Biomedical subjects

D A Atchison

Publications and source records attributed to D A Atchison.

At least 55 records · Page 3Linked to original sources

PubMed1992

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)

Color Perception↗

PubMed1992

Modeling the power of the aging human eye.

A hypothesis is presented that may explain why the aging eye does not become myopic with age. The power of the eye lens is predicted with a modeling approach to determine how the form of the refractive-index gradient within the lens can change to maintain a constant power in spite of age-related curvature increase. Methods used include published age-dependent data on the optical parameters of the eye, a mathematical model of the lens based on elliptical isoindicial contours, and a refractive-index profile that can be expressed as a power series in the distance from the lens center. The kinds of change in profile required to prevent the eye from becoming myopic as its lens grows are shown.

Aging↗

PubMed1992

Tolerance to spherical aberration induced by rigid contact lenses.

We have investigated the effects of spherical aberration (SA) on visual performance by fitting subjects with rigid lenses of varying front surface asphericity. Three pairs of lenses were worn by each subject with front surface asphericities of Q = 0 (conventional spherical surface), Q = -0.26 (similar to the average corneal asphericity) and Q = -0.51. The calculated average change in spherical aberration induced by each pair of contact lenses in photopic conditions was +0.02 D (Q = 0), -0.49 D (Q = -0.26) and -0.94 D (Q = -0.51). In mesopic conditions these same lenses induced an average change in spherical aberration of +0.01 D (Q = 0), -1.18 D (Q = -0.26) and -2.16 D (Q = -0.51). In this double-blind study, 12 adapted, low to moderate myopic rigid lens wearers wore each pair of lenses for one week and noted visual performance. With each pair of lenses we also measured the subject's visual acuity with high and low contrast logMAR charts. All measurements were conducted using the right eye at distance in photopic and mesopic conditions. When forced to select the lenses they would accept as compensation for participation in the study, most subjects (9 out of 12) chose the Q = 0 lenses, 3 subjects those the Q = -0.26 lenses and none of the subjects chose the Q = -0.51 lenses. Visual performance measures showed little difference between the Q = 0 and Q = -0.26 lenses. However, the Q = -0.51 lenses significantly reduced visual acuity in mesopic conditions with the low contrast acuity chart.

Contact Lenses↗

PubMed1992

The use of colour difference vectors in diagnosing congenital colour vision deficiencies with the Farnsworth-Munsell 100-hue test.

Colour difference vector analysis provides useful and meaningful information in scoring the Farnsworth-Munsell (FM) 100-hue test. However, the FM 100-hue test is limited in its ability to diagnose type and severity of congenital colour vision defect. Type classification for all subjects is incorrect in 21% of cases, and for deuteranomals the misclassification rate is 38%. Visual inspection of the plots yields a similar misclassification rate and classification of plots with few errors (under 180) is generally less reliable. The FM 100-hue test has a limited ability to separate dichromats from anomalous trichromats. A test protocol based on joint D15 and FM 100-hue tests should pass 36% of anomalous trichromats and 26% of all colour defectives yet fail all dichromatic observers. We conclude that administering the FM 100-hue test is of less value than a combination of D15 panels (Standard D15 and L'Anthony's desaturated D15) in the clinical diagnosis of congenital colour defective observers. Our results for the FM 100-hue panel are similar to those reported previously by other investigators.

Adult↗

PubMed1991

Optical design of low index intraocular lenses.

The effect of shape of low index (1.43) intraocular lenses on optical performance of pseudophakic eyes was investigated. A range of image quality criteria was used. The optimum lens for on-axis vision was biconvex, with the more curved surface facing the cornea. For small off-axis angles, optimum lens shape varied with the ocular parameters. A recommended range of intraocular lenses is plano-convex (with the curved shape facing the cornea) to equi-convex. Compared with conventional poly(methyl methacrylate) lenses (1.492 index), optimum lens shapes of low index lenses were less sensitive to changes in eye parameters and lens fitting position. The magnitudes of aberration of pseudophakic eyes varied more with changes in lens shape and lens position in low index lenses than in conventional lenses.

Humans↗

PubMed1991

Quantitative scoring methods for D15 panel tests in the diagnosis of congenital color vision deficiencies.

Farnsworth's standard D15 and L'Anthony's desaturated D15 panel tests were administered to 99 congenital red-green color defective subjects. The results were analyzed in three ways: (1) by summing the color differences between adjacent caps according to Bowman, (2) by averaging color difference vectors (CDV) according to Vingrys and King-Smith, and (3) by visually inspecting and counting the crossings. The Bowman measure was highly correlated with one of the CDV measures but provides less information regarding a cap arrangement. The desaturated D15 test can be expected to misclassify 5% of dichromats by type (protan/deutan) compared to a type misclassification rate of less than 0.1% for the D15 panel test. The correct diagnostic rates for type were 45% for the standard D15 test and 58% for the desaturated D15 test. However, the improvement in correct diagnostic rate for the latter test was accompanied by an increase in the misdiagnostic rate from 2 to 10%. The main value of the desaturated D15 test in congenital color vision diagnosis would seem to be in providing classification of those subjects who pass the standard D15 test. Quantitative scoring of the tests provides a good estimate of severity of defect. Visual inspection provides a similar diagnostic rate to CDV analysis, but has a lower type misdiagnostic rate at the desaturated D15 test and is more likely to be correct when the two methods disagree. We suggest that quantitative scoring techniques are of limited benefit for the clinical diagnosis of congenital color vision defects but that they are of use in clinical trials or for the monitoring of changes in color vision over time.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1991

Use of the Humphrey Lens Analyzer for off-axis measurements of spectacle lenses.

