Retinal scanning laser polarimetry and methods to compensate for corneal birefringence.

Scanning laser polarimetry (SLP) was developed to provide objective assessment of the retinal nerve fiber layer (RNFL), a birefringent tissue, by measuring the total retardation in the reflected light. The birefringence of the anterior segment of the eye, mainly the cornea, is a confounding variable to the RNFL measurement. Anterior segment birefringence varies over a wide range among individuals. This paper reviews the principle of SLP and methods to measure and compensate for anterior segment birefringence as implemented in a commercial SLP system, GDx VCC. Anterior segment birefringence is measured from the macular retardance profile. It can be neutralized with a variable retarder in the GDx VCC and the measured retardance directly represents the RNFL retardance. Alternatively, a bias retarder can be introduced in the measurement beam path with approximately vertical slow axis, SLP measures the combination of the RNFL and the bias retarder, and RNFL retardance is then mathematically extracted from the measurement. The latter has the advantage of improved signal-to-noise ratio. With the combination of a visual RNFL image and rapid, objective, and reproducible assessment of the RNFL, GDx VCC provides an attractive clinical tool in glaucoma management.