Mid-1980s Philips SOXC 22-26W

The 1980s was a very creative decade at Philips on the low-pressure sodium lamp front. Following the introduction in 1982 of the SOX-E family of highly efficient lamps, the race was on to develop the next step in the technology. Research activities focused on two main areas: efficiency and size. It was felt that sodium lamps could be made in more compact form factors and with higher lumen efficacies leading, among other things, to the famous ELS, the first and only electrodeless SOX lamp. There were several projects running in parallel and whose main goal was to improve the design of low-pressure sodium lamps by applying innovations in lamp making techniques that were developed for other light sources.

The 1980s was also the decade compact fluorescent lamps came into being and ingenious methods were developed to fold fluorescent tubes into very small form factors. This approach to making compact discharge lamps was naturally applied to the SOX platform, resulting in a wide variety of experimental sodium lamps with unusual burner shapes. A key innovation in glass tube assembly which led to the smallest CFL configurations possible is the cut-and-kiss method, also known as "hot kiss", developed at Philips in the early 1980s. This technique makes the formation of a short bridge between closely-spaced glass tube sections possible, a method which led to the mass production of Philips's groundbreaking pi-light (PL) lamps. Naturally, this technique was applied with great difficulty (different glass type) to make a low-pressure sodium version of the PL, resulting in the highly unusual SOXC shown here. Folding the discharge in such a compact form factor allows a increase in power dissipation of up to 44 %, depending on which ballast is used, without changing the overall size of the lamp (the basic platform is that of a SOX 18W here). Moreover, the presence of cold spots at the extremities of all four tube sections makes the use of sodium-retaining dimples redundant.

Not everything was positive with this new SOX design however. The first and potentially most serious problem was that the closure and the bridging of the glass tubes certainly caused a significant degradation of the thin internal borate glass layer that protects the burner from the hot sodium vapor. The consequence of such degradation is a severe reduction of the lamp's service life, combined with a rapid reduction of the light output. The other issue with such a lamp configuration is that a significant amount of the resonant light emitted by the sodium discharge is re-absorbed by adjacent burner limbs, which has a negative impact on the lumen efficacy. These issues, combined with the more difficult handling of the two-ply glass tubes used in the production of SOX lamps (borate is hygroscopic) led to the decision to abandon such a design approach for low-pressure sodium light sources.