ELECTRODELESS BULB

The present invention relates to an electrodeless bulb.

In our International Patent Application No PCT/GB05/005080, dated 23 ^rd

December 2005 and now published under No WO 2006/070190, we have described and claimed a method of making an electrodeless bulb, the method comprising the steps of:

• providing a bulb enclosure of quartz glass, • forming an adjacent neck having a bore less than a transverse internal dimension of the bulb enclosure either:

• integrally with the bulb enclosure or

• in a branch tube opening into the bulb enclosure,

• inserting at least one pellet of excitable material into the bulb enclosure through the adjacent neck,

• evacuating the bulb enclosure through the adjacent neck and

• sealing the bulb.

The object of the present invention is to provide an improved electrodeless bulb of ceramic material.

According to the invention there is provided an electrodeless bulb of ceramic material, the bulb comprising:

• a hollow bulb envelope of ceramic material, • a stern of ceramic material extending from the bulb,

• a plug of ceramic material sealingly received within the stem and

• a charge of excitable material.

Whilst it is envisaged that the stem and plug can be of different materials - albeit with the same coefficient of thermal expansion - normally they will be of the same materials.

The ceramic material can be translucent or transparent. A example of the former is polycrystalline alumina and one of the latter is polycrystalline Yttrium Aluminium Garnet - YAG. Other possible materials are aluminium nitride and single crystal sapphire.

Normally the fill will be of a noble gas, typically xenon or krypton and a metal halide such as indium bromide. Nevertheless, other volatile substances that are known to emit light when excited as a plasma can be used.

Whilst it is possible to fabricate the bulb from a first tube of green ceramic providing the hollow bulb, a green plug inserted in one end thereof and a green smaller diameter tube inserted in the other, these components being fired together to unify the bulb; in the preferred embodiment, the components are moulded as one of green ceramic and then fired. The moulding can be made around a lost wax core or by slip casting in a porous mould.

Whilst it is envisaged that the plug can be sealed within the stem by local melting of the material of the stem and/or the plug as by a laser; preferably the plug is sealed in place with a separate fusible material having a coefficient of thermal expansion compatible with that of the material of the bulb, including the plug. The fusible material can be:

• a frit glass or a mixture of metal oxides not already fused together and ground down as frit is or an inert metal such as platinum applied in powder or foil form; and can be

• inherently or compounded to be resistant to attack by halides, where the excitable material is or includes a halide, or by other excitable material used to charge the bulb; and can be

• fused by means of a laser or an inductively or resistively heated furnace.

Normally the bulb will be used in a lamp, in combination with a ceramic waveguide, in which the bulb is mounted, and a microwave radiator positioned within

the waveguide and from which microwave energy is transferred via the waveguide to the bulb for its light emitting excitation in use.

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 bulb in accordance with the invention; Figure 2 is a cross-sectional side view of the bulb; Figure 3 is diagrammatic view of the bulb in combination with a ceramic wave guide;

Figure 4 is a diagrammatic side view of a bulb being sealed;

Figure 5 is a view similar to Figure 2 of another bulb of the invention.

Referring to the drawings a bulb 1 of polycrystalline alumina has a main bulb tube 2, an end plug 3, a neck 4 and a plug 5. The pieces are all formed, as by moulding, in the green state and then fired. The pieces 1 to 4 are fired after assembly together unifying them into a unitary construction, whilst the plug is fired separately.

With reference to Figure 4, the bulb is finally assembled in a chamber, not shown, which is evacuated and then filled typically with xenon or krypton. A pellet 6 of metal halide, typically indium bromide, possibly with traces of other halides to adjust the spectral distribution of emitted light, is deposited in the bulb. This is supported in a cooled copper block 11, with the neck protruding. An open-centred, cylinder of graphite 12 is placed around the neck. The plug, which has a leg 14 and a head 15 has a ring of halide resistant frit 16 - typically yttrium alumina silica (Y ₂ O ₃ - Al ₂ O ₃ - SiO ₂ ) - placed on the leg against the head. The leg is then inserted in the neck. The graphite cylinder is encircled by an RF coil 17. For compactness of the evacuated chamber, the latter can be provided within a quartz bell jar, not shown but provided immediately around the graphite cylinder, with the coil 17 arranged outside the bell jar. On excitement of the coil, the graphite absorbs the energy supplied and heats to red heat, passing radiant heat to the neck. The frit melts and is drawn by capillary action into the neck along the leg 14. To stop the metal halide evaporating prematurely the copper block is kept cooling by cooling water flow and cools the bulb. On switching off of the RF feed to the coil the bulb neck cools and is sealed.

This process is capable of sealing many bulbs at once, with many recesses 18 in the copper for many bulbs and many openings 19 in the graphite.

In use, the bulb is placed in a bore 21 in a ceramic wave guide 22, with a microwave feed 23. The sealed neck is received in a bore 24 in a back metal plate 25. On excitation, the metal halide vaporises and emits light. To allow light that is radiated from the main bulb tube, as opposed to from the end plug, to be used, the bore 21 can be slightly tapered 26 as shown or it can be a straight bore. Further as shown, the tip of the bulb can extend beyond the ceramic wave guide for enhanced usefulness of emitted light.

In Figure 5 is shown an alternative bulb 31, sealed in the same way but having a smooth stress relieving shape produced by slip casting of the bulb in the green state.

The above described bulbs have advantage over our existing quartz bulbs in that they are able to operate at a higher temperature. Thus size for size they can be driven at a higher wattage - or be smaller and driven at the same wattage. Operation at a higher temperature results in a higher specific light output, i.e. a higher light output per watt of drive energy.

The invention is not intended to be restricted to the details of the above described embodiment. For instance, as shown in Figure 5, the bulb can be shaped as desired for best optical efficiency, e.g. with a lens shaped end. Also other ceramic materials are expected to be found to be suitable.