||While the high efficacy and long life of the standard HPS lamp make it eminently suitable for exterior and industrial lighting, its relatively poor colour rendering excludes it from uses where better colour fidelity is required. Nevertheless in the 1980s it was not unusual to find ordinary HPS lamps in such applications as office or even shop window lighting, to achieve higher light levels at low cost. Naturally the results were quite unsatisfactory, and efforts began on improving the colour rendering properties of this light source.
The first such lamp from Philips was the SON-T Comfort in this 250W rating, launched around 1987. It employs the well-known principle that an increase in sodium vapour pressure results in the spectrum being further broadened, which leads to a higher colour rendering index. The vapour pressure of the sodium has been increased in this design by a combination of applying heat-reflectors of niobium around the arc tube in the area behind the electrodes, and reducing the arc length. Ordinarily such changes would result in a somewhat greenish colour light, so to bring the colour point back to the blackbody locus the mole fraction of sodium in the amalgam has been adjusted to about 80%.
High-loaded lamps of this type are especially susceptible to loss of sodium during life, which results in a change in the amalgam composition and the resulting light colour. To minimise this problem the diameter of the arc tube is larger than normal, and the electrodes are coated with a different emissive material which has a reduced rate of reaction with the sodium. To minimise efficacy loss, the xenon pressure in this lamp is higher than normal. Starting is facilitated with the aid of an auxiliary ignition antenna.