Early-1980s Philips SON-T 15W

The SON-T 15W shown here is certainly one of the smallest standard high-pressure sodium lamp ever built. This peculiar model was developed for security lighting applications, specifically in areas were the 1,800 lm output from the 18 W SOX is deemed too high. While smaller SOX lamps were developed and tested in the early 1980s at Philips and at Thorn for that specific purpose, the Dutch investigated also the possibility of applying the HPS technology to that problem as it delivers a better light color quality in a smaller lamp package.

The miniature 15 W SON lamp is one result of such research. The lamp is built around a low-voltage sodium burner with a 2-cm electrode gap length for an operation at 54 V on the standard series-choke 20 W fluorescent lamp ballast (i.e., the same impedance as that used for the SOX 18W). In order to limit heat losses and ensure an optimum operating temperature, the discharge tube is placed inside a quartz sleeve coated with an IR-reflecting dichroic mirror. End losses in the burner are reduced further with the use of single-wire feedthroughs instead of the usual tube seals, a design that Philips began using at the end of the 1970s in some of its experimental ceramic discharge lamps. Finally, lamp ignition is ensured by a glow-switch starter connected in parallel with the burner’s terminals. There is no need for a switch to disconnect the glow bottle during operating because it cannot be energized due to the low operating voltage of the arctube.

While this creative lamp concept works and produces about half the light output of the 18W SOX (i.e. 850 lm), it has several critical shortcomings, such as a deep orange light color that is not too dissimilar from that of SOX lamps. This arises from a discharge power load of 6.8 W/cm only, which is significantly lower than that of standard high-pressure sodium lamps and results in a particularly low arc temperature. The other issue is a lumen efficacy limited to a meager 55 lm/W (31 lm/W at system level!), caused in part by optical absorption losses in the coated quartz sleeve. Such performance is way too low compared to that of the compact fluorescents lamps in the same lumen class that were already on the market (i.e., the integrated Philips SL 18W, 50 lm/W) or were about to be introduced (i.e., the non-integrated Philips PL-S 11W, 82 lm/W). As a result, the SON-T 15W concept was never developed into a commercial product.