Radiators that are meant to produce as final outcome ultraviolet radiation as a result of an electrical discharge, generated between two electrodes, in a low- or high-pressure mercury, that is contained in a transparent bulb or tube.
Many different ultraviolet radiators are produced for industrial applications such as photochemical processes, drying and hardening of materials, lithography, and reprography; for disinfection purposes and insect repellents; and for phototherapy and sun tanning. In these applications, visual effects play no role and are therefore in this encyclopedia not dealt with. Another type of ultraviolet radiators is produced to make fluorescent substances in materials directly visible to the eye. These types are dealt with here. They are usually called black-light lamps and used for artistic and decorative reasons (especially in theaters, discos, and bars) and for making fluorescent dyes visible in bank notes, stamps, and documents to detect counterfeits and for making fluorescent bacterial infections visible for medical purposes.
Since they are not used for general lighting purposes, only the fundamentals of their working principle and of their properties are described in this encyclopedia.
Working Principle and Properties of Black-Light Lamps
Low-pressure mercury lamps (both in tubular and compact shape)
High-pressure mercury lamps
The only difference compared with the lamp executions that produce visible light is the type of glass and the type of fluorescent powder used. As discussed in the relevant lamp chapters, these lamps produce ultraviolet radiation as result of the discharge between two electrodes. The glass of the discharge tube exists either of Wood’s glass that transmits ultraviolet and infrared radiation but blocks visible radiation (with the exception of purple around 400 nm and deep red around 800 nm) or of normal glass coated with a filter that transmits ultraviolet and blocks visible radiation. The final result is a weak, purple-bluish glowing lamp that emits UV-A radiation with a spectrum depending on the type of fluorescent powder used on the interior of the discharge tube. Either barium disilicate (industrial designation “BSP”) with a resulting peak wavelength at 349 nm or strontium tetraborate (industrial designation “SBE”) with a peak wavelength at 368 nm is used. The high-pressure mercury type of black-light lamp exists in fluorescent-coated (SBE) and clear versions. The fluorescent version has a higher radiant flux in UV-A, while the clear version has a more concentrated light-emitting area making it more suitable for use in reflector type of luminaires. The high-pressure mercury black-light lamps are far more compact than the low-pressure mercury lamps when comparing a same ultraviolet radiation output. They can therefore be used in floodlight type of luminaires as used for theatrical and concert performances. They are also available in higher output versions.
The low-pressure mercury versions are available in the wattage range from 4 to 36 W with UV-A radiation output power between some 1 and 10 W. The high-pressure versions are available in the wattage range from 100 to 1000 W with UV-A radiation output between some 50 and 500 W.