Colin Fowler

Do tinted lenses or filters improve visual performance in low vision? A review of the literature.

This is a review of studies that have investigated the proposed rehabilitative benefit of tinted lenses and filters for people with low vision. Currently, eye care practitioners have to rely on marketing literature and anecdotal reports from users when making recommendations for tinted lens or filter use in low vision. Our main aim was to locate a prescribing protocol that was scientifically based and could assist low vision specialists with tinted lens prescribing decisions. We also wanted to determine if previous work had found any tinted lens/task or tinted lens/ocular condition relationships, i.e. were certain tints or filters of use for specific tasks or for specific eye conditions. Another aim was to provide a review of previous research in order to stimulate new work using modern experimental designs. Past studies of tinted lenses and low vision have assessed effects on visual acuity (VA), grating acuity, contrast sensitivity (CS), visual field, adaptation time, glare, photophobia and TV viewing. Objective and subjective outcome measures have been used. However, very little objective evidence has been provided to support anecdotal reports of improvements in visual performance. Many studies are flawed in that they lack controls for investigator bias, and placebo, learning and fatigue effects. Therefore, the use of tinted lenses in low vision remains controversial and eye care practitioners will have to continue to rely on anecdotal evidence to assist them in their prescribing decisions. Suggestions for future research, avoiding some of these experimental shortcomings, are made.

Liquid crystal lens review

Liquid crystal lenses are possibly the spectacle lenses of the future for presbyopic corrections. They present a variable refractive index which can be used to produce the necessary addition needed for presbyopia. The variability of the refractive index is due to the fact that these materials are optically anisotropic. This effect is obtained by the application of an electric or magnetic field through the liquid crystal layer. A review of the literature relating to the evolution, the preparation and the operation of these lenses is given. A discussion is also included of the practical problems and drawbacks, which at present limit their use, and possible solutions.

Evaluation of the Eyesys Model Ii Computerized Videokeratoscope. Part Ii: The Repeatability and Accuracy in Measuring Convex Aspheric Surfaces

Purpose. To evaluate the precision and repeatability of a Placido disc-based computerized videokeratoscope, using convex surfaces of varying eccentricities and apical radii designed to simulate the range of topographical variations of the human cornea, rather than the purely spherical surfaces used in most previous studies. Methods. Form Talysurf analysis was used to verify the exact form of 12 Perspex convex surfaces. The EyeSys model II videokeratoscope was used to measure the sagittal radii of curvature twice at known points on each surface. The raw data tables were analyzed to assess the repeatability and accuracy for both central and peripheral points on each surface. The relationship between these factors and the eccentricity was investigated. Results. For central radii the instrument showed high correlation (r=0.996) between actual and measured values. There was a small instrumental bias of +0.042 mm and the 95% limits of agreement were narrow (+0.121 to -0.037 mm), indicating clinically acceptable accuracy. The accuracy decreased slightly as the p-value decreased (greater peripheral flattening). For peripheral radii, the overall accuracy compared well to central radii, with an average bias of +0.022 mm and maximum error in 95% of cases 0.083 mm (bias +1.96 x SD). However, for surfaces where p=0.50, the bias was +0.049 and maximum error in 95% of cases 0.110 mm. Repeatability for the aspheric surfaces was shown to be high (SD ±0.01 mm in all quadrants). Conclusions. The accuracy of the EyeSys corneal analysis system (CAS) in measuring central and peripheral radius of curvature was shown to be dependent on the shape of the surface to be measured. For more rapidly flattening surfaces, a decrease in accuracy was found for both central and peripheral radius of curvature, which in clinical terms is thought acceptable.

Review of past and present techniques of measuring corneal topography

Various methods of measuring corneal topography are described. The advantages and disadvantages of the principles used in the measurement of corneal shape by the various techniques are discussed. The corneal surface may be described in a number of ways; some researchers have used conic sections while others have used more complex polynomial expressions. Computer algorithms have also been developed to calculate quantitative measures of corneal topography to augment the information obtained from topographical maps. These descriptors are discussed in this article.

