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normalized in the diagram. We are actu-
ally about 20 percent more sensitive to
the green curve than the red, and about 40 times less sensi-
tive to the blue curve.
As the wavelength of light varies, the probability that a
cone will absorb that light depends on its spectral response,
but all light absorbed by the same cone contributes equally
to its response regardless of wavelength. The relative
amounts of light collected by the three cone types, our tris-
timulus response, determines how we perceive a particular
color. This makes human color perception trichromatic
(three-color). The sum of the three responses determines
our perception of brightness, while the ratio between the
three responses determines our perception of hue and sat-
uration, the chromatic properties of light.
Interestingly, different spectral distributions can be per-
ceived as the same color if they provide the same tristimu-
lus response. Also notice that there are areas at the
extremes of the visible range where only a single type of
cone has any response. That means colors in those areas
will be perceived the same, since there is a response from
only one type of cone.
Colorblindness
About 0.003 percent of people can’t see color at all. About 8
percent of males and 0.5 percent of females are color blind,
which means they don’t see color the way most of us do.
About 2.5 percent of males see reds and greens as the same
color. The other 5.5 percent of colorblind males match col-
ors differently than the rest of us and differentiate small
color differences less well. This hasn’t much to do with
video, but it was too interesting to leave out.
3. Color Concepts for Video
Additive Color
All video display systems create colors by adding together
three primary colors of light, which is equivalent to adding
together their spectral distributions. Red, green, and blue
are used by video systems as primary colors because they
can create a wide gamut of visible colors.
To be a primary color, it is only necessary
that no primary can be created by a combination of other
primaries. It is also important to our video system that
adding the three primaries in some portions will create a
reference white color.
In video projectors, the three light sources are mixed by
overlaying them on the projection screen. In direct-view
monitors, phosphor dots or stripes of the primary colors
are arranged so closely together that the eye perceives the
light coming from a single location. The eye’s visual acuity
(ability to see detail) to color is related to the separation of
the cones on the retina.
Luminance
Luminance is a measure of our sensation of brightness. It
depends on the spectral sensitivity of human vision. Colors
closer to the center of the visible wavelength range (yellow-
green at 550 nm) are perceived as brighter, and therefore
have higher luminance than other colors with the same
energy.
Hue and Saturation
Hue is what we commonly refer to as red, green, yellow,
greenish-yellow, and so forth. It is related to the dominant
wavelength of a color.
Saturation is the purity of the color, what might be
described as its vividness or depth of color. The more pale
the color, as a pastel, the less saturated it is. A color can be
desaturated by adding white. If a color is formed by adding
portions of three primaries, some portion of white that con-
sumes one primary can also be formed. That portion of
white can be thought of as desaturating the color formed by
the remaining portions of the other two primaries.
The Color of White
It may seem that the color of white is unique, a black and
white matter. But of course there are many colors of white.
Compare the pages of this book, writing paper, or anything
else you normally define as white. They all have a distinc-
tive hue. In video systems, the color of a reference white is
crucial to generating all other colors. So it is critical to have
a precise method to specify the color of white required. The
physicist Max Planck determined that carbon heated to
extreme temperatures emitted light with broad spectral dis-
tributions (i.e., shades of white) determined by their tem-
perature. In physics these are called blackbody radiators.
Standard illuminants are defined by the temperature of a
blackbody radiator that most closely matches their color.
This is called the correlated color temperature, which is
measured in absolute degrees Kelvin (K). So the color of
white can be specified by a temperature. All of our current
video systems use a standard illuminant called D65, at a
correlated color temperature of 6500 K.
4. The CIE Color System
It is far too complex in practical applications like video to
specify colors by their spectral distributions. The CIE
(Commission Internationale de L’éclairage - International
V I D E O
Fig. 2