Philips SON-T

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Standard High Pressure Sodium Lamp with tubular bulb for outdoor lighting with 50W, 70W (E27 base) and 100W, 150W, 250W, 400W (E40 base)


  • Clear tubular outer bulb
  • Internal high pressure sodium with ceramic discharge tube
  • E27 or E40 base
  • Color rendering index Ra of 25 (on all lamps)
  • Typical lifetime of 28.000 to 16.000 hours (lowest on highest wattage lamps)
  • Luminous efficacy of 68 to 130 lumens/watt (highest on highest wattage lamps)
  • Warm white colour designation (2000 K colour temperature)
  • Universal operating position

Lamp control gear

  • Requires external gear and appropriate wiring:
    • Reactive inductive ballast rated as the lamp, different from mercury lamp types
    • Suitable ignitor for 1800-3500 Volt pulses (except "I" designated 50 and 70W lamps)
    • May require power factor correction capacitor for balancing line load

Philips SON-T 70W

Best retro purpose

Sodium vapour lamps are still widely used on street lighting as well as industrial, commercial and residential outdoor security applications. Most of these uses cases are already slowly transitioning to LED lamps that offer much better performance with lower energy consumption and easy servicing. There are still some specific uses that are specific to this technology such as greenhouse lighting but special LEDs are also replacing them. Light appearance is golden yellow a bias towards greens and reds with very low ultraviolet emission.

Exterior lighting is a good application but most gardens and outside areas look rather dull. The lamp themselves do release some heat meaning that closed fixtures are not well suited if not specifically built for this purpose. A good high pressure sodium vapour fixture is flexible, being possible to repurpose it for mercury vapour or metal hallide lamps with some minimal rewiring and gear change, being a very popular and cheap option as sodium vapour fixtures have been manufactured in millions until recently and large stocks are still available worldwide. Modern lamp gears are more versatile as older ones, being able to operate all sodium vapour lamps of the specified wattage. Note that typical sodium lamp ballasts have three connections using the semi-parallel ignition scheme rather than the parallel ones as on metal hallide lamps.

While products that have an internal ignitor are easier to operate, they are quite rare and are unavailable for higher wattage lamps. Also, they may be less reliable than standard lamps that do not include internal ignitors since the ones being supplied are built to a lower quality standard and made to a set cost and space constraint.

Understanding manufacturer data

Lamp light output is always measured in lumens. This is a way of measuring by averaging light output at a distance of 1 meter in an integrating sphere. This was carried out in lab environments and this information was mentioned in lamp datasheets and catalogues. While this information is accurate it should be considered only across similar lamp types.

Lamp life is presented in thousands of hours. It does not point at a specific moment when an installed lamp will not operate anymore but a statistical point at which some of the lamps may not operate, giving a rough estimate of useful life.

Ra8 colour rendering index, or simply colour rendering index is a way of expressing the typical colour rendering capability of a lamp. While it may be non-intuitive this is a computed average of brightness of certain coloured samples that are light by the lamp. The brighter they are, the more efficient is the lamp in this task. Most lamps do not have a continuous colour spectra so only some specific colours might look very bright and others look very dull. Colour samples are not intense reds, greens and blues but intermediately vivid colours that are focused on human skin colours and some fabrics or surfaces, meaning that a only very high colour rendering indexes are desirable for indoor home lighting. Sun light renders colours almost perfectly, having a value of 100 while typical lamps have a rendering index between 50 and 80, a good value being above 80 and a poor one below 50.

Colour temperature is another important detail. The value is presented in Kelvins and follows a theory that boils down to the fact that light can be produced by heating a metal up to a specific temperature. A camp fire releases light as the flame reaches around 1500-2000 degrees Celsius and a slightly higher value expressed in Kelvin. The designation is warm white for a value of around 2700 Kelvin, natural white with a value of around 3200-3500 Kelvin, cool white for a value of around 4000-4500 Kelvin, daylight for a value between 5000 and 5500 Kelvin, and cool daylight for a value above 6000-6500 Kelvin. There are cultural preferences that make some warm colour temperatures preferred in colder climates and cool colour temperatures in warmer climates. The most popular home lighting worldwide is mostly warm white, due to a comfortable, pleasant atmosphere that is close to the old incandescent lamp.

