DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

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Figure 22.1 Additive reproduction. This diagram illustrates the physical process underlying
additive color mixture, as is used in video. Each primary has an independent, direct path to the
image. The spectral power of the image is the sum of the spectra of the primaries. The colors of
the mixtures are completely determined by the colors of the primaries; analysis and prediction
of mixtures is reasonably simple. The SPDs shown here are those of a Sony Trinitron monitor.
not determined by the colors of the individual primaries,
but by their spectral properties.
Additive reproduction is employed directly in a video
projector, where the spectra from a red beam, a green
beam, and a blue beam are physically summed at the
surface of the projection screen. Additive reproduction
is also employed in a direct-view color CRT, but through
slightly indirect means. The screen of a CRT comprises
small phosphor dots (triads) that, when illuminated by
their respective electron beams, produce red, green,
and blue light. When the screen is viewed from
a sufficient distance, the spectra of these dots add at
the retina of the observer.
The widest range of colors will be produced with primaries
that individually appear red, green, and blue. When
color displays were exclusively CRTs, RGB systems were
characterized by the chromaticities of their phosphors.
To encompass newer devices that form colors without
using phosphors, we refer to primary chromaticities
rather than phosphor chromaticities.
CHAPTER 22 COLOR SCIENCE FOR VIDEO 235