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The orthophosphates were the last family of phosphors to be introduced before the present material yttrium vanadate came into service. They are all activated with tin, which must be in the divalent state to give luminescence.
All tin-activated phosphors are characterised by a their very broad spectral energy emission. Typically they cannot achieve such a deep red emission as is possible from previously mentioned materials, but their quantum efficiency is remarkably high, as much as 87% for magnesium orthophosphate. They are colourless so the light transmission is high, and because the emission is at wavelengths to which the human eye is more sensitive, lamps with remarkably high luminous efficacy can be fabricated. They do, however, leave a lot to be desired in terms of their colour rendering properties, but from the early 1960's until the late 1980's many manufacturers offered two different kinds of mercury lamp - types with good colour rendering, and types with high efficacy at the expense of colour properties.
Strontium orthophosphate, (Sr,Mg)3(PO4)2:Sn2+ modified with Mg to adjust the crystal lattice structure was the most commonly employed material in high efficacy lamps. Its emission peak lies at 630nm at room temperature, decreasing to shorter wavelengths quite dramatically at lamp operating temperatures but the light emission remains high. By 330°C the red emission peak has shifted to 570nm so it is clearly evident that colour rendering of these lamps suffers. While a magnesium germanate 400W lamp gives a red ratio of about 7.5%, the strontium orthophosphate lamps only manage about 5%.
Similar performance was attained from calcium orthophosphate modified with Mg and this material found preference with some of the eastern European manufacturers. It has slightly higher efficiency again, but red ratio falls further to 4.4%, really no better than the first MAF type lamps.
Meanwhile Sylvania in the USA developed materials based on calcium-zinc orthophosphates, and a particular 400W lamp attained an efficacy of 59.5lm/W. This was excellent by comparison with the 49lm/W attainable from red-emitting phosphors at the time, and is still impressive compared with the 55lm/W of present-day lamps with vanadate based coatings.
When Vanadate coatings became available in the late 1960's though, they entirely superseded the high-efficacy line of mercury lamps in all countries except the USA, where a demand curiously continued for the old-fashioned inefficient coatings for nearly twenty years more. The vanadate phosphors offered 10% higher luminous efficacy and considerably enhanced colour rendering properties as well while also being pure white in colour, for the first time satisfying all of the requirements of a mercury lamp phosphor.
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