a similar resistivity (and plasma conductivity) to mercury is required for the radiation source to operate, meaning use of an alternative metal is preferred. However, a problem with using an alternative metal to mercury is obtaining a sufficiently high vapour pressure. For example, whereas mercury produces a vapour pressure of 100 kPa at approximately 350 °C, zinc only does so at 900 °C.

However, it is known that metal halides have suitably high vapour pressures compared to those of the corresponding metals. The halide salt may evaporate and once in the gas phase, the plasma temperature is high enough to result in dissociation. The disassociated metal and halogen then emit radiation, at least some of which is in the germicidal UV region.

A metal halide is therefore used instead. This enables the working materials to evaporate due to being heated to temperatures which would not be hot enough to evaporate a pure metal.

In place of mercury, it has been found the formation of both zinc (II) iodide (Znl₂) and tin (II) iodide (Snl₂), in particular, result in plasma conductivity comparable to that of mercury. Figure 3B shows a mercury-free gas-discharge lamp with a filling comprising zinc iodide and argon or xenon as the starting gas.

In the present invention, the working material may comprise a gas, or one or more substances that can react to form a gas, having plasma conductivity comparable to that exhibited by mercury.

Possible metal halide candidates include:

• magnesium (II) iodide (Mgl₂)

• zinc (II) iodide (Znl₂)

· tin (II) iodide (Snl₂)

• antimony (III) iodide (Sbl₃)

In some embodiments a mixture of two or more metal halides is used. Of the different metal halides listed above, radiation sources incorporating zinc (II) iodide were experimentally found to show the highest germicidal efficacy (i.e. UV radiation in the wavelength range 180nm to 320nm, as defined by Meulemans). Typically, with cold spot temperatures below l OOCO, operational vapour pressures for a radiation source contained in a lamp are in the range of approximately 10² to 10⁵ Pa (1 mbar to 1 bar).

Figure 4 shows an electrical discharge of a radiation source within a lamp in which zinc (II) iodide has been used as the working material exhibiting turbulent behaviour and substantial emissions in the visual spectrum.

In use, the metal halide evaporates and disassociates due to heating by the ignited starting gas; for example, zinc iodide dissociates into elemental zinc and iodine. This presents a problem in that the iodine forms part of the plasma together with the zinc, only with emission in the visible to infrared parts of the spectrum rather than in the desired UV range. This was found to decrease the efficiency of the lamp. In addition, at the working gas pressures required to produce a suitable power density output, it was found that the discharge became turbulent as a result of the high amount of elementary iodine incorporated within the plasma, again decreasing the efficacy of the radiation source. This is a particular problem for radiation sources contained in larger diameter lamps, because of the higher working gas pressures required. Excessive iodine may also cause the radiation source to be difficult to ignite. Use of a non-stoichiometric ratio

This problem has been solved by providing separately a suitable metal and the metal halide. This allows the metal and the halogen to be supplied in non-stoichiometric proportions, such that there is an excess of the metal and hence the mass of halogen in the radiation source is thereby reduced. In the case where zinc and diodine (l₂) are provided separately in a non- stoichiometric proportion, the amount of elementary iodine incorporated within the plasma is reduced in operation.

Figure 5 shows a graph plotting the mass proportion of zinc and iodine when provided separately in a radiation source against germicidal efficacy (ideally, between 200nm and 300nm) and maximum power, based on experimental data. Providing more zinc than the stoichiometric ratio of zinc to iodine of zinc (II) iodide (i.e. where the atomic mass of Zn = 65.38 and of I = 126.9; resulting in a ratio Zn:l = 0.2576 for Znl₂) provides a more stable plasma and allows emission from elemental zinc to dominate the emitting behaviour of the radiation source, increasing the efficiency and therefore the achievable efficacy of the radiation source at the cost of a reduction in power density. It is necessary to have a high enough concentration of diiodine in the radiation source to allow for sufficient evaporation (i.e. 'transport' into the gas phase) of elementary zinc. Advantageously, elementary iodine can be 'reused' for this purpose, because free iodine reacts with excess zinc present at the lamp walls. By this mechanism, the zinc is transported into the vapour phase while the amount of iodine is reduced, allowing emission from the radiation source within the lamp to be dominated by the emitting behaviour of zinc.

