Editing Incandescent light bulb (section)

==Construction==
Incandescent light bulbs consist of an air-tight glass enclosure (the envelope, or bulb) with a filament of [[tungsten]] wire inside the bulb, through which an [[electric current]] is passed. Contact wires and a base with two (or more) conductors provide electrical connections to the filament. Incandescent light bulbs usually contain a stem or glass mount anchored to the bulb's base that allows the electrical contacts to run through the envelope without air or gas leaks. Small wires embedded in the stem in turn support the filament and its lead wires.

An electric current heats the filament to typically {{convert|2000|to|3300|K|°C °F}}, well below tungsten's melting point of {{convert|3695|K|°C °F}}. Filament temperatures depend on the filament type, shape, size, and amount of current drawn. The heated filament emits light that approximates a [[black body|continuous spectrum]]. The useful part of the emitted energy is [[visible light]], but most energy is given off as heat in the near-[[infrared]] wavelengths.

===Bulbs===

Most light bulbs have either clear or coated glass. Coated glass bulbs have [[kaolin]] clay blown in and electrostatically deposited on the interior of the bulb. The powder layer diffuses the light from the filament. Pigments may be added to the clay to adjust the color of the light emitted. Kaolin diffused bulbs are used extensively in interior lighting because of their comparatively gentle light. Other kinds of colored bulbs are also made, including the various colors used for "party bulbs", [[Christmas tree]] lights and other decorative lighting. These are created by [[Glass coloring and color marking|coloring the glass]] with a [[dopant]]; which is often a metal like [[cobalt]] (blue) or [[chromium]] (green).<ref>{{cite web|url=http://www.geconsumerandindustrial.com/environmentalinfo/documents/msds/msds_incandescent_lamps.pdf|title=Lamp Material Information Sheet – Incandescent Lamp|access-date=20 May 2013|url-status=usurped|archive-url=https://web.archive.org/web/20130521005210/http://www.geconsumerandindustrial.com/environmentalinfo/documents/msds/msds_incandescent_lamps.pdf|archive-date=21 May 2013}}</ref> Neodymium-containing glass is sometimes used to provide a more natural-appearing light.

{| border="0"
|
[[File:Incandescent light bulb.svg|frameless|upright=1.25|center]]
|
#Outline of Glass bulb
#Low pressure inert gas ([[argon]], [[nitrogen]], [[krypton]], [[xenon]])
#[[Tungsten]] filament
#Contact wire (goes into stem)
#Contact wire (goes out of stem)
#Support wires (one end embedded in stem; conduct no current)
#Stem (glass mount)
#Contact wire (goes out of stem)
#Cap (sleeve)
#Insulation ([[vitrite]])
#Electrical contact
|}

The glass bulb of a general service lamp can reach temperatures between {{convert|200|and|260|°C|°F}}. Lamps intended for high power operation or used for heating purposes will have envelopes made of hard glass or [[fused quartz]].<ref name="GETP110" />

If a light bulb envelope leaks, the hot tungsten filament reacts with air, yielding an aerosol of brown [[tungsten nitride]], brown [[tungsten dioxide]], violet-blue [[tungsten pentoxide]], and yellow [[tungsten trioxide]] that then gets deposited on the nearby surfaces or the bulb interior.

===Gas fill===

Most modern bulbs are filled with an [[inert gas]] to reduce [[evaporation]] of the filament and prevent its [[oxidation]]. The gas is at a pressure of about {{convert|70|kPa|atm|abbr=on|1}}.<ref name="argon1">{{cite web |url=http://www.uigi.com/argon.html |title=Argon (Ar) Properties, Uses, Applications Argon Gas and Liquid Argon |website=Gas Properties, Uses, Applications |publisher=Universal Industrial Gases, Inc. |archive-url=https://web.archive.org/web/20120204105153/http://www.uigi.com/argon.html |archive-date=4 February 2012 }}</ref>

The gas reduces evaporation of the filament, but the fill must be chosen carefully to avoid introducing significant heat losses. For these properties, chemical inertness and high [[Relative atomic mass|atomic]] or [[Molecular mass|molecular weight]] is desirable. The presence of gas keeps the tungsten below its vapor pressure, allowing it to be operated at higher temperature without reducing its life (or, for operating at the same temperature, prolongs the filament life). On the other hand, the presence of the gas leads to heat loss from the filament—and therefore efficiency loss due to reduced incandescence—by [[heat conduction]] and [[heat convection]].

Early lamps used only a vacuum to protect the filament from oxygen. The vacuum increases evaporation of the filament but eliminates two modes of heat loss. Some small modern lamps use vacuum as well.

The most commonly used fills are:<ref name="ChemArtLight">{{cite book|isbn=978-0080933153|url=https://books.google.com/books?id=eaj8BAAAQBAJ&pg=PA263|title=The Chemistry of Artificial Lighting Devices|publisher=Elsevier Science|last=Ropp|first=Richard C.|date = 22 October 2013|url-status=live|archive-url=https://web.archive.org/web/20171206142310/https://books.google.com/books?id=eaj8BAAAQBAJ&pg=PA263&dq=tungsten+wire+drawing&hl=en&sa=X&ei=zzfRVI-9F4HqUOX7gfgJ&ved=0CE0Q6AEwBQ#v=onepage&q=tungsten%20wire%20drawing&f=false|archive-date=6 December 2017}}</ref>

* [[Vacuum]], used in small lamps. Provides best [[thermal insulation]] of the filament but does not protect against its evaporation. Used also in larger lamps where the outer bulb surface temperature has to be limited.
* [[Argon]] (93%) and [[nitrogen]] (7%), where argon is used for its inertness, low [[thermal conductivity]] and low cost, and the nitrogen is added to increase the breakdown voltage and prevent arcing between parts of the filament<ref name="argon1"/>
* Nitrogen, used in some higher-power lamps, e.g. projection lamps, and where higher breakdown voltage is needed due to proximity of filament parts or lead-in wires
* [[Krypton]], which is more advantageous than argon due to its higher atomic weight and lower thermal conductivity (which also allows use of smaller bulbs), but its use is hindered by much higher cost, confining it mostly to smaller-size bulbs.
* Krypton mixed with [[xenon]], where xenon improves the gas properties further due to its higher atomic weight. Its use is however limited by its very high cost. The improvements by using xenon are modest in comparison to its cost.
* [[Hydrogen]], in special flashing lamps where rapid filament cooling is required; its high thermal conductivity is exploited here.
* [[Halogen]], a small amount mixed with inert gas. This is used in halogen lamps, which are a distinct type of incandescent lamp.

The gas fill must be free of traces of water, which greatly accelerates bulb blackening (see below).

The gas layer close to the filament (called the Langmuir layer) is stagnant, with heat transfer occurring only by conduction. Only at some distance does convection occur to carry heat to the bulb's envelope.

The orientation of the filament influences efficiency. Gas flow parallel to the filament, e.g., a vertically oriented bulb with vertical (or axial) filament, reduces convective losses.

The efficiency of the lamp increases with a larger filament diameter. Thin-filament, low-power bulbs benefit less from a fill gas, so are often only evacuated.

Early light bulbs with carbon filaments also used [[carbon monoxide]], [[nitrogen]], or [[Mercury (element)|mercury]] vapor. However, carbon filaments operate at lower temperatures than tungsten ones, so the effect of the fill gas was not significant as the heat losses offset any benefits.