**4. Strengths and limitations [7]**

perpendicular to the fiber layer. This difference in the speed between the two beams is called retardation and is proportional to the RNFL thickness. The scanning laser beam used is 785nm.

**Figure 5.** Polarized light that travels parallel to the RNFL slows down. This retardation is proportional to the thickness

Because the corneal also exhibits birefringence the GDx has a variable corneal compensator (VCC) in order to subtract the retardation from the cornea and the only retardation measured is that derived from the RNFL. The newer GDx machines have an enhanced corneal compen‐ sator that offers better reproducibility of the measurements and is more accurate in the

**Figure 6.** Printout of the RNFL analysis with the GDx VCC. The colored images at the top of the printout are the fundus photos. Below them is the thickness map. It is a color coded representation of the thickness of the RNFL within a 20° × 20°

diagnosis of glaucoma [10]. The transverse resolution of the GDx is 45 µm.

of the RNFL

300 Glaucoma - Basic and Clinical Aspects

SLP can only measure data from RNFL. Areas of peripapillary atrophy give false information about the RNFL. A minority of the eyes examined show atypical retardation patterns (APRs) which are overcome by the GDx-ECC machines [11]. Atypical patterns are those which do not follow the normal histological distribution of the RNFL with the supero- and inferotemporal sectors being the thickest. APRs give falsely high RNFL measurements [12]. Newer SLP models are not compatible with the older ones. On the other hand RNFL analysis with SLP does not require a reference plane.