Technical details

Designation Base Lamp wattage Luminous flux Colour appearance Colour Temperature Colour Rendering Index
Service Life
(to 50% failures)
SON-T PIA Plus 50W/220 E27 SLV E27 50 W 4400 lm Warm White 2000 K 25 28000 hours
SON-T PIA Plus 70W/220 E27 SLV E27 70 W 6600 lm Warm White 2000 K 25 28000 hours
SON-T PIA Plus 100W/220 E40 SLV E40 100 W 10700 lm Warm White 2000 K 25 32000 hours
SON-T PIA Hg-Free 100W/221 E40 SLV E40 100 W 9000 lm Warm White 2150 K 25 32000 hours
SON-T PIA Plus 150W/220 E40 SLV E40 150 W 17500 lm Warm White 2000 K 25 32000 hours
SON-T PIA Hg-Free 150W/221 E40 SLV E40 150 W 15000 lm Warm White 2150 K 25 32000 hours
SON-T PIA Plus 250W/220 E40 SLV E40 250 W 33200 lm Warm White 2000 K 25 32000 hours
SON-T PIA Hg-Free 250W/221 E40 SLV E40 250 W 28000 lm Warm White 2150 K 25 32000 hours
SON-T PIA Plus 400W/220 E40 SLV E40 400 W 56500 lm Warm White 2000 K 25 32000 hours
SON-T PIA Hg-Free 400W/221 E40 SLV E40 400 W 48000 lm Warm White 2150 K 25 32000 hours
SON-T PIA Plus 600W/220 E40 CRP E40 1000 W 90000 lm Warm White 2000 K 25 28000 hours

Note: some lamps have an integrated internal starter and do not require an external ignitor, being designated with an I in the designation; CL means clear bulb;

Durability and Repair-ability

Generally, sodium vapour lamps were very durable and most modern products were released with even longer typical lifetimes. This is mainly due to the fact that the main market of street lighting pressured producers to cot servicing costs which means that reliability improvements were mandatory. Unfortunately, due to the way that almost all lamps are designed, they age in a different way compared with other light sources. Sodium slowly permeates the ceramic arc tube meaning that the lamp voltage raises as the lamp ages, making it harder to remain in stable operation after startup. This creates the well known on-and-off lamp cylcles that repeats every couple of minutes, accelerating as the lamps is close to demise. Eventually, the lamp does not start but this happens after a long cycling and gear stress that might span over months or even a whole year.

Compared with modern LED lamps, sodium lamps compare favourably but they have a much poorer light output and require careful wiring. In most cases operational cost is lower as less powerful LED luminaries can be used since all useful light is directed downwards compared with the inefficient fixtures used on most sodium vapour lamps. Ballast and ignitors can survive tens of years if properly protected against direct rain or snow exposure. A complex circuitry means that lamp servicing is difficult and there are major risks due to improper wiring on servicing as large voltages may be present. 70, 100 and 150 watt lamps are very well built and they were highly popular. Ignitors can be potentially repaired but the lack of schematics means that they can be repaired only by experienced persons and are, thus, mostly discarded when not working.

Greatest features & flaws

Features Flaws
High light output in very compact package Serious risk of glare
High efficiency and very long life Not suited for main interior light
Mostly omni-directional light Poor colour rendering with orange glow
Quick lamp start and restart Requires bulky, heavy, complex lamp gear


These high pressure sodium vapour lamps are compact light sources that are viable for wide area lighting. Their poor colour reproduction and large output means that they are entirely suited to outdoor lighting or greenhouses as grow light. Compared with fluorescent lamps they were very compact, having great performance at low temperatures. A very long lifetime, that was around 16 to 25 times higher than tungsten lamps, and almost double that of some fluorescent lamps, meant that these lamps are well suited for use in places where lamp replacement was costly, as in high bay lighting, street lighting, tunnel lighting.

