The present invention relates to a UV discharge lamp with improved operating life, particularly the utilisation of two dissimilar pairs of electrodes in a medium pressure UV discharge lamp, operable over an extended power range. Background to the Invention

A medium pressure (MP) UV discharge lamps' operation relies on an electrical discharge (or arc) forming between two electrodes supported in a fused silica (quartz) envelope. This envelope contains the chemicals that are required for initiating/sustaining the arc and also for producing the desired wavelength of UV radiation. The electrical discharge is initiated by applying electrical energy to the electrodes such that it causes them to be of opposite polarity, resulting in the gases inside the envelope being ionised. During this process electrons emitted from the tip of one electrode flow across the lamp to reach the opposing electrode.

MP lamps typically utilise electrodes manufactured from Tungsten which is a refractory metal suitably resistant to wear and temperature, since it has the highest vaporisation temperature (boiling point) of all the elements. Nevertheless, the constant high energy electron emission occurring in MP lamps removes material from the emitting surface in a process called 'sputtering'.

This sputtering effect is greatly affected by the electrode operating temperature. The increase in electrode temperature predominantly comes from the conversion of electrical energy in the gas discharge and opposition to the electron flow as it passes through the electrode (electrical resistance), both of which are directly affected by the amount of electrical current flowing through the lamp.

Generally, the end of life failure indication for MP lamps is that sputtered tungsten becomes deposited on the inside of the lamp envelope. This directly blocks the required radiation from leaving the lamp (and hence is no use to the external process the lamp is required for) and also chemically combines with other material essential for the efficient production of UV wavelengths. Tungsten deposits also lead to a higher absorption of radiated energy in these areas, increasing the envelope wall temperature and accelerating other failure modes. From the foregoing it can be seen that optimising electrode operation is essential in achieving an acceptable lamp life. Operators of MP lamps are primarily interested in the maximum power output of the system in order to achieve the necessary process use requirements. In broad terms the lamp output power is the product of the operating voltage and current available. Therefore, to achieve the desired specific lamp power, the lamp designer has to know the current available from the power supply to be used and then designs the lamp operating voltage accordingly. It is impractical for electrodes to be designed for individual operating currents, so they tend to be operable over a range of currents. It is down to the knowledge of the designer to determine the correct specification.

It is common practice for designers of MP lamps to select an electrode based on the maximum lamp current because, otherwise, overheating will occur, therefore it operates closer to the vaporisation temperature where the amount of material sputtered from the electrode is greatly increased. However, if a high current electrode is selected and the operating current is too low then the electrode is operated in an overcooled state. This changes the mechanisms for the emission of electrons and, again, results in more electrode material being sputtered into the lamp. Deviations in lamp current from a nominal value in both directions can therefore cause blackening of the lamp envelope resulting in a reduction of UV transmission through to the process.

MP UV lamps are being increasingly operated using electronic power supplies which usually have a lower open circuit voltage than traditional wire wound power supplies. The lower open circuit voltage results in increased running current to achieve a similar lamp power density. The increased operating current typically means that electrode sizes have had to be increased to stay within acceptable electrode running temperatures to ensure good lamp life.

Discharge lamps do not have the ability to be turned off and on quickly due to difficulty in reinitiating the discharge whist the lamp is hot from previous operation. Therefore, it is common practice to run lamps in a 'standby' setting achieved by reducing the current supplied from the power supply. This allows the intensity of the lamp to be changed rapidly to accommodate the process without the issue of restarting a hot lamp.

As system designers strive to use less total energy there is a tendency to reduce the 'standby' power of the lamp by significant amounts, for example in some cases as low as 20% of the maximum lamp current. This has the benefit of effectively reducing the duty cycle of the total system resulting in less energy to be consumed overall. However, the reduced 'standby' power results in the electrodes within the MP lamp electrode being run in a cold state. This practice will shorten the lamps useful life, depending on the amount the current is reduced by and the operating time at this level.

Despite best efforts, the general degradation of UV (and visible) output from a lamp is shown in Figure 1. A steady decline of percentage output (100% being unimpeded output) can be seen over time, which becomes significant after several thousand hours of operation.

