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Biomedical subjects

D A Atchison

Publications and source records attributed to D A Atchison.

66 records · Page 4Linked to original sources
PubMed1987

Effect of defocus on visual field measurement.

Defocus increased visual field contrast thresholds for stimuli equal to or smaller than Goldmann perimeter target III (26 min arc diameter). Beyond 30 degrees-40 degrees from the centre of the visual field, defocus had little effect on contrast thresholds. For accurate clinical perimetry, ametropia and presbyopia should be corrected for small targets (less than or equal to target III) within 30 degrees-40 degrees of fixation with either a spectacle lens (if required power less than +/- 10 D) or a contact lens. Larger stimuli are little affected by defocus. Outside this central 30 degrees-40 degrees field, correction with contact lenses should be provided if fixation is poor due to ametropia. Scotomata for which no cause is evident should be further investigated to find whether they have a refractive basis.

Contact Lenses↗
PubMed1987

Spectacle lenses and third-order distortion.

Third-order formulae are developed for calculation of distortion in thin spectacle lenses, when one or both surfaces are conicoid aspherics. Solutions which allow correction of rotatory or peripheral distortion, when one lens surface is a conicoid aspheric, are illustrated. A study of these solutions shows that one of the off-axis power errors (e.g. oblique astigmatism) and one of the distortions can be simultaneously eliminated, but the lens forms required are too curved to be cosmetically feasible.

Eyeglasses↗
PubMed1986

On the description of zonal aspheric surfaces.

It is shown that, in general for a rotationally symmetric surface, the sagittal curvature is a function of the first derivative and tangential curvature is a function of the first and second derivatives of the surface equation. Smith and Atchison developed an "off-set" model for zonal aspheric surfaces with continuous gradients (or first derivatives) at the boundaries between zones of the surface, and discontinuous second derivatives at the boundaries. This model predicts continuous sagittal power errors but discontinuities in tangential power errors. However, measurements on commercially available zonal lenses by Atchison and Smith showed that both sagittal and tangential power errors were continuous across zone boundaries. Thus it is likely that the off-set model is incorrect. Zonal aspheric surfaces are more likely to be made according to a "co-axial" model which is described.

Humans↗
PubMed1984

Clinical trial with commercial aspheric aphakic lenses.

A clinical trial with 11 subjects was conducted to compare the Armorlite Multiple Drop, the Signet Hyperaspheric, the Sola Hi-Drop, and the American Optical Fulvue types of aspheric aphakic lenses. Reasonable off-axis visual acuities were obtained with the Signet, Sola, and Fulvue types, but poor visual acuities were found with the Armorlite type. Similar visual field limits were found with the armorlite, Signet, and Sola types. Considerably larger limits were found with the Fulvue type than with the other types. However, subjects failed to appreciate these larger limits; this was attributed to the extremely poor image quality near the edge of the visual field with this lens type. Some subjects disliked the poor off-axis image quality with the Armorlite lens type, and it had poor acceptance relative to the other types. Concerning subjective perception of weight, cosmetic appearance, and visual field size, and subjective evaluation of performance of everyday activities, there were no significant differences between lens types. It was concluded that reducing lens off-axis power errors, corresponding to the rotating eye, should remain the most important principle of aphakic spectacle lens design.

Aged↗
PubMed1984

Third-order theory and aspheric spectacle lens design.

Third-order equations are developed giving sagittal and tangential power errors corresponding to the eye rotating to look through the periphery of a spectacle lens. The equations are applicable for conicoid aspherical surfaces. From these, other equations are derived for elimination of oblique astigmatism and mean oblique error. Graphical examples show how aspherizing either lens surface affects the lens bendings required to eliminate oblique astigmatism.

Astigmatism↗
PubMed1983

Construction, specification, and mathematical description of aspheric surfaces.

Rotationally symmetrical aspheric surfaces of spectacle lenses are constructed as either "zonal aspherics" or "continuous aspherics." Zonal aspherics consist of annular zones surrounding a central zone with each zone being nominally spherical with progressively lower surface power the farther the zone is from the surface vertex. Aspheric surfaces are often specified by the radial drop in surface power from the center to the edge of the lens (e.g., Welsh Four-drop), but for assessment purposes the surface shape must be specified more precisely. The formulas for the description of continuous aspherics can be manipulated into different forms. Mathematical descriptions are given or developed which will enable theoretical assessment of the performances of all lenses with rotationally symmetrical aspheric surfaces.