Progressive addition and variable focus lenses: a review

This paper reviews recent developments in the field of progressive addition spectacle lenses and variable focus lens systems design. The study is a sequel to the previous analytical work of von Rohr (1916), Graham (1942), and Bennett (1970). A brief history of the work documented by the aforementioned authors is presented. Comparative assessment is undertaken of the most recent patents of progressive addition and variables focus spectacle lenses. A largely verbal description is employed because patent literature is often devoid of mathematical models.

Evaluation of the EyeSys Model II Computerized Videokeratoscope. Part I: Clinical Assessment

Purpose. To compare the agreement and repeatability of keratometry measurements with simulated keratometry measurements from the EyeSys (model II) corneal analysis system (CAS). Furthermore, to establish any relationship between the repeatability of peripheral corneal measurements and the spatial location at the cornea. Methods. Measurements using a Bausch & Lomb keratometer in 19 eyes (19 subjects) were compared with the simulated keratometry values using the EyeSys CAS. Repeatability of the keratometer (14 eyes, 14 subjects) and the EyeSys simulated keratometry values (10 eyes, 10 subjects) was assessed by comparing the difference of two measurements. Peripheral repeatability of the EyeSys CAS was compared in a similar manner in a sample of 10 eyes (10 subjects). Results. The 95% confidence limits (—0.529 to +0.149 D) showed a lack of agreement between both instruments. With respect to the repeatability, both the keratometer (SD=±0.103 D) and the EyeSys (SD=±0.072 D) were found to exhibit approximately similar degrees of repeatability. The repeatability of peripheral corneal measurements using the EyeSys CAS showed changes in repeatability dependent on the corneal meridian and the distance from the point of alignment. Measurements along the superior and nasal meridians showed poorest repeatability. Repeatability was also found to deteriorate away from the point of alignment. Conclusions. EyeSys simulated keratometry values were not interchangeable with the keratometer. This could be due to differences in the method of alignment or measurements from different areas on the corneal surface. Repeatability of both the Bausch & Lomb keratometer and the EyeSys CAS was found to be similar. The spatial dependency of peripheral corneal radius repeatability measurements could be attributed to interference of the ocular adnexa in those areas of measurement.

A comparison of three methods for the measurement of progressive addition lenses

Three methods of measuring aberrational astigmatism in progressive addition spectacle lenses are described. Measurements are presented from each method made on an Essilor Varilux 2 lens, which illustrate the small differences between the methods for low power lenses.

Recent trends in progressive power lenses

Despite having been first marketed in the 1950s, new designs of progressive addition spectacle lens continue to appear. Some of the recent patent literature is reviewed on the design of such lenses. As well as a number of improvements to general purpose designs, specifically to include aspheric surfaces for the distance portion of progressive lenses, the literature includes a recent patent on an improved version of the Alvarez lateral translation lens system. The optimisation of single vision lens forms in order to extend the depth of field for early presbyopes is also discussed.

A gradient‐index ophthalmic lens based on Wood's convex pseudo‐lens

Although the Varilux type varifocal has been and still is very popular, research has been carried out on other methods of providing a progressive addition lens. This paper deals with the field of gradient‐index optics. It includes a review of the literature related to the production of a gradient‐index element and the theoretical analysis of concentric varifocals. A procedure is described to construct a varifocal lens, after that described by Wood in 1905, to produce lenses known as ‘non‐homogeneous cylinders’ or ‘pseudo‐lenses’. The method is based on a diffusion process, taking place in half of the lens, and creates steps in the refractive index of the material. Experimental data are also given on the change of the spherical and cylindrical power, which indicate that the lens constructed is an optical system having a rotational symmetry in power, around a fixed point.

Automatic measurement of varifocal spectacle lenses

An instrument is described for the automatic measurement of lens power across the whole aperture of a varifocal lens. The apparatus is based on an electronic focimeter controlled by an external computer, which also positions the lens by two stepper motors. Methods of display of the data are discussed, and the repeatability of measurement is illustrated.