Apple Final Cut Pro 7 - Manual (Page 1300)

The RGB color model is additive, which means the red, green, and blue channels combine
to create all the available colors in the system. When all three primary color values are
the same, the result is neutral, or grayscale. For example, if all three primary colors are 0
percent, the result is black. If all three primary colors are 100 percent (the maximum
value), the result is white.
When all three primary color channels are nearly the same strength, the result appears
neutral with a slight color cast, depending on which channel is the strongest. For example,
if the value of the red channel is higher than the value of the blue and green channels,
the result is a slightly red image. Secondary colors are combinations of two primary colors:
red plus green is yellow, green plus blue is cyan, and blue plus red is magenta.
Y
′
C
B
C
R
Color Model
In the RGB color model, all three channels contribute to the perception of brightness. In
the early 1950s, this was a problem when developing a three-channel color television
system that would be compatible with existing black-and-white televisions. The solution
was to encode a single channel that represented luminance—light intensity as perceived
by humans—which existing black-and-white televisions could decode. Color televisions
would receive the same luminance channel and two additional color channels that could
be decoded back into RGB color for display.
Luminance
Luminance is a measure of physical light intensity modified by the spectrum sensitivity
measured in human vision. In other words, red, green, and blue contribute to the
perception of brightness, but not equally. Since two of the cone cell types in the eye favor
light in the green portion of the spectrum, the luminance channel is weighted to mostly
represent the green color channel. This makes the luminance channel appear to have
the equivalent brightness that we see in a color image.
The Y
′
C
B
C
R
color model derives luminance from the Commission Internationale de
l’Eclairage (CIE) XYZ color system, in which the Y component represents luminance (X
and Z represent color components).
In video systems, a gamma adjustment is applied to the RGB color channels to make
efficient use of the bandwidth available for carrying and recording signals. The
gamma-adjusted channels are called R
′
G
′
B
′
, and the derived black-and-white channel is
called Y
′
, or luma. For more information about gamma, see “About Gamma.”
1300 Chapter 78 Measuring and Setting Video Levels