The Black Body Locus
The Black Body Locus is measured in Kelvin. Indicated along the Black Body Curve are
the color temperatures for candlelight or sunrise (approximately 1800K), and household
incandescent lamps at 2848K. Most theatrical lamps are 3200K, but with line voltage
drop and dimming you tend to get less. Also shown is 5400K which is nominal
daylight. Daylight color temperature will range from sunrise at 1800K to north sky
at 28,000K.
A Black Body Radiator is a theoretical source which when chilled to minus 273ºC
(absolute 0) emits no energy. As the theoretical Black Body's temperature is raised,
it begins to glow. At approximately 950K (1223ºC) the Black Body will emit a small
deep red glow which is visible to the human eye. As we raise the temperature
to about 1800K, it will have a very warm glow much like fire, candlelight or sunrise.
Incandescent light sources used in our home are approximately 2848K. Many theatrical
light sources are designed to be 3200K unless they are long life lamps which have
a lower Kelvin temperature, usually around 3000K. If we do not apply the proper
voltage or the lamps are dimmed, their Kelvin temperature will drop. As the
Black Body Radiator's temperature is raised, it will get bluer in its content,
and will have more and more blue energy in relation to its red energy.
5400 Kelvin has been designated as nominal noon day sun but in reality the color
temperatures of the sky can be much higher, depending on weather conditions and
time of day. The higher the Kelvin temperature, the bluer its content.
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