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Peripheral refraction measured from through-focus double-pass imaging


Purpose : Peripheral refraction is a relevant information in different areas of clinical optics, such as myopia or cataract. We have developed and validated an optical instrument and a corresponding method for the in-vivo assessment of peripheral defocus and astigmatism from the recording of through-focus double-pass images.

Methods : An optical setup was developed to acquire non-cycloplegic, double-pass through focus images of the eye at 0, 15 and 30 degrees of visual angle. A 2-mm diameter infrared laser diode (780nm) beam was projected on the ocular fundus with the appropriate relay optics via a tunable lens capable of inducing +5D to -18D of defocus. The ocular fundus was then imaged through the full natural pupil of the eye and the same relay optics and the tunable lens on a CMOS sensor. A series of through focus images was acquired at a step of 0.1D and best focus was established by finding the image where peak intensity value was maximum. The magnitude of astigmatism was determined by finding the two defocus images where the ratio of the ellipse’s minor axis in their shape was minimized. Astigmatism was then defined as the dioptric difference between those two images and the axis of astigmatism was defined as the axis of the image with the highest defocus value.

Results : Using appropriate off-axis fixation of the subject, the refractive measurement was performed at 0, 15 and 30 degrees of visual angle at a temporal horizontal visual field. The procedure was validated in-vivo in 2 healthy subjects and each measurement was repeated twice to determine the repeatability of the measurement. Repeatability in central and peripheral refraction measurement was within 0.25D sphere and cylinder for both subjects. Both subjects exhibited a myopic periphery with respect to the foveal refraction (-0.7 and -1.4D at 30 degrees). The difference of astigmatism between the 30 degrees field angle and central vision was in the myopic direction for both subject (-3.3 D and -2.6D) respectively where the more refractive axis was horizontal. At each eccentricity the measurement lasted a few seconds.

Conclusions : An instrument and an associated method was developed for the fast, in-vivo assessment of central and peripheral refraction. The refraction obtained from this method pertains to the actual refraction optimizing the double-pass images of the eye and is free of errors associated to refractometry and wavefront sensing at elliptical pupils.

This is a 2021 ARVO Annual Meeting abstract.

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