1 Road Safety And Automobile Association Patrol In Great Britain

FLUORESCENT LAMPS


Fluorescent lamps are based on passing an electric current through an evacuated tube containing a little mercury vapor and argon gas at very low pressure. The pressure of the mercury vapor is about 1/100 000 atmosphere. When this is done, the mercury atoms give out ultraviolet radiation and a little light.
a) The ultraviolet radiation through the plain glass. The result if the tube has clear, plain glass. In 1OOW mercury lamp, about 35W of power is lost as heat. Some of this is conducted away along the tube and its end supports, and some is lost by convection currents of warmed air flowing up past the warm tube. Most of the power comes out as ultraviolet radiation, about 62W. The lamp is therefore extremely dangerous, especially to eyes. About 3W of power emerges as light.
Some chemicals absorb ultraviolet radiation and then give out light; they are called fluorescent substances. If a thin layer of a fluorescent powder is put on the inside of the glass of a mercury lamp, the powder absorbs all of the ultraviolet radiation; the powder then gives out infra-red radiation and light. The tube is now a fluorescent lamp.
As producers of light, fluorescent lamps are more efficient than filament tamps, and therefore their energy costs are lower. A filament lamp is cheap and has a life of about 1000 hours; fluorescent lamps are more expensive, but they can be expected to give very good service for at least 7000 hours. Fluorescent lamps need electrical control gear, including capacitors and coils, so the cost of installing them is higher than for filament lamps, which only need a lamp socket.
To reduce the overall costs of lighting, lighting engineers look for ways of making lamps more effective in their use of power, and they try to invent and develop new types of lamp. Other ways of producing light are by using high-pressure mercury lamp, low-pressure sodium lamps and high-pressure sodium lamps. Low-pressure sodium lamps are used for road lighting. They are the most effective for turning electrical power into light.

Topic # 13