Azimuthal brightness of a birefringent crystallite ($I\propto {\sin }^2(2\alpha )$)

Rotating a uniform crystallite around a fixed propagation direction changes the crossed-polar intensity factor ${\sin }^2(2\alpha )$. Extinctions repeat every90degrees and maxima occur45degrees away. Similar interference colour can coexist with different brightness if retardation is similar but azimuth differs.