Color Produced by Incandescence
Dagmar Kamenářová
Fakulta chemická, Vysoké učení technické v Brně
Purkyňova 118, 612 00 Brno
e-mail: [email protected]
INTRODUCTION
All the colors in the universe originate from a mere fifteen fundamental physical causes.
These fifteen causes of color derived from a variety of physical and chemical mechanisms are
summarized in five groups. Vibrations and simple excitations explain the colors of
incandescence (e.g., flames), gas excitations (neon tube, aurora), and vibrations and rotations
(blue ice and water).
Incandescence refers to light produced by an object solely by virtue of its temperarure. The
range of colours produced by incandescence is limited to the continuously grading sequence:
black, red, orange, yellow, white, and blue-white.[1] The light produced consists of photons
emitted when atoms and molecules release part of their thermal vibration energy.
Light is often said to have a color temperature. What this means is that the color of the light
is the color of light radiated by a so-called black body (an idealized radiating object) which is
at that temperature. Color temperature is measured in Kelvins or degrees of Celsius and the
higher the color temperature the bluer the light. In practice the actual temperature is not the
same as the color temperature. [2]
The color temperatures of some common light sources are shown in Table 1.
Table 1
Approx.
20,000 K
Open sky
6,500 K
5,400 K
3,780 K
3,400 K
2,865 K
1,930 K
Overcast
sky
Direct
sunlight
Carbon arc
light
Photoflood
bulb
100Watt
tungsten
bulb
Candle
flame
Although we can employ incandescence to produce color, it is used primarily to supply a
light as close to “white” daylight as possible so that color matching, color photography, and
similar activities can be coducted independently of the sun.
INCANDESCENCE
If an object is heated its color changes as its temperature rises. We use these colors to describe
temperature colloquially: “red hot”, “yellow hot,” and so on. The definition of “white hot” is
conditioned by daylight as derived from the sun, the surface of which has a temperature of
about 5700°C. Some stars are even hotter than this and radiate bluish white light.
The increasing temperature derives from the increasing energy in the object emitting the
light. The hotter the object, the more energetic the vibrations of the atoms in the oject and the
higher the energy and therefore the frequency of the light in equilibrium with it. This
parallelism is shown in Figure 1, which ilustrates the spectrum and three ways in which the
spectral colors can be designed: frequency in hertz, wavelength in nanometers, and energy in
electron volts.
Figure 1. Spectrum with three ways of numerically
specifying the colors.
As shown in Figure 2, a barely red-hot
object at 700°C radiates mostly in the
infrared beyond 1000 nm. The next curve
at 1700°C shows a peak near 1500 nm,
but with significant light radiated
throughout the entire spectrum. The
resulting percived color of this curve is
an orange. This curve does not actually
cross the 700°C curve; it is much higher
in position but height has been reduced to
remain in the diagram. The next two
curves, additionally reduced, give the
calculated emission for the 5700°C
surface temperature of the sun and also
the curve corresponding to actual
daylight. The alignment of the daylight
curve with the spectral sensitivity curve
of the eye gives us the definition of
“white”.
BLACKBODY RADIATION
The curves in Figure 2 are called ideal or
blackbody curves, since they imply an
idealized body that does not reflect or
transmit light but absorbs light completely
at any energy, which would naturally
result in a black appearance. Such an
object is also able to emit light of any
energy, but does so only in a way
governed by its own temperature.
An incandescent object which is not a
blackbody, that is, any real object
whatever its temperature, will have a light
emission that deviates from the calculated
curves. Whatever the deviation, it is
always possible to assign a color
temperature, namely, that the temperature
of a true blackbody emitter which, to the
eye, best matches the color of the actual
incandescent object.
The continuous sequence of colors
Figure 2. Radiation curves for blackbodies at 700,
exhibited by an incandescent body can be
1700, and 5700°C, curve for sunlight and the
displayed as a curve on the chromaticity
sensitivity curve of the eye.
diagram of Figure 3. The sequence
includes black, red, orange, yellow, white, and blue-white as the temperature increases.
However high the temperature, the last point on the curve marked ∞ for infinity cannot be
exceeded.
Since the color emitted by an object varies with its temperature, it is also possible to
measure the temperature by determining the color. An optical pyrometer frequently functions
by comparing the color with that of an electrically heated filament.
Figure 3. Blackbody colors shown on the chromaticity diagram.
INCANDESCENT LIGHT SOURCES
All early human light sources were based on radiation from heated objects, and many modern
still are. The reddish glow from a wood or coal fire and the orange light from a candle or
kerosene flame both derive from hot objects.
The science of a candle flame was reported by Michael Faraday
(1791-1867). Color tells us about the temperature of a candle flame.
The outer core of the candle flame is light blue (1400°C). That is
the hottest part of the flame. The color inside the flame becomes
yellow, orange and finally red. The further you reach to the center
of the flame, the lower the temperature will be. The red portion is
around 800°C. The reason there is this variation in a candle´s flame
color is because air convection pulls the warmer gasses upwards. It
is shown in Figure 4.
