Discharge lighting

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Discharge lamp on pole 4326-3.jpg
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Sodium discharge lights are used for outdoor lighting. With high energy efficiency and low light quality, they're widely used as street lights.

Metal halide offers good quality white light, but is not as energy efficient as sodium.

General Information

The advantages of sodium are extreme energy efficiency and very long lamp life. The disadvantages are poor light quality and slow warm up.

Mercury is much more energy efficient than halogen lamps, but less than sodiums. It has become obsolescent due to CFL, LED & metal halide.

Metal halide lamps are costly, but total cost is still less than filament lamps such as halogen, due to their much better energy efficiency.

Discharge lamps (light bulbs) are typically relatively expensive to buy, but cheap and highly energy efficient to run. They are very long lived, which minimises relamping.

Sodium & mercury lamps take minutes to warm up. They can not be used on PIR detectors.

Discharge lights must be run from a suitable ballast, never connect a bulb direct to the mains. The ballast is built into the fitting.

Sodium lights are available from 18w upwards. An 18w lamp will light more than the average drive or yard, providing a similar light level to a 150w filament lamp.

Light types

Metal Halide

  • Good quality white light
  • Available in warm white 2700K and daylight 4000-5000K
  • Much better light quality than sodium & mercury lamps
  • Relatively high purchase cost
  • Total cost of ownership per 1000 hours is less than linear halogen due to much better energy efficiency.
  • Colour of light tends to shift as lamp ages
  • Electronic ballasts cost more but give longer lamp life & less colour shift
  • Lamps are prone to exploding on failure. Relamping before end of life can avoid this.
  • These lamps use a very high strike voltage, several kilovolts, so care is required if modifying a luminarie.
  • Won't restrike while hot after the power's interrupted

Low pressure sodium (SOX)


  • Extreme energy efficiency upto 180 lumens per watt
  • Pure yellow light
  • Start time in the region of 9 minutes
  • Often used to light motorways
  • No colour perception from SOX light

High pressure sodium (SON)

The golden light of high pressure sodium lamps

  • Gives an orangey / golden / pink light.
  • Start time in the region of 2 minutes
  • Often used as town centre street lights.
  • Highly efficient

Modified Sodium Lamps


These modified sodium lamps give slightly better CRI and output than standard SON lamps.

  • Fit in standard SON luminaires

White Sodium

These are another modification of the high pressure sodium lamp. They produce white light of 2700K and 85 CRI.

  • Much better light quality than SON
  • Less efficient & long lived than SON
  • Require their own type of control gear


Slightly improved SOX lamp

SOX Economy

Improved energy efficiency over standard SOX when run on hf control gear


  • Obsolescent
  • White light, but cold, harsh & not pleasant
  • Start time under a minute
  • Efficiency better than halogen but poorer than sodium, metal halide, LED & CFL.
  • Was often used as street or car park lights. Most have been converted to sodium, but some are still in use.

Colour Perception

White Filament Lamp spectrum
Low Pressure Sodium

Under low pressure sodium light, since only one colour is produced, vision is monochrome, so no perception of an object's colour is possible. The viewer's perception of colour under these lights is mainly due to light from other sources, and to some extent from memory, and the awareness that greens and blues will look dark while orange and yellow appear light.

High Pressure Sodium

High pressure sodium produces a wider spectrum, colour perception is possible but not accurate.

Mercury discharge lamps produce white light, but the spectrum is not high CRI, and the relative brightness or intensity of different colours is not natural. Although this is the closest of the 3 to white light, it is perceptually less pleasant than high pressure sodium.

The combination of both mercury and high pressure sodium lamps gives a better quality of light than either lamp alone, but metal halide has become the norm where light quality matters.

Metal halide lamps give a true white light.

More information on the spectra of these lamps and the implications for vision:

Luminous Efficacy

Typical figures, in lumens per watt. Filament lamp performance is included for comparison.

  • 180 Low pressure sodium
  • 110-150 High pressure sodium
  • 50-125 Metal halide
  • 35-60 Mercury discharge
  • 10-17 Filament


Control gear is built in to the fitting.

  • Sodium lamps run on sodium lamp control gear
  • Mercury lamps run on mercury lamp control gear
  • Metal halide lamps run on metal halide ballasts. Some used sodium control gear, some used mercury lamp control gear


Discharge lighting fittings typically look more industrial than domestic. 18w SOX fittings that look like bulkhead lights are available. If you want something that looks better, the contents of an ugly fitting can be transplanted to a fitting of your choice.


  • Metal halide uses very high starting voltages, and fittings must be able to contain exploding bulbs.
  • The wattage rating of the light fitting should exceed that of the lamp being used.
  • Don't use a fitting with a mirror finish reflector, this can cause lamp overheating. Light scattering reflectors are ok.
  • Don't try to operate discharge lamps from PIR sensors. Timers and dawn to dusk sensors are suitable.

For SON and mercury:

  • Keep the wire between ballast and bulb short.
  • This wire will have to cope with high voltage starting pulses with some of these lamp types, so use a heavy mains flex.


Some lamps have internal ignitors, some external. The 2 lamp types can not be swapped in fittings.

  • Letter I in a triangle means internal igniter
  • Letter E in a triangle means external


Discharge lights are mostly used for lighting large yards and warehouses, and of course street lighting.

The absence of low output discharge lamps and poor light quality makes them unsuitable for pretty much all indoor lighting.

Discharge lamps are also used for plant lighting. Sodium is used where its light will be mixed with daylight. When sodium is used for plant lighting, high light levels, strong flicker, poor colour perception and low visual contrast make working under the lighting for a length of time unpleasant. When long working times are expected, coloured glasses can improve this by blocking some of the yellow light, thus improving colour, excess brightness and contrast. The ideal glasses colour is blue-purple.

Lighting Options

For domestic use PIR operated LEDs are generally the best option.

Mounting a high output light high up reduces glare. Mounting it within reach of a window makes relamping easier.

If a PIR light would spend a lot of time on in your situation, a low power sodium lamp on a timer or better a dawn to dusk sensor may produce lower total energy use and less bulb changes. The price is lower light quality.

See also