 Secondary spectrum

The secondary spectrum is residual chromatic aberration after an achromatic lens brings two selected wavelengths to the same optical focus. For example, equal red and blue focal lengths can coexist with a different green focal length. The Abbe number fixes one F–C relative difference, not the complete shape of optical dispersion between or beyond those wavelengths. The perfectly flat sum in the achromatic doublet power balance diagram deliberately omits this residual effect. ^[ref].