Achromatic waveplate is similar to Zero-order waveplate except that the two plates are made from different materials, 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. Hence, the retardation of the resulting waveplate can be made to low sensitivity to wavelength change. Based on the contact methods of two plates, we classify the achromatic waveplate to two types: Airspaced and Cemented.
