Incandescent Filament Bulb

      The construction of the GLS lamp is shown in Fig. and a variation of this form of light source, known as the ''striplight'', is shown in Fig 1.

Fig. 1    GLS lamp

Fig. 2  striplight
      The GL incandenscent lamp produces light as a result of the heating effect of an electric current flowing through a tungsten filament wire. Tungsten has a melting point of 3380 °C and if the conditions are correct it can function in the incandenscent mode within a few hundred degree of this temperature. However, where considerable lasting power is required, such as for incandenscent lamps that have an average life of 1000 hours, the filament is kept to a much lower operating temperature of around 2500 °C.
     Tungsten filaments that are exposed to air at normal operating temperature of 2500 °C will quickly evaporate. For this reason the filament is situated inside a glass envelope (bulb) that has all the oxygen extracted.
      VACUUM type incandenscent lamps tend to blacken after a long period of operation due to the evaporation of the filament and the consequent deposit of metal particles on the glass wall of the lamp. This loss of tungsten from the filament will eventually cause it to burn out.
      Reduction the rate of evaporation of tungsten can be achieved by increasing the pressure on the filament by filling the lamp with an inert gas. Argon and nitrogen are the gases that are generally used. However, the introduction of the gas filing will tend to transfer heat more rapidly from the filament to the lamp wall. If a straight conductor filament was used, the gas would have a distinct cooling effect on the conductor, resulting in reduction of light output.
      To overcome this problem the filament is wound into a fine helical coil and the radiation between each turn of the coil raises the overall temperature and improves the light output of the lamp. The efficiency can be increased still further to around 15% when coiled coil filaments are used (Fig. 3)
Fig. 3 Coiled coil filament

GLS lamp data
. efficacy ranges between 10 and 18 lumens per watt
. good colour rendering
. pearl or clear bulb
. can be operated in any position
. average life 1000 hours for standard GLS lamps
. suitable for dimmer circuits
. no control gear required – connected direct to the source of voltage
. low initial cost
. applications – domestic, commercial and industrial uses
. lamp designation – GLS
     Further variations of the incandenscent lamp include:
. decorative lamps, for example candle type
. reflector lamps
. special purpose lamps for use in control panels, electric heaters and fires


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