Color - Vision with Direction - page 27

2

μ Z ( λ )

1.8

1.6

1.4

1.2 μ X ( λ )

μ Y ( λ )

1

0.8

0.6

0.4

0.2

0

400 450 500 550 600 650 700

Wavelength λ

Fig. 2.3. The functions used in projecting the wavelength distribution to CIE XY Z space

The aim of the CIE diagram is to model color, e.g., those generated by a TV set,
as if generated by mixing three types of light sources, each composed of photons
with different ranges of wavelengths. This is called additive 2 color model . Each of
the threee light sources alone will produce a different color sensation, corresponding
to the three “primary” colors, i.e., three points in the CIE diagram. A new color
is produced by changing the relative amount of light emitted by the primary light
sources. An example of such a color triplet is marked as R , G and B in the copy
of the CIE diagram represented by Fig. 2.4. If these three colors are appropriately
placed by the manufacturer of the device, then most colors will be reproducable.
The points R , G , B on the diagram will define a triangle, so that any new color
made by mixing these three (primary) colors will be within the triangle. It should,
however, be emphasized that it is impossible to find three such points so that all
perceivable colors of the CIE diagram can fit into the corresponding triangle, since
the form of the diagram is not strictly triangular. In consequence, there will always
be a fraction not included in the triangle, if the CIE diagram is to be approximated
by three points. The colors included in the triangle are called the gamut of the three
primaries. As a special case, one can produce a limited range of “color” by mixing
only two primaries. In this case the produced colors will be limited to those to be
found on the line joining the two primaries, the gamut of them.
The point marked as W in Fig. 2.4 is the color white. Note that we have three
color components, XY Z , but these are normalized to yield xy coordinates. 3 As a
result, the colors in the CIE diagram are normalized so that colors differing by only

2 The subtractive color model, e.g., used by painters, also exists and achieves the same result.
3 Because x + y + z =1, computing z is not useful in practice.