Diffractive Bifocal Contact Lenses Research Articles

Chromatic changes in vision with diffractive ophthalmic optics.

Diffractive optics is a valuable technique for designing presbyopia-correcting lenses, but its effectiveness is wavelength-dependent. This study investigates the spatio-chromatic alterations in visual resolution associated with diffractive multifocal lenses by using non-invasive, removable diffractive bifocal contact lenses. The study combines theoretical analysis, numerical simulation, and clinical intra-observer experiments to assess visual acuity under various lighting conditions. Results demonstrate the introduction of spatio-chromatic asymmetry and a change in visual acuity under red and blue lights, depending on the operating diffraction order employed in the lens design. The energy distribution of the diffractive contact lens studied favors resolution under red illumination at far distances and under blue illumination at near distances. These findings are consistent with computational simulations and provide insights into the visual changes induced by diffractive ophthalmic lenses.

Problems in the Measurement of Wavefront Aberration for Eyes Implanted With Diffractive Bifocal and Multifocal Intraocular Lenses

To determine the origin of the wavefront aberration as measured in eyes fitted with diffractive intraocular lenses (IOLs). Wavefront aberration was measured by Shack-Hartmann aberrometry in 2 eyes fitted with diffractive bifocal contact lenses and 10 eyes implanted with Acrysof ReSTOR multifocal IOLs. Problems were encountered as a result of the multiple wavefronts generated by the diffractive lenses and their dependence on wavelength. These led to ambiguities in the positions of the Shack-Hartmann spot images and in the form of the derived wavefronts. Aberrometry may not provide reliable information on the wavefront aberration associated with either the distance or near power of diffractive IOLs.

An exploration of modified monovision with diffractive bifocal contact lenses

A group of eight presbyopic subjects wore in succession four types of correction. One of these (A) consisted of a conventional pair of diffractive bifocals in which equal amounts of light contributed to the distance and near images in both eyes. Two (B and C) were modified monovision corrections using non-standard diffractive bifocal lenses. The diffractive designs were such that in the dominant eye a greater fraction of light was sent into the distance image and less into the near image and in the non-dominant eye a greater fraction contributed to the near image. The last lens combination (D) was a conventional monovision arrangement, with a single-vision distance correction on the dominant eye and a single-vision near correction on the non-dominant eye. Monocular and binocular high-contrast acuity, contrast sensitivity and stereopsis were assessed at both distance and near with each of the four lens combinations. It was hoped that the modified monovision arrangements (B and C) might yield improved distance and near acuities in comparison with the standard diffractive bifocals (A) while still maintaining good stereopsis in comparison with the single-vision monovision combination (D) and thus allow wearers to achieve better all round visual performance. Although these expectations were at least partly fulfilled, the modest advantages offered by the diffractive modified monovision approach did not appear to compensate for the disadvantages of its greater complexity.

Understanding the diffractive bifocal contact lens.

How a diffractive bifocal contact lens works is not intuitively obvious. This is because the diffractive bifocal operates by an unusual blend of refraction and interference. The present article seeks to clarify how it achieves its two focal powers. Most of this article, however, is devoted to the conceptually simpler case of diffraction gratings in order to develop an intuition for how a double focus is produced. Single slit, double slit, and multiple slit gratings are considered using two formalisms: Feynman's "arrow" method (also called phasors) and the Fourier method. When dealing with linear diffraction gratings we are interested in the light intensity in the focal plane. By a simple redefinition of variables, all of the mathematics for diffraction gratings can be applied to circular lenses. For this latter case instead of looking at the light intensity in the focal plane, we look at light intensity on the optic axis. The appearance of two focal points is a simple consequence of a combined interference and diffraction pattern. The goal of this article is to present an introduction to Fourier optics and an analysis of the bifocal contact lens in a reader-friendly manner.

Diffractive properties of the Diffrax® bifocal contact lens

We have verified that the Pilkington Diffrax lens behaves optically as a diffractive bifocal contact lens. The longitudinal chromatic aberration of the lens is similar to that predicted by theory, both on and off the eye. The over-refraction with the lens on the eye is similar to the distance subjective refraction, which is also as predicted.

Optical performance of decentered bifocal contact lenses.

Contact lenses frequently do not center over the pupil. The potential changes in optical performance introduced by poor centration of bifocal contact lenses have only been described previously from theoretical considerations. The present study measured the modulation transfer function (MTF) of rigid concentric-design refractive and rigid diffractive bifocal contact lenses with varying apertures and amounts of decentration. Decentration was shown to have a greater effect on the MTF of the refractive than on the MTF of the diffractive bifocals. The MTF of the refractive bifocals was related to the proportion of the aperture covered by the central optic zone (COZ). Unexpectedly, the MTF of the distance focus of the diffractive bifocal contact lenses was reduced at moderately high spatial frequencies. The practitioner should be aware of the effects of decentration of bifocal contact lenses.

