Collector Spotlight - Peter Van den Bossche
About the Author: Peter Van den Bossche
Peter is an ardent sox fan! No, not the famous Chicago White Sox. He'd rather go for the Cubs - but SOX (low pressure sodium) lighting.
Build Used in This Experiment
1. A threatened species
SOX is the trade name of low pressure sodium lamps by Philips, but this brand has diluted into a generic name for such lamps. The lighting division of Philips (which now calls itself Signify while still using the well-trusted Philips brand name) has ceased production of these light sources however and has closed its sox factory in Hamilton, Scotland. Demand for SOX in fact declined facing the competition by LED¹.
SOX has become a threatened species, being replaced by LED on a large scale. The wholesale conversion to LED often takes place before the end of the projected lifespan of SOX fixtures (thus creating a waste of resources), and this LED craze is in many cases to be considered a mere fashion statement and a greenwashing. Purchase policies in public lighting circles are often politically motivated, supported by a virulent antisoxism where LED is hailed as the ultimate lighting solution, having none of the perceived drawbacks of SOX. Many typical arguments against SOX can however be rebutted:
Energy Consumption
At up to over 200 lm/W, SOX is still leading in luminous efficacy. The highest efficacies are obtained with the larger SOX lamps, particularly the SOX-E version, used with electronic ballast. The energy efficiencies of LED are gradually improving, reaching the level of the smaller SOX bulbs but few of them match the efficacy of a full-size sox.
Color Rendition
The narrowband spectrum and its associated color rendition is often cited as the main disadvantage of SOX. However, the low-light conditions typically encountered in nighttime public lighting promote the use of scotopic vision where the rendition of colors is of a lesser priority anyway. Furthermore, roadlighting with LED typically comes in a bright white (4000 K) hue which may have a good color index of 80 but which creates a harsh and artificial atmosphere at night. The choice for white light is often supported by local politics, lighting being one of the few domains today where one can express a preference for whiteness without being called politically incorrect or worse².
The bichromatic³ SOX on the other hand is uniquely atmospheric, promotes road safety through enhanced contrast observation undistracted by colors, and causes much less light pollution disturbing flora and fauna. The effects of such light pollution on insects has been extensively studied in recent scientific publications, highlighting the benefits of narrowband spectrum light such as offered by SOX.
On the LED front, new components are becoming available which combine a high luminous efficacy of 157 lm/W with a narrowband spectrum of just 16 nm around 625 nm. This is close to the 589 nm of sox and these LED could become a worthy successor of sox for public lighting applications, providing the white dominance in lighting planning would be reduced. It often takes some development time in fact to provide a worthy successor of a technology. When the Europeans banned incandescent GLS lamps starting from 2009, the only alternative were CFL which were a poor replacement, while in the meantime filament led lamps have emerged as a worthy successor comparable in aspect and performance.
Warm-up Time
The longer warm-up time of SOX is complementary to the twilight phase and thus reflects natural phenomena, representing a more harmonious approach to life than the immediacy of an impulsive on-off culture. The ability for dynamic road lighting through immediate switching is often cited as an advantage for led, however it can be stated that dynamic operation of outdoor lighting adds a particular form of light pollution, such sudden changes in illuminance not occurring naturally and presenting an extra burden on all kinds of creatures.
Dimming
Dim as in dim-witted? This is also a form of dynamic lighting with the same potential drawbacks. Furthermore, the power quality issue of dimming circuits has to be taken into account. SOX is uncompromising and does not deal with dimming.
Industrial Heritage
Taking into account the primordial role of sox in the development of public lighting, its value as a piece of industrial heritage shall be recognized. In this framework, a selected stretch of public road shall have its SOX lighting protected as a listed monument, maintaining the installations in the future, seemingly small details like public lighting being also part of our heritage.
2. Project
2.1 Introduction
Now that SOX outdoor lighting is getting rarer to find on the street, it was proposed to develop a SOX based indoor light fixture, allowing to be introduced in the world of SOX and to enjoy and experience the unique qualities of sox.
2.2 SOX Lamp
For this project, the smallest sox bulb was chosen: the 18W SOX-E. With a size of just 216 × 52 mm and a burning position of vertical ±110◦ it can be accommodated easily and its luminous flux is appropriate for indoor applications. The aspect of such a lamp is shown in fig. 1.
Figure 1: 18W sox-e lamp
SOX lamps are available from various online vendors. With the mainstream products switching to LED however, one may have more chance from small specialist firms than from the big lighting wholesalers. Several types are available:
- Original Philips-branded product ”Master SOX-E”. As the production of SOX lamps by Philips in Scotland ceased end 2019⁴ , these are from remaining stocks, if they are gone, they are gone.
