1 The Director of Photography – an overview

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Colour temperature
What is colour temperature?
Colour temperature is important to the cinematographer for a number
of reasons. Technically, it is vital that any light source that is to
appear neutral in the final rendition of the scene is emitting light of the
same colour balance as that for which the film has been designed.
Artistically, it is important that the cinematographer has an understanding
of the psychological and emotional effects of the colour of
light within a scene and that they have complete control over this colour.
In order to evaluate and control anything, we must have a unit of
measurement and a means of measuring. We measure differences in
colour temperature in degrees Kelvin (°K) using a colour temperature
meter.
This unit of measurement is named after Lord William Thomson
Kelvin, a British physicist who, at the end of the nineteenth century,
developed a means of quantifying that which makes colours different.
His experiment was quite simple. He took a beam of daylight coming
through a hole in his shutters and caused it to pass through a simple
glass prism so as to form a rainbow effect on the opposite wall. He
then placed a standard mercury thermometer in the path of each separate
colour and noted that the reading was very slightly different.
Thus, he discovered the relationship between the colour of light and
the temperature that relates to it.
Kelvin did not leave his discovery there, but went on to propose that
if a black body, i.e. one having no reflective surfaces and which can
therefore only emit light, is heated to a glowing state then the light
coming from it will have a different colour depending on the temperature
the body is heated to. The classic way to illustrate this is to imagine
a poker being put into a fire. Initially, the poker is black iron, but as
it warms it first glows deep red then a straw colour and as it becomes
hotter still it appears white. If the fire were hot enough, say something
akin to a blacksmith’s furnace, then eventually the iron would appear
to be blue/white.
Here we must guard against a simple confusion. At lower temperatures
the colour emitted by a heated black body is red and at higher
temperatures blue, but we think of red as warm and blue as cold.