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Unlike other phosphors employed in mercury lamps, the barium-strontium-lithium silicate material developed by Osram-GEC in England fluoresces with a range of colours comprising red, orange and blue along with intermediate shades of these colours. It consists of a mixture of barium lithium silicate and strontium lithium silicate and by varying the proportions of each, a continuous series of solid solutions can be formed with a wide variety of luminescent properties. It functions in much the same way as the halophosphate group of materials which were also invented by the GEC for use in fluorescent tubes.
A double activator is employed in this group. Trivalent cerium is the primary activator and this leads to the blue emission, but also acts to sensitise the secondary activator, manganese. As the manganese content is increased the intensity of the blue emission falls in favour of increased orange-red emission that is characteristic of manganese.
If just the barium lithium silicate component is first considered, the manganese emission is rich orange, but this can be made significantly redder as strontium atoms replace barium. This is an important addition because when raised to the lamp operating temperature the emission shifts towards the yellow, but this can of course be minimised by starting with a higher strontium concentration.
Unlike the germanate and arsenate phosphors already described, the silicate materials are almost completely snowy-white in body colour and consequently their light absorption is low, and lamp efficacy can be quite high. There is a UV absorption peak at 360nm, conveniently very close to the 365nm radiation of the mercury discharge, and this diminishes almost entirely to zero by 420nm, hence the very small blue absorption. The colour temperature of lamps made with the optimum material is typically around 4400K, the chromaticity co-ordinates being x=0.36 and y=0.38. It is not so far shifted towards the green as for germanate and arsenate phosphors but its colour temperature is slightly higher on account of the blue emission of the phosphor.
Application of Silicate Phosphors to the Bulb
Sulphide, Germanata and Arsenate phosphors are traditionally applied to the glass bulb as a suspension in a nitrocellulose binder, as used to be standard practice for coating tubular fluorescent lamps. Subsequently the coated bulbs are baked in air at 500°C to burn out the binder, but the required bake-out temperature is too high for the silicate phosphors. The cerium activator is oxidised in air at such high temperatures with most undesirable effects on the efficiency of the phosphor.
A technique was therefore developed which allowed the lamp manufacturer to spray or dust the dry powder onto the inner surface of a bulb which had previously been wetted a film of dilute phosphoric acid. Following a reduced temperature bake-out at 300°C in an inert atmosphere the coating was then dried out and adhered to the bulb wall without degradation of the cerium activator.
When any phosphor is applied to the bulb it is desirable that the coating should be sufficently thick that it absorbs all of the UV for conversion into visible light. With germanate and arsenate coatings this is not possible, because the blue absorption then becomes so high that relatively more of the mercury green line is transmitted and the lamp imparts an unpleasant greenish hue to all that it illuminates. Coating weight therefore has to be a compromise at about 3mg per cm2 at which there is still some blue absorption. With the silicate phosphors this same coating weight is sufficient to absorb all of the UV and there is no significant blue absorption. This is one of the primary advantages of the silicate phosphor - it may not be quite so efficient, but it allows lamps to be made which have a more natural white light emission.
Just like with Philips' magnesium arsenate lamps, it was found that the red ratio of silicate lamps depreciated rather rapidly, but that once again this could be eliminated with a carbon dioxide gas filling. The reason for this was never determined. In the Philips lamps it works by providing a tiny partial pressure of oxygen which prevents the tetravalent manganese activator being reduced to the divalent species. However since the manganese activator starts out as divalent with the GEC silicate phosphor, the mechanism must be different. Furthermore an oxidising atmosphere is known to be detrimental to the primary activator of cerium and yet this does not appear to be degraded as a result of the CO2 bulb filling.
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