Operating Principle

The incandescent lamp is based on the principle of temperature radiation - in that objects emit light when they are heated to high temperatures. It is a well known phenomenon observed everyday, for instance from the glow of coals in a fire or the bright yellow radiation of molten steel being cast into a mould. The incandescent body in the modern lamp consists of a fine metallic wire, and it is heated by passing an electric current through it. The friction of the flow of electrons being forced along the narrow wire causes its temperature to be increased, and the higher the electric current the greater the degree of heating.

The filament will begin to glow dimly if heated above about 500°C, but to obtain a useful amount of light it is necessary to operate at temperatures in excess of 2500°C when it glows white hot. The actual temperature attained by the filament is depending on the power, or wattage that it dissipates. The wattage in turn depends on the electrical resistance of the filament wire and the voltage of the electricity supply. Lamps of various ratings are thus made by providing them with filaments having differing electrical resistances.

Figure I6 illustrates the classical Wien's Displacement Law, which reveals the spectral power distribution of filaments heated to various temperatures (shown by the different curves in Kelvins, K). It will be observed that the higher the filament temperature the greater the amount of energy it radiates, represented by the area under each curve.  The dashed line indicates a general trend that the hotter the filament, the further its peak emission wavelength is shifted from the infrared towards the visible region. It will thus be appreciated that the most efficient lamps are those having the highest possible filament operating temperatures.

Most metals are molten at the desired filament operating temperature, and there are very few materials which make suitable lamp filaments. Swan and Edison chose carbon because it has the highest working temperature of any of the known chemical elements – it sublimes at about 3825°C. However its vapour pressure is rather high and before this temperature is reached, it evaporates away rapidly. To reduce the rate of evaporation and obtain satisfactory life performance, carbon must be operated at considerably lower temperatures. Nevertheless it served as the only mass-produced filament material until the introduction of osmium in 1906 (melting point 3033°C) and tantalum in 1907 (melting point 3017°C). Despite having lower melting temperatures, these metals feature much lower vapour pressures than carbon. Consequently for a given rate of filament evaporation, and hence lamp life, they can be operated at higher temperatures than carbon. Owing to the higher temperature they produce more light as already explained with Wien's Displacement Curves. In 1907 lamps having pressed tungsten filaments were introduced, having a melting point of 3417°C and a still lower vapour pressure. Tungsten remains the metal of choice today, but the fragile filaments made from pressed tungsten powder were superseded in 1909 by drawn tungsten wire, which is stronger and easier to handle.

Despite the very high operating temperature that the tungsten filament permits, the efficacy with which it converts electrical energy into visible light remains pitifully low. The energy balance diagram in Figure I7 to the right shows how the energy of a modern 100W GLS lamp is dissipated. Only some 5% is converted into visible light, this representing an efficacy of 13.4 lumens per watt. A massive 83% of the energy is wasted as radiant heat, while the remaining 12% is lost as heat conducted through the base and glass envelope. Indeed, these lamps are better described as very efficient electrical heaters which waste a small amount of their energy in the form of light!