The higher the
temperature
of a light source,
the more light it emits. These differences in
temperature and quantity influence the color
composition of the light.
The
incandescence
of any
light source
depends on the type of material it is made of.
The filament of tungsten lamps will begin
to emit light at a
temperature
of
about 600O C (873O K) .
The relationship between the
temperature
of
a
light source
and its spectral makeup,
i.e. its color, was formulated by Lord William
Thomson
Kelvin
(1824-1907).
Kelvin conceived a
temperature
scale beginning
at 0 degrees K at the theoretical absolute zero,
i.e. -273.16O degrees C, and placing the boiling point of water,
100OC at 373.1O K.
Kelvin's
color temperature
model is based on a theoretical black body which absorbs all light falling on it and reflects no light at all. When this body's
temperature
is absolute zero, or 0 K, it emits no
radiation
at all . When the body is heated, it radiates electromagnetic radiation, first of long wavelengths, such as
infrared
radiation. As the
temperature
rises, some of the wavelengths are emitted in the form of colored light, as is the case when a rod of iron is heated over a fire . It begins to glow, turning from red to yellow red to warm white light, and then to a cold, bluish white light .
Thus psychological concepts such as cool light and warm light do not reflect the
color temperature
of the light. A cool blue light such as daylight has high color temperature, while a warm orange light such as candlelight is of a low
color temperature
(6) .
The
color temperature
of a luminaire can be changed by placing a
color filter
in front of the lens. Certain filters have been designed specially to match different light sources, such as filters #201, #202 and #203 which adapt the
color temperature
of a halogen
lamp
so that it matches that of various degrees of sunlight. This and other series of filters were developed for the film industry, where the matching of color temperatures and film emulsions is of utmost importance.
