LAMP

The present invention relates to a lamp.

In our patent International application No WO2006/129102, we have disclosed a lamp having an electrodeless bulb, which is excited by a microwave wave guide. The bulb is short and contained within the depth of the wave guide.

In another patent application, as yet unpublished, we have disclosed a bulb that protrudes slightly from the wave guide. It does not protrude by the depth of the wave guide.

In US Patent No 5,072,157, in the Thorn name, a launcher or surfatron is disclosed for exciting a plasma in a discharge tube. In the words of this patent, the surfatron "generates an oscillating electric field parallel to the longitudinal axis of the discharge body". This excites the plasma to emit ultraviolet radiation.

US Patent No 4,810,933 describes an improved lamp of the surfatron and discharge tube type. It warns against surfatron arrangements which create standing waves in the discharge tube.

We have discovered that we can excite a plasma discharge in a discharge tube using a microwave guide of the type described in out patent. This is surprising, particularly in view of the electrical fields - albeit at microwave frequency - of a surface wave in the discharge being axial of the tube and the electric fields of the wave guide being orthogonal to the tube, with the wave guide being arranged with the tube passing through it.

According to the invention there is provided a lamp comprising: • a microwave wave guide of solid dielectric material, the wave guide being a body having: • thickness between two opposite sides,

• a discharge tube aperture extending into the body of the wave guide from one side and

• a transverse dimension between two opposite edges, which dimension is an integer and/or fractional multiple of a wavelength of microwave oscillation in the wave guide at a drive frequency between the edges, which edges are electrically grounded,

• a drive probe received in the wave guide for exciting the microwave oscillation and

• a discharge tube: • having a length greater than twice the thickness of the wave guide,

• extending from the wave guide on at least one side thereof and into the aperture in the wave guide for coupling the microwave oscillation from the wave guide to the discharge tube and

• containing a material excitable by the microwave oscillation to produce a light emitting plasma in the discharge tube.

Normally, the thickness of the wave guide will be less than its transverse dimension.

The two opposite edges can be plane, parallel edges, as where the wave guide is cuboid or they can be oppositely and convexly curved as where the wave guide is round or elliptical.

Preferably, the solid dielectric material is chosen from the group comprising: alumina, quartz, plastics materials including polytetrafluoroethylene, barium tetratitinate and titanium oxide; and the discharge tube is of quartz, glass or polycrystalline alumina.

At this stage in the development of the lamp, we have tested a number of discharge tubes, mostly of the order often times the thickness of the wave guide. We expect discharge tubes as short as twice the wave guide thickness to operate. However such a short tube is likely to be on the limit of usefulness in that the light emitting discharge in these tubes involves collision of high energy electrons with cold

gas molecules as opposed to collision with hot gas molecules in the case of the lamps of our patent application No WO2006/129102. The latter emit visible light from excitation of metal halide in noble gas carriers.

The discharge tubes of our present invention emit visible and/or ultraviolet

(UV) light in accordance with their fills. Where the fills are of rare gases and/or free halogen gases and/or include mercury, they emit UV light. Such fills are well known as excimers. A lamp emitting UV light can find use where UV light is required as in water sterilisation, or the discharge tube can be coated with fluorescent material of producing visible light. Where the fills are of rare gas with metal halides, metal oxyhalides and metal hydrides, they emit visible light. Suitable compounds are the iodides, bromides and chlorides of titanium, tin, indium and thallium Niobium oxy chloride and magnesium hydride are also suitable.

It should be noted that all halogen containing excimers and the above metal halides, metal oxyhalides and metal hydrides would be expected to attack the electrodes of discharge lamps having electrodes, which the discharge tubes of the lamps of the invention do not have.

Despite the teaching of US Patent No. 4,810,933, to avoid the formation of standing waves, in particular by use of non-coherent launchers, we have discovered that beneficial effects can be obtained by use of a discharge tube of a length to cause standing waves of radiation from the plasma discharge to be established.

Again we have surprisingly discovered that, by driving the lamp with the same frequency and different lengths of discharge tube, differing wavelengths of standing waves are excited in that the inter-nodal distance of the radiation can vary. At this stage, we do not understand the mechanism by which this occurs.

Further we have driven long discharge tubes at comparatively low power, such that the discharge decays exponentially from the drive point at the wave guide and does not occur along the full length of the tube. This is less satisfactory in terms of usage for instance in sterilising a liquid stream, where nodal variation is unlikely to be

a problem but decay from one end of the tube to the other could allow part of the liquid stream to escape irradiation.

To help understanding of the invention, a specific embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of a lamp of the invention; Figure 2 is a diagrammatic side view of the lamp with a short discharge tube; Figure 3 is a similar view of the lamp with a long discharge tube.

Referring to the drawings, Figure 1 shows a discharge tube 1 of quartz, filled with a low pressure halogen / noble-gas mix, passes through a Vz λ wave guide 2 of alumina at an aperture 3 1/4 λ from one end 4, i.e.centrally. The wave guide is silver plated to establish resonant oscillation between its opposed ends 4,5. This mode of resonance is independent of the thickness of the wave guide, which can therefore be thin, say 1/20 λ. An antenna/probe 6 is provided in another aperture 7, driven via a matching circuit 8 from an amplifier 9. The matching circuit can be of the comb filter, bandpass type, as described in our International Application No PCT/GB2007/001935. The amplifier can be a solid state device. Alternatively, a magnetron can be used.

