Editing Incandescent light bulb (section)

===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.