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What
is Photographic Colour Temperature?
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In
order to establish a standard for defining the colour content of
light, temperature is used and quoted in degrees Kelvin.
An apocryphal story told to me in the early days of my film career
explained that the colour temperature of light was originally derived
from a boiling crucible of carefully defined metals, heated to such
an extent as to emit a light which matched daylight, as observed
at mid-day during mid-summer.
The measured temperature at this point was 5600 degrees Kelvin.
This was then established as the standard daylight colour temperature.
The light emitted from a tungsten halogen lamp would match the light
emitted from the crucible of metal when it had cooled to 3200 degrees
Kelvin, a domestic tungsten filament lamp would match it at 2700
degrees Kelvin, and so on.
Fig.16 Colour Temperature
When light sources of varying Kelvin temperatures
are being used together, they should be colour corrected as required
to conform to a single colour temperature output. In general you
will be matching for either daylight at 5600 degrees Kelvin or artificial
light at 3200 degrees Kelvin. A range of colour correction filters,
are available for this task. A camera balanced for 3200 degrees
Kelvin will reproduce images lit by a 5600 degree light source with
a blue tint, or conversely a camera balanced for 5600 degrees Kelvin
will produce images lit by a 3200 degree light source with an orange
tint.
Colour temperature, as defined in a photographic context,
is only concerned with the characteristics of the recording medium,
and this is measured according to the ratio of light between the
blue and red region of the spectrum. In this respect it differs
when applied to physics and colorimetry measurements. Here it refers
to the temperature at which a theoretical ideal blackbody radiator
would emit light of the same colour (having the same chromaticity)
as that of the light being measured, but without melting, boiling
or incinerating.
This theoretical model can therefore be extended to cover the electromagnetic
spectrum, from radio waves down through the visible spectrum to
gamma rays, for analytical and scientific measurement purposes.
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What
is White Balance?
Within limits, all electronic cameras have the
ability to balance available light to render white as white. A correct
balance of all colours in the visible spectrum is required in order
to produce white, so it follows that if the camera 'white balance'
is correctly set up, all colours recorded under the same lighting
conditions will also be correct. Having shots that are correctly
colour graded makes viewing the final cut sequence of shots a far
more pleasant experience. This also saves a great deal of time at
the post-production stage, matching and correcting shots.
Fig.
17 Primary colours of light
It is advisable to obtain
and keep your own white reference target as not all whites are the
same. Some whites contain a touch of blue to make them look whiter
to the eye. Just aiming the camera at something in the room that
looks white is not a reference to a constant and is most unprofessional.
To test this, just gather a number of pieces of paper from around
your office that you consider to be white. Now compare them to each
other in the light of the office. Note the one you think looks the
whitest, then take them into daylight and compare them again. Some
camera operators use a white reference card that they refer to as
a "warm card"; this white card contains a little blue
in order to cheat the camera towards red and thereby produce a warmer
image.
On a two-camera shoot it is advisable to use the same white reference
target for both cameras.
It is not good practice to "white-balance" a camera in
a situation where the illumination is being derived from various
sources that differ in colour temperature. An example here would
be a mixture of daylight, fluorescent light, and tungsten light,
shown in Fig. 18. Remember that light travels in straight lines
and although you may have achieved a "white balance" on
the white reference target, when you introduce your subject, with
all its natural contours, the shadowed areas formed by each light
will lack the correct mix of light required to produce the right
colour tones in those shadowed areas. Although not visible to the
eye, you will notice a green caste and strange pastel tones in the
shadows of the image on the screen.
Reflected light from tinted paint on nearby walls can have
an undesirable influence on white balance set ups, so this should
be avoided. In such a situation, it is better to balance for the
light source only and avoid the mix of reflected light; this will
preserve the correct colour balance and skin tones in the lit frame
area. The reflected light will also be its correct colour and be
perfectly acceptable if its reason for being there appears within
the shot.
In the case of theatre lighting, where coloured gels are being used
throughout, select one unfiltered light, away from the scene, and
use this to set the white balance. The colours will then be represented
as the audience sees them.
If a continuous shot between an interior and exterior is required,
the interior lighting should be daylight compatible or colour corrected
for daylight by using the appropriate blue filters, the camera should
then be balanced for daylight, and accordingly the passage between
the two will appear as natural to the viewer, as it would to the
eye in real life.
The use of a colour temperature meter helps ensure consistent results.
It is not easy for the human eye to detect gradual changes in colour
temperature because the brain naturally applies a degree of correction.
This process does not occur in the same way when viewing a sequence
of shots from cameras that have not been set up correctly; confronted
by direct comparison, the differences can be most distracting.
Although we often refer to daylight colour temperature being 5600°K
as a guide, I am afraid that's all it is. The colour temperature
of each day from sunrise to sunset can vary dramatically. Once a
camera has been set-up to produce white, it may not continue to
do so if the light source has changed during the day. If in doubt,
re-white-balance, especially if the camera has been moved to a new
location where the lighting may be different. |
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Fig.18 The result of setting the white
balance to compensate for a mix of light sources, including ambient
fluorescent light. |
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