Tungsten is a metallic element which is also known as Wolfram. Since tungsten melts at 3383°C, it can be heated to such a high
temperature
that it emits white light. Tungsten filaments are formed into coils minimizing the radiating surface so that the
light source
will be as small as possible. The luminous efficiency of tungsten lamps may vary and is about 22 lumens per watt, although short-life photoflood lamps reach 35.8 lumens per watt.
All incandescent lamps are thermal radiators, which generate light by heating a solid body to extremely high temperatures. The incandescent material must be able to reach such temperatures without melting and to maintain them over long periods of time at a low rate of vaporization.
The higher the burning
temperature
of the lamp,the brighter its light output will be. This will however be achieved at the cost of a shorter lamp life.
Consequently, lamps which have a high light output tend to have shorter rated
lamp
life.
A wide variety of lamps are manufactured to suit the exact requirements of various luminaires on the market. The structure of the filament and consequently the shape of the
lamp
are adapted to the shape of the luminaire and its optical system.
An incandescent filament at minimum power.
There are three main categories of tungsten filaments used in theatre luminaires:
Flat structure filaments. These can be monoplane, having a single incandescent surface, or biplane with a double incandescent surface. The biplane filament concentrates the
incandescence
within a smaller area, making the optical systems of the luminaires more effective.However, the light radiating from the
reflector
onto the filament may generate a great deal of heat that will eventually shorten the
lamp
life.