Twelve-month success rates with a hydrogel diffractive bifocal contact lens.

In this study we assessed success rates over 12 months with a hydrogel diffractive bifocal contact lens. A total of 108 presbyopes were fitted with lenses, and after 12 months 46% were still wearing the lenses. Subjects with previous presbyopic lens-wearing experience achieved greater success (58% of those fitted) than neophyte lens wearers (33% of those fitted, and 17% of all neophyte presbyopes initially expressing an interest in contact lenses before screening). More neophytes failed for nonvision-related reasons than experienced subjects (31% vs. 12% of those fitted). Consequently, when nonvision-related failures were excluded from the calculation of success rates, 59% of those fitted with lenses (49% of neophytes and 66% of experienced subjects) were still wearing the lenses at 12 months. Most discontinuations from lens wear occurred in the first month of the study, the major reason for discontinuation being poor near vision (blur and/or ghosting; 47% of discontinuations).

Chromatic aberration and optical power of a diffractive bifocal contact lens.

Although diffractive contact lenses have been well documented in theory, no definitive experimental data have been reported which confirm that the near image is in fact created by diffraction rather than by refraction. We have tested the diffraction hypothesis for one type of diffractive contact lens (the Hydron Echelon bifocal) experimentally by measuring its longitudinal chromatic aberration in isolation and when worn on the eye. The basis of this test is that, according to theory, diffractive lenses should have chromatic aberration which is opposite in sign to that measured for the eye. Objective measurements of chromatic aberration were made with a focimeter when the lens was in a wet cell. Subjective measurements were made with a Badal optometer when the lens was worn on the eye. Four control experiments were conducted to provide baseline measurements of the eye's chromatic aberration, against which we compared the results obtained for the diffractive contact lens. The data were also compared with conventional measurements of refractive error obtained by standard subjective techniques and by an automated infrared refractor. Our results showed that the longitudinal chromatic aberration of the diffraction image of the Echelon bifocal lens was about one-half that obtained under the four control conditions: for the naked eye, for the nondiffraction image of the Echelon lens, or for either image of a refractive bifocal contact lens (CIBA Bisoft). These results are consistent with the theoretical prediction that the negative chromatic aberration of a diffractive contact lens should partially cancel the positive chromatic aberration of the human eye.(ABSTRACT TRUNCATED AT 250 WORDS)

Diffractive bifocal contact lenses in aphakia and pseudophakia

Diffractive bifocal contact lenses function by diverting incident light to two focal points. Light from near objects is focused by diffraction while that from distant objects is focused by conventional refraction. Both processes occur simultaneously throughout the full aperture of the lens. Fourteen aphakic and pseudophakic contact lens wearers were fitted with diffractive bifocal lenses and observed for an eight week period. Six (42.8%) were satisfied with the visual result. Five subjects (35.7%) discontinued lens wear complaining of poor quality of near vision. We conclude that these lenses may help selected aphakic patients to discard their reading glasses but the low acceptance rate suggests that this type of optic is probably not suitable for implantation as an intraocular lens.

Theoretische und praktische Sehqualität mit diffraktiv-multifokalen Linsen

I investigated the discrimination of objects with diffractive bifocal contactlenses, when objects in different distances are presented simultaneously. Computer assisted considerations suggested an impairment of discrimination as compared to monofocal correction for far as well as for near. The theoretical loss of image information has been shown to occur also in a visual test with steric arrangement of object points. However, the experiments suggested, that this kinds of visual quality impairment does not play a dominant role in the qualification of diffractive bifocals, since a rather sophisticated procedure was needed to demonstrate the expected effect.

Monovision vs. soft diffractive bifocal contact lenses: A crossover study

The relative clinical performance of monovision and the Hydro® ECHELON® soft diffractive bifocal contact lens were evaluated in a 1 -month, two-period, open-label crossover study. Wearers' subjective impressions were also elicited. Monovision provided superior low contrast acuity and the bifocal better stereopsis. No difference was found for high-contrast visual acuity or physiological signs. Although monovision was subjectively preferred for reading, no difference was found for other subjective variables. These included visual quality ratings at far and intermediate viewing distances, under dimly lit conditions, when driving at night, and for overall preference. Similiar patient acceptance can be expected for both modalities. The results of this study reflect a significant improvement in performance of the diffractive bifocal lens over previous simultaneous vision bifocal contact lens damage.

A new diffractive bifocal contact lens

Anew bifocal contact lens uses the simultaneous vision principle but divides the incident light in a novel way. Instead of the usual division of the aperture into near segment and distance segment this new lens uses diffraction to divide the amplitude of the light. Both images therefore use light from the whole aperture restricted only by the natural pupil. This ‘full aperture’ bifocal contact lens is capable of excellent visual acuity at near and distance over a wide range oflight levels. The optical principles and visual characteristics of the lens will be described.