DVD Demystified

92
Chapter 3
The human retina has three types of color photoreceptor cells, called
cones. 3 Each is sensitive to different wavelengths of light that roughly correspond
to the colors red, green, and blue. Because the eye perceives color
as a combination of these three stimuli, any color can be described as a
combination of these primary colors. 4 Televisions work by using three electron
beams to cause different phosphors on the face of the television tube
to emit red, green, or blue light, abbreviated to RGB. Television cameras
record images in RGB format, and computers generally store images in
RGB format.
RGB values are a combination of brightness and color. Each triplet of
numbers represents the intensity of each primary color. As just noted, however,
the eye is more sensitive to brightness than to color. Therefore, if the
RGB values are separated into a brightness component and a color component,
the color information can be more heavily compressed. The brightness
information is called luminance and is often denoted as Y. 5 Luminance is
essentially what you see when you watch a black-and-white TV. Luminance
is the range of intensity from 0 percent (black) through 50 percent (gray) to
100 percent (white). A logical assumption is that each RGB value would
contribute one-third of the intensity information, but the eye is most sensitive
to green, less sensitive to red, and least sensitive to blue, so a uniform
average would yield a yellowish green image instead of a gray image. 6 Consequently,
it is necessary to use a weighted sum corresponding to the spec-
3Rods, another type of photoreceptor cell, are only useful in low-light environments to provide
what is commonly called night vision.
4You may have learned that the primary “colors” are red, yellow, and blue. Technically, these are
magenta, yellow, and cyan and usually refer to pigments rather than colors. A magenta ink
absorbs green light, thus controlling the amount of green color perceived by the eye. Since white
light is composed of equal amounts of all three colors, removing green leaves red and blue, which
together form magenta. Likewise, yellow ink absorbs blue light, and cyan ink absorbs red light.
Reflected light, such as that from a painting, is formed from the character of the illuminating
light and the absorption of the pigments. Projected light, such as that from a television, is formed
from the intensities of the three primary colors. Since video is projected, it deals with red, green,
and blue colors.
5The use of Y for luminance comes from the XYZ color system defined by the Commission Internationale
de L’Eclairage (CIE). The system uses three-dimensional space to represent colors,
where the Y axis is luminance and X and Z axes represent color information.
6Luminance from RGB can be a difficult concept to grasp. It may help to think of colored filters.
If you look through a red filter, you will see a monochromatic image composed of shades of red.
The image would look the same through the red filter if it were changed to a different color, such
as gray. Since the red filter only passes red light, anything that is pure blue or pure green will
not be visible. To get a balanced image, you would use three filters, change the image from each
one to gray, and average them together.