||The original Blacklight lamps were based on the high pressure mercury discharge, and employ an outer bulb fabricated of Woods glass. This unusual black-coloured glass is doped with the oxides of nickel and cobalt, and has a relatively narrow transmission band in the UV-A spectrum. This allows passage of longwave UV radiation which causes fluorescence effects in many materials, while absorbing shorter wavelength and more dangerous UV-B and UV-C rays. Of great importance is that it also absorbs practically all visible light. That ensures that the fluorescence effects are not diminshed by an excess of visble glare from the lamp.
During the mid to late 1950s, more convenient and lower power blacklight lamps were developed based on the low pressure mercury fluorescent lamp. The first to be commercialised by Philips was a 4-foot 40W tube, being joined soon after by this 2-foot 20W model.
The construction is similar to Philips ordinary fluorescent lamps of the era, with the envelope material being changed to Woods glass. The lamp is notable for its great weight, the glass tubing presumably having been hand-drawn and having greater wall thickness than usual. Since the low pressure mercury discharge radiates primarily short-wavelength UV-C radiation, a fluorescent coating is employed to convert this to longer wavelength UV-A. It is believed that this early lamp employs barium disilicate activated by lead, emitting at about 355nm. Owing to the shape of the transmission curve of the glass, the peak emission of the complete lamp is around 365nm. It is dimensionally and electrically compatible with ordinary fluorescent lamps.