Scanning laser polarimetry (SLP), a technology for glaucoma diagnosis, uses imaging polarimetry to detect the birefringence of the retinal nerve fiber layer. A simple model of SLP suggests an algorithm for calculating birefringence that, unlike previous methods, uses all of the data available in the images to achieve better signal-to-noise ratio and lower sensitivity to depolarization. The uncertainty of the calculated retardance is estimated and an appropriate averaging strategy to reduce uncertainty is demonstrated. Averaging over a large area of the macula of the eye is used in a new method for determining anterior segment birefringence.