What is INCANDESCENT BULB?
An incandescent bulb is any electric light bulb that directly emits light (photons) by means of heating a filament to the point of incandescence. Confusingly, it is now generally accepted in the lighting industry that Incandescent bulbs are specifically vacuum filament bulbs or inert gas filament bulbs EXCEPT for halogen gas bulbs and xenon gas bulbs.
Typical parts of an incandescent bulb are; glass, filament, base, exhaust tube, stem press, lead-in wires, fuse, heat-deflecting disc, button, support wires.
All oxygen gas must be removed from the bulb or it will quickly burn out. Either air is removed by vacuum then bulb is sealed where bulb is under vacuum it's entire life, or all oxygen is replaced by an inert gas (argon/nitrogen mixture is commonly used). To remove the rest of the oxygen a "getter" is used. Inert gas is used on nearly all bulbs 40 watts and over.
When a filament is heated, a small amount of tungsten metal particles boils off which condense as black soot on glass areas and internal supports which are cooler.
Over time the light output of the bulb is reduced because of the dark sooty build-up of tungsten particles on the glass partially blocking light from escaping the glass envelope. An interesting fact is that light output actually increases a little during the first part of bulbs life (before the soot build-up).
In vacuum bulbs, there isn't any convection current so bulb sooting is more evenly dispersed.
In inert gas filled bulbs, a convention current takes place and the majority of soot condenses directly above the heated filament.
On bulbs that are designed for use in any burn position, when operated "base-up" position, convection current will deposit a majority of the soot in the neck, leaving the main bulb chamber much clearer.
Some gas filled bulb include metal screens or grids placed directly above the filament designed for capturing soot so the glass stays clearer.
Some specialty bulbs purposely include the addition of tungsten granules allowing the user the ability to physically shake the bulb clean by mechanically cleaning (sanding) away most of the soot (powdered soot and tungsten granules lay at the bottom harmlessly.
As tungsten metal boils off, the filament gets thinner and an effect called "notching" occurs.
The filament doesn't randomly get thinner. Tungsten vapor boils off more rapidly from the hottest sections of the filament thereby causing those areas to get thinner quicker, which then causes those sections to raise in temperature. In other words, the thinnest parts of a filament are the sections that keep getting thinner the quickest.
As the filament gets thinner, it eventually burns out.
Also, it should be obvious that the thinner a filament gets, the more fragile and susceptible to physical (and electrical) shock which may cause the ruin of the bulb prematurely.
Tungsten filament bulbs can be specially designed to take advantage of some special physical and chemical reactions with halogen (Tungsten halogen cycle, or Halogen Bulb) or xenon gas (xenon bulb) to improve performance in certain applications.
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references to this page: CANDLEPOWER, COLOR RENDERING INDEX, GLASS, HALOGEN BULB, INCANDESCENT, LUMEN, LUMENS PER WATT, VOLT