Since the very beginnings of mercury lighting it had been realised that a significant portion of the energy radiated by all mercury discharges is in the ultra-violet part of the spectrum. In the case of the low pressure discharge, more than half of the total energy supplied is radiated in the short-wave UV region at 253.7nm. High pressure lamps radiate about 10% of their energy in the long-wave UV region at 365.0nm, but an appreciable amount is also radiated at shorter wavelengths. In general lighting applications this UV is wasted since it is invisible to human eyes. But it was known from the outset that certain materials called phosphors exist, which are capable of fluorescing under ultra-violet radiation and generating light of other colours.
In the mid 1930's such materials were put to use in the development of the fluorescent tube, based on converting the plentiful shortwave UV radiation from a low pressure mercury discharge into visible light. The phosphor coating was applied to the inside of the glass tube in which the mercury discharge took place.
However the phosphors developed for the fluorescent tube are not suitable for use with higher pressure mercury lamps. Firstly, they are tailored to respond to shortwave UV, whereas the high pressure discharge radiates a mixture of both short and longwave UV. But of more importance, high pressure mercury lamps run hotter than their low pressure counterparts - the bulb wall temperature is typically of the order of 150 to 250°C. Rather inconveniently, the phosphors known at the time had the awkward habit of greatly losing their efficiency at such high operating temperatures.
Thus it did not take long to draw up a set of requirements for new phosphors to be developed for the high pressure mercury lamp, viz:
High efficiency at the elevated bulb wall temperature
Chemical stability at the high temperature and under intense UV radiation
Sensitivity to both short and long wave UV from the arc tube
Light absorption to be as little as possible
Ideally, the phosphor should emit red light for colour-correction
Overall light generation should be white, not too far from the blackbody locus
In the early days, some phosphors were discovered which could match one or two of the above criteria, but not all of them! Thus mercury fluorescent lamps could be had in two main types - colour-corrected types where the phosphor contributed a small amount of red light to the spectrum, or high efficacy types where the phosphor contributed a much larger quantity of blue or green light. It was not until the late 1960's that the problem was satisfactorily solved and a new phosphor satisfying all of the above requirements was developed. The following pages in this section chart the development of the five principal families of phosphors which have been employed in mercury lamps.
Basic Phosphor Science
Before considering each phosphor in detail, it will be helpful to introduce some of the basic principles governing their properties and performance. One of the most fundamental laws which applies to nearly all luminescent solids, is that the wavelength of the emitted radiation is always longer than that of the absorbed radiation. For instance, short-wave UV can be converted into light of any colour, since the wavelength of each colour of light is much longer than UV. Short-wave blue light can similarly be down-converted into longer wavelength red light. But long-wave red light cannot be converted back up to short-wave blue light (except under exceptional circumstances). This general principle which virtually all phosphors follow is called Stokes' Law.
Incidentally when the names of the various phosphors are introduced throughout this section, it will be noticed that they nearly always consist of the host material plus a so-called activator. The activator is generally a minute trace of impurity and curiously, it is generally the activator which determines the wavelength of light emission from each phosphor. When the names are the phosphors are written it is standard practice to write the chemical symbol of the activator after the formula of the host material. For instance, zinc sulphide activated with manganese is written as ZnS : Mn.