Technology Corner: Randy Hoffner
What Is White?
When we looked at color and color mixing, one of the
things we examined was the fact that when red, green and blue lights
are mixed in equal proportions, the result is neutral gray light.
If we consider a range of grays from no light to maximum brilliance
– otherwise known as a gray scale – the most brilliant
form of gray is white and the least brilliant form of gray is black.
The answer to the question, "What, exactly, is black?"
is straightforward: It is the absence of all light. The answer to
the question, "What, exactly, is white?" is less straightforward.
What "white" means in a given situation
is closely related to the concept of color temperature. The color
temperature model is based on a theoretical standardized material
called a black body radiator, which absorbs all incident radiation.
It does not reflect any energy but rather, all the energy it emits
is generated by heating the material.
This black body radiator is theoretical, but its behavior
is closely approximated by many metals. The color temperature of
a black body radiator is determined by the energy distribution of
the visible light it emits when heated to a given temperature. Color
temperature is expressed in Kelvins.
ABSOLUTE ZERO
The Kelvin temperature scale – named for the
father of thermodynamics, Lord Kelvin – determines absolute
temperature, based on the average kinetic energy of a substance’s
molecules due to heat agitation. Zero Kelvin is absolute zero, that
point at which molecular motion has ceased.
The Kelvin scale is based on centigrade degrees (also
known as degrees Celsius) – one-centigrade degree being 1/100
of the difference between the temperature of melting ice and that
of boiling water. Ice melts at 0 degree Celsius and water boils
at 100 degrees Celsius. Zero Kelvin (not zero degree Kelvin) corresponds
to -273.15 degrees Celsius, so ice melts at +273.15 K and water
boils at +373.15 K. We note in passing that there is an absolute
temperature scale in the English system as well – the Rankin
scale – that is based on Fahrenheit degrees.
To get back to our black body radiator – if it
is heated to a temperature of about 900 K, the spectrum of visible
light it emits will cause it to glow a dull red. Heated to between
1,500 and 2,000 K, it emits a yellowish-red color. At 3,000 K the
black body radiates a yellowish-white color, and at 5,000 K a bluish-white
color is apparent.
In scientific terms, the color temperature of a light
source may be defined as the value of the absolute temperature of
a black body radiator when the radiator’s chromaticity matches
that of the light source. Although some light sources – a tungsten
filament, for example – behave quite similarly to a black body
radiator, emitting a continuous spectrum of light, others less closely
approximate the chromaticity of a black body.
A fluorescent lamp, for example, emits spikes of color.
This type of radiator has a "correlated" color temperature,
arrived at through a calculated chromaticity. The concept of color
temperature – and the associated concept of a "white point"
– are important in both film and video photography, and in
video displays as well.
HERE COMES THE SUN
The sun behaves as a textbook black body radiator.
Its internal temperature reaches millions of degrees and its surface
temperature is about 6,000 K. The sun, therefore, intrinsically
radiates bluish-white light, and that is what a passenger in the
space shuttle sees.
Before we earthbound observers see the sun’s
light, however, it must pass through the earth’s atmosphere,
which filters, diffuses and reflects it – often radically altering
its correlated color temperature, giving us blue skies and red sunsets.
The coordinated color temperature of outdoor daylight may range
from about 2,000 K to 30,000 K, depending on many factors –
among the more important of which are the mix of direct sunlight
with diffused and reflected light (skylight), the degree of refraction
through the atmosphere and the contents of the atmosphere (dust,
water vapor, etc.).
How can the correlated color temperature of daylight
reach 30,000 K, when the color temperature of the sun is only 6,000
K? As the lower frequency components of the light are absorbed,
the remaining bluer components predominate, shifting the correlated
color temperature upward.
In the early morning and late evening, sunlight passes
through the atmosphere in a direction nearly parallel to the earth’s
surface. It therefore passes through a relatively thick layer of
atmosphere, causing a large refraction. This emphasizes the redder
components and results in a low-correlated color temperature.
A chart of approximate correlated color temperatures
from Kodak shows sunrise or sunset at about 2,000 K, a little higher
than a candle flame (1,850 K), while average summer sunlight at
noon in Washington, D.C. is about 5,400 K. At noon, the sun is almost
directly overhead and sunlight travels nearly perpendicularly to
the earth’s surface, causing it to pass through a relatively
thin layer of atmosphere and minimizing refraction, generating a
higher correlated color temperature.
