In a radial gradient, the refractive index profile is normally represented by a polynomial with only even powers of r. If, however, odd powers of r are present in the representation, the effect on aberrations can be serious. A Wood lens serves as an example.
© 1990 Optical Society of America
Erich W. Marchand, "Axicon gradient lenses," Appl. Opt. 29, 4001-4002 (1990)