Spectral extinction between crossed polarizers ($\Gamma =m\lambda$)

A birefringent plate with retardation $\Gamma =t\Delta n$ gives $I/I_P={\sin }^2(2\alpha ){\sin }^2(\pi \Gamma /\lambda )$. At non-extinction azimuths, wavelengths satisfying $\Gamma =m\lambda$ are removed by a crossed analyser. At a principal-axis azimuth every wavelength is extinguished.