High pressure sodium lamps had higher light output, almost double, of mercury vapour lamps while having very low quality colour reproduction. This means that mercury vapour lamps were still suited for areas where colour reproduction was moderately important and there was still an advantage from compact light sources. However, street lighting was a very important market as the lamp's efficiency ensured cost savings in labour and electricity use, after the 1970s oil crisis. There were some advances in colour reproduction with different lamp technology but standard lamps were not favourite of industrial, workshop lighting commercial premises. The light spectra was, however, highly beneficial for plant growth.

The inherent flicker of mains voltage lamps is very low even with conventional gear due to the fact that a large amount of light happens due to thermal emission and balance of the temperature of internal arc tube. There were some electronic medium frequency gear similar with ones used on fluorescent lamps but they were very costly. However, sodium and metal hallide gears are generally compatible, meaning that most sodium lamps can be easily replaced with compatible wattage metal hallide lamps that have a much better colour appearance, sacrificing some energy efficiency due to different lamp technology. The requirement for ignitors along ballasts while it raises total cost and increases complexity, as well as raising the risk due to passing thousands of volts through some wires, means that fixtures have a clear advantage of maximum useful light and restart in case of power failure in almost one minute.

Fixtures are generally compact, considering the light output made available, but are generally heavy due to the bulky electromagnetic gear being used. There are directional and omni-directional fixtures that are suited to various purposes. In street lighting, the well known cobra-head style lamppost was used, and they may still be available for purchase. They were only moderately effective as light covered a large area but a considerable amount of light was lost due to the fixture directing some of it upwards, with no control. Omni-directional fixtures were used in residential areas and parks, where ambience was more important than efficiency. Indoor fixtures were centered on the typical high-bay or low-bay types due to their application in greenhouses where light has to be directed entirely downwards. Other fixtures were compact, having a boxy design, with a symmetrical or asymmetrical lamp placement that was suited for flood lighting. These type of lamps were used outdoor, for floodlighting. There were no home designed fixtures as these lamps were never used in home environments.

Nowadays, sodium vapour lamps are still very widely used as outdoor lighting. As the number of available fixtures is massive, availability is high and will diminish no early than 2030s, meaning they are economically effective to purchase except and they can be repurposed for mercury vapour of metal hallide lamps if proper lamp gear is used. Free standing lamps are quite capable on their own so if you need a compact light source that is highly durable and colour output is not important. They have the additional advantage of attracting less insects during the summer, if used as outdoor light. They are especially well suited for security lighting. Of special interest is greenhouse lighting, as was mentioned before, even if specialized lamps have also been designed for these purposes, the standard lamp is also appropriate.


These lamps are somewhat better suited for frequent switching but starting is still a major stress. They need less than two minutes to reach maximum light output and less than that to restart after being turned off. The light colour and output changes from start-up to normal operation from white to orange. White light at startup and then greenish light midway to warm-up suggests mercury is also used on the lamp. This slightly improves colour rendering but affects slightly reduces lamp life while the ones that do no change colour are either new or do not use mercury at all have a yellower light appearance. Lamps without mercury are more difficult to start but they have longer lifetimes and slightly higher efficiency.

The lamp requires a conventional or electro-magnetic gear of the same power output as the lamp, connected in series with a lamp and an ignitor that must be connected as is required in the circuit. Connecting the lamp directly to mains damages it instantly as there is no current limitation. Improper connection of the ignitor is also dangerous and proper care must be exerted during installation and servicing. A power factor capacitor that has a specific value might be used to correct the power factor and current requirements on the circuit. The lamp itself is not influenced by the presence or absence of this capacitor. When used sparingly in homes or some premises where not a lot of these lamps are located, power factor correction is not important but large industrial or commercial clients were required to maintain a specific power factor due to electricity distribution contracting.


The best lamps in terms of useful lifetime, colour reproduction, light output, are major branded lamps. Chinese branded lamps are much worse in all performance and quality metrics with shorter useful life and efficiency as they do not use modern ignitors and gears. Coated lamps have less glare and more beam uniformity than clear ones but they are less efficient in fixtures that use reflectors.