- White-label SOX-E lamps sold by various vendors. The lamp itself is unbranded, it comes in a plain cardboard box with a vendor-specific label. Interestingly enough these bulbs bear both CE and UKCA certification markings; since the latter is a post-Brexit thing, this lamp must be of a quite recent manufacturing. These lamps are made by a company called QS Lighting who is still manufacturing them today. A relief for SOX-lovers worldwide!
- For the 18W SOX-E, typical prices are €50-70 / $60-80 but they are also sometimes offered at prices exceeding €100 / $115. With remaining stocks depleting, prices may be expected to rise further.
2.3 SOX Ballast
Like every gas discharge lamp, the SOX needs an appropriate ballast. Electronic ballasts are the way to go as they are simpler to use and more efficient than the conventional electromagnetic ballast, and do not need additional components like an ignitor or capacitor. One just connects the mains at one end and the lamp at the other end.
The original Philips-branded SOX electronic ballasts are hard to find these days, but alternatives are available from specialist vendors. These are also white-label products, coming in a plain white box and bearing no origin, brand name or manufacturer (fig. 2). Expect to pay €50-80/ $60-90 for a 18W SOX-E electronic ballast.
Figure 2: sox-e 18W electronic ballast
2.4 Luminaire Selection
In selecting the appropriate luminaire to be equipped with SOX, there are several constraints to be considered:
- Lamp position: the SOX shall be vertical, base up, ±110◦; this excludes base down positions which cause harmful sodium deposits on the base seals shortening lamp life ⁵
- Ballast: the ballast shall be properly accommodated on the luminaire
- General build: the style of the luminaire shall be compatible with the aspect of sox and if possible represent a typical application scene of SOX
An interesting solution was found with the Hektar luminaire by Ikea. This is a 180cm high standing luminaire with a large cap, reminiscent of the old fixtures used in industrial and street lighting and typically accommodating discharge lamps such as SOX, SOX or HPL. Such luminaire would also look nice with a blended ML bulb, without need for ballast. Such lamps come at 160W and up; being developed in the LED era, the Hektar is rated however a puny 53W maximum, less even than the typical 60W incandescent which would traditionally equip such fixture in a domestic setting. But no problem here as we go for an 18W SOX-E.
Two changes have to be made to the luminaire in order to accomodate the SOX:
- The sox lamp base being B22, an adapter is needed for the Hektar’s E27 fitting (fig. 3)
Figure 3: B22/E27 adapter
- The ballast is mounted on the bottom plate of the luminaire6, and the lamp wiring connected through it (fig. 4)
Figure 4: Ballast mounted on bottom plate
2.5 Assembly
The Hektar fitted with SOX will henceforth be called SOXTAR. Figure 4 shows the assembled luminaire in test operation under daylight.
3. Performances
3.1 Illuminance
It takes about 10-15 minutes for the SOX after switching on to reach full power. Some lux measurements (horizontal illuminance) were performed. The illuminance measured under the luminaire is around 120 lx, in the room surrounding the luminaire 50 to 70 lx. Stray light from the sox gives 1 up to 10 lx in adjacent rooms, ideal for nighttime security lighting.
These values are within expectation considering the rated luminous flux of 1800 lm7 and the geometry and arrangement of the luminaire.
3.2 Color Rendition
In order to visualize the typical uniform color rendition by sox, a still life was composed featuring a red tomato, orange clementine, yellow banana, green apple and blue brush arranged on a traditional Belgian dwell-cloth. Figure 6 shows it under daylight, fig. 8 under sox illumination⁷. With 100 lm/W this is at the lower end of the sox efficacy range, larger wattage SOX scoring significantly better.
Figure 5: SOXTAR assembled luminaire
Figure 6: Still life under daylight
Figure 7: Still life under the SOX
3.3 Electrical Characteristics
The electrical characteristics of the SOX-E 18W with its electronic ballast have been measured. Results are given in table 1. The electronic ballast is quite well-behaved with low extra energy consumption, unity power factor and reasonable distortion. After several hours of operation, the ballast is still cool to the touch.
Table 1: Electrical characteristics
| Voltage | 235V |
| Current | 0,082A |
| Active power | 19,5W |
| Power factor | 1,0 |
| Current THD | 15% |
4. Indoor Deployment
Although the beneficial properties of sox for outdoor lighting are well documented, its use indoors, as the intended outcome of this project, brings particular challenges.
