Azimuthal brightness of a birefringent crystallite
= Azimuthal brightness of a birefringent crystallite
{title2=$I\propto\sin^2(2\alpha)$}
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.