Not all light sources within street-lighting and floodlighting are the same.
Low Pressure Sodium Lighting (LPS)
One cannot have missed the bright yellow lights that are a common feature of the UK road lighting system. This light source is from a Low Pressure Sodium (LPS) discharge tube. Whilst it is relatively cheap to run in electricity terms, it requires more frequent changes of discharge tube as the life of these tubes is relatively short when compared than other types of light source. The tube is relatively long, and can be any length up to almost 1 metre and consequently the light output is difficult, if not almost impossible to control.
Therefore much of the light output (perhaps up to 30%) is 'spilt' which then contributes to the yellow glow seen above our towns and cities. Additionally, LPS lighting is not favoured by the police as true colour rendition is impossible when viewed in yellow light. Additionally in urban areas where architectural features or high amenity is required a white light is often preferred over the yellow wash that LPS gives to the urban landscape. (see diagram)
High Pressure Sodium Lighting (HPS)
This light source produces a warm pinky white light, often seen on Motorway, Major Trunk Roads and frequently seen on much of the new lighting being installed today. The discharge tube in HPS is much more compact, a few centimetres, and the luminaire (the light fitting) is consequently quite small. This enables the light output to be easily controlled with the reflectors inside the luminaire. Although the HPS discharge tube uses more electricity per unit of light produced, the discharge tubes have a longer design life than LPS. As all the light can be controlled and placed onto the road surface, these light sources are considered to be more efficient than LPS lights, especially when one takes into account the percentage of LPS light that is spilt and the cost of replacing the discharge tube more frequently. The cost of closing major trunk road or motorway lanes is enormous!
This is the light source and luminaire favoured by many environmentalists and astronomers as the light output is so well controlled when used in flat glass luminaires.
Mercury Vapour Lighting
Mercury Vapour lighting produces a bluish white light frequently seen in sports stadia where there may be the use of cameras for broadcast TV. The blue/white lighting enables colour recognition more easily especially when viewed on a television screen. There is also some limited use of Mercury Vapour lighting within street lighting in the UK. The Mercury Vapour discharge tube is similar in size the HPS and so compact luminaires can be used, and so controls effectively the light distribution.
These are used extensively in the DIY market for floodlighting. They give a white light output. Often the source of the lighting, the halogen discharge tube, is visible and gives rise to intense glare causing discomfort and poor visibility as a result.
This light source provides a slightly greenish white light. Often seen around the home and in shops and factories, these light sources are generally not used as exterior lighting, although there have been experiments with their use as road lighting in the UK. There is greater use of this style of lighting in the Benelux countries on the continent. Generally they are efficient to operate in the home, but as described with LPS lighting, their size inhibits good exterior lighting control.
This is an example of your good old fashioned domestic light bulb. This light source provides a reddish white light. Out of the examples of light sources provided here, the tungsten bulb gives the lowest light output per unit of electricity used. It is rare to find this source of lighting used in UK road lighting today.