Incandescent light bulb#Filament

A filament, and the filament or filament is a coiled wire of tungsten or tungsten alloys, which converts its resistance by an electric current in the radiation and heat energy. In contrast to a filament heater coil is operated at higher temperatures, so that a higher proportion (about 3-15 %) of the radiation is emitted as visible light. The physical basis for improving the light yield by a filament is due to the Langmuir layer around the filament.

The picture shows the filament as a double helix in a light bulb for mains voltage at reduced voltage. Good to see the heat losses on power cable and - much lower - to the eyes of the retaining wire.

A filament represents a typical thermistor, i.e. its resistance is less than the moment of starting the operation. This results in a higher current flow during switching, which in turn explains the phenomenon that filaments burn out usually when turning.

Molding

• Single-coiled filaments (up to about 24 volts) used for incandescent lamps of lower operating voltage
• A double helix is ​​a spiral, which in turn is formed into a spiral again: by the denser packing in the coiled coil, the coils heat up each other, resulting in the same power at a higher temperature and thus higher light yield.
• A flat spiral is a projection lamps in common, transverse to the optical path standing flat coil having a rectangular shape.

Related Topics

The color temperature of incandescent light is about the temperature of the filament. Color temperature and light output to rise with increasing operating voltage. Projection lamps have particularly high temperatures of the filament, but lifetimes of only a few hundred hours. The color temperature can be by coating the glass envelope (mostly interference filter, as in blue or bright yellow car light bulbs ) or dichroic mirror (eg for halogen dichroic lamps ) influenced later.

The heat balance of filaments can be affected in addition to the double helix shape also by the back-reflection of radiation to the filament. Projection lamps are therefore adjusted so that the image of the underlying spherical mirror comes to lie adjacent to the filament - filament from ( flat spiral ) and its image is obtained composed the rectangular shape of the illuminated field of view. In so-called IRC halogen lamps, however, the glass bulb is a dichroic coating so that the proportion of infrared radiation is reflected on the filament. Characterized the light output is increased up to 45 %.

The image at right shows a close-up of a halogen filament. It shows not only that the filament is burned out. On one side of the interruption a small molten tungsten tip can be seen. Consequently, the lamp has failed during operation at high temperature of the coil.