Low-pressure mercury discharge lamps are used at present for germicidal applications. Mercury vapor in an elongate glass envelope is excited by a longitudinal electric field and so caused to emit ultraviolet radiation (UV). W in the germicidal region from 200 to 280 nm results in the inactivation of most microorganisms. When using mercury discharge lamps the germicidal effect is principally due to an emission line at 254 nm.

Mercury low pressure discharge lamps, while very widely used, suffer from significant disadvantages. Most importantly, their power is limited to approximately 30 Watts/metre. Additionally the presence of electrical connections and electrodes through the fused glass forming the envelope gives a limitation on the lifetime of such lamps.

Medium and high pressure mercury lamps produce output powers greater than 30W/m, but suffer from high temperatures and low efficiencies.

Furthermore there is a recognised need for a powerful UV source for use in the production of ozone. Exposure of oxygen to LJV of appropriate wavelength (the 254 nm line produces ozone but a wavelength of 185 nm is more suitable) produces ozone, which is itself a powerful germicidal agent. Increased UV power is also desirable when using UV for water sterilisation.

The inventors have devised an ultraviolet lamp which makes use of microwave power for excitation of gas in an envelope. Very brief details of the physical construction, and some aspects of the performance, of such a lamp have been described in a paper by Al-Shamma'a, Pandithas and Lucas (J. Phys. D: Appl. Phys. 34 (2001) 2775- 2781) entitled"Low-pressure microwave plasma ultraviolet lamp for water purification and ozone applications".

The lamp has been the subject of considerable experimental and theoretical study by the inventors and it is now recognised that effective and reproducible operation of the lamp requires that the physics of the lamp be understood in detail and that this understanding be applied to the choice of operating conditions of the lamp.

Broadly stated, an object of the present invention is to provide an effective ultraviolet lamp using microwave excitation.

An additional or alternative object of the present invention is to provide for the efficient operation of an ultraviolet lamp using microwave excitation.

In accordance with a first aspect of the present invention there is an ultraviolet ("W") lamp comprising a microwave-resonant cavity, a microwave source arranged to supply microwave power to the cavity and an envelope containing gas, the envelope comprising material which is at least substantially transparent to UV radiation at an emission frequency of the gas and projecting into the resonant cavity whereby microwave energy is coupled into the envelope providing an electric field within the envelope, the microwave source being driven at a power which is such that in a discharge zone within the resonant cavity plasma discharge takes place, while in a UV emission zone of the envelope outside the cavity the kinetic energy imparted to an electron due to acceleration by the electric field is less than the ionisation energy of the gas.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:- Figure 1 illustrates a UV lamp embodying the present invention; Figure 2 is similar to Figure 1 except that part of a lower section of the lamp is cut away to expose certain internal details; Figure 3 illustrates a resonant cavity forming part of the lamp in section, electrical field lines within the cavity being shown; Figure 4 is a graph of experimental data showing how power due to microwave excitation is dissipated in Xenon gas ; p Figure 5 is a perspective illustration of a gas-containing envelope of a further W lamp embodying the present invention; Figure 6 is perspective illustration of still a further UV lamp embodying the present invention; Figure 7 is a schematic illustration of a switching circuit for providing a modulated power input to the lamp ; Figures 8 and 9 are schematic illustrations of doubler circuits used to drive a magnetron used with the lamp; Figure 10 is a schematic illustration of a complete circuit used to drive the lamp; Figure 11 is a schematic illustration of an ozone generator incorporating the lamp; Figure 12 is a schematic illustration of a food treatment arrangement for utilizing ozone from the generator illustrated in Figure 11 ; Figure 13 is a representation of the chemical reaction that occurs during the purification of water using UV and the photocatalyst titanium dioxide; Figure 14 (a) is a schematic illustration of the interior of an arrangement for water treatment; Figure 14 (b) is a graph of experimental data showing how the kill rate of E. coli varies with intensity of UV used; and Figure 15 is a schematic illustration of a modified atmosphere food packaging line.

