The measurement results show that for all subjects, except one (#4), the strength of the RBS signal during central fixation increased after adding the 61° (133 nm) wave plate oriented at 144° to the optical system, as predicted with the RBS computer model. Note that subject #4 already yielded a remarkably high foveal fixation signal without the wave plate, compared with the signal strength measured without the wave plate for the other tested right eyes, even higher than the foveal signal for subjects 1, 3, and 5 measured after the 61° (133 nm) wave plate had been added to the optical system.
In accordance with RBS model predictions, foveal signal strength is even further increased for the test subjects (except subject #2) with the wave plate oriented at the individually optimized azimuth. This confirms the hypothesis that for our small group of tested subjects, a fast axis orientation of about 66° is better suited for the right eye measurement with the monocular eye fixation monitor, significantly enhancing foveal fixation detection.
For subject #3, RBS signal strength is in general low, which might be explained by the astigmatism present in the subject's right eye, causing less useful light to return to the detector after the double-pass through the ocular system. The eye serves as an efficient retro-reflector only when the retina and the source of light are situated in conjugate planes, in other words with the eye being properly focused on the light source. This assumption is included in the RBS computer model, but it is certainly not valid for the astigmatic right eye of subject #3.
There are possible limitations of the RBS computer model, and possible errors in the assumptions used in its development, that may be contributing to deviations between measurements and model predictions. First, even though polymer retarders, such as the 61° (133 nm) wave plate used, are much less sensitive to changes in retardance with changes in the angle of incidence (if tilted about its fast or slow axis), the 10° tilt, minimizing specular back reflections, could have introduced significant errors with the fast axis rotated to its optimized azimuth. Also, the tilt changes the retarder's fast axis azimuth slightly with respect to the beam reference system. In other words, the actual fast axis orientation will differ from the manually adjusted azimuth on the rotary mount, more precisely it will be shifted towards lower angles. This explains the observed deviation of 2° between measured and predicted fast axis azimuth that yields maximal foveal signal strength for subject #1. The theoretically predicted value, 62°, was 2° less than the measured result, 64°.
Second, different head positioning during the assessment with the GDx instrument and the monocular eye fixation monitor could cause differences in measured and actually present corneal birefringence during data acquisition with the monocular RBS-based eye fixation monitor. In fact, measurements with the GDx are limited to the central cornea, whereas with the large exit pupil used in retinal birefringence scanning, to allow relative freedom of the subject's head, the entire cornea overlying the pupil is used. Thus non-uniformity of corneal birefringence across the pupil of real eyes can be a major confounding factor potentially resulting in inconsistent agreement between model predictions and measurement results. Such irregularity is undoubtedly present with the large pupils induced during the assessment with the monocular RBS-based eye fixation monitor by turning off room lights to allow more light to enter the eye.
Another potential source of error includes the assumption that the fundus acts as an ideal retro-reflecting surface, modeled by the Müller matrix of an ideal mirror. In real eyes only a small portion of the light incident on the retina is reflected (about 1/10,000 to 1/1000 of the light is reflected) [1
], which varies across individuals.
Despite the potential sources of errors, the preliminary validation experiments with human subjects using the monocular RBS-based eye fixation monitor showed that the RBS computer model is capable of assessing the influence of varying corneal birefringence on the strength of the differential RBS signal during foveal fixation. The data obtained confirmed the model's ability to predict an appropriate double-pass wave plate, which, when added to the optical system, improves RBS signal strength during central fixation.
In conclusion, the RBS computer program described in this paper allows assessment of the effect of varying corneal birefringence on the strength of the RBS signals. It also provides a means of optimizing RBS using wave plates and other optical components. “Wave-plate-enhanced” RBS enhances recognition of foveal fixation by minimizing the deleterious effects of corneal birefringence. We are currently completing a binocular RBS system incorporating spinning and fixed wave plates [32