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K. Twietmeyer, R. Chipman, N. Beaudry, A. Elsner, R. Park; Polarization States of Light Delivery Path of Laser Diagnostics GDx Nerve Fiber Analyzer . Invest. Ophthalmol. Vis. Sci. 2005;46(13):278.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To determine the polarization states of the light exiting the GDx Nerve Fiber Analyzer system and entering the eye. The GDx system utilizes polarized light for diagnosis of glaucoma via the measurement of retinal nerve fiber layer (RNFL) thickness. Methods: The GDx scanners are disabled so that a single beam of light exits the instrument, coincident with the optic axis. The polarization state of the beam for each rotational position of the GDx half wave plate is measured with a Stokes polarimeter. This polarimeter consists of a fixed linear polarizer and a retarder which rotates from 0° to 360° in 30 increments. Beam power through the polarimeter is measured at each of the 30 steps, and these data are converted to a Stokes vector. Results: By plotting the polarization states on the Poincare sphere, it is seen that the states fall on a circle that is tilted relative to the equator, with average ellipticity of about 2° and range of ellipticity of ± 28°. This data fits the model of a rotating nearly half wave plate followed by a fixed retarder with axis at approximately 15° and retardance of approximately 18°. The Degree of Polarization varies from 96% to 100%. Conclusions: Laser Diagnostic Technologies GDx Nerve Fiber Analyzer utilizes a rotating half wave plate to steadily rotate the delivered polarization state, together with a fixed retarder for compensation of corneal birefringence. As the corneal compensator has fixed axis and retardance, some error is introduced into the measurement of absolute RNFL thickness if the subject’s corneal birefringence is along a different axis and/or of different magnitude. It is further possible that the polarization state varies across the field of view due to polarization effects of the scanning mirrors. Our results illustrate the polarization states utilized by the GDx instrument, and point toward the direction and size of polarization–based error sources.
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