A luminous flame is a burning

flame A flame (from Latin ''flamma'') is the visible, gaseous part of a fire. It is caused by a highly exothermic chemical reaction taking place in a thin zone. When flames are hot enough to have ionized gaseous components of sufficient density they ...

which is brightly visible. Much of its output is in the form of

visible light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...

, as well as heat or light in the non-visible wavelengths. An early study of flame luminosity was conducted by

Michael Faraday Michael Faraday (; 22 September 1791 – 25 August 1867) was an English scientist who contributed to the study of electromagnetism and electrochemistry. His main discoveries include the principles underlying electromagnetic inducti ...

and became part of his series of

Royal Institution Christmas Lectures The Royal Institution Christmas Lectures are a series of lectures on a single topic each, which have been held at the Royal Institution in London each year since 1825, missing 1939–1942 because of the Second World War. The lectures present sci ...

, ''

The Chemical History of a Candle ''The Chemical History of a Candle'' was the title of a series of six lectures on the chemistry and physics of flames given by Michael Faraday at the Royal Institution in 1848, as part of the series of Christmas lectures for young people found ...

''.

Luminosity

In the simplest case, the yellow flame is luminous due to small

soot Soot ( ) is a mass of impure carbon particles resulting from the incomplete combustion of hydrocarbons. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolysed ...

particles in the flame which are heated to

incandescence Incandescence is the emission of electromagnetic radiation (including visible light) from a hot body as a result of its high temperature. The term derives from the Latin verb ''incandescere,'' to glow white. A common use of incandescence is ...

. Producing a deliberately luminous flame requires either a shortage of combustion air (as in a

Bunsen burner A Bunsen burner, named after Robert Bunsen, is a kind of ambient air gas burner used as laboratory equipment; it produces a single open gas flame, and is used for heating, sterilization, and combustion. The gas can be natural gas (which is main ...

) or a local excess of fuel (as for a

kerosene Kerosene, paraffin, or lamp oil is a combustible hydrocarbon liquid which is derived from petroleum. It is widely used as a fuel in aviation as well as households. Its name derives from el, κηρός (''keros'') meaning "wax", and was regi ...

torch). Because of this dependency upon relatively inefficient combustion, luminosity is associated with

diffusion flame In combustion, a diffusion flame is a flame in which the oxidizer and fuel are separated before burning. Contrary to its name, a diffusion flame involves both diffusion and convection processes. The name diffusion flame was first suggested by S. ...

s and is lessened with

premixed flame A premixed flame is a flame formed under certain conditions during the combustion of a premixed charge (also called pre-mixture) of fuel and oxidiser. Since the fuel and oxidiser—the key chemical reactants of combustion—are available throughou ...

s. The flame is yellow because of its temperature. To produce enough soot to be luminous, the flame is operated at a lower temperature than its efficient heating flame (see

). The colour of simple incandescence is due to

black-body radiation Black-body radiation is the thermal electromagnetic radiation within, or surrounding, a body in thermodynamic equilibrium with its environment, emitted by a black body (an idealized opaque, non-reflective body). It has a specific, continuous spect ...

. By

Planck's law In physics, Planck's law describes the spectral density of electromagnetic radiation emitted by a black body in thermal equilibrium at a given temperature , when there is no net flow of matter or energy between the body and its environment. At ...

, as the temperature decreases, the peak of the black-body radiation curve moves to longer wavelengths, i.e. from the blue to the yellow. However, the blue light from a gas burner's premixed flame is primarily a product of molecular emission (

Swan band Swan bands are a characteristic of the spectra of carbon stars, comets and of burning hydrocarbon fuels. They are named for the Scottish physicist William Swan, who first studied the spectral analysis of radical diatomic carbon (C2) in 1856. S ...

s) rather than black-body radiation. Other factors, particularly the fuel chemistry and its propensity for forming soot, have an influence on luminosity.

Bunsen burner

One of the most familiar instances of a luminous flame is produced by a

. This burner has a controllable air supply and a constant gas jet: when the air supply is reduced, a highly luminous, and thus visible, orange 'safety flame' is produced. For heating work, the air inlet is opened and the burner produces a much hotter blue flame.

Combustion efficiency

Efficient combustion relies on the complete combustion of the fuel. Production of

represents a waste of fuel (the soot could have been burned further) and may also be a problem for its build-up around jets and burners. Heating burners are thus usually designed to produce a ''non-''luminous flame.

Oil lamps

Lamps for illumination rather than heat may use a deliberately luminous flame. A more efficient method overall uses a mantle instead.A Brief History of the Incandescent Mantle Pressure Lamp
Like the incandescent soot in a luminous flame, the mantle is heated and then glows. The flame does not provide much light itself, and so a more heat-efficient non-luminous flame is preferred. Unlike simple soot, a mantle uses

rare-earth element The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silve ...

s to provide a bright white glow; the colour of the glow comes from the

spectral line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...

s of these elements, not from simple black-body radiation.

Flame testing

When performing a

flame test A flame test is an analytical procedure used in chemistry to detect the presence of certain elements, primarily metal ions, based on each element's characteristic flame emission spectrum (which may be affected by the presence of chloride ion ...

, the colour of a flame is affected by external materials added to it. A non-luminous flame is used, to avoid masking the test colour by the flame's colour.

References

{{Reflist, colwidth=30em Combustion engineering Fire Light