Summary of the Invention The present invention seeks to address problems observed in the prior art, for the purposes of improving MP lamp operating life.

In broad terms according to the invention a medium pressure lamp fitted with four electrodes, instead of the usual two, can be operated in a way that prolongs the life of the lamp. The invention is broadly defined according to the appended claims. Particularly, one pair of electrodes is selected to run at the maximum operating current as determined by the UV system and power supply (hereinafter referred to as "running electrodes") while another pair of electrodes is selected according to the smallest operating current specified for the standby setting (hereinafter referred to as "standby electrodes"). Each end of the MP lamp requires one of the pair of running electrodes and standby electrodes respectively. These two electrodes are electrically isolated from each other by a dedicated connection through the lamp seal.

The advantage of the invention is that each pair of electrodes will be subjected to a smaller current variation, keeping within a more comfortable operating window for respective designs. Accordingly, the invention will result in each electrode pair being run closer to their optimum temperature, reducing the sputtering effect and therefore extending the useful operating life.

In preferred embodiments the medium pressure lamp has an operating internal pressure that is greater than or equal to 2Bar. In preferred embodiments the medium pressure lamp has an operating internal pressure that is less than or equal to 9Bar, and preferably less than or equal to 8Bar.

In preferred embodiments the maximum operating current for the first electrode pair is less than or equal to 30Amps. In preferred embodiments the maximum operating current for the first electrode pair is greater than or equal to 3Amps.

A typical operating range for the first electrode pair is 50% to 100% of the maximum operating current for the first electrode pair. It has been found that operating the second electrode pair in this range helps to address the sputtering problem.

In preferred embodiments the maximum operating current for the second electrode pair is less than or equal to 15Amps. In preferred embodiments the maximum operating current for the second electrode pair is greater than or equal to 1.5Amps.

A typical operating range for the second electrode pair is 20% to 50% of the maximum operating current for the first electrode pair. It has been found that operating the second electrode pair in this range helps to address the sputtering problem. In preferred embodiments the first electrode pair and second electrode pair are located in parallel through the wall of the sealed chamber, each with electrically isolated terminals for electrical connection exposed outside of the chamber.

According to another aspect of the invention there is provided a UV lamp system including a UV lamp according to any configuration described herein, including a switch means for activating/energising the first and second electrode pairs independently and/or alternatively as needed. The switch means can be controlled by a control means.

The control means can be arranged to cause the switch means to activate the first electrode pair when the lamp is operating above a defined threshold current and activate the second electrode pair when operating below the defined threshold current. The defined threshold current is typically 40% - 60% of the maximum operating current rating of the first electrode pair, and preferably is around 50% of the maximum operating current rating of the first electrode pair.

According to another aspect of the invention there is provided a method for curing a UV curable material, including subjecting the material to UV light emitted from a UV lamp according to any configuration described herein, or a UV lamp system according to any configuration described herein.

According to another aspect of the invention there is provided a method for sterilizing material, including subjecting the material to UV light emitted from a UV lamp according to any configuration described herein, or a UV lamp system according to any configuration described herein.

Brief Description of the Drawings

Figure 1 shows a graphical representation of UV degradation throughout service life as observed in the prior art;

Figure 2 illustrates an end of a UV lamp made according to the invention; and Figure 3 shows a UV lamp system according to the invention, including a control system for controlling operation of the UV lamp.

