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Chromaticity is the quality of a color as determined by its "purity" and dominant wavelength.

"Purity" in this context is roughly equivalent to the term "saturation" in the HSV color model. The dominant wavelength is commonly thought of as equivalent to the property "hue" as used in general color theory and in specific color models such as HSV or HSL.

Another way to look at this is as the distance of the color from the axis in the L*ab color space. A color in LAB can be described in two ways: an L* value, describing the lightness of a slice plus a and b in cartesian form, or an L* value, a chromaticity, and a hue angle, the latter two forming a length and vector.

There are many parallels between accelerator physics and optics. Since a bunch of charged particles has a tendency to disperse over time, it is important to include numerous magnets of different types along the beam line in order to keep the beam well controlled, and tightly bunched. When quadrupole magnets are used, this is known as beam focusing. Focusing the beam in this way, however, can lead to problems if the bunch contains particles of differing energy. In this case, the low energy particles will be focused much more tightly than high energy particles -- exactly in the same way that longer wavelengths of light (i.e. the lower energy photons), will be brought to a focus more quickly than short wavelengths.

In the case of a storage ring, a high degree of chromaticity can lead to instabilities in the beam's motion, which will result in large movements of the beam. This will eventually cause the beam to hit the wall of the chamber and be lost and/or damage the machine. For this reason it is advantageous to correct the chromaticity introduced by bending and focusing magnets. This can be done with sextupole magnets.

Thus it can be seen that the chromaticity of a beam is an indication of the energy spread of its constituent particles, in much the same way that the chromaticity of light is an indication of the energy spread of its constituent photons.

• Chromaticity diagram

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