Some Halophosphates Phosphors
The Halophosphate phosphor is defined as a phosphor with the apatite mineral structure. Almost all commonly found tubes on the global lighting market employ an internal coating of calcium halophosphate materials (generally known simply as ‘Halophosphate’
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Some Halophosphates Phosphors
6.1 Introduction The halophosphate phosphor is defined as a phosphor with the apatite mineral structure. Almost all commonly found tubes on the global lighting market employ an internal coating of calcium halophosphate materials (generally known simply as ‘Halophosphate’ tubes). This revolutionary material was invented in 1942 by a group led by A. H. McKeag of Osram-GEC in London, and succeeded in almost doubling lamp efficiency. This breakthrough was responsible for propelling the fluorescent business into the vast market. However, by modern standards, halophosphate materials are relatively inefficient and deliver inferior lighting quality compared to newer technologies of fluorescent phosphors. Although fluorescent tubes have low initial purchase cost, this was rapidly offset by the increased electrical power consumption required to generate a given amount of light. Owing to their lesser energy efficiency, halophosphate tubes are being phased out and will shortly be replaced by other more efficient fluorescent phosphor materials. These phosphors are blends of two different materials which radiate broadly in the blue and orange parts of the spectrum, respectively. By changing the ratio of the two components a full range of warm to cool white hues can be achieved. Halophosphate phosphors generally have the formula such as, Ca5 (PO4 )3 (F,Cl). The halophosphate materials may contain various activator ions which impart the phosphor property. For example, a europium (Eu)-activated halophosphate phosphor absorbs ultraviolet (UV) emission (i.e., exciting radiation) from the mercury plasma in a fluorescent lamp and emits blue–green visible light. The phosphor efficacy and lumen maintenance of the halophosphate phosphor was improved by adding cadmium to the phosphor. The addition of a few percent of cadmium to the halophosphate phosphor induced a strong absorption of the 185 nm damaging component of the mercury plasma, which reduced the intensity of this component of the plasma. Consequently, the density of the color centers created in the phosphor was reduced by adding cadmium to the phosphor. The decrease in the density of color centers in the phosphor increased the efficacy and lumen maintenance of the K. N. Shinde et al., Phosphate Phosphors for Solid-State Lighting, Springer Series in Materials Science 174, DOI: 10.1007/978-3-642-34312-4_6, © Springer-Verlag Berlin Heidelberg 2012
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6 Some Halophosphates Phosphors
phosphor. However, the use of cadmium was later eliminated in phosphors manufactured in the United States and Japan for public health reasons. Therefore, it was desired to obtain a halophosphate phosphor with an improved efficacy and lumen maintenance, preferably without adding cadmium to the phosphor. Europiumactivated strontium chlorophosphate, i.e., Sr 10 (PO4 )6 Cl2 :Eu2+ is probably the first useful narrow band blue emitting phosphor. The emission is a rather narrow band at 450 nm and it can be used in the triband lamp. Blue emitting phosphors that are commonly
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