“…Unfortunately, the crystallographic orientations of the optic axes in biaxial crystals are generally wavelength dependent, so any particular direction chosen for measurement will only coincide with an optic axis for a single wavelength in the spectral region. 26,27 The latter delivers the entire polarization transfer matrix, the 4 × 4 Mueller matrix, without any moving optical parts, a boon for the minimization of instrumental errors that can be as large as the light intensity changes from CB or CD. 18 This is why, even in transparent crystals, measurements of OR along low symmetry directions are extremely challenging, 19,20 with a number of the credible literature examples coming from crystals with accidental linear anisotropy compensation, that is, crystals with LB = 0 at particular frequencies.…”