It is well known to generate ultraviolet light using a microwave energisable light source. Such light sources are described, for example, in GB-A-2336240 and typically comprise an ultraviolet-transparent envelope (typically formed from quartz) which contains a pressurised gas fill (typically of mercury and a noble gas such as argon) which when energised at microwave frequencies emits light through the envelope walls from the plasma gas fill.
As has been noted in the prior art mentioned above (and the prior art discussed in the introduction thereto) there are two significant problems which must be overcome in order to make practical use of such microwave energisable lamps.
The first of these problems is that of microwave leakage. Generally speaking, microwave radiation is hazardous and therefore it is necessary to ensure that the microwave energy used to energise the bulb is contained. This, however, is usually in direct conflict with the need to allow radiation of the ultraviolet energy.
The second problem is that of even illumination of the quartz envelope. This is, particularly important for adhesive and paint curing applications in which is undesirable to over or under expose adjacent portions of the paint or adhesive. It may also be critical in germicidal applications although in practice, over exposure of articles to ultraviolet radiation for germicidal applications is not as critical as it is for curing applications.
The problem of even illumination becomes particularly acute when it is desired to illuminate over a large area. For example for areas having a minimum dimension of 150 mm or more.