A source of W radiation, referred to herein as a"UV lamp", will firstly be described. Briefly explained, the lamp uses microwave energy to create a UV emissive plasma. Microwave energy is coupled into a resonant cavity 2. A gas-containing envelope projects into the cavity and surface wave excitation takes place therein. The envelope contains, in this embodiment, a mixture of argon and mercury at low pressure which, when stimulated by the microwaves emits radiation including UV.

Looking at the lamp in more detail, a microwave source is schematically indicated by a box 6 in Figure 1. The microwave source in this embodiment is a magnetron. The unit used in trials to date has been of a type known for use in a microwave oven, operating at 2.45 GHz and providing a power up to lkW. This is a well knows, widely available and inexpensive type of unit. Other types of microwave source could be used. At higher magnetron frequencies, theory predicts that the W output power per unit length increases as the square of the frequency as shown in the following equation P=k. freq'. d' where d is the diameter of the cylindrical envelope 4 and k is a normalising constant. As industry is now aiming to provide a 5. 8GHz magnetron, this may be the preferred source when high powers are required. A low loss, high power, electric cable 7 conducts the magnetron's output signal to the resonant cavity 2. Alternatively a waveguide structure may be used to transport the microwaves to the resonant cavity along with an appropriate launcher. In the illustrated exemplary embodiment the cable is connected to a loop antenna 8 within the cavity 2, the antenna serving to couple microwaves into the cavity.

The resonant cavity 2 is formed by a conductive container 10. In the illustrated embodiment this has walls of copper construction. The cavity is a tunable short-gap or re-entrant type operating in the TEloo (transverse electric field) mode, although other cavity geometry may be chosen to suit the particular application. In order to obtain resonance precisely at the magnetron frequency, and to provide the desired high Q factor, the cavity has a tuning stub 12 at its base and also a fine tuning screw 14 in its side wall, both projecting an adjustable distance into the. cavity 2. It must be understood that the illustrated construction has been used for experimental trials and the facility for adjustment of the cavity characteristics has been useful in this context. Such adjustment may be. dispensed with in production versions of the lamp by virtue of optimised design of the cavity. Field lines within the cavity have been calculated using commercial modeling software and are represented at 16 in Figure 3.

The discharge tube 4 has a portion which projects into the resonant cavity. The remainder of the discharge tube is exposed to provide UV emission to the exterior. The wall of the discharge tube 4 is formed of a dielectric material. In the present embodiment it consists of a fused silica tube (chosen for its transparency to the relevant UV frequencies, among other factors) containing argon and mercury vapor at a pressure of approximately 1 Pa (0. 01 Torr).

In the illustrated embodiment the cavity has dimensions of 100 mm diameter and 80 mm length. The envelope 4 is inserted approximately 10 mm into a gap in the cavity, and is 25 mm in diameter. However the materials and dimensions used in this experimental arrangement may be altered. The position of the end of the discharge tube in the cavity is adjustable to give maximum transfer of microwave power thereto, but this adjustment too is likely to be dispensed with in a production version, once the design has been optimized.

In operation microwave excitation of the contents of the discharge tube produces a plasma therein. This plasma is found to be very stable and entirely reproducible. A standing wave pattern is observed in the light intensity along the length of the discharge tube (as a pattern of bright and dark regions) and is interpreted by the inventors as an \ indication that a surface wave, which propagates in the tube, carries the microwave power along the tube away from a coupling region within the cavity, and simultaneously provides the field required to sustain the plasma. A plasma column can propagate waves such that the electric field has a velocity parallel to the tube axis, which has a maximum value at the discharge tube/plasma interface. Such"surface waves"are known in the academic literature. The plasma discharge can be sustained by the surface wave under standing or traveling wave modes but it is the latter which allows an increase in power per unit length to be achieved. The microwave power flow associated with such a wave is partly in the plasma and partly in the dielectric tube. The relative proportions of the power depend on the microwave frequency, tube diameter, lamp constituent pressure and power absorbed per unit length.

