Aspheric Contact Research Articles

Optical surface optimization for the correction of presbyopia

Presbyopia, the gradual loss of accommodation that accompanies aging, can be corrected by creating asphericity in the optical path of the eye. Bifocal and aspheric contact lenses, intraocular lenses, spectacle lenses, and laser refractive surgery are all widely used to alleviate the symptoms of presbyopia. These types of corrective surfaces try to concentrate vision in limited peaks over the full range of vergences. The methodology described in this paper is designed to correct presbyopia by optimizing vision over the entire target range of near to distant. A corrective surface was created by employing an iterative function minimization algorithm to optimize an optical metric. In most cases, it is possible to obtain an optical surface that will optically compensate for presbyopia.

Verification of aspheric contact lens back surfaces.

To suggest a tolerance level for the degree of asphericity of aspheric rigid gas-permeable contact lenses and to find a simple method for its verification. Using existing tolerances for the vertex radius, tolerance limits for eccentricity and p values and were calculated. A keratometer-based method and a method based on sag measurements were used to measure the vertex radius and eccentricity of eight concave progressively aspheric surfaces and six concave ellipsoidal surfaces. The results were compared with a gold standard measurement made using a high-precision mechanical instrument (Form Talysurf). The suggested tolerance for eccentricity and p value and is +/-0.05. The keratometer method was very accurate and precise at measuring the vertex radius (mean deviation +/- SD from Talysurf results, -0.002 +/- 0.008 mm). The keratometer was more precise than and similar in accuracy to the sag method for measurement of asphericity (mean deviation of keratometer method results from Talysurf results, 0.017 +/- 0.018; mean deviation of sag method results from Talysurf results using five semichords, -0.016 +/- 0.032). Neither method was precise enough to verify the asphericity within the suggested tolerance. The keratometer can be efficiently used to verify the back vertex radius within its International Organization for Standardization tolerance and the back surface asphericity within an eccentricity/p value tolerance of +/-0.1. The method is poor for progressive aspheres with large edge blending zones. Deriving the eccentricity from sag measurements is a potential alternative if the mathematical description of the surface is known. The limiting factor of this method is the accuracy and precision of individual sag measurements.

Measuring ophthalmologic surfaces by means of moire´ deflectometry

A new technique for measuring ophthalmologic surfaces such as aspheric contact lenses and the human cornea is proposed in this paper. The method is based on the principle of moire´ deflectometry. In moire´ deflectometry, two fringe patterns (deflectograms) are generated, each providing a piece of information about the measured wavefront. Compared with interferometric techniques, interpretation of the fringe patterns obtained is more difficult. Therefore synthetic deflectograms for surfaces with known parameters are generated. Computer simulation of deflectograms in the case of aberrations introduced by an aspherical object is also performed. Experimental results for various curved surfaces, including contact lenses and the human cornea, are demonstrated.

Aberrations associated with rigid contact lenses.

A rigid contact lens on an eye can produce levels of spherical aberration very different from those produced by a spectacle lens in front of the eye. These levels are considerably affected by contact lens surface asphericity. Change in longitudinal spherical aberration associated with aspherizing a contact lens surface is well predicted by a simple equation for change in sagittal power of the surface. Displacing an aspheric contact lens on the eye can produce considerable defocus, which is well predicted by simple equations for change in sagittal and tangential surface powers. The best refractive correction with contact lenses can be determined only by overrefraction with a patient wearing a contact lens of power and characteristics similar to that which will be prescribed. An aspheric contact lens that moves to a considerable extent on the eye will cause more unstable vision than will a spherical lens that moves to the same extent.

The effects of spherical and aspheric rigid gas permeable contact lenses: Corneal curvature and topography changes

The purpose of this study was to evaluate the performance of a rigid, posterior surface aspheric contact lens compared with a spherical contact lens. A total of 13 subjects were fitted with an aspheric lens in one eye and a spherical lens in the other eye and were evaluated for changes in corneal curvature and corneal topography over a 6-month period. The eyes fitted with the aspheric lenses demonstrated a trend for greater corneal flattening and a greater reduction in corneal toricity compared with their spherical counterparts. These results are believed to be related to differences in posterior sagittal lens depth, posterior surface design, mechanical molding effects, and vertical lens displacement.

Spherical Aberration of Aspheric Contact Lenses on Eye

It is important to assess aspheric rigid contact lenses not only for fit, but also for optical performance. Using ray tracing techniques, on-eye total longitudinal aberration induced by aspheric contact lenses and the variation of the tangential focus with ray height were determined. Both the contact lens and tear lens were taken into account. The front and back surface profiles of the contact lens and the corneal profile were modeled as conic sections. On-eye aberration was found to become strongly more positive as the p-value of the contact lens front surface increased. It became more negative as the back surface p-value increased, but the tendency was weaker. These calculations indicate that selection of a contact lens back surface p-value according to fitting considerations without choosing a front surface p-value to minimize aberrations could, in some circumstances, lead to a reduction in visual performance.

Aspheric contact lenses used to control convergence excess

A 15-year-old female patient presented wearing progressive addition spectacle lenses to help control a condition of convergence excess. She was fitted with aspheric, multifocal, rigid gas permeable contact lenses for cosmetic reasons. The lenses were fitted 1.5 dioptres steeper than the flatter keratometer reading, and incorporated an eccentricity value of 0.7. Visual acuities through these lenses were 6/7.5 at both distance and near in each eye. Phorias through the lenses were measured using two methods. The cover test revealed 1 D of esophoria at 6 m and 3 D of esophoria at 40 cm. The Maddox rod also yielded 3 D of esophoria at 40 cm. (Clin Exp Optom 1993; 76.4: 141–142)

Fitting of Aspheric High Gas-Permeable Rigid Contact Lenses to Scarred Corneas

Scarring of the cornea often results in an irregular corneal surface, which causes scattering in light perception. Therefore, the impaired visual acuity cannot be adequately corrected by spectacles in most cases. In this study, high oxygen-transmissible aspheric rigid lenses were fitted, with computer assistance, in 26 scarred eyes of 23 consecutive patients. In 15 of 26 eyes (57.7%), a successful fitting with good vision, no complications, and a sufficiently long wearing time was accomplished. The main lens-related complications included fluorescein-staining epithelial defects in five of 26 eyes (19.2%) and epithelial edema in two of 26 eyes (7.7%). Computer-aided fitting was of limited value because keratometer readings were not measurable in 50% of the cases. The results of this study indicate that the application of high oxygen-transmissible aspheric rigid contact lenses may obviate corneal surgery.

Multifocal Aspheric Contact Lenses in the Management of Accommodative Esotropia

The successful clinical use of multifocal aspheric contact lenses in the management of accommodative esotropia is reported. The patient was a 13 year old boy.

Inspection of back surface aspheric contact lenses.

Two methods of clinically inspecting and verifying rigid back surface aspheric contact lenses were investigated. A modification of the Radiuscope was found to be an accurate method for making such measurements. Measurement data of a number of aspheric lenses obtained by using this procedure are presented.