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While the high efficacy and long life of the HPS lamp makes it eminently suitable for exterior and industrial lighting, its relatively poor colour rendering has excluded it from uses where better colour fidelity is required. The first notable improvement was not achieved until Westinghouse's 1978 introduction of its 250W Ceramalux-4 Deluxe Colour lamp. GE was swift to follow, launching its 250W Lucalox Deluxe in 1980. Whereas competitors quickly added 150W and 400W ratings, GE instead offered an unusually small 70W model in 1987. GE's Deluxe range was not completed until 1990 when 150W and 400W models were finally introduced.
These lamps employ the principle that the spectrum can be further broadened by operation at still higher sodium vapour pressure than in regular HPS lamps. An increase from about 10kPa to 45kPa enlarges the D-line self-reversal width Δλ from around 100Å to 250Å, and substantially more light is produced in the red and blue wings of the sodium spectrum. This boosts the CRI from 20 to 50-70, and CCT from 1900 to 2200K. Since the eye is relatively insensitive to the deep red and blue wavelengths, lamp efficacy is somewhat reduced.
Higher vapour pressure is achieved by shortening the gap behind the electrodes and applying heat reflectors to raise amalgam temperature. Arc tube diameter is also increased, leading to further self-absorption of resonance radiation and still better CRI. Ordinarily such changes would result in a greenish light, which has been avoided by a small increase in the mole fraction of mercury in the amalgam which leads to proportionally more red radiation. Owing to the increased sodium pressure, which reacts more vigorously with the arc tube seals, life is reduced to around 10-15,000 hours. |