The lamp can be thought of as comprising a pair of coupled resonators, one formed by the cavity 2 and the other formed by the envelope 4.

A particularly important advantage of the present lamp, as compared with conventional mercury discharge lamps, is that in the former the electric field acting on the UV emissive material is transverse, whereas in the latter it is longitudinal. This transverse field is provided by virtue of the aforementioned surface wave. The transverse field generated in the present lamp can have a large potential gradient-ie a large electric field strength-because of the small dimension across which the field is created-the transverse dimension of the discharge tube. Compare this with the conventional mercury discharge lamp in which the field is generated between electrodes at either end of the tube, the large separation of the electrodes creating a relatively small electric field. This transverse field direction is one of the factors which enables the present lamp to produce greater power per unit length than the conventional lamp.

Remarkably it is found that microwaves are confined by the cavity 2 and the envelope 4. That is, the lamp does not emit significant microwave energy to its surroundings. This is an important safety factor.

The physics of UV generation within the discharge tube have been extensively studied by the inventors, using among other tools a unique computer model of"Monte Carlo"type. The principles will now be explained. Generation of a plasma (ionised gas) is necessary to the function of the lamp since it is charged particles which are excited by the microwaves, being accelerated by the electric field. Because the field is rapidly time varying it is the free electrons of the plasma which, due to their low mass, are sufficiently accelerated by the field to receive the majority of the energy. The more massive positive ions achieve relatively low velocities in the time varying field. The desired emission of W results largely from inelastic collisions between electrons and the mercury in the discharge tube, bound electrons in the mercury being promoted by the collisions to higher orbitals and releasing, upon their relaxation to lower orbitals, a photon of UV light. Several factors affect the frequency of these desirable collisions:- 1. the majority of collisions are inelastic. That is, most collisions do not result in energy being imparted to the mercury to achieve the required excitation by electrons.

This is not necessarily problematic in itself since inelastic collisions do not result in substantial loss of energy by the electrons.

2. mercury has"metastable states"-very long lived states in which bound electrons are in elevated orbitals. The excitation within the discharge tube leads to mercury atoms adopting metastable states. A mercury atom in a metastable state is not available for UV production. Over time, due to the long lifetime of the metastable states, the tendency is for the proportion of mercury particles in such states to increase as is experienced with conventional dc or ac lamps. The frequency of collisions which yield W photons consequently tends to decrease over time, as the population of mercury atoms available for UV production decreases.

3. the collision cross sections of the particles are energy dependent.

4. excessively energetic collisions cause ionisation of the mercury, rather than the desired excitation of electrons from one bound state to another. Hence, with respect to the frequency of UV producing collisions, it is not desirable that the electron population should be so energetic as to cause a high frequency of ionising collisions.

However it is necessary for ionising collisions to take place in order to create the plasma.

These considerations are potentially conflicting.

The construction of the lamp presented herein, as well as the manner in which it is operated in accordance with the present invention, permit these potentially problematic and conflicting factors to be reconciled.

With reference to item 4 in the above list, it should be understood that two different regimes prevail within the envelope 4. The portion of the envelope 4 received in the resonant cavity 2 is subject to a relatively strong electric field. Electron energy in this region is consequently relatively high. Specifically the electron energy in this region is sufficient to provide frequent ionising collisions. Plasma generation is therefore concentrated particularly in this region of the discharge tube. The remaining portion of the discharge tube 2 is subject to a weaker but uniform electric field. Operating conditions are chosen such that in this outer portion of the discharge tube 2 the electron energy is suitable to promote the desired collisions yielding UV photons. The energy required to ionise a mercury atom is in the region of 12.6eV. Hence the energy distribution of free electrons in the outer portion of the discharge tube should be such as to provide only a small proportion of free electrons with this much energy.

