Philips Master SDW-T

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Higher colour rendering High Pressure Sodium Lamp with tubular bulb for outdoor and indoor lighting with 35W, 50W and 100W power rating


  • Clear tubular outer bulb
  • Internal high pressure sodium with ceramic discharge tube
  • PG12-1 base
  • Color rendering index Ra of 83 (on all lamps)
  • Typical lifetime of 15.000 hours
  • Luminous efficacy of 26 to 50 lumens/watt (highest on highest wattage lamps)
  • Warm white colour designation (2500 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
    • May require power factor correction capacitor for balancing line load

Philips Master SDW-T 50W

Best retro purpose

High colour rendering sodium vapour lamps are a specialty product lamp highly suited for indoor lighting, due to the warm and pleasant light output. Although the lamp is not very efficient, the unique light output that leans strongly on warm tones creates a cosy atmosphere that highlights furniture and other items. The lamp and associated gear is not as suited for outdoor lighting as the cost of the lamp, service lifetime and efficiency is not as high as on standard high pressure sodium lamps.

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 but a suitable lamp base is quite difficult to find as they were quite rare. Fortunately, a way of connecting the lamp can be easily improvised using a typical flourescent lamp starter base, even if the lamp itself may require some support to avoid improper contact. All sodium vapour lamps gear of similar power rating can be easily used. 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)
Master SDW-T 35W/825 PG12-1 SLV PG12-1 35 W 1300 lm Warm White 2500 K 83 15000 hours
Master SDW-T 50W/825 PG12-1 SLV PG12-1 50 W 2300 lm Warm White 2500 K 83 15000 hours
Master SDW-T 100W/825 PG12-1 SLV PG12-1 100 W 5000 lm Warm White 2500 K 83 15000 hours

Durability and Repair-ability

Generally, sodium vapour lamps were very durable but these special lamps are less durable due to their higher discharge tube stress. As sodium permeates the discharge tube and electrodes are worn quicker, the lamp's output and colour rendering degrades faster than on standard high pressure sodium vapour lamps.

Compared with modern LED lamps, these sodium lamps compare favourably but they have a much poorer light output, efficiency 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. No lamp wattage seem to have been particularly popular as the whole range was used sparingly in commercial centers. 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 Moderate risk of glare
Pleasant warm light, highly suitable for indoor use Not very high efficiency and service life
Mostly omni-directional light Long restrike time, hard to find lamp base
Reasonable lamp start time Requires bulky, heavy, complex lamp gear


These high pressure sodium vapour lamps are compact light sources that are viable for indoor lighting. Their very good colour reproduction and warm light appearance made them very well suited for shopping areas. Even if home applications could have been viable, the high cost of lamps, associated gear and a lack of attractive fixtures meant that this market was never represented. Creating an even warmer atmosphere than even typical incandescent lamps, with a better showcase of reds and oranges, this light source fared very well in terms of visual comfort. The only downside of SDW-T lamps was the lack of an instant restrike design, which meant that this lamp should not be switched on or off too frequently, as this would also affect light. On the few commercial applications were they were most likely applied, they fared very well. Groceries such as vegetables, bread and other food products were very well and appealingly presented. The lamps themselves and gears were quite long lived. There was also a more compact SDW-T lamp design that had similar dimensions to low-wattage ceramic metal hallide lamps that was especially attractive in the early 2000s as the entire fixture was very compact and light control was very good. Of course, the advent of LED lamps that could create any light appearance with lower maintenance, heat and better energy control meant that SDW-T lamps were not required.

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. Fixtures were compact, having a boxy design, with a symmetrical or asymmetrical lamp placement that was suited for flood or spot lighting. Generally, due to the cost of the lamp and application, the few fixtures that were designed or adapted from other compact metal hallide ones, had very good optical systems, with high quality mirrors and lenses.

This type of lamps, due to their technology and way of operation could be successfully used both with cheaper conventional magnetic gears as well as high performance electronic ones. Light is mostly created as a result of thermal emission of hot sodium vapours, which translates into a high inertia that hides most flickering and light variations that are unavoidable in high pressure discharge lamps. This meant that SDW-T lamps were excellent for indoor uses even on inexpensive lamp gear. Despite an overall excellent lamp, the only drawbacks were high lamp cost and lack of highly marketable fixtures that detracted even from some residential luxury lighting applications as well as general perception that was mostly shaped by the typical street lighting application of cheaper lamps, setting lower expectations about the technology. Of course, manufacturers were also to blame as there was no advertising on this lamp and its benefits except occasional installation showcases.


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.


Due to the very small market share that this lamp technology had and the amount of required research and development to produce such lamps, only major manufacturers were involved in it and the products were of very high quality.

The choice of wattage depends on application but, as usual, lower powered lamps are more compact. 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. Typical ceramic metal hallide gear is quite well adapted to the power requirement of lamps. It is not advisable to use lower than lamp rated gears as this affects light output quality while higher powered ones damage the lamp due to over-stressing and is 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 previously developed high pressure sodium lamps, in the early 70s. It was noticed that increasing the pressure of sodium vapours in a discharge improves light output by creating a spectrum that is more appealing. As the emission broadens to reds and greens, colour reproduction improves tremendously, while efficiency slightly drops. Of course, highly resistant ceramic tubes had to be used to contain the sodium vapour discharge and its intensity. Both heat and the corrosive, permeating nature of sodium pose challenges to long service life and good operation. In order to further increase pressure and temperature to improve colour rendering the discharge tube is broadened and shortened. This increases the intensity of the discharge but increases thermal and electrical losses of electrodes and the design of the lamp gets more complex. As usual, sodium still leaks out of the discharge tube at high temperatures so the lamp design had to compensate for these losses with a special construction and gas filling. All these requirements meant that producing such a lamp was highly costly and good manufacturing consistency was mandatory.

The market for such lamps was small, favouring some indoor aplications especially in colder climate countries such as Great Britain, where sodium lamps were already appreciated. In commercial applications they were more successful when the typical tungsten halogen lamps very low energy efficiency and simple use was no longer acceptable, in late 90s and early 2000s. Development of improved arc tubes was still possible but was considered not viable as the market was not expanding to cover such research and production expenses compared with the more flexible application compact ceramic metal hallide lamps. These lamps were much more energy expansion and have much broader appeal than high pressure sodium lamps.

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 very good quality and consistency and Japanese manufacturers such as IWASAKI deserve a special mention as they were creating excellent lamps. Philips lamps, however, were much more popular and are easier to find.