A
luminous flame is a burning
flame which is brightly visible. Much of its output is in the form of
visible light, as well as heat or light in the non-visible wavelengths.
An early study of flame luminosity was conducted by Michael Faraday and became part of his series of Royal Institution Christmas Lectures, The Chemical History of a Candle.
Luminosity
In the simplest case, the yellow flame is
luminosity due to small
soot particles in the flame which are heated to
incandescence. Producing a deliberately luminous flame requires either a shortage of combustion air (as in a
Bunsen burner) or a local excess of fuel (as for a
kerosene torch). Because of this dependency upon relatively inefficient combustion, luminosity is associated with
and is lessened with
.
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 Bunsen burner). The colour of simple incandescence is due to black-body radiation. By Planck's law, 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 () 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
Bunsen burner. 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
soot and/or
carbon monoxide represents a waste of fuel (further burning was possible) the potential problem of soot build-up in 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
gas 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 elements to provide a bright white glow; the colour of the glow comes from the
of these elements, not from simple black-body radiation.
Flame testing
When performing a
flame test, 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.