In Figure 4 the relative significance of different modes of dissipation of energy can be seen over a range of average electron energies E/N. Figure 4 relates to xenon rather than mercury, although the principle is the same for either material. The present invention makes possible use of xenon as the UV emissive material, in place of mercury, a point which will be considered further below. Xenon has an emission line at 172 nm.

At low energies, losses due to elastic collisions, e. g. with the walls of the discharge tube, predominate. At high energies, power dissipation due to ionisation becomes significant.

Energy also goes into excitation of bound electrons into higher states, not yielding the desired radiation at 172 nm. Between these two extremes of low and high energy is a desirable condition in which power usage in the exposed part of the discharge tube is predominantly accounted for by excitation of the xenon to provide the required 172 nm radiation. Suitable selection of the input microwave power allows this desirable condition to be achieved.

The problem of metastable states is addressed, in accordance with the present invention, by providing a time varying or"modulated"microwave input power. The inventors have found that by modulating the input power, the proportion of atoms in metastable states is greatly reduced, even without any reduction in the average power input to the lamp. In the lamp system currently under consideration the power modulation takes the form of pulsing-rapidly and repeatedly switching the power on and off. Circuitry for providing this pulsed input power will be described below.

The frequency of the power modulation is chosen to be high enough that the plasma discharge is sustained through the low power part of the cycle. The required frequency can be determined empirically. It is observed that too low a frequency causes the luminous gas to retreat along the discharge tube-that is, the luminosity is not observed in the part of the tube remote from the resonant cavity 2. A modulation frequency in the region of lMHz has been determined by the inventors to be highly suitable. Frequencies down to 100 I (Hz have been successfully used in trials. However the appropriate frequency depends on such factors as lamp design and lower-or indeed higher-frequencies may be used in practice.

The mark space ratio of the power modulation has a bearing on the incidence of metastable states. The inventors have determined that a ratio of 1: 3 (i. e. high power periods one third as long as the intervening low power periods) is highly suitable.

The combination of power modulation and appropriate choice of power level makes it possible to operate the lamp at high efficiency and high operating power (>300W/m). In trials the illustrated lamp has been found to be 80 percent efficient in converting microwave energy into UV radiation. The 20 percent loss is due to a variety of causes including elastic and metastable collision losses, reflection, dielectric heating and W absorption and conversion.

The inventors have observed a further advantage of input power modulation which is that the germicidal effect of the lamp is improved. For a given average input power, a lamp having a modulated input is found to have a higher germicidal (e. g. bacterial) "kill rate". The inventors have conjectured that this is because the high instantaneous power provided by virtue of the modulation causes, where the UV output is directed onto a surface to be sterilised, some ablation and boiling at the surface which results in microorganisms being de-activated.

Running the magnetron on a modulated power input also increases its working life.

Power modulation also provides for control of the lamp's operating temperature.

It is found that the proportion of energy in different spectral lines varies with temperature. If operated at 30 degrees centigrade the illustrated mercury-based lamp is found to emit 90 percent of the W radiation at 254 nm and 10 percent at 185 nm. At an operating temperature of 60 degrees centigrade. it produces 40 percent at 254 nm and 60 percent at 185 nm.

As noted above, lamps embodying the present invention need not necessarily use mercury as the UV emissive material. Conventional electrical discharge lamps have difficulty in producing the electron energy needed to ionise gases such as xenon. The much stronger electrical field provided by the present lamp removes this limitation, however, and makes the use of a broader range of materials possible. The W emissive material can be chosen to provide a required frequency. Light produced by xenon at 172 nm is particularly effective in production of ozone, making this a potentially important example.

The concentration of W emissive material-i. e. its pressure-can also be increased as compared with conventional discharge lamps due to the greater electric field strength, offering increases in UV power output.

