||The poor colour rendering of HPS lamps has been improved in the "Deluxe" family, invented by Westinghouse in 1978. GE followed in 1980, Osram-GEC in 1981, and Thorn in 1982. Thorn's range began with 150W & 250W diffuse ovoid lamps, being joined by 400W and tubular models in 1983.
Deluxe HPS lamps feature a broader spectrum, thanks to operation at still higher sodium vapour pressures. An increase from about 10kPa to 45kPa widens the D-line self-reversal Δλ from around 100Å to 250Å, and more light is produced in the red and blue wings of the spectrum. This boosts the CRI from ~25 to 50-70 and CCT from 1900 to 2200K, but at the expense of efficacy since the eye is relatively insensitive to the deep red and blue wavelengths.
Sodium pressure is increased by shortening the gap behind the electrodes and applying niobium heat screens around the ends, to raise amalgam temperature. Owing to the very high loading, these are graded in steps of 0.1mm length and matched to individual arc tubes. CRI is further increased by using wider arc tubes, which amplifies the self-absorption of resonance radiation. The increased diameter requires a shorter arc to maintain optimum thermal balance. Ordinarily such changes would result in a greenish light, and this is counteracted by increasing the sodium ratio in the amalgam. At such high sodium pressures the chemical reactions in the arc tube are accelerated, which leads to more rapid voltage rise and lamp life is approximately halved. This arc tube has a 3-piece monolithic construction, with additional ceramic buttons to increase the seal length. It is of Thorn's most recent design, in which the diameter of the niobium tubes has been reduced to just 1.75mm to increase seal reliability.