Achromatic wave is similar to zero order waveplate except that the two plates are made from different birefringent crystals, such as crystal quartz and magnesium fluoride.
Since the dispersion of the birefringence can be different for the two materials, it is possible to specify the retardation values at a wavelength range. From the curve, you can see that the bandwidth of such achromatic wave is very wide, while the achromatic wave remain a nearly constant retardance over a range of wavelength.
This article comes from wavelength-tech edit released