Impact of contact lens zone geometry and ocular optics on bifocal retinal image quality

Abstract

Purpose To examine the separate and combined influences of zone geometry, pupil size, diffraction, apodisation and spherical aberration on the optical performance of concentric zonal bifocals. Methods Zonal bifocal pupil functions representing eye + ophthalmic correction were defined by interleaving wavefronts from separate optical zones of the bifocal. A two-zone design (a central circular inner zone surrounded by an annular outer-zone which is bounded by the pupil) and a five-zone design (a central small circular zone surrounded by four concentric annuli) were configured with programmable zone geometry, wavefront phase and pupil transmission characteristics. Using computational methods, we examined the effects of diffraction, Stiles Crawford apodisation, pupil size and spherical aberration on optical transfer functions for different target distances. Results Apodisation alters the relative weighting of each zone, and thus the balance of near and distance optical quality. When spherical aberration is included, the effective distance correction, add power and image quality depend on zone-geometry and Stiles Crawford Effect apodisation. When the outer zone width is narrow, diffraction limits the available image contrast when focused, but as pupil dilates and outer zone width increases, aberrations will limit the best achievable image quality. With two-zone designs, balancing near and distance image quality is not achieved with equal area inner and outer zones. With significant levels of spherical aberration, multi-zone designs effectively become multifocals. Conclusion Wave optics and pupil varying ocular optics significantly affect the imaging capabilities of different optical zones of concentric bifocals. With two-zone bifocal designs, diffraction, pupil apodisation spherical aberration, and zone size influence both the effective add power and the pupil size required to balance near and distance image quality. Five-zone bifocal designs achieve a high degree of pupil size independence, and thus will provide more consistent performance as pupil size varies with light level and convergence amplitude.

Effect of age, decentration, aberrations and pupil size on subjective image quality with concentric bifocal optics

“…Figure shows that the two‐zone optical profile is much more influenced by the pupil diameter than the eight‐zone profile. This result was already observed recently by Bradley et al . The change of pupil diameter on a two‐zone optic modifies the ratio between the areas dedicated to correct near and distance vision.…”

“…However, the optical design of contact lenses is very often based on much wider sizes. For example, Acuvue Bifocal ® has five optical zones up to 6 mm in the periphery . For a 4.5 mm pupil size only three zones and a part of the fourth are in front of the pupil.…”

“…Spherical aberration is known for having a double impact on the presbyopes' vision quality: it improves near vision quality while decreasing distance vision quality . Integrating spherical aberration in a bifocal design turns it into a multifocal . Plainis et al .…”

Which Ratio of Areas Improves Vision Quality in Simultaneous Focus Optics?

“…Consequently, bifocal designs are highly dependent on pupil diameter. 9 However, these results do not take into account subjective quality of vision, which can be useful to assess differences between subjects. Studies including these last two parameters did not find that equal portions of area for distance and near vision balanced near and distance vision 9,10 ; they respectively calculated that a central optical zone area representing 36 and 39% equalized distance and near vision.…”

Correction of presbyopia: old problems with old (and new) solutions

“…In these lenses, a half wavelength (pi) phase shift between each zone produces a lens with two powers, a bifocal and other phase step combinations can be used to generate trifocal designs . These diffractive designs can be distinguished from the previously described concentric refractive lenses because every location across the lens contributes to each power, and thus these lenses are often referred to as ‘full aperture’ designs, in which the relative amount of energy in the distance and near images does not necessarily vary with pupil size, as it does with concentric refracting designs . Diffractive optics have been incorporated into several IOLs, but have had limited commercial success as contact lenses (for example, Echelon and Diffrax).…”