A model is presented which predicts HPS lamp efficacy to within about 5 1m W-1 over a wide span of design space. When the input wattage is fixed and the cold-spot temperature of standard colour HPS arc tubes is varied, a universal curve of normalised lamp efficacy versus relative electric field is followed. The curve peaks where the spectral power distribution is optimum for luminous efficacy. Published lumen ratings and physical measurements were used to determine an arc thermal conduction loss of 6.6 ± 0.5 W cm -1 for commercially available products.
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