Automated focimeters can be used to make quick, precise measurements of off-axis power and prismatic effects corresponding to an eye rotating behind a spectacle lens. An automated focimeter, the Humphrey Lens Analyzer, was assessed in this regard. The Humphrey Lens Analyzer can be used to give a valid measure of off-axis power of lenses with low power, but not of lenses with moderate to higher power (greater than 3 D). For 3 D spherical lenses discrepancies of the order of 0.1 D occur at 30 degrees rotation, and 6 D spheres give discrepancies of 0.5 D at the same rotation. Small discrepancies were found for measurements of prism. The Humphrey Lens Analyzer was also used in a mode where the lens being tested is rotated about the center of curvature of its back surface. This is the mode often used to assess aberrations and prism of progressive-addition lenses. In this mode, the instrument provides reasonable accuracy in estimating off-axis power corresponding to eye rotation for lenses with low power, but not for lenses with moderate to higher power (greater than 3 D). However, it provides accurate values of the variation in off-axis surface power for low powered lenses with aspheric front surfaces. There were considerable systematic errors associated with the measurement of prism. A simple raytracing method was developed to predict the results of measurements with the Humphrey Lens Analyzer. Predictions of off-axis power were good when lenses were rotated about a position corresponding to the center-of-rotation of an eye, but were poorer when lenses were rotated about the center of curvature of their back surfaces. Predictions of primatic efforts were good in both situations. A method by which the Humphrey Lens Analyzer should provide an accurate measurement of off-axis powers corresponding to eye rotation behind a spectacle lens is described, but has not been tested.

Eyeglasses↗

PubMed1991

Design of aspheric intraocular lenses.

The value of aspherizing intraocular lens surfaces for the purpose of improving retinal image quality of pseudophakic eyes was investigated. Surfaces were made as conicoids. Changes in primary wave aberrations and refractive errors were found to be proportional to changes in surface asphericity. The optimum asphericity depends markedly on ocular parameters, lens fitting position and how well lenses are centred within the eye. If lenses are not well centred, the asphericity that eliminates spherical aberration may result in poorer performance than that occurring with spherical surfaces. It is suggested that there is no general benefit in aspherizing surfaces of intraocular lenses.

Humans↗

PubMed1991

The optical modelling of the human lens.

The effect of varying lens shape factors and refractive index distribution on two lens performance characteristics (equivalent power and spherical aberration) are considered, using a mathematical model of the human lens which is based upon ellipsoidal iso-indical contours. Values of radii of curvature based on the work of various authors are used to determine asphericity and these inferred values compared with previous measurements. Discrepancies are found and although asphericity has no effect on equivalent power, it does affect the spherical aberration. Based on preliminary findings of the refractive index distribution in the equatorial plane of human lenses, the index distribution is described by a polynomial in the Y (the distance from the optical axis in the equatorial plane), the coefficients of which are polynomials in Z (the distance along the optical axis). The shape of the index profile was found to have a significant effect on the equivalent power and spherical aberration of the lens. The results indicate that more information on surface asphericity and accurate measurement of the index profile in the sagittal plane are required for more accurate modelling of the human lens.

Humans↗

PubMed1990

Optical design of poly(methyl methacrylate) intraocular lenses.

The effect of poly(methyl methacrylate) intraocular lens shape on optical performance of pseudophakic eyes was investigated. Image quality criteria included wave aberrations, spot diagrams, longitudinal aberrations, the modulation transfer function, and an optimization procedure. For eyes with corneal asphericities encompassing most of the population, the optimum lens form for on-axis vision is close to plano-convex, with the more curved surface facing the cornea. This applies whether lenses are correctly centered, tilted, or decentered. For small off-axis angles, optimum lens shape varies with the ocular parameters. Good intraocular lens shapes range from the plano-convex to the equi-convex. Plano-convex lenses with the curved surface facing the retina are not recommended.

Humans↗

PubMed1990

Investigation of central visual fields in patients with age-related macular changes.

The effect of early macular pigmentary and drusen changes on the central visual field was investigated in elderly patients with normal visual acuities. Visual field measurements were taken with the Humphrey Field Analyser using its 24-2 and 10-2 full threshold programs. No significant differences were found between two patients groups, one with and one without the macular changes. We conclude that fine pigmentary changes and hard drusen do not cause changes in visual functioning and can be accepted as normal age-related changes.

