Guide to UV Measurement
Bulbs with Different Spectral Outputs
Additive materials such as metal halides can be included in the fill gas of medium-pressure UV lamps in small, precisely measured quantities. These additives, when ionized and driven with the plasma, emit their own characteristic wavelengths. In addition, short wavelength photons emitted by the mercury have sufficiently high energy so that when a photon-molecule collision occurs within the bulb, the additive material will re-emit at its characteristic wavelengths. The emission from the additive shows as an enrichment of the spectral output in longer UV wavelengths and is the basic principle of formulation of "special fill" bulbs.
Different bulb manufacturers use different designations for additive-type bulbs. From manufacturer to manufacturer, the internal pressure and concentration of additives will vary. For this reason, there is no such thing as a “standard” medium pressure bulb.
The most common types of additives are salts of iron or gallium. Medium-pressure mercury lamps are strong in the short wavelengths (240-320 nm). Because additive bulbs also contain some mercury, they emit some - but less - short wavelength energy. Iron-additive bulbs are stronger in the 350-400 nm range, while gallium-additive bulbs are rich in the 400-450 nm range.