And polarization-preserving reflector (PPR). Firstly, the retardation (i.e., the change in polarization) is proportional to the RNFL thickness. In this instrument, there are four retarders in the measurement beam’s path: 1. The first two linear retarders have equal retardance and form a VCC. The third linear retarder is the combination of cornea and lens—the anterior segment  3. The fourth linear retarder, with radially distributed axes, is the retinal birefringent structure (RE; either peripapillary RNFL or the Henle fiber. As polarized light passes through a form-birefringent medium, one of the two component waves traveling at 90 to each other becomes retarded relative to the other. The degree of the resulting phase shift is directly proportional to the number of microtubules the light passes through, which in turn, is directly proportional to RNFL thickness. The figure above illustrates this process. The RNFL isn't the only form-birefringent structure in the eye. Anterior segment structures, such as the cornea, also phase-shift polarized light. So the latest instrument includes a compensating device or compensating corneal which is designed to remove the portion of the signal generated by the anterior segment. This device consists of two optical retarders, which when rotated relative to each other, allow the operator to set the compensator to any value between 0 nm and 120 nm. Rotating the device to any axis can compensate for anterior segment birefringence in any orientation up to 120 nm in magnitude. The slow axis of R was oriented radially, and distance around R was measured from the horizontal nasal meridian by angle β. At each point, therefore, the fast axis of R was R = β + 90°. Radial variation in retardance was not analyzed. The measuring beam was reflected at a deeper layer and traveled back through the three retarders to the ellipsometer. Reflection from the ocular fundus exhibits a high degree of polarization preservation, and the reflector in the model (polarization-preserving reflector [PPR]) was assumed to preserve completely the polarization state of the incident beam, except for a 180° phase change due to the reversal in direction. Each optical component in the model experienced a double pass of the measuring beam. Birefringent is relared or characterized as a double refraction. In this picture we can see calcite crystal laid upon a paper with some letters showing the double refraction.