Discharge lighting

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Sodium & Mercury discharge lights are used for outdoor lighting. They are widely used as streetlights, and are effective for always-on lighting of large areas.

They are highly energy efficient, but the quality of light is poor.

Energy efficiency and Cost

Their advantage is extremely high energy efficiency. The disadvantages are poor light quality and slow warm up.

The lamps (light bulbs) are relatively expensive to buy, cheap and highly energy efficient to run, very long lived, and take minutes to warm up. Hence they can not be used on PIR detectors.

Discharge lights must be run from a suitable ballast, never connected direct to the mains.

Sodium lights are available from 18w upwards.

Light types

  • Mercury lights give an ice cold white light
  • Low pressure sodium gives a pure yellow light
  • High pressure sodium gives an orangey light, often described as golden or pink.

Colour Perception

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

High pressure sodium produces a wider spread of colours, but still an incomplete spectrum. More colour perception is possible.

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 a perceptually less pleasant light than high pressure sodium.

The combination of both mercury and high pressure sodium lamps gives a better quality of light than either lamp alone. To be visually acceptable, the 2 lamps must be in the same place, hence this strategy is best suited to high positioned lamps lighting large areas.

Luminous Efficacy

Typical figures, in lumens per watt:

  • 150 High pressure sodium
  • 180-200 Low pressure sodium
  • ? Mercury discharge

See also