Lighting in the UK-

Legislation on street lighting in the UK was first introduced in 1415, when all dwellings along the highway were to provide an outside lantern between All Hallows Eve (Oct.31) and Candelmas (Feb. 2), except for the nine nights after the moon had passed the first quarter. An encyclopaedist called Krunitz explained that this was because "The grand light that rules the night can light the streets sufficiently, and lanterns are superfluous".

Unfortunately this approach to lighting has been long abandoned, and overlighting is common, since so many people believe "more is better". However, by using the recommended type of streetlights with appropriate bulbs and wattages this tide can be reversed.

When thinking about lighting we must first consider which bulb we are going to use and after that we can decide the appropriate luminaire for the job.

There are four main types of light:

• Incandescent
• Metal Halide
• Low Pressure Sodium
• High Pressure Sodium

As can be seen from the table above incandescent bulbs are the least energy efficient, and have the highest cost per year, remembering that a light fixture is a minimal amount (4%¹) of the total cost of a lamp. They take the most energy to produce the same amount of light as other bulbs. For example, a 55W low pressure sodium bulb produces the same number of lumens as a 100W high pressure sodium bulb, and three times as many as a 150W incandescent bulb.

Low Pressure Sodium bulbs are the most efficant, use the least energy in producing light and have very low running costs. The light given off by this lamp is monochromatic; in other words all the light has the same wavelength. Because the wavelength is in the yellow part of the visible spectrum the light appears to be yellow. This is an advantage to astronomers since it makes the light easy to filter out, but makes colour rendition very poor. Because of this Low Pressure Sodium lighting is only suitable for places like streetlights, security lighting or floodlighting where colour definition is not important. However it is excellent for these purposes since, when well shielded, it produces very little sky glow or glare. The eye also sees yellow light better than any other light, making it a safe choice for roads.

High Pressure Sodium lighting has a long life (20000-25000 hours¹), consistent performance and is economic to run. It is very efficant, being second only to Low Pressure Sodium Lights. Its light quality, though better than Low Pressure Sodium, is still not very good. It’s used for the same purposes as Low Pressure Sodium, but isn’t quite as efficant or cost effective.

Metal Halide Lamps produce "white light" i.e. light, which contains all wavelengths in the visible spectrum. This makes the colour definition from it good, but the light very hard to filter. These are suitable for floodlighting and sports lighting where high colour definition is essential. However they have several drawbacks: they have a long strikeback time after a power interruption, as the lamp ages the colour tends to shift and they are difficult to dispose of since they contain mercury, which is poisonous.

Now that we have considered the bulbs we can decide what would be the best luminaire to use.

The biggest problem with security lighting is that most people use too high a wattage for the job (500W instead of the maximum of 150W recommended by the ILE for halogen lights) causing blinding glare. Sky glow is caused by the incorrect positioning of the luminaire so the majority of the light goes horizontally or upwards instead of on the ground where it’s needed. To stop this, lights must use lower wattages and be fitted with shields to direct all light at the intended source and minimise glare.

Here are some examples of bad security lighting:

You will notice that these are all badly shielded and will direct most of the light away from the ground where it is needed. Compare these images to some examples of good security lighting, shown on the next page:

You will see that all these features are fully shielded, directing all the light ownwards. The bulbs in all these features are high up, allowing the shielding to have its full effect, not low where some of the light would still escape sideways.

These are two good examples of security lighting. As well as the shielding on these you can see the motion detectors underneath, so they only switch on when they detect movement. The detector on the left is particularly good since it can be moved independently of the luminaire, allowing it to be positioned correctly so that passing animals or trees swaying in a breeze don’t turn it on unnecessarily.

If a more decorative light was required one of these might be more appropriate:

When considering streetlighting we must not only consider the bulb and the luminaire but also the height of the mount and the distance they are apart.

All the light from a streetlight should be directed at an angle of less than 70⁰ to the vertical. You can see that the streetlights in these diagrams⁴ are going to allow light to be directed on and above the horizontal.

It is obvious that to direct the light downwards the bulbs need to be deeply embedded into the fixtures like in the streetlights below⁵.

You will also notice that these luminaires all have glass that is flat. If the glass is positioned parallel to the ground this will give full cut-off shielding. However, if the lights are positioned nearer ground level it is possible to get away with a wider spread of light from a slightly curved glass bottom that is still effective in lighting the ground, allowing fewer lights to be used.

Both the luminaires shown above use high pressure sodium lights. The best streetlights are like the ones below with a fully cut-off light (flat glass) and a low pressure sodium light. It would be especially suitable for highways carriageways or town lighting where colour definition is not important.

Floodlighting and spotlighting are probably the two worst offenders for light pollution. People use lights that point upwards and are far too bright, creating problems with glare, light trespass and skyglow, not to mention being an eyesore. The Royal Fine Art Commission said about spotlighting "Subtle focused lighting of buildings is to be praised. Too much light bleaches out architectural detail, obliterates colour and pollutes the air."

The important thing is to use the minimum amount of light needed to do the job, and to use properly shielded lights. Below are examples of good and bad⁴ lighting.