Notes

¹The value of the azimuthal angle is measured anticlockwise from the horizontal direction within the image captured by the CCD camera.

²The value of the birefringence, and thus the optical retardance and the phase difference, takes a positive value when the sign of the optical indicatrix is positive and a negative value when the sign of the optical indicatrix is negative.

³However, some X-ray techniques such as topography do provide spatially resolved images.