Eyeglasses↗
PubMed1983

Laboratory evaluation of commercial aspheric aphakic lenses.

Four types of commercial aspheric aphakic lenses were assessed with respect to mass, spectacle magnification, off-axis power errors, and peripheral distortion. The aspheric lenses have better distortion and off-axis imagery properties than spherical flat back lenses, but the improvement in distortion properties is small for those lenses whose design form specifies a flattish back surface. The claims concerning the distortion properties of these lenses are exaggerated. The shape (bending) of lenses has a large effect on spectacle magnification, peripheral distortion, and off-axis imagery.

Aphakia, Postcataract↗
PubMed1983

Aspheric high positive power lenses and third order theory.

To correct third order (primary) oblique astigmatism of thin spectacle lenses outside an approximate lens power range of -22 D to +7 D, one surface must be aspherized. Aspherizing is of particular importance in the high positive power range used for the correction of aphakia. Graphs are presented showing the required front surface asphericities and power drops required to correct oblique astigmatism in this range. The results are a guide to the required asphericities in thick lenses.

Astigmatism↗
PubMed1983

Effect of conicoid asphericity on the Tscherning ellipses of ophthalmic spectacle lenses.

One of the criteria in ophthalmic spectacle lens design is the elimination of oblique astigmatism. For a range of equivalent powers, Seidel (primary or third-order) astigmatism can be eliminated, and the solutions of back- (or front-) surface power are commonly displayed graphically in the form of ellipses (Tscherning ellipses). The Tscherning ellipses apply only to lenses constructed from spherical surfaces. If one or both surfaces are made aspheric, the solutions for zero astigmatism are no longer in the form of ellipses. If one surface, usually the front surface, is made as a conicoid aspheric, the solutions for zero astigmatism can be presented graphically similarly to the Tscherning ellipses. For any given equivalent power, there are two or no solutions for spherical lenses. However, there is always one and up to three solutions for conicoid aspheric lenses.

Eyeglasses↗
PubMed1980

Prismatic effects of spherical ophthalmic lenses.

Exact prismatic effects were calculated for stock spectacle lenses (-6 D to +10 D) and trial refracting set lenses (-10 D to +10 D) using a range of visual field eccentricities on the 0.30 m Goldmann perimeter, and on the 1- and 2-m tangent screens. Slightly larger prismatic effects were found for the tangent screens than the perimeter, but the difference in results for the two tangent screen distances was negligible. For low power ophthalmic lenses (-2 D +2 D), the prismatic effects were smaller than the variation in apparent target eccentricity caused by variation in patient fixation distance. Paraxial estimates of prismatic effects using both thick and thin lens theory were calculated, but found to become more inaccurate as target eccentricity increased. Data are presented which can be used by clinicians to predict the alteration in the size of kinetic perimetric isopters when ophthalmic lenses are used to correct patient defocus resulting from ametropia or presbyopia.

Eyeglasses↗
PubMed1979

The effect of pupil size on visual acuity in uncorrected and corrected myopia.

The effect of pupil size on the relation between Snellen visual acuity and corrected and uncorrected myopia was examined for 22 young subjects with degrees of myopia ranging from 0.75 D to 7.5 D. Effective pupil size was varied by inducing mydriasis and then placing artificial pupils of between 1.0 and 8.0 mm diameter before the eye. Both a constant chart luminance of 120 cd/m2 and a constant retinal illuminance of 2150 trolands were used. There was little difference in results for the two lighting conditions. For the corrected myopes considered as a group, maximum visual acuity occurred for 2--3 mm diameter pupils, but larger pupils reduced acuity only marginally. For the uncorrected myopes, variation in pupil size produced a large variation in visual acuity, and for refractive errors greater than about 1.5 D, the optimum pupil diameter was less than 1 mm. For uncorrected myopes of 3.0 D or less, visual acuity was nearly as good with a 1-mm pupil as for corrected myopes. The presented data are a useful guide to the clinician.

Adult↗