Aged↗

PubMed1989

Refractive errors induced by displacement of intraocular lenses within the pseudophakic eye.

Simple methods were developed to estimate refractive errors when intraocular lenses are not fitted optimally within pseudophakic eyes. The accuracy of these methods was determined by comparing results obtained with them to results obtained by raytracing through a model eye. Accuracy was good for longitudinal displacement and tilting, and reasonable for transverse displacement. Refractive errors are related linearly to the magnitude of the longitudinal displacement, and are related to the square of the magnitude of tilt or transverse displacement. The refractive error upon transverse displacement is quadratically dependent upon lens shape.

Lenses, Intraocular↗

PubMed1989

Optical design of intraocular lenses. I. On-axis performance.

The theoretical effect of intraocular lens shape on on-axis performance of model pseudophakic eyes was investigated. Image quality criteria included the magnitude of spherical aberration (both wave and longitudinal), spot diagrams, and the modulation transfer function. For eyes with corneal asphericities encompassing most of the population (Q greater than or equal to -0.5), the optimum lens form is close to being plano-convex, with the more curved surface facing the cornea.

Lenses, Intraocular↗

PubMed1989

Optical design of intraocular lenses. II. Off-axis performance.

The effect of intraocular lens shape on off-axis performance of model pseudophakic eyes was investigated. Image quality criteria included an optimization procedure, spot diagrams, and the modulation transfer function (MTF). The optimum lens shape varies with lens fitting position, corneal asphericity, and pupil size. An earlier investigation showed that the optimum lens shape for on-axis imagery is approximately plano-convex, with the more curved surface facing the cornea. In this investigation, optimum lens shapes for a 10 degrees off-axis position ranged from near this shape to near the reversed shape, i.e., the more curved surface facing the retina. Because the on-axis case should carry more weight and because conditions favoring the reverse form include small pupils for which image quality is fairly insensitive to variations in lens shape, it is suggested that good forms for intraocular lenses range from plano-convex (curved surface facing cornea) to equi-convex.

Equipment Design↗

PubMed1989

Optical design of intraocular lenses. III. On-axis performance in the presence of lens displacement.

The effect of intraocular lens shape on on-axis performance of model pseudophakic eyes was investigated, when lenses were displaced by tilting or decentration. Image quality criteria included an optimization procedure, spot diagrams, and the modulation transfer function (MTF). Displacement adversely affects image quality. With conventional lens shapes, the deterioration is either small or retinal image quality can be nearly restored to predisplacement levels by spectacle correction. Optimum lens shapes in the presence of decentration are close to being plano-convex, with the more curved surface facing the cornea. Based on the results of this and previous studies, it is suggested that preferred shapes of intraocular lenses range from this plano-convex form to equi-convex.

Equipment Design↗

PubMed1989

Third-order aberrations of pseudophakic eyes.

The aberrations of pseudophakic eyes were analysed by third-order theory assuming thin intraocular lenses. Third-order aberrations with spherical surfaced lenses are quadratic functions of lens shape. Spherical aberration and coma are generally the most rapidly changing aberrations, at small field positions, with change in lens shape. Optimum designs probably lie within the shape factor range at which these attain minimum absolute values (+1.1, spherical aberration; -0.9 to -3.4, coma). The importance of spherical aberration declines relative to that of off-axis aberrations as pupil size decreases. The introduction of a level of corneal asphericity alters each aberration by a constant amount for any lens shape.

Cornea↗

PubMed1989

Colour vision in a family with Sorsby's dystrophy.

Fraser & Wallace (1971) investigated a family with Sorsby's dystrophy, and found a high prevalence of a mild red-green colour deficiency. Using a battery of tests, I re-examined the colour vision of members of the family at risk of developing the disease. I conclude that the mild colour vision defect is classical sex linked recessive deuteranomaly occurring adventiously in the family; its high prevalence is due to its being introduced by at least 3 persons marrying into the family. Two alternative hypotheses of the colour vision deficiency are that it is an acquired disorder which is a prodromal sign of the dystrophy or that it is inherited in an autosomal dominant manner. However, these hypotheses were rejected because the colour vision deficiency had the characteristics of a typical sex linked disorder. These findings emphasize that it is important to exclude any co-existing inherited colour vision defect when defining the clinical features of any retinal dystrophy by careful evaluation of unaffected family members.

Adolescent↗

PubMed1988

Laboratory evaluation of blended lenticular aphakic lenses.

Six types of blended lenticular aspheric lenses for the correction of aphakia were assessed with respect to mass, spectacle magnification, off-axis power errors, peripheral distortion, visual field size and cosmetic appearance. Relative to spherical surfaced lenses, these lenses showed considerably reduced mass, reduced peripheral distortion, increased visual field size and improved cosmetic appearance. All but one type showed considerably reduced off-axis power errors, but none of the lenses showed much reduction in spectacle magnification. In general, the manufacturers' claims made in favour of the blended lenticular lenses were supported.

Evaluation Studies as Topic↗