For a Zn:l ratio above 1 .5, the efficacy of the radiation source appears to saturate, possibly due to insufficient transfer of zinc into the vapour phase caused by the lack of available iodine. To achieve a suitable trade-off between efficiency and the desired resistivity of the plasma, a metakhalogen mass ratio of 2:1 is therefore preferred.

It will be appreciated that such a non-stoichiometric proportion may be provided for metal halides used as working gases that are not species of zinc iodide. In particular, species of tin iodide have been identified as an alternative.

However, the output of radiation emitted in the germicidal (UV) spectral range from radiation sources comprising these materials is significantly lower than the desired germicidal radiation output by pure mercury. Use of additives / dopants

To overcome this limitation, it has been found that additives (also referred to herein as dopants or "emitting substances") can be added to the plasma formed by the working gas, which causes the additive to emit in a required region of the spectrum. These emitting substances may (also) comprise a halide in order to provide suitable vapour pressures for operation and will be discussed in detail further on.

As a result, emitting substances may be added to the filling in order to increase the UV output of the radiation source. In use, these emitting substances act as dopants within the plasma and act to populate the desired UV range with further emission lines, increasing the overall output of the radiation source in the desired (germicidal) range. The emission from the emitting substances may form the bulk of the output in the germicidal UV range, with the working gas having a minor radiative effect. The main purpose of the working gas is to provide the efficiency benefits of a LTE, then to form a suitable plasma to provide the heat to evaporate the emitting substances.

The emitting substances are, ideally, provided as solids, similarly to the materials which form the working gas. Typically, best results are achieved when the amount of emitting substance is maximised such that all of the emitting substance is evaporated - an approximate ratio of weight of working materials to emitting substance of 2:1 is suitable for most fillings. Failure to evaporate all of the emitting substance (i.e. where saturation has occurred) may cause the discharge to become unstable.

Initial investigations considered inter alia: MN, Ge, Mg, Lu and Sn - preferably as halides complementary to the metal halide working gas.

The materials considered for use as emitting substances were chosen for their ability to produce atomic line radiation in the UV region at a cold spot temperature of around 800 °C, together with their relatively high vapour pressure. However, these vapour pressures need not be as high as the working gas, because it is acceptable that they only evaporate as the radiation source gets up to its working temperature, rather than following the ignition of the starting gas. Additionally, it is required that any emitting substance material will not have significant deleterious effects on the discharge, such as reducing its size or preventing ignition.

Figure 6 is a chart showing spectral irradiances of different emitting substances. In particular, it shows the spectral irradiances of different possible emitting substances in a lamp filling comprising 21 mg of zinc (II) iodide as the working gas and argon at 40mbar as the starting gas, wherein the emitting substances were as follows: L25 is gallium (III) iodide, L26 is tellurium, L27 is geranium (IV) iodide and L28 is a gallium-indium alloy.

Figure 7 is a chart showing a comparison of lamp (germicidal) efficacy of different emitting substances, in particular it shows the efficiency of mercury-free gas-discharge lamps having fillings comprising the same emitting substances (L25, L26, L27 and L28) as described above.

- Elements

It was found that tellurium (Te) could be used in elementary form as a suitable emitting substance. Tellurium shows vapour pressures of approximately 0.1 MPa (1 bar) at temperatures around l OOCO and emits UV radiation in the desired spectral region. Alternatively, other elemental materials which may offer potentially high UV line radiation may be used such as gold, platinum, gallium and lead. Alkali metals such as caesium (Cs) and rubidium (Rb) were investigated for use as emitting substances due to their characteristic radiation in the desired UV range and their low ionisation energy, which produced the advantage that the density of free electrons incorporated within the plasma could be increased, which could be bound to iodine ions. By this means, the amount of iodine provided in the radiation source could be increased, increasing the pressure within any containing lamp and thereby lowering the conductivity of the plasma. However, the use of these materials as emitting substances was eventually discounted due to their high reactivity, which is deleterious to radiative output and causes problems with igniting the radiation source. - Halides

Other possible emitting substances may preferably be provided as halides to allow for suitable vapour pressures. More particularly, they should preferably be provided as iodides to prevent possible cross reactions with the working gas, although it may also be possible to use bromides in order to reach higher UV efficacies. It will be appreciated that these metal halides may be provided non-stoichiometrically, as previously described with reference to the working gas, and with similar advantages.

