The birefringence (as well as the average refractive index) will generally be dispersive, that is, it will vary as a function of optical frequency (wavelength).
One method of elemental analysis that is important in forensic analysis is energy-dispersive X-ray spectroscopy performed in the environmental scanning electron microscope.
The plus sign in the second term in parentheses refers to a positive dispersive configuration while the minus sign refers to a compensating configuration.