Figure 4.
Burning in limited air in the dark center of the flame, some
carbon particles (soot) are produced by reactions occuring at 800 to 1000° C, such as
C17 H 35 CO 2 H + 11 O 2 → 9 H 2 O + 5 CO + 8 C + 9 H 2
Coplete combustions occurs in the outermost regions of the flame at 1200-1400° C:
2 CO + O 2 → 2 CO 2
2 C + 2 O 2 → 2 CO 2
2 H 2 + O 2 → 2 H 2O
A candle flame is classified as a diffusion flame, since significant quantities of oxygen must
diffuse inward through the reaction products to reach the burning zone.
When coal gas became available, early fishtail burners produced a light similar in quality
to that from a candle or kerosene flame but much more intense. A similar light source is the
carbide lamp, which has been used widely by cave explorers. Its soot-rich acetylene flame
(the gas is produced from the reaction of calcium carbide with water) gives off an intense
light.
Limelight was discovered in 1816. It is the very brilliant light emitted by lime. About 70
years later the Wellsbach mantle was invented. It is based on burning cotton gauze which
was soaked in a mixture of thorium and nitrates. These mantles are still employed in lanterns
used for camping or for emergency purposes, now usually fueled by propane.
With the advent of electrocity came the carbon arc. On touching two carbon rods
connected to a powerful electric current source, a spark forms and heats and vaporizes some
of the carbon. The carbon vapor continues to conduct the electric current. Both the carbon rod
ends as well as particles of carbon in the vapor become very hot and emit an intense, almost
white light with a color tmeperature of about 3700°C.
Direct incandescence from heated filament was invented independently and almost
simoutaneously by Thomas Alva Edison and by Joseph Swan about 1878. At first a carbon
filament was used in an evacuated glass bulb. Later metal filaments were used, finally ending
up with tungsten. Tungsten has lower melting point of 3380°C than the 3550°C of graphite. It
has also a much lower rate of evaporation (one of the lowest of any metal) which permits
tungsten to be used at a higher temperature. The use of a coiled filament, the addition of
nonreactive gas (nitrogen at first, but later argon) and finally, the use of a frosted bulb,
copleted the major development of the incandescent light bulb as we know it today, with the
filament operating at about 2500°C.
Higher temperatures are available in quartz-halogen light bulbs. Here a bulb is made of
fused quartz and a small amount of the halogens bromine or iodine is present along with the
inert gas filling.
PYROTECHNICS
Incandescence is a major ingredient in pyrotechnics, in both entertainment fireworks and
safety flares, and in a variety of military uses. Most of these brilliant illuminations are based
on combustion of magnesium powder. The color can be modified by the use of sodium nitrate
for an orange color, strontium nitrate, possibly combined with potassium nitrate, for red,
barium nitrate for green, and copper nitrate or copper arsenic compounds for green and blue.
In the attractive burning sparklers, the major igredients again are nitrates or chlorates, but
slower-burning aluminium powder and/or iron fillings are used insted of magnesium.
An important incandescent light source is the photographic flash. The flashbulb was first
used in about 1925 and it is the glass envelope containing shredded zirconium metal and
oxygen. On electric ignition, the burning produces molten zirconium and zirconium oxide,
which radiate light with a color temperature of about 4000°C.
THE NATURE OF INCANDESCENT LIGHT
Light from an incandescent source is disordered in four different ways. Two of these
disorders can, in theory, be corrected, but the other two are inherent in the way the light is
produced. Consider the hot, light-radiating filament in the light bulb of Figure 5. Different
parts of the filament emit light at different wavelenghts in different directions and
intermittently in time. A point on the filament that has emitted a photon has lost some energy
and will have cooled a little. The energy is soon replaced from the electrical heating. The next
photon emitted from that same point could have the same energy or could have a different
energy.
Even if made parallel by a lens and monochromatic by a filter, it is still spatially incoherent,
as at B and C, and temporally incoherent, as at D and E.
Figure 5. Light from an incandescent lamp contains quanta of various wavelengths
emitted in various directions.
SUMMARY
Incandescence is produced by any material solely because it is at a high temperature, so that
the atoms and molecules emit part of their energy of vibration as photons. With increasing
temperature, the color sequence black, red, orange, yellow, white, and blue-white is produced.
The incandescent objects can deviate from the ideal blackbody incandescent curves, but it is
always possible to assign a color temperature. The color temperature is the temperature of
blackbody which to the eye most closely matches the color.
Photons can be out of step with each other in four ways: in direction, in energy, in spatial
incoherence, and in temporal incoherence.
LITERATURE
[1] Kurt Nassau: The Physic and Chemistry of Color, The Fifteen Causes of Color
[2] Web Exhibits, http://webexhibits.org/causesofcolor/index.html