Sony G90 manual CIE Chromaticity Diagram, Display Color Accuracy RGB Primaries The Color Triangle

Commission on Lighting) was created in	V I D E O		created by mixing two other colors will
1927, and in 1931 established a colorime-			lie on a straight line connecting them.
try system to describe colors using a simple system of		The distance along the line where the new color appears is
numerical coordinates. I won’t delve into the details behind		inversely related to the proportions of the two colors. Since
that system other than to say it is based on the tristimulus		all single-wavelength visible colors lie on the Spectral
response principles discussed earlier and experiments that		Locus, all visible colors must be inside the locus. Further-
were done with human observers. A result of their work		more, it follows that all colors made from a combination of
was the creation of a two-dimensional (x,y) chart called the		three primary colors must lie inside a triangle formed by the
CIE Chromaticity Diagram. (Figure 3)		three colors. Colors outside the triangle cannot be repro-
		duced because that would require negative amounts of light
		from one or two primaries.
		The red, green, and blue primaries of video systems are

chosen to enclose a reasonably large triangle that deter-

mines the entire gamut of colors that can be displayed. If

two displays have slightly different sets of red, green, and

blue primaries, the overlapping color triangles demonstrate

that the two displays cannot produce exactly the same

gamut of colors. (Figure 4)

Fig. 3

The CIE Chromaticity Diagram

The horseshoe-shaped curve in the diagram is called the

Spectral Locus, and consists of all colors with only a single

wavelength. The lowest wavelength color, ultraviolet at 380

nm, is located at the bottom left of the horseshoe, and the

wavelengths increase moving around the Spectral Locus to

infra-red, at 780 nm, on the far right. The Line of Purples

connecting the ends of the locus represents colors that can-

not be created with any single wavelength.

The line shown in the middle is the called the Plankton

Locus, or blackbody radiation curve. The color temperature

is infinite at the left end and drops to several thousand

degrees as the Plankton Locus converges with Spectral

Locus in the reds. The important D65 white reference, at a

correlated color temperature of 6500K, is at (0.3127, 0.329)

expressed as CIE (x,y) coordinates. It is a “correlated” color

temperature because it lies just off the Plankton Locus.

Brightness is not shown on the diagram, only saturation

and hue. The hue of totally pure colors is determined by

their position on the Spectral Locus, or Line of Purples. The

saturation is determined by how far the color lies from the

reference white point near the center of the diagram. The

pastel colors lie close to the center and the pure, fully satu-

rated colors lie on the horseshoe.

5. Display Color Accuracy

RGB Primaries – The Color Triangle

One of the properties of the CIE diagram is that any color

Fig. 4

Equally important, since the primaries are at different

locations, mixing the primaries with the same proportions

of light will generate different colors. Hence we see that to

reproduce the colors specified by video signals, it is critical

that the primaries be matched to the video system stan-

dards.

The NTSC primaries were specified in 1953 based on

phosphors available at that time. But over the years, TV dis-

play manufacturers continually used newer phosphors that

provided higher light output than the original NTSC phos-

phors. This created serious color errors. Finally, in 1971 a

new set of primary phosphors, called the SMPTE “C” phos-

phors were selected. (The SMPTE C phosphor specification

has been revised since then with very slight changes.)

These are the phosphors used in all professional broadcast

monitors. Unfortunately, consumer CRTs are still using

slightly different phosphors, and different phosphors from

one product to another. (The SMPTE C phosphor values are

given in Table 1.)

The obvious result of not using the standard phosphors