Film Term:

A measurement of the color of light, and important in that film is much more sensitive to color temperature than our eyes are. Is measured on scale that takes its name from the scientist Lord Kelvin.

Glossary of Film Terms - http://homepage.newschool.edu/~schlemoj/film_courses/glossary_of_film_terms/
reprinted with permission

Color temperature has a specific physical meaning. If we can measure the spectrum of a radiating object, the color temperature is the temperature of a black body radiating most of its light at the same wavelength of the object.

When you plot a black body spectrum on a brightness versus frequency plot, it looks like a bell curve, steeper on the high frequency side than on the low. When you change the temperature of a black body, the shape of the spectrum doesn't change, but the frequency and height of the bell curve do. Thus if you measure the frequency of the peak, you can estimate the temperature of the object. This is described by Wien's Displacement Law,

νpeak = 5.88 × 1010 Hz × T (kelvin)

or

λpeak = 0.29 cm / T (kelvin)

If the object doesn't have a black body spectrum, or you can't precisely measure the spectrum over a wide range of frequencies, you simply estimate the color temperature based on the brightest frequency of light. A (relatively) cool object will have a black body peak in the red end of the visible spectrum, while a very hot one will have a peak in the blue end. The Sun is very near the middle of the visible spectrum, with a peak in the yellow. (Remember that blue light corresponds to very high temperatures, rather than the low temperatures we intuitively associate with water and ice.)

In photography, color temperature describes the lighting conditions under which a photograph was taken. According to the source on the net I found, the photographic color temperature scale roughly follows the physical one. However, the photographic scale is based upon our perception of the light's hue, rather than a physical temperature. The photographic scale ranges from very red light (T = 1000 kelvins) of candles, to warm yellow light (T = 5500 kelvins) of bright sunlight, to blue-white light (T = 10,000 kelvins) of heavy overcast, to very deep blue light (T = 20,000 kelvins) found at high elevation. This doesn't quite follow a physical black body since sunny days are much brighter than heavily overcast ones. But the progression of peak frequencies is the same, and the important point is the perceived hue of the illumination, rather than its intensity.

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brightness temperature -- color temperature -- effective temperature
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Sources:
Radiative Processes In Astrophysics, G. Rybicki and A. Lightman, Wiley Interscience
http://cybaea.com/photo/color-correction.html
Class lecture notes

The color temperature within photography is the apparent color of the light and measure the red/blue balance. This does not mean that the actual temperature of the source of the light is that hot. The sun is the source of light for sunrise, dusk, daylight, overcast and hazy sky and yet has a range from 3,100 K to 10,000 K.

Often color temperature is measured as "MIRED Value" which stands for MIcroREciprocal Degree. This can be calculated as
1,000,000 / Color Temperature

source      Color Temp (K)  MIRED Value
Candlelight   1,930          518
 75 Watt      2,800          357
100 Watt      2,900          345
200 Watt      3,000          333
Sunrise/set   3,100          323
Tungsten      3,200          312
Photolamp     3,400          294
Dawn/dusk     3,600          278
predawn/dusk  4,000          250
"Daylight"    4,500          222
  fluorescent
Carbon arc    5,200          192
Daylight      5,500          182
Camera flash  6,000          167
Overcast sky  7,000          143
Hazy sky     10,000          100
sunless blue 11,000           91
  sky
clear sky in 20,000           50
  mountains

Most film is balanced for daylight at 5500 K. Using a light source that is warmer (lower color temperature) or cooler (higher color temperature) requires a filter to readjust the light. For example, if you are taking a picture without a flash of a person with daylight film under a 200 watt bulb the photograph will appear very warm (the color balance would be shifted to the red). Likewise, a photograph on a bright hazy day will appear much bluer.

Filter  Conversion (K)  MIRED
80A     3200 to 5500    -131 |  Cooling
80B     3400 to 5500    -112 |  Filters
80C     3800 to 5500    - 81 |
80D     4200 to 5500    - 56 |  (blue)
82C     2800 to 3200    - 45 |
82B     2900 to 3200    - 32 |
82A     3000 to 3200    - 21 |
82      3100 to 3200    - 10 |
---------------------------------------
81      3300 to 3200    +  9 |
81A     3400 to 3200    + 18 | Warming
81B     3500 to 3200    + 27 | Filters
81C     3600 to 3200    + 35 |
81D     3700 to 3200    + 42 | (amber)
81EF    3850 to 3200    + 53 |
85C     5500 to 3800    + 81 |
85      5500 to 3400    +112 |
85B     5500 to 3200    +131 |

It is easy to see that these filters mostly deal with daylight balanced film (5500K) and tungsten-balanced film (3200K). Furthermore, slide film (typically the only place you find the tungsten balance) is much more accurate with its color renditions and thus the increments are 100K rather than the 200K or 400K units with print film.

The MIRED shift value above is defined as:

1000 * (1000/T2 - 1000/T1)
Where T1 is the color temperature present and T2 is the desired color temperature (most often that of the film). This value is often called the Light Balance Index of the filter.

The using the light balance for a filter is not specific to that color and can be used for other color shifts. Furthermore, it is additive so that stacking two filters will add the values together.

Take for example you are photographing a person with daylight film at dusk and you want the skin tone rendered correctly. Looking at the chart you see that the color temperature is about 4000K, but alas there is no 4000K to 5500K shift. So, you whip out your calculator:

1000 * (1000/5500 - 1000/4000)
A color shift of -68. Glance at your filters and... oh no! You've got a -81 color shift, and a -56 and a whole bunch more - but nothing that is -68. Relax, You can stack a 82 (LB index of -10) on top of the 80D (LB index of -56) for a shift of -66. Or you could put a 82A (LB index of -21) on top of a 82C (LB index of -45) also for a shift of -66.

A common complaint with flash photography is that the light is too cool - and it is. The camera flash is 6000K while the film is at 5500K often giving a blueish cast to things. This requires some warming. Once again the table has nothing that outright says "6000K to 5500K"... so...

1000 * (1000/5500 - 1000/6000)
A +15 color shift. In most cases, people recommend an 81A for work with a flash.


http://cybaea.com/photo/color-correction.html
The Nikon Field Guide

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