Light pollution is artificial light where it is either unnecessary and a waste of energy or a nuisance that could even be downright dangerous eg dazzle from an undipped car headlight or a badly installed high powered "security" lamp. A nearby sorts ground used to be such a blaze of light that it was difficult to see the road.

  The impact on Astronomy is such as to hinder and discourage completely any form of observation of the sky: buying a bigger and better telescope makes not the slightest difference if the sky is blotted out by fog, cloud or man-made glare reflected by the dust particles in the atmosphere. At my previous home there was so much over-enthusiastic and badly directed street lighting that it was possible to see the Moon but not much else. I moved to a village where most of the lights went off around midnight but the local council opted for cheapness and sensors were fitted that kept them on all night on the pretext of security—as though these streets were busy late at night. In six years I counted only one late drunk going home (and probably lost anyway). These lamps direct their light all over the place, including upwards; it is fortunate that they are of the low pressure sodium variety since this light can be blocked by a special filter. There is a lamp in front of my upstairs bedrooms at the front: I prevailed on the electricity board to paint part of the glass black. Fortunately no street light is visible from my back garden and the telescope that had been for years "mothballed" has now been installed. However, there is still a ridiculous amount of "sky-glow" from neighbouring towns.

  It seems ironic that a government that requires school children to have some knowledge of astronomy in the mandatory National Curriculum permits lesser authorities to blot out the night sky, not only in London but elsewhere. This makes trying to teach anything much rather difficult. Mention the Milky Way—"Please sir, you can buy it at the sweetshop". What irritated me most about Fifth year Nuffield Physics was the long arid stretch of theory concerned with the Solar System, devoid of real observation. Theory should surely come after observation and what is more inspiring than the sight of the sky on a truly dark night, especially during winter when it is dark around teatime! How many youngsters ever do get the chance to do just this? They are denied on the grounds of safety (as if illuminating the sky makes for safety) and security (as though the more energy wasted in all directions the more secure we become).

  The whole issue of security has been thoroughly befuddled in the interest of sales. Thieves need light, that is why most household burglaries take place in the daytime because it allows a thief to check that the house really is empty, to make a fast and silent entry, decide if there is anything worth stealing and make a speedy exit. A neighbour of mine has twice suffered in this way. At night a torch out of doors is amazingly remarkable but friendly street lighting assists in the robbery of sheds and outhouses, eg those containing valuable bicycles. The sight of a security light in the daytime suggests something worth stealing inside. At night if the house is known to be empty then a blaze of light more often than not just blinds any potential witness. Once inside the wall of light it is the thief who is secure.

  There are "full cut-off" luminaires that send their light downwards on the ground, where it is meant to be; South Oxon Council has already started installing them in some places. Where control seems lacking completely is in domestic security lighting, some of which is dangerous, especially the sort that comes on suddenly. Noise nuisance seems to be dealt with rather quickly by the police—recently a man was killed by a motorist who was blinded by bright lighting outside a public house.

  Light can be measured in direction and intensity. Photographers' meters do this but the scale is marked off in terms of exposure time and aperture. The physical standard source is a cylinder of thorium maintained at the temperature of solidifying platinum: one square cm of this produces 60 units, called "candelas". A point source radiating uniformly in all directions and of intensity one candela delivers into one unit solid angle one unit of luminous flux, called a "lumen". (There are 4 pi unit angles in the surface of a whole sphere). One lumen per square metre is the unit of illumination, called a "lux" and this is what a light meter measures. Light incident on the front surface of a (cheap) barrier-type photovoltaic device causes electrons to flow through the device, usually in the shape of a thin slab, and these register as a current in a microammeter, the scale of which can be calibrated directly in lumens per square metre, or lux. The candela was introduced in 1948 and the factor of 60 to keep it as close as possible to the international "standard candle". It is a laboratory standard from which secondary standard light meters are then calibrated, and these can be used again to produce cheaper light meters. It is just not possible to claim that the intensity of a light is a matter of opinion any more than the loudness of a noise is a matter of opinion, Legally enforceable regulatory controls are most certainly possible—what is needed is the will to do this.

24 April 2003