Other possible emitting substances include:

• iodides, including: magnesium (II) iodide, germanium (IV) iodide, iron (II) iodide, gallium (III) iodide, lead (II) iodide, bismuth (III) iodide, antimony (III) iodide

• bromides, including: lead (II) bromide, bismuth (III) bromide.

Further materials, such as thulium (III) iodide and thallium iodide, are also candidates, although production of working radiation sources may be hampered by material impurities.

Experimental tests found that the most effective emitting substances were bismuth (III) bromide, lead (II) iodide, and tellurium. However, as mentioned, it should be appreciated that many materials may be suitable for use as an emitting substance. Other considerations

- Filling

The mercury-free lamps described above have been found to be more sensitive to impurities than mercury-based lamps. Particularly important is to ensure that oxygen and moisture are not introduced into the lamps during filling. Typically, a glove-box is therefore used when the working gas material is introduced into lamp (in solid form). The lamp is then evacuated, back-filled with the starting gas (eg. argon, to a pressure of -10 mbar) and sealed.

- Toxicity

An additional requirement for the substances constituting the radiation source and the emitting substances is that they should have low toxicity, and not be harmful to humans in the used dosages, unlike mercury. Care must be taken because this toxicity may vary between phases, and in particular may be more of a problem in the plasma phase due to the presence of free radicals and the possibility of reactions between components of the plasma and the quartz wall of a lamp or other lamp components.

Of the three most effective emitting substances, lead (II) iodide is toxic, which may limit its usage, bismuth (III) bromide is non-toxic but corrosive , and tellurium is mildly toxic, although some of its salts are toxic.

- Ignition

The use of tellurium improves the efficacy of the radiation source relative to other emitting substances, but may cause difficulties with ignition (potentially due to the presence of impurities) and may cause the radiation source to operate at a higher working temperature, which may cause any lamp containing the radiation source to bend.

As with mercury-based radiation sources, the radiation source requires a starting gas, which, upon ignition, provides heat to the lamp walls and causes evaporation of the working gas. Enough heat should be provided such that the cold spot is hot enough to evaporate the material used as working gas. When the radiation source is contained in a gas-discharge lamp, this cold spot is typically found at the ends of the lamp. The ignition of this gas is governed by the breakdown voltage of the gas, which depends non-linearly on gas pressure and the electrode gap. Typically, the pressure of the starting gas for a given electrode gap length (between two electrodes disposed in a lamp) must be relatively low. This is to allow for a low enough mean free path of electrons within the gas that ionisation, and thereby ignition, can take place at a reasonable strike voltage (the minimum voltage required to start the lamp). Under some circumstances, this may necessitate that that radiation source is cooled (for example, using a freezer spray) prior to ignition in order to reduce the pressure of the starting gas. This cooling may be achieved with nitrogen or alternatively any inert or noble gas, preferably a non-flammable gas, potentially with compressed air or water.

However, very low pressures lead to low particle collision frequencies and consequently a high conductivity, resulting in very high currents being required to provide the power to cause the radiation source to heat up. The starting gas fillings are all at a very low gas pressure; a high conductivity will only remain until lamp fillings vaporise.

- Starting gas

Noble gases such as argon, neon or xenon are suitable for use as starting gases. When a radiation source is provided as a relatively high volume of material within a lamp, neon is preferred due to its lower breakdown voltage at higher pressures. Xenon has lower ionization energy than neon and so may improve ignition behaviour; however, argon is well known for use as a starting gas in radiation source containing mercury - and is also relatively cheap.

To allow for a lower pressure of starting gas with a reduced requirement to cool the radiation source, a Penning mixture may be used instead of a pure inert gas, resulting in a starting gas which may ignite at a lower burning voltage. A Penning mixture is a mixture of a first inert gas with a very small amount of a second inert gas, the second inert gas having a lower ionisation energy than the first inert gas enabling increased ionisation due to inert gas interactions which consequentially reduces the strike voltage of a lamp.