Average summer shade is listed at 8,000 K, with summer
skylight varying from 9,500 to 30,000 K. A 100-watt incandescent
lamp is about 2,865 K. Tungsten lamps of the type used for most
television and film photography are in the 3,200 K range. Some other
correlated color temperatures to note are average summer sunlight
plus skylight at 6,500 K and average summer shade at 7,100 K.
SHOT IN THE DARKER, LIGHT
What does all this have to do with television? Any
photographer knows film that is intended for daylight or arc lamp
shooting is balanced for a color temperature of about 5,500 K, while
film intended for shooting under tungsten light is balanced for
about 3,200 K. 5,500 K light is much bluer than 3,200 K light, and
if outdoor film is shot using tungsten illumination, the pictures
that result will have a distinctly yellow cast – particularly
apparent in the areas that should look white.
Conversely, if tungsten film is used to shoot outdoors,
the resulting pictures will have a blue cast. These colorimetry
errors can be compensated using appropriate filters to alter the
color balance of the light striking the film. A daylight film, for
example, can be used under tungsten illumination with the proper
filter over the camera lens.
In addition to altering the color balance, the filter
will of course reduce the amount of light reaching the film, and
thus the exposure index or speed of the film. Most color negative
film used for television production is balanced for tungsten light
at 3,200 K, so that it may be used for indoor shooting with no compensation.
When shooting outdoors, a filter is placed over the
lens to reduce the correlated color temperature of the light striking
the film. This also reduces the exposure index of the film –
but this is not typically a big problem when shooting in sunlight.
An example is Kodak Vision 500T, a high-speed color
negative film used in shooting for television. This film is balanced
for a color temperature of 3,200 K, and has an exposure index of
500. When used in 5,500 K daylight, Kodak recommends the use of
a No. 85 gelatin filter, which reduces the exposure index to 320.
This filter effectively establishes the proper white point: It will
cause a white sheet of paper to look white and not blue.
Video cameras are subject to the same principles of
color balance as film cameras, and shooting is usually done under
lighting conditions similar to those used for film shooting –
although white-balance is typically adjusted electronically rather
than with gelatin filters.
MONITORING THE SITUATION
Displays have correlated color temperatures too, of
course. Professional television picture monitors use a standardized
set of phosphors and are adjusted for a color temperature of 6,500
K. Adherence to this specification is particularly important when
evaluating or correcting color. Anyone, viewing the pictures anywhere,
must see the same color balance as anyone else.
If a master tape is produced using monitors employing
SMPTE phosphors set at 6,500 K, it would not do to apply subsequent
color correction using a monitor that was set at 9,300 K, because
the pictures – as viewed on the 9,300 K monitor – would
appear to have a yellow-orange cast. Correcting to the 9,300 K monitor
would result in a product that was much too blue.
Although professional monitors are set at 6,500 K,
other factors – including the desire for a brighter image –
cause the color temperature of consumer television sets to be set
at around 7,100 K in the United States. This higher color temperature
produces a brighter, but somewhat bluer, picture.
Preferences differ in other parts of the world. European
consumer sets are typically closer to 6,500 K than U.S. sets, while
sets sold for use in Japan are set at 9,300 K. Computer monitors
are also typically set at 9,300 K. This color temperature had its
origin in the black-and-white TV days, when the best phosphor available
for monitors (a yellow-blue combination) had a color temperature
of 9,300 K.
Black-and-white monitors used in television production
and broadcast today are 6,500 K monitors because they are intermixed
with 6,500 K color monitors.
We have spoken by implication mostly of CRT displays,
but other technologies – such as LCD, DLP and plasma –
have their own sets of color temperature considerations. It is interesting
to ponder momentarily that all of what we might call "light
transducers," from the simplest box camera to the most cutting-edge
micromirror projector, are subject to the same fundamental physical
laws.
Randy Hoffner is manager of technology and strategic
planning at ABC, New York, N.Y. The views expressed in his column
are his own, and not necessarily those of ABC. Write to him c/o
TV Technology.
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