Typically the drive frequency is 2.4GHz, at which frequency, λ in alumina with its dielectric constant of 10 is 100mm and the wave guide at λ/2 is 50mm long. The wave guide and antenna apertures are centred 12.5mm from the respective ends of the wave guide and the wave guide is 5mm thick.

As shown in Figure 2, with the wave guide close to one end, plasma discharge occurs in the short 200mm tube with three standing wave nodes N along the length of the tube at 50mm spacing. The wave guide is acting to couple microwave energy from resonance in the wave guide to surface waves in the discharge tube. We refer below to the wave guide as a "coupler". The dielectric constant of the tube is not 10 and it is believed to be coincidental that the nodes are at the same distance in the tube as the λ/2 length of the wave guide. The tube being of quartz, its dielectric constant is

3.78. Typically it is of 6mm outside diameter, with a lmm wall thickness. We have used a fill pressure of 2 torr (2mm Hg). At this pressure the light emitting plasma discharge occurs throughout the gas fill, whilst we believe that the energy dissipation from the microwave, surface-wave, electric field to the fill occurs close to the outside of the column of the gas fill, generating high energy electrons, which travel into the body of the fill, where they strike the gas particles and cause emission of light. We have noted that as the fill pressure is increased, there is a tendency for the emission, i.e. the plasma to be more concentrated along the axis of the gas fill. We do not expect to use a fill pressure above one atmosphere.

Accordingly for increased light dissipation, we prefer to increase the tube diameter and retain the low fill pressure, typically below 10 torr. Figure 3 shows a practical way in which this can be achieved with a 25mm tube diameter, which is comparable with some of the wave guide dimensions and large in comparison with the aperture 3. (Making this too large would disturb the oscillation mode in the waveguide.) The tube shown there has a 10mm neck 21 of 6mm diameter and a 100mm large diameter portion 22 of 25mm outside diameter.

Figure 4 shows a 600mm long discharge tube with the coupler ¹ A of its length along. For the strength of driving microwave signal, the plasma discharge occurs with nodes in its short portion, but no nodes in its longer portion, where the excited surface wave decays along its length, not least due to the ultra violet emission from the plasma. Thus there is not enough microwave radiation reaching the far end of the tube for reflection to establish a standing wave.

In Figure 5 there is shown a practical installation of a lamp of the invention in a water main 101 for irradiation of the water W with UV light from the lamp for sterilisation of the water. At a bend 102 in the main, a glass tube 103 projects on the central axis of an upstream portion of main. The tube has a closed upstream end 104 is sealed by an O-ring 105 against leakage of water at a fitting 106 in the elbow. The fitting has a mouth 107, closed in use by a light-tight plate 108, which prevents escape of UV light. The lamp has its discharge tube 109 extending into the glass tube, with coupler 110 and drive circuitry 111 accommodated in the mouth. UV light from tube irradiates the water flowing in the main. It should be noted that in this embodiment,

the discharge tube aperture 112 does not extend through the coupler 110 and the discharge tube extends on one side only of the coupler.

A lamp of the invention configured as a strip light is shown in Figure 6. It comprises a central coupler 201 , with a matching circuit and microwave source 202 in a ceiling housing 203. From each side of the housing and integrally moulded therewith extend downwardly open reflectors 204. These and the inside of the housing are metallised 205, for reflection and as part of Faraday cage, completed by a wire mesh 206 held by lips 207 of the reflectors. Extending laterally from the coupler 201 is the lamp's discharge tube 208 with equal portions in the two reflectors.

Another lamp is shown in Figure 7, in the form of a luminaire having an wall housing 301 and a transparent cover 302. At an upper region of the housing, the lamp's coupler 303 extends radially with the matching circuit and microwave source 304 arranged centrally. The discharge tube 305 is annular, having no separately discernible ends , looping down within the housing around the source. A wire 306 is wound around the tube as a Faraday cage.

A further lamp, as a filament bulb replacement, is shown in Figure 8. It has a moulded housing 401, with a screw fitting 402. Forwards from the housing extends a transparent enclosure 403, which is metallised 404 to be reflective except at its forward end from which light radiates and which has a pattern of metallisation lines 405 to complete the Faraday cage here. A coupler 406 and a microwave source 407 are mounted in the housing. A discharge tube 408 passes through the coupler and is doubled back on itself to accommodate its length within the enclosure. The ends 409 of the tubes abut for mutual support alongside the coupler, but are not continuous internally and are merely physically connected to the outside of the coupler.

The invention is not intended to be restricted to the details of the above described embodiments. For instance, provided the fill used is not aggressive to borosilicate glass, it can be used in place of quartz for the discharge tubes. Further alternatives are polycrystalline alumina. Other materials are possible but unlikely on economic grounds, e.g. YAG and artificial sapphire. Where it is desired to use a large diameter discharge tube with a small diameter portion extending through the wave

guide and to have the discharge tube extending on both sides of the wave guide, the tube can be waisted and the wave guide can be provided in two parts, closed around the waisted portion of the tube.