- Cold spot temperature

To operate the lamps, higher cold spot temperatures are necessary in comparison with mercury containing ones. A solution to this is the use of smaller lamp diameters and/or an infra-red reflective coating, preferably gold or platinum, at each end of the tube or lamp.

- Lamp geometry

The geometry of the lamp (more specifically, the diameter of the lamp) used has a significant effect on the performance of the radiation source. For a given power output, smaller diameter lamps have walls which are close to the high temperature plasma. Although quartz has a relatively high melting temperature, it may start to deform above 1 l OCO. To vaporise and disassociate fully, metal halides require a working temperature of above 1000°C, resulting in a small operating region in which the lamp can operate effectively without deforming.

This problem can be alleviated by the use of a larger diameter lamp, which for a given power output increases the separation distance between the walls and the hottest part of the plasma.

However, in general the temperature of the radiation source (and therefore the temperature of the lamp) determines the lifetime of the lamp, with a lamp containing high temperature plasma having a short lifetime. Generally, the higher the power density the higher the temperature; higher power density means higher photon flux and greater quartz degradation. Additionally, higher internally temperatures can mean higher electrode component assembly temperatures also reducing life. Excessively high internal temperatures could directly mean electrode degradation

Smaller diameter lamps are however more efficient in terms of UV output due to the discharge being stabilised by the wall, so there is a trade-off between efficiency and lamp life expectancy, necessitating careful choice of lamp geometry given intended application.

Smaller diameter lamps are more efficient in a high pressure lamp as the distance of the wall from the emissive part of the lamp (being the arc, which is generally in the centre of the tube) is reduced so there is less reabsorption by non-emitting gases.

For the present lamp configuration the benefit of a smaller tube means there is less offset on the arc and the higher temperatures discussed earlier in reference to halogen interaction. Two side effects were identified as being caused by the small lamp geometry:

i) Corrosion due to the electrodes being stressed through the typical high current load during ignition of the starting gas. This was addressed by using larger electrodes and matching design criteria to the lamps electrical characteristics.

ii) A voltage increase is typically observed as the working gas starts to evaporate, which may cause the lamp to overpower. As a result, electronic control gear (ECG) and electrical ballast may be used to control and limit current through the lamp where necessary. - High-pressure operation

Plasma temperature depends on current amplitude of the plasma, which in turn depends on power load. Therefore, a higher burning voltage may be used to reduce plasma temperature for a given power load. This presents several issues, so rather than increase the burning voltage the conductivity of the plasma is lowered so as to lower the current amplitude for the same burning voltage. This has the additional advantage that the current load on the electrodes (a limiting factor on the lifespan of the electrodes) is reduced. To lower the conductivity while keeping the same radiation output, it is necessary to increase the pressure within the lamp, thereby increasing the overall particle density in the lamp and so reducing the conductivity. An increased amount of filling (maintaining the same proportions) may thereby increase the output of the lamp (due to the plasma having a lower temperature, allowing wall stabilisation) but has the trade-off of hindering ignition, which benefits from having a lower pressure. As a result, a large amount of cooling may be required to sufficiently reduce the pressure to allow ignition.

Ideally, the power density of the discharge is between 80W and 130W per cm of arc length. Ideally, the germicidal efficacy of the lamp is between 22% and 30% by Meuleman weighting. Ideally, the lamp may operate for more than 8,000 hours with less than 30% loss in UV output.

- Tube material

Optionally, the lamp is made of a ceramic which is transparent to UV rather than quartz. This material may have better thermal properties and so reduce described problems with deformation at high temperatures.

In summary, a mercury-free ultraviolet radiation source for a gas-discharge lamp is provided. Standard UV discharge lamps are commonly based on mercury, radiating when excited in gaseous form. Replacing mercury directly with another metal is problematic because they tend to have comparatively higher melting points / lower vapour pressures, which would require the lamp to be heated to unfeasibly high temperatures. Instead, the appropriate metal halides are used, which have significantly lower melting / higher vapour pressures. These dissociate into the constituent metal and halogen on heating, allowing the metal to enter the gaseous state more readily. In some instances, the gaseous halogen has detrimental effects (radiating in the visible region, making the discharge plasma unstable). This is solved by providing an excess of the metal (for example, by introducing separately - as solids into the lamp - the metal and the metal halide), thereby resulting in use in a non- stoichiometric ratio of the metal : halogen. Also, additives (dopants) are used to enhance the emission in the desired wavelength range. Specific non-stoichiometric ratios are identified for particular metal halides (namely Znl₂). Suitable additives for UV are also identified. Other considerations are also discussed.