Whereas the lamp illustrated in Figures 1 to 3 has a tubular discharge envelope 2, another of the advantages of the type of lamp presented herein is that the envelope can be designed with any number of different shapes to suit a specific application. Figure 5 illustrates an alternative shape 20 for the envelope having a narrow stem 22 for insertion into the resonant cavity and a broad, shallow UV emissive portion 24. The small depth of this portion (perpendicular to the plane of the paper) results in the desired high potential gradient.

Another variant of the lamp design is illustrated in Figure 6 and differs from the version shown in Figure 1 in having two resonant cavities 26,28, both supplied with microwave energy, arranged at either end of a gas envelope 30, providing higher LTV output power. Another possible variant (not illustrated) would have a set of discharge envelopes being driven from a single resonator. Careful cavity design and positioning of the envelopes would be required to ensure maximal power transfer to the envelopes.

A circuit 50 for switching the power input to the lamp to provide the required power modulation is illustrated in Figure 7. The circuit is capable of handling 1900 volts.

It uses a DC to DC converter 52 receiving a 5 V DC supply to provide a 12 V DC supply for the switching circuit itself. The high voltage switch is controlled by a signal generator 54 which provides a logic signal to the input of an opto-isolator 56. The signal generator is used, in this prototype system, for the sake of flexibility. A more simple oscillator could of course be substituted. The output of the opto-isolator 56 is fed to an input of a high speed, integrated circuit MOSFET/IGBT driver 58 whose output is connected through a 10 ohm resistor 60 to the gate of a high voltage IGBT (integrated gate bipolar transistor) 62. The IGBT serves to switch the voltage applied to the magnetron. The component used in trials is capable of handling 2500 volts although a series combination of zener diodes 64 clamps voltage across the IGBT at no more than 1900 volts.

Two or more circuits of this type can when necessary be cascaded to handle larger voltages, the opto-isolator inputs being connected-typically in series-to switch concurrently. Two stage circuits have been used to control a magnetron voltage of 3kV.

A refinement would be to add a resistor/capacitor network across each IGBT to aid voltage sharing.

Circuitry used to provide the high voltage required to drive the magnetron is illustrated in Figures 8 and 9. Figure 8 shows a voltage doubler circuit 70 for use with a single phase A. C. supply which is applied to primary windings 72 of a step-up transformer 74 whose secondary windings 76 are connected on one side directly to earth and on the other side via a doubler capacitor 78 and diode 80 to earth. During the capacitor charging time there is no voltage to the magnetron 82. Rather than take a path through ground and up to the plate of the magnetron, the current swings up through the diode. The voltage across the capacitor rises with the transformer secondary voltage to the peak supply voltage (2800 volts). As the transformer secondary voltage begins to decrease from its maximum positive value the diode prevents further capacitor discharge and the doubler capacitor 78 remains at the peak supply voltage.

Subsequently the transformer secondary (output) voltage swings into the negative half-cycle and increases in a negative direction to the negative peak of the supply voltage (2800 volts). The transformer secondary and the charged doubler capacitor are now essentially two EMFs in series. The 2800 volts across the transformer winding adds to the 2800 volts stored in the capacitor and the sum voltage of 5600 volts is applied to the magnetron cathode to drive the magnetron.

There are two fundamental characteristics of this high voltage output that should be noted. First, because a voltage doubler is also a rectifier, the output is a DC voltage.

Second, the resulting output voltage that is applied to the magnetron tube is actually a pulsed DC voltage. This is because the doubler generates an output only during the negative half-cycle of the transformer's output (secondary) voltage. So, the magnetron tube is, in fact pulsed on and off at the supply frequency (e. g. 50Hz, in the case of a domestic UK mains supply).