The best choice of wattage are the 70 and 100 and 150 watt lamps as lower wattages have quite a low light output and larger powered lamps are too powerful for most purposes. The above lamp wattages are also cheaper as they were highly popular. Electro-magnetic ballasts or conventional gears are plentiful and can be used interchangeably with all manufactured lamps for a specific wattage although some ignitors may not be optimal in some scenarios. Lower rated power output gears can be used, making the lamp operate dimly but higher powered ones damage the lamp due to over-stressing and is highly dangerous.

Used lamp gears might work but old lamps are not as desirable. Rusted ballast may still operate but are a potential hazard while old, used lamps, may not have much useful life left and they may already exhibit some colour light emission issues. New old-stock lamps are the best choice, if available.


The lamp technology was an improvement of previous lamps that were used in larger luminaries, the low pressure sodium types. Looking similar to fluorescent lamps, sodium lamps had a totally different operation. They output monochromatic lamps compared with a pleasant fluorescent white light but they were very well suited for traffic as this light was very little affected by fog and contrast was perfect on roads. Very poor colour reproduction was a very important limitation in market adoption, and it was noticed that increasing sodium pressure broadened light spectra contributing reds and greens to the light output. Many years there was no stable arc tube material that was able to withstand high temperature and aggressive sodium.

When a suitable ceramic arc tube was developed and manufactured the lamp's high performance was confirmed in applications. Early lamps were attempted to be released with no external additional gear but durability was very poor so ignitors were developed for this specific application. The wattage and voltage of sodium lamps was not compatible with mercury lamps so an operational compromise had to be used to allow operation with mercury vapour lamp gear. This was especially important many years ago as mercury lamp installations were dominant. There was also research that culminated in specialty, even higher pressure sodium lamps, that have improved colour reproduction and a pleasant whiter appearance but they were very costly, had a shorter life and efficiency. They were never popular due to fluorescent lamps being much cheaper and easier to use. These lamps can be easily noticed by seeing their rather thicker internal arc tubes or the higher colour temperatures in datasheets.

Later developments focused on releasing optimized ballasts and ignitors that followed slender and more efficient internal arc tubes. Efficiency was improved significantly, making 20 to 30 thousands our lifetimes a certainty, increasing servicing intervals, lowering early failures and making high pressure lamps a very reliable lighting system. Electronic gears were not popular due to high costs although they were entirely possible from a technological perspective and they offered simpler wiring along with higher system energy efficiency. Only ignitors made the switch to advanced electronics as they were products with frequent replacements. The last technological advancement was made by using Xenon and removing Mercury from the arc tube, improving lamp life and efficiency but requiring higher starting voltages and new ignitors. These lamps have a few degrees lower colour temperatures expressed in Kelvin.

Overall, sodium high pressure lamps, as is the case of metal hallide lamps, differ very much in terms of performance, even for similar types. Using a datasheet is mandatory if the circuit diagram and lamp being used is not the one that was supplied with the fixture, making operation and servicing a higher skilled job than with previous mercury vapour lamps. Modularity was ensured by making whole fixtures easy to replace and restricting servicing, including cleaning, to offsite premises, avoiding electrocution and prolonged labour at height risks.

Manufacturing specificities

Most manufacturers have been releasing lamps for many decades. Lamps produced in the 70s and early 80s are probably not commonly found today. In the 90s and early 2000 Philips was manufacturing lamps in Holland and other countries, while in the last years they were doing the same thing in China. Quality was not severely affected as manufacturing quality was maintained and applications were demanding, used in wealthier as well as poorer countries. It should be noted that sodium vapour lamps were mass manufactured and considered mainstream products, meaning that quality and reliability had to be consistent.

Products manufactured by Philips were similar to General Electric and Osram, with a quality and consistency much above some East-European manufacturers in terms of colour rendering, light output and life. However, Philips was manufacturing lamps with shorter lifetimes than typical Osram ones, as the datasheets and installations performance shows. These differences are maintained even nowadays. Philips lamps, however, were much more popular and are easier to find.