"White" light exists mostly in theory; at any given time, the light we think of
as "white" is actually somewhere between a pale yellow and a pale blue.
This range is referred to as "color temperature".
It can best be demonstrated by setting the "white balance"
on a digital camera to "daylight" and taking a picture under incandescent
light. The resulting image will be slightly yellow, because incandescent
light is yellower – has a lower color temperature – than
does daylight. Likewise, setting the white balance to "tungsten" and
shooting a scene lit by daylight will yield a photo that is slightly bluish,
since daylight has a higher color temperature than does tungsten light.
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For this photograph, the white balance
was set to
"daylight" (or "outdoor").
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For this photograph, the white balance
was set to
"tungsten" (or "indoor").
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Color temperature is measured in "degrees Kelvin," abbreviated as "K."
As mentioned above,
the bluer a light is, the higher its color temperature will be. Below
is a table of approximate color temperatures for common light sources:
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Light Source
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Color Temperature
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Overcast Daylight
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6500° K
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Typical Daylight
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5500°–6000° K
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Theatre Lighting / Photofloods
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3050°–3400° K
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Standard Incandescent
Theatre Lighting
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3000° K
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Household Incandescent
Light Bulb
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2700°–3300° K
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Sun at Sunrise
or Sunset
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2000°–3000° K
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Candle Flame
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1850° K
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Match Flame
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1700° K
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So what does this mean? From what are these numbers derived? The
temperature is referenced to that of a standard "black body" –
a block of carbon which, when heated to specific temperatures, emits
light of specific colors.
The Kelvin scale (named after British physicist William Kelvin) is a
variation of the Centigrade/Celsius scale, except that rather than its
being keyed to the freezing point of water, it is based on "absolute
zero". If you subtract 273 from a Kelvin temperature, you will have
its Celsius equivalent.
You may be thinking that none of the colors above looks like "white"....
and you're right. First of all, they are merely approximations and
not, as such, accurate. Secondly – and more important –
the eye (or, more accurately, the brain) adapts. We tend to automatically
"white balance" ourselves to the dominant light source in our environment
at any given time. If, for example, you spend a significant amount of
time outdoors during the daylight hours and then go into your house or
apartment, the interior lighting may look yellowish for several hours,
until your brain "white balances" to its new environment. This is
something you should remember, by the way, when you're writing the
first few cues of your show.
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