Solar Cells - Energy Efficient Lighting

Fluorescent lamps

The fluorescent lamp is a gas discharge tube whose output of light is so increased by special means that it can be used for lighting purposes.

 The inner surface of the wall of the tube is coated with a light-emitting substance - usually fluorescent or phosphorescent metallic salts (calcium tungstate, zinc sulphide, zinc silicate). The tube is filled with mercury vapour at extremely low pressure. The electrons ejected from the incandescent electrodes collide with the mercury atoms and cause these to emit radiation which consists for the most part of ultraviolet rays, which are invisible.

The visible portion of the mercury vapour rays is situated in the green and blue range of the spectrum and gives a pale light. The ultraviolet light strikes the fluorescent substance with which the wall of the tube is coated and causes this substance to emit radiation with a longer wavelength in the visible range of the spectrum - i.e., the coating transforms the invisible rays into visible light. By suitable choice of the fluorescent substance, this light can be given any desired colour. The lamp has to be operated with a choke, which prevents a harmful rise in voltage and serves to ignite the lamp. For this purpose a small auxiliary glow lamp provided with a thermal contact is connected in parallel with the main lamp. When the current is switched on, the glow lamp first lights up (the bimetallic thermal contact is then open).

This causes the bimetallic strip to warm up and close the contact, with the result that the glow lamp is short-circuited and the cathodes of the main lamp receive the full current that makes them incandescent.

The bimetallic strip cools and breaks the contact. Through the agency of the choke this interruption of the circuit produces a voltage surge which is high enough to initiate the discharge in the fluorescent lamp itself. Because it is bypassed by the main lamp, the small auxiliary lamp then ceases to function. The bimetallic strip keeps the contact open.

The cathodes of the main lamp are kept glowing at white heat by the impingement of positive mercury ions, and the lamp thus continues to function and emit light in the manner described. The light of a fluorescent lamp is not produced by an incandescent body (such as the filament of an ordinary electric lamp), but is emitted as a result of the excitation of atoms (namely, those of the mercury vapour and the fluorescent coating) and is extremely economical.

Because of the large light-emitting area, a fluorescent lamp gives a pleasant light which produces only soft shadows.



Is China the future?

China, which is expected to double its demand for energy in the next 15 to 20 years, currently consumes 45% of the world’s fossil fuels. This will mean that an additional 45% of oil, gas, coal, and mineral resources will have to be found. In the 1970’s oil was changing hands at around $10 per barrel. Today, new fracking techniques (getting oil, rather than blood out of a stone) will cost at least $80 per barrel, and this cost will increase as the oil rich reserves are quickly used up.


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