It will be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.

Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.

Reference numerals appearing in the claims are by way of illustration only and shall have no limiting effect on the scope of the claims.

Claims

1 . A radiation source for a gas-discharge lamp, the radiation source comprising:

a metal halide, comprising a metal and a halogen; and

an additional source of the metal;

wherein the metal halide dissociates, in use, whereby in gaseous form the ratio of metal to halogen exceeds the stoichiometric ratio of the metal halide.

2. A radiation source according to claim 1 , wherein the metal is zinc, magnesium, tin or antimony.

3. A radiation source according to claim 1 or 2, wherein the halogen is iodine or bromine.

4. A radiation source according to any preceding claim, wherein the metal comprises zinc, the metal halide comprises zinc iodine and the ratio by mass in gaseous form of zinc to iodine is in excess of 0.2576, preferably in excess of 1 .5, more preferably approximately 2.0.

5. A radiation source according to any preceding claim, the radiation source further comprising an additive, wherein in use the additive emits electromagnetic radiation in substantially the same spectral range as the metal.

6. A radiation source according to claim 5, wherein the additive comprises an elemental metal, preferably tellurium, gallium or iron or an alkali metal, preferably caesium or rubidium.

7. A radiation source according to claim 5, wherein the additive comprises a halide, preferably the halogen of the halide additive being the same as that of the metal halide, more preferably an iodide of magnesium, germanium, iron, gallium, lead, bismuth, antimony, thulium or thallium or a bromide of lead or bismuth.

8. A radiation source according to claim 5, wherein the additive comprises an alloy, preferably gallium-indium.

9. A radiation source according to any of claims 5 to 8, further comprising a combination of additives.

10. A radiation source according to any preceding claim, further comprising a gas mixture comprising a noble gas, preferably argon, xenon or neon.

1 1 . A radiation source according to claim 10, wherein the gas mixture is a Penning mixture.

12. A radiation source according to any preceding claim, adapted to emit electromagnetic radiation in the ultraviolet UV spectrum.

13. A radiation source according to claim 12, wherein the UV radiation has a wavelength that is germicidal.

14. A radiation source according to claim 13, wherein the UV radiation has a wavelength of between 200 and 300 nanometers, preferably between 250 and 280 nanometers, more preferably 265 nanometers.

15. A radiation source according to claim 12, wherein the UV radiation has a wavelength suitable for curing or drying.

16. A radiation source according to claim 15, wherein the UV radiation has a wavelength of between 200 and 400 nanometers.

17. A radiation source according to any preceding claim, which does not contain any mercury.

18. A gas discharge lamp comprising a radiation source according to any preceding claim.

19. A gas discharge lamp according to claim 18, further comprising a tube arranged to contain the radiation source, preferably wherein the tube comprises a ceramic material configured to be transparent to UV radiation.

20. A gas discharge lamp according to claim 19, further comprising two electrodes disposed within the tube in a spaced arrangement, preferably with an electrode positioned at either end of the tube, more preferably wherein the lamp has an inner diameter of around 9mm and the electrode gap is around 250mm.

21 . A gas discharge lamp according to claim 19 or 20, further comprising an infra-red reflective coating, preferably of gold or platinum, at each end of the tube.

22. A fluid treatment apparatus incorporating a gas-discharge lamp according to any of claims 18 to 21 .

23. Curing apparatus incorporating a gas-discharge lamp according to any of claims 18 to 21 .

24. A radiation source substantially as herein described with reference to the accompanying drawings.

25. A gas discharge lamp substantially as herein described with reference to the accompanying drawings.

26. A method of operating a gas-discharge lamp substantially as herein described with reference to the accompanying drawings.