Sony G90 Colorblindness, Color Concepts for Video Additive Color, Luminance, Hue and Saturation

normalized in the diagram. We are actu-	V I D E O		can create a wide gamut of visible colors.
ally about 20 percent more sensitive to			To be a primary color, it is only necessary
the green curve than the red, and about 40 times less sensi-		that no primary can be created by a combination of other
tive to the blue curve.		primaries. It is also important to our video system that
As the wavelength of light varies, the probability that a		adding the three primaries in some portions will create a
cone will absorb that light depends on its spectral response,		reference white color.
but all light absorbed by the same cone contributes equally		In video projectors, the three light sources are mixed by
to its response regardless of wavelength. The relative		overlaying them on the projection screen. In direct-view
amounts of light collected by the three cone types, our tris-		monitors, phosphor dots or stripes of the primary colors
timulus response, determines how we perceive a particular		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.

Fig. 2

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

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