||Since the commercial introduction of the first high pressure sodium lamps in the mid 1960's, much effort has been devoted to improving the colour rendering properties of this light source. It was well known that increasing the sodium vapour pressure inside the arc tube led to improved colour rendering, but this also results in a loss of luminous efficacy.
Standard SON-Deluxe lamps have shorter arc tubes than normal and contain a greater concentration of mercury to ensure that the volt drop is the same across the shorter tube. The smaller arc tube, which has the same power loading naturally attains a higher operating temperature and hence increases sodium vapour pressure. But efficacy falls for two reasons - firstly because more light is generated at wavelengths to which the eye is not so sensitive, and also because light generation efficacy is greatest for a longer arc - just like with the low pressure sodium lamp.
In this experimental design Thorn was able to maintain the same arc gap as the standard high-efficacy 400W SON lamp, but also boost sodium vapour pressure by raising the wall temperature. A standard 400W arc tube is surrounded by a thin walled quartz tube, having a hot mirror interference film applied to its outer surface. The coating is largely transparent to visible light but is an excellent infra-red reflector and it served to reflect heat back onto the arc tube, which would otherwise be lost.
Unfortunately, due to the small light absorption of the coating it was not as efficient as might have been expected. The cost of the dichroic coated sleeve was also rather high and as a result production of this lamp could not be justified.