Oblique effects with multisegment spectacle lenses: 2. Ray tracing to determine power corrections.

Purpose: Part 1 of the study investigated image quality associated with oblique incidence of light on a multisegment lens (Hoya MiyoSmart) intended to treat myopia development. Part 2 investigates power corrections associated with oblique incidence.

Methods: Modelling and ray tracing were carried out with lenses of -4 D distance power and, to a lesser extent, +0.25 D. Ray tracing simulations were done for the lens by itself, an eye model by itself and the combination. These simulations were for the static situation of peripheral vision when the eye looks through the lens centre and for central (foveal) vision when the eye rotates to look at objects away from the lens optical axis. The outcome was power correction of the optics, that is, the difference between the nominal power of the distance correction provided by the carrier lens under specific conditions and the actual power. This was determined across the field to about 45° and across the pupil for certain field angles. Most investigations were performed for distance vision, but some were for near objects with an accommodating version of the model eye.

Results: Ignoring intended multisegment effects, the quality of optics associated with peripheral vision was poor. There was considerable astigmatism (cylinder) across the field and high variation in astigmatism across the pupil. The added effects of the lens and eye were similar to those of their combination. For the accommodated eye model with an object at 250 mm, results were similar to those obtained with the unaccommodated model viewing a distant object. For foveal vision with the rotating eye, optics were relatively good with lower levels of astigmatism than for peripheral vision.

Conclusion: The results of Part 1, finding considerable effects of the obliquity of incidence associated with peripheral vision and with foveal vision for the rotating eye, were supported by the power corrections.