Sphero-cylindrical error for oblique gaze as a function of the position of the centre of rotation of the eye

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Abstract

Purpose

New designs of ophthalmic lenses customised for particular wearing conditions (e.g., vertex distance or wrap tilt angle) have emerged during the last few years. However, there is limited information about the extent of any improvement in visual quality of these products. The aim of this work was to determine whether customisation according to the centre of rotation of the eye (CRE) improves visual quality for oblique gaze in monofocal spherical lenses.

Methods

Conventional spherical lenses were designed by numerical ray tracing with back vertex powers (BVP) ranging from +8 to −8 dioptres (D) and base curves from 0 to 8 D. The wavefront error at oblique gaze (40°) was computed for each design with CRE positions from 20 to 35 mm. Sphero-cylindrical (SC) error was calculated using wavefront Zernike coefficients, considering only monochromatic aberrations. Visual acuity in logMAR was estimated following the Raasch empirical regression model.

Results

SC error and visual acuity maps were calculated for each BVP as a function of base curves and CRE in a graded colour scale. From SC error maps maximum spherical and cylindrical errors (MSE and MCE) of 1.49 D and −1.24 D respectively were found for BVP from 0 to −2 D, 2.27 D and −1.90 D for BVP from −2 D to −4 D, 2.59 D and −2.20 D for lenses from −4 D to −6 D and 2.63 D and −2.28 D for lenses from −6 D to −8 D. Concerning positive lenses, we obtained MSE and MCE of 0.37 D and −1.35 D respectively for lenses from 0 D to +2 D, 0.39 D and −2.23 D for lenses from +2 D to +4 D and 0.36 D and −2.73 D for lenses from +4 D to +6 D. Regarding visual acuity maps for 40° oblique gaze, significant loss of visual acuity (>0.30 logMAR, Snellen 6/12, 20/40, decimal 0.50) was found for BVP as low as −2 D. Clinically negligible high order aberration levels (equivalent spherical power <0.25 D) were found for all cases.

Conclusions

We calculated SC error and visual acuity maps as a function of base curves and CRE position for a set of spherical lenses at oblique gaze. These maps could be useful as a clinical guide to select the best base as a function of CRE and BVP for each patient. It was found that negative lenses with low BVP provided high SC error when they were designed with low bases. However, high BVP negative lenses with low SC error were found for medium bases and low CRE or for low bases and high CRE positions. In the case of positive lenses, the dependence of SC error with CRE position was less noticeable. Finally, high order aberrations did not have a significant influence over visual quality.

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