To remove the supply frequency pulsing of the high voltage, a three phase supply can be used with the circuit 100 illustrated in Fig. 9. Similarly to the previous voltage double circuit, this uses a step-up transformer 102 one side of whose secondary 105 is led to ground via a doubler capacitor 104 and diode 106. A bleeder resistor 108 is incorporated in the circuit 100, in parallel with the double capacitor 104. The inclusion of a second diode 109, connected in series between the capacitor 104 and the output 112 through which high voltage is supplied to the magnetron, prevents the common voltage point from affecting the charge/discharge cycle to the voltage doubler and so enables use with a three phase supply.

A substantially constant DC high voltage output is provided by the doubler circuit 100. The entire magnetron drive circuit 120 is schematically indicated in Fig. 10, incorporating the high voltage switching circuit 50 and the three phase doubler circuit 100 to drive the magnetron, labelled 122 in this drawing. The three phase supply is indicated at 124.

The lamp can be applied in sterilisation and in killing bacterial growth and mould. The high power it provides allows more effective UV sterilisation than was possible with electrical discharge lamps.

Exposure of microbiological systems to W light within the germicidal region from 200 to 280nm wavelength results in their inactivation. Hence one manner of use of the lamp is to expose material to the lamp's UV output. Water sterilisation can be achieved in this manner. The same technique can be used with some foodstuffs, particularly those such as sliced meat which present a flat surface. Direct W treatment does not work in shadowed areas and so is less well suited for example to use with bread, in view of its porous crumb structure.

Ozone gas can be generated by use of UV light and has been considered for use in food sterilisation. Doses in the region of 2-5 parts per million produce significant micro-biocidal effect even in short exposure times consistent with modern high speed production lines. The lamp described above may thus be used, for food treatment purposes, either (1) by direct exposure of the foodstuff to UV, (2) by generating ozone for food treatment or (3) through a combination of UV exposure and ozone generation.

Figure 11 illustrates an ozone generator utilizing the lamp 200. An enclosure 201 is provided around the envelope 202 of the lamp and is formed of UV opaque material. Aluminium is used in the illustrated example.

A gas in-feed 204 receives gas from a source 206 and supplies it to the enclosure interior, wherein exposure to UV from the lamp produces ozone. The gas used in the present embodiment is air although other gases including pure oxygen could be used.

Ozone-bearing gas leaves the enclosure through outlet 208 and passes via a flow valve 210 and ozone sensor 212 to a spray outlet 214. A computer or other electronic control unit 216 receives inputs from various sensors via a bus 218 and controls the process accordingly. Other inputs to the control unit 216 come from a pressure sensor 220 and a UV sensor 222 within the enclosure 201, the latter serving to indicate whether the lamp is functioning. A sparking system for lamp starting is also indicated at 224. In tests, ozone concentrations of 50-100 parts per million have been achieved using air at pressures in the range 0.1-0. 3 Mpa (1-3 Bar). Higher concentrations could be achieved, if necessary, using oxygen.

Figure 12 is a schematic representation of an arrangement for utilizing the ozone in food sterilisation. Ozone-bearing gas from the generator illustrated in Figure 11 is passed through an input conduit 250 to a spray hood 252 disposed adjacent to, and directed toward, a confined food chamber 254. As well as containing food, the chamber has an ozone sensor 256 which passes on indication of the ozone level to control electronics including an ozone concentration display 258, enabling the process to be controlled in dependence upon the measured ozone concentration in the vicinity of the food. The food chamber may form part of a conveyor on a food packaging line.

Particular advantages stem from incorporating the lamp and a photocatalyst, such as titanium dioxide, into a water purification system.

The lamp is placed in a water pipe with water to be treated travelling through the pipe. The rate of flow of the water to be treated is dictated by the quality of the water and the hydraulic design. However, the flow rate at which the water is treatable is directly proportional to the amount of germicidal UV produced by a UV source. The water is initially filtered and then exposed to the UV. light. The treatment of fine residual particulates is preferably achieved by combining the action of W with the oxidising and reducing actions of UV irradiated titanium dioxide. The emitted UV alone will directly kill microorganisms but the combination of IN and photocatalyst will remove other micro contaminants and increase the kill rate of microorganisms. For compact systems lamps having an output of at least IkW per single lamp are desirable.