SOX as Narrowband Light
The color rendition of SOX’s narrowband spectrum (see fig. 7) makes it suitable for areas where the detection of objects has a higher priority than the discernment of separate colors. The sox can thus be recommended as sole lighting for halls, corridors and entryways, where it provides safety and security. When complementing wide spectrum light sources in an association setup, sox adds its atmospheric detail to the general lightscape.
SOX as Session Light
The SOX is not a lamp to just casually switch on or off. SOX has a relatively long warm-up time, and once it is lit, it is not recommended to switch it off soon after, as submitting it to short cycling is detrimental to a long lamp life. For best results it shall remain lit at least a few hours at a time.
This makes the SOX a session light, lit at sunset in the evening and put out late at night (or even early morning), not unlike public lighting which is in fact SOX’s main operating theatre. SOX complements the sun, providing its proper light, fit for the night, rather than trying to emulate daylight. With SOX, one can not abide by the classical energy-conscious drill of switching the light off if one leaves the room. Such long sessions may not seem energy-efficient, but SOX's high efficacy partially compensates this. SOX provides security lighting, but of a permanent kind rater than triggered by external events such as motion detectors.
This apparent lack of control over SOX’s immediate light output may at first seem a drawback but has in fact a deeper value as it reminds the operator of his position within the universe, promoting humbleness and thoughtful handling over arrogant selfishness and abuse of power, contributing to making one a better human being.
SOX in the House
The selection process of an appropriate location for the SOXTAR luminaire developed in this project came out to the house entry hall, where the presence of sox light will present several opportunities:
- The creation of the sox-infused atmosphere presenting an unique welcoming to those entering
- The continuous lighting of this space providing a safe illumination for transit
- The stray light from the sox providing background and security night lighting in adjacent rooms
- A drawback however was the lack of a socket-outlet in the selected locale, necessitating the use of an extension cord
The aspect of the space under the SOX light is shown in fig. 8.
The illuminance obtained in the selected setup make the sox particularly suitable for atmospheric lighting. The visual effect turned out to be much more agreeable than initially expected, with the 18W SOX-E performing well as ambient light, even creating its own kind of cosiness beyond expectation.
5. Conclusion
The deployment of SOX for indoor lighting in this project was intended as a tribute to this great technology and a conservation of its heritage. Even with this successful implementation it is clear however that indoor lighting with SOX is not for everyone. Defining a suitable location and mode of operation for the SOX is not always easy, and many will find sox inappropriate for indoor domestic use due to its impractical long warm-up time and highly uncosy colorscape, or just because it is not considered a consumer product, being labeled for professional use only⁸. SOX is not a product for the faint of heart, nor for heralds of the indefinite article.
SOX will be appreciated though by discerning intellectuals who are knowledgeable about lighting, particularly about sox and its characteristics. They recognize the role of sox in lighting development, and implement it as a tribute, ready to abide by its uncompromising nature. Above all, they consider lighting as a higher order phenomenon exceeding its mere practical application.
Light, as the visible emanation of the electromagnetic spectrum, is in fact a basic element in our universe, and the origin of light is a key element in both scientific and mythical interpretations of cosmogenesis and creation, as illustrated in fig. 9 by a 17th century alchemist and hermeticist Robert Fludd’s Utriusque cosmi historia, showing the light coming out of the darkness. (Let this hermetic figure thus be a source of deeper reflection and serve as inspiration for our future endeavors, under the following motto: Work hard, since you know what has to be known ⁹.
Figure 9: The light coming out of the darkness (R. Fludd)
Notes:
¹The name SOX is still used today to market replacement white led lamps for street lighting with the same form factor.
²One could argue that white light actually embraces diversity since is made up of a full frequency spectrum, but such discussion goes beyond the scope of this document.
³SOX is actually bichromatic: there are two distinct lines in the sox spectrum, even if they are close together!
⁴Datasheets for sox lamps were however still being published by Signify (Philips Lighting) dated 2023!
⁵This applies for the smaller sox, 18 or 36W; the bigger ones need to be positioned horizontally ±20◦
⁶ The arrangement presented is for demonstration purposes, for production an enclosure for the ballast would be advisable.
⁷With 100 lm/W this is at the lower end of the sox efficacy range, larger wattage SOX scoring significantly better.
⁸Such mention may come from a genuine safety concern that the equipment is properly installed — e.g. with the correct ballast — by skilled (BA5) or instructed (BA4) personnel; one shall however also consider the discrepancy in approach and communication regarding professional vs. consumer lighting applications, which often turns out somewhat patronizing for the latter.
⁹ From the essay The light coming out of the darkness by itself by Marc-Antonio Crasselame, 17th century alchemist.