||Mercury vapour lamps have traditionally been offered in two variants - ordinary versions in clear outer bulbs which are generally tubular in shape, and fluorescent versions having improved colour rendering which typically have ellipsoidal shaped coated outer bulbs. The large dimensions of the ellipsoidal types is necessary to reduce the temperature of the phosphor below what it would have to endure if applied to a tubular bulb envelope, and this leads to a considerable increase in efficacy and colour quality.
Of course the sodium lamp does not require any fluorescent coating and a clear tubular bulb is technically sufficient. However when installed in luminaires of the kind that had been developed for coated mercury lamps, the great decrease in the size of the optical source led to undesirable changes in the light distribution pattern from these luminaires, or a troublesome increase in glare. The HPS lamp was therefore also offered in two versions - clear tubular to replace the clear mercury lamps, and coated ellipsdoidal to replace the fluorescent mercury lamps. Of course the coating on the sodium ellipsoidal lamps is just a diffusing material having no fluorescent properties.
In the interests of standardisation it would be interesting if only one bulb shape could be used, preferably tubular, which has advantages in its reduced dimensions for packaging and distribution. Since the coatings used on HPS lamps are not temperature sensitive, they can equally well be applied to tubular bulbs and this experimental lamp was created by Philips in the mid 1980s. However owing to the fact that cost pressures on HPS lamps were not especially strong at that time, it was not launched for production.