Figure 14 (a) represents the interior of such a water pipe and shows the envelope 275 of the UV lamp which is axially mounted within the tube. The direction of water flow is indicated by arrow 277. TiO2 particles 279 are confined by upstream and downstream protective members 281,283. The pipe is filled with water 285.

Figure 13 shows the chemical reaction that occurs due to the photocatalyst.

Acting as a semiconductor when exposed to W, the titanium dioxide supplies an electron current consisting of electrons in the conduction band and holes in the valence band. The electrons reduce material and the holes oxidise material, hence providing a strong action for the decomposition of organic particulates and other water contaminants (pesticides, oil, oxide of sulphur and nitrogen etc.). The necessary electric field is provided by the microwaves.

In waste water processing the large organic particulates are removed by the use of filters. The availability of inexpensive UV sources allows a combination of UV and titanium dioxide photocatalyst to be used for the removal of fine organic particulates from the water, which would cause discoloration and odour if not removed. The reaction rate is destruction of 1. 0m mole/(m3kW) of organic particles. This process enhances the water quality and permits further recycling of the water to be obtained, lowering the processing cost.

A prototype of this technique has been successfully completed in the laboratory using a combined action of UV to kill E. Coli bacteria and a combination of W and titanium dioxide to oxidise and reduce fine particulate matter not captured by the filtration system. The 99.9% kill rate for E. Coli requires 66 J/m2 (see Figure 14 (b) ) and since the above described embodiment of the present invention produces 1000 J/m2 the target of 66J/m2 is achieved almost instantaneously. The reaction of particulates with titanium dioxide is slower. However, the kill rate for E. Coli was doubled in the presence of titanium dioxide for the same strength of LTV irradiation.

Existing water purification systems require the order of 5000W of germicidal UV light. Hence if conventional mercury discharge lamps are used then a cluster of at least 200 lamps with a length of 1 metre are required to purify drinking water at the rate of 400m3/hr (140 litres/sec), while with the microwave plasma system described above it requires no more than 5 lamps to achieve the same rate of purified water. This gives many benefits to the water system engineers for example less electrical cabling, easy access to lamps for cleaning, electrical connections only for the top end and these are not immersed in water.

Particular advantages stem from incorporating the lamp and/or ozone generator in a modified atmosphere packaging apparatus. Such apparatus provides an atmosphere other than ambient air inside a food package and can extend shelf life and improve the environment of the food while inhibiting bacterial growth. Modified atmosphere packaging is well known to those skilled in the food packaging art. The atmosphere used is selected to suit the particular food but typically contains some proportion of CO2, N2 and 0,. Non-permeable packaging films can be used to ensure that the modified atmosphere pack remains stable and sealed against entry of ambient air. Suitable micro- biocidal treatment of the food prior to packaging is called for and chlorine washing, a currently used technique, is expected to be discontinued for organic produce.

A modified atmosphere packaging apparatus utilizing the lamp is illustrated in highly schematic form in Figure 15 and has a set of food trays 300 which are moved from right to left on a conveyor. At filling stations 302 and 304 food is placed in the trays, which are subsequently passed beneath an ultraviolet lamp 306 of the type already described herein. The lamp is mounted above the conveyor and is similar to the lamp illustrated in Figure 1, having an elongate cylindrical envelope which extends transversely to the conveyor. A reflector 308 maximizes W light utilization. After passing beneath the lamp 306 in a guarded area 312 of the apparatus, a top web is applied to the trays from a roller 314 and then sealed onto them by a sealing unit 316. The process takes place under the required modified atmosphere. A photocatalyst may be used in combination with the UV lamp to aid the process and is particularly useful for packaging fruit and vegetables. The photocatalyst may be titanium dioxide.