The principal dispersion is a rather crude measure for chromatic dispersion of an optical material in the visible spectral region, which was developed in the early years of optics. It is simply the difference of refractive indices between two specific standard spectral lines of hydrogen:
- the F line at 486.1 nm (blue)
- the C line at 656.3 nm (red)
The principal dispersion nF − nC appears in the definition of the Abbe number.
In photographic optics, the principal dispersion is often based on two somewhat different spectral lines of cadmium:
- the F' line at 480 nm (blue)
- the C' line at 643.8 nm (blue)