||For as long as mercury lamps have been available, they have been recognised as being valuable sources of ultra-violet as well as visible radiation. By employing outer bulbs of various different glasses, particular spectral regions of interest can be isolated for transmission. The Purple-X is the first longwave type, introduced by GE in 1941.
This lamp is the type known as the Blacklight Mercury, having an outer bulb fabricated from Woods Glass. That is a soft glass heavily doped with nickel oxide, which renders it almost completely opaque to the visible wavelengths of the electromagnetic spectrum, but it has a high transmission of both ultra-violet and infra-red rays. The only visible wavelengths transmitted are those close to each end of the visible range, i.e. the violet and deep red. The usual iron oxide impurities found in soft glasses prevent the transmission of ultra-violet below about 320nm, so these lamps do not emit a significant amount of dangerous shortwave UV, and are completely safe to observe when lit provided the time of exposure is kept within reason. Vision may appear blurred but this is not harmful, it is simply due to fluorescence of the eye itself which can make focussing difficult. The UV output is almost monochromatic and centred around the 365.5nm line.
Blacklight has a broad range of applications, nearly all of which involve its employment to stimulate fluorescence. Some of these include mineralogy, the detection of forged documents and bank notes, identification of materials which have been marked with UV-fluorescing ink, observation of UV markings on stamps and posted letters, the detection of invisible stains on fabrics, entertainment lighting, and a host of other applications that is far too long to reproduce here.