Detailed Description of a Preferred Embodiment of the Invention

Figure 2 illustrates a section view of an end of a UV lamp 10 with an envelope wall 11 forming a sealed chamber 12 containing gas in the known way. The wall 11 is preferably formed from fused silica, although any suitable material could be contemplated. According to the invention, electrodes 13 and 14 protrude through a distal end of the sealed chamber 12, set into the silica material. The larger electrode 13 corresponds to a running electrode that is preferably selected to run at the maximum operating current as determined by the UV system and power supply. The maximum operating current for the larger electrode 13, is typically in the range 3 to 30A. The smaller electrode 14 corresponds to a standby electrode chosen according to the smallest operating current specified for the standby setting. It will be apparent to a skilled person that the electrodes 13/14 are paired with a matching electrode located at an opposite end of the lamp 10. This would appear as a mirror image of the view illustrated by Figure 2. The construction of an electrode is generally known in the art and it will be apparent that the "business ends" 15/16 of the respective running and standby electrodes 13/14 are located within chamber 12 where an arc can be formed across the lamp, toward a second end (not seen in Figure 1). The electrodes are electrically connected to an external device mounting (not illustrated) via terminals 17/18 respectively. The mounting is part of an apparatus including a control system for operating the lamp. In practice there will likely also be a ceramic cover protecting each end of the lamp to assist mounting in the UV control device/power supply.

The control system 20 includes an electronic controller 22 and a switch device 24 (see Figure 3). The electronic controller 22 controls operation of the switch device 24, and causes the switch device 24 to switch between operating the running electrodes 13 and standby electrodes 14.

According to practical implementation of the invention it is likely that the running electrode pair 13 would be activated by the controller 22 at operating currents above a certain threshold, for example when the current rises above 50% of the designed tolerance. In a preferred arrangement running electrode pair 13 is activated when the current rises above 50% of the maximum operating current for the running electrode pair 13. Thus the running electrode pair 13 operates in the range 50% to 100% of maximum operating current for the running electrode pair 13. The standby electrode pair 14 is activated below this threshold. In the preferred arrangement mentioned above the standby electrode pair operates when the operating current is below 50% of the maximum operating current for the running electrode pair 13. Typically the standby electrode pair 14 operates in the range 20% to 50% of the maximum operating current of the running electrode pair 13. Alternative configurations are possible following experimental testing and optimisation of the inventive concept.

The invention has means, within the control system 20, to switch the current flow between the respective pairs of electrodes, in line with the lamp operating current required at that time. The benefit of such an arrangement is that each pair of electrodes will be subjected to a smaller current variation, keeping within an optimum operating window for respective designs. Accordingly, the invention will result in each electrode pair being run closer to their optimum temperature, reducing the sputtering effect and therefore extending the useful operating life. The inventors have determined that by providing a lamp having standby electrodes 14, the lamp is operable at relatively low power (down to around 20% of the maximum operating current) without causing unacceptably accelerated lamp deterioration. It should be noted that a conventional UV lamp without standby electrodes according to the invention typically has to operate at at least 50% of the maximum current or unacceptable lamp deterioration occurs.

The advantageous aspects of the invention are particularly important when fitted into a UV lamp system that has very low standby power settings or when the process requirement dictates longer periods of time with the lamp being run in a standby mode.

It will be apparent that a UV lamp according to the invention can be constructed from available materials, e.g. Tungsten electrodes and fused silica chamber, and manufacturing techniques, so long as the provision is made for a second pair of electrodes suitable for standby mode use. As described above, the invention is embodied by a UV lamp and/or system comprising same, where a running electrode is configured to run at a relatively high operating current, compared to a standby electrode having a low operating current. The running electrode is activated to form an arc and ionise gas within the chamber in the known way in order to create a source of UV wavelength emissions for use in an industrial process. The standby electrode is activated (and the running electrode deactivated) during a standby mode in order to maintain a "ready-to-use" state. In practice the service life of the lamp is extended because each electrode pair is run closer to its optimum temperature, reducing the sputtering effect detrimental to operation. In preferred embodiments the medium pressure lamp has an operating internal pressure that is, typically, greater than or equal to 2Bar and less than or equal to 9Bar. In some embodiments, the operating internal pressure is less than or equal to 8Bar. In other embodiments the operating internal pressure is less than or equal to 7Bar. When manufactured, a medium pressure lamp has a non-operating internal pressure that is less than atmospheric pressure. For example, a medium pressure lamp may have a non-operating internal pressure that is less than 300mBar, preferably less than 200mBar and more preferably still less than lOOmBar.

The UV lamp and system according to the invention is particularly suited for use in a UV curing process or a UV sterilization process.