The incandescent lamp is a thermal radiator. The filament wire begins to glow when it is heated to a sufficiently high temperature by an electric current. As the temperature increases the spectrum of the radiated light shifts towards the shorter wavelength range – the red heat of the filament shifts to the warm white light of the incandescent lamp.
Depending on lamp type and wattage the temperature of the filament can reach up to 3000 K, in the case of halogen lamps over 3000 K. Maximum radiation at these temperatures still lies in the infrared range, with the result that in comparison to the visible spectrum there is a high degree of thermal radiation and very little UV radiation.
Lack of a suitable material for the filament means that it is not possible to increase the temperature further, which would increase the luminous efficacy and produce a cool white luminous colour. As is the case with all heated solid bodies – or the highly compressed gas produced by the sun – the incandescent lamp radiates a continuous spectrum.
The spectral distribution curve is therefore continuous and does not consist of a set of individual lines. The heating of the filament wire results from its high electrical resistance – electrical energy is converted into radiant energy, of which one part is visible light.
Although this is basically a simple principle, there are a substantial number of practical problems involved in the construction of an incandescent lamp. There are only a few conducting materials, for example, that have a sufficiently high melting point and at the same time a sufficiently low evaporation rate below melting point that render them suitable for use as filament wires.
Nowadays practically only tungsten is used for the manufacture of filament wires, because it only melts at a temperature of 3653 K and has a low evaporation rate. The tungsten is made into fine wires and is wound to make single or double coiled filaments. In the case of the incandescent lamp the filament is located inside a soft glass bulb, which is relatively large in order to keep light loss, due to deposits of evaporated tungsten (blackening), to a minimum.
To prevent the filament from oxidizing the outer envelope is evacuated for low wattages and filled with nitrogen or a nitrogen-based inert gas mixture for higher wattages. The thermal insulation properties of the gas used to fill the bulb increases the temperature of the wire filament, but at the same time reduces the evaporation rate of the tungsten, which in turn leads to increased luminous efficacy and a longer lamp life.
The inert gases predominantly used are argon and krypton. The krypton permits a higher operating temperature – and greater luminous efficacy. Due to the fact that it is so expensive, krypton is only used in special applications.
A characteristic feature of incandescent lamps is their low colour temperature - the light they produce is warm in comparison to daylight. The continuous colour spectrum of the incandescent lamp provides excellent colour rendition.
As a point source with a high luminance, sparkling effects can be produced on shiny surfaces and the light easily controlled using optical equipment. Incandescent lamps can therefore be applied for both narrow-beam accent lighting and for wide-beam general lighting.
Incandescent lamps can be easily dimmed. No additional control gear is required for their operation and the lamps can be operated in any burning position. In spite of these advantages, there are a number of disadvantages: low luminous efficacy, for example, and a relatively short lamp life, while the lamp life relates significantly to the operating voltage.
Special incandescent lamps are available with a dichroic coating inside the bulb that reflects the infrared component back to the wire filament, which increases the luminous efficacy by up to 40 %