Prismatic Effect Research Articles

Anisophoria and aniseikonia. Part I. The relation between optical anisophoria and aniseikonia.

Part I of this publication demonstrates and explains the close relation between aniseikonia and anisophoria induced by spectacles. It discusses the clinical implications of this relation by discussing certain aspects of aniseikonia theory, prismatic effects during oblique gaze through spectacles as for reading, and a simple formula that presents a comprehensive description of all prismatic effects and prismatic differences produced by a pair of spectacles. It also describes an easy method of specifying iseikonic lenses, as well as some conventional methods of measuring aniseikonia and anisophoria. Part II will deal with the correction and management of anisophoria when induced together with aniseikonia. Parts I and II, together, will convey a new approach toward the management of anisophoric spectacle corrections.

Influence of lens vertex distance changes on prismatic effect and decentration.

Changing the lens vertex distance alters the effective power of a lens, both in refractive and prismatic effect. For spherical lenses under paraxial conditions, it was found that though the amount of incorporated prism should be modified in the correcting lens in cases of vertex distance changes, the actual amount of decentration should be kept constant.

Prism in pairs of progressive power lenses

The off-axis prismatic effects (prism) of five types of progressive power lenses were measured on a modified vertometer. The lens types assessed were the Essilor (Hoya) Varilux II, the American Optical Truvision Omni, the SOLA Graduate, the Rodenstock Progressiv R, and Zeiss Gradal HS. Conventional flat top bifocal lenses were also assessed. Progressive power lenses have smooth changes of prismatic effects, free of sharp jumps associated with bifocals. Differential prism of progressive lens pairs matched for power mainly result from a failure of some manufacturers to design for individual right and left lenses. This failure can produce up to 1.0Δ vertical and horizontal differential prism. We recommend that reduction of differential prismatic effects be carefully considered in future progressive lens design.

Visual postural performance in ametropia and with optical distortion produced by bifocals and multifocals.

Visual stabilization of posture depends strongly on the performance of the visual system and declines with experimentally induced reduction of visual image quality. We studied postural performance in subjects with median and high grade hyperopia or myopia. While body sway of median grade ametropic subjects increased slightly when measured without glasses, high grade ametropia showed no significant differences between postural performance with glasses on and off. We concluded that the increase in visual acuity when wearing glasses is outweighted by concurrent dioptric distortions such as size change and prismatic effects. In addition we could not find any difference between bifocal and multifocal lenses in their effect on sway when investigated in normals with either steady fixation or saccades or foveal pursuit.

Spherical lens decentration errors by Prentice's rule.

Sources and magnitude of errors in applying Prentice's rule in calculating decentration for spherical lenses were investigated. It was found that spherical aberration can be the only source of error in applying the rule. The overall magnitude of errors in effective prismatic effect is, however, clinically insignificant.

Prismatic Effects

Prismatic Effects

TWO SIMPLE CALCULATING SCHEMES FOR USE IN OPHTHALMIC OPTICS‐I. TRACING OBLIQUE RAYS THROUGH SYSTEMS INCLUDING ASTIGMATIC SURFACES

A ray tracing scheme using paraxial approximations is described to allow oblique rays to be traced through lenses or optical systems in which some or all of the surfaces may be astigmatic, even at different axes. It is based on consideration of the prismatic effects introduced at each surface. A numerical example reveals some lesser known properties of the circle of least confusion of astigmatic pencils.

THE EFFECT OF PRISMATIC SPECTACLE AND CONTACT LENS CORRECTIONS ON OCULAR ROTATION

The effect of prismatic spectacle corrections on ocular rotation for different object distances is determined and evaluated. The same principles are applied for a correction fitted on the eye, that is, a contact lens correction. The latter must be considered as a thick lens system. The effects of different types of prismatic contact lenses on ocular rotation are considered. Assuming the contact lens to remain stationary and the eye behind the lens to rotate, it is shown that the theoretical effect on ocular rotation of prismatic contact lenses is practically equal to their prismatic effect in air. The effect for high-powered contact lenses is discussed.

The effect of prismatic spectacle and contact lens corrections on ocular rotation

Abstracts— The effect of prismatic spectacle corrections on ocular rotation for different object distances is determined and evaluated. The same principles are applied for a correction fitted on the eye, that is, a contact lens correction. The latter must be considered as a thick lens system. The effects of different types of prismatic contact lenses on ocular rotation are considered. Assuming the contact lens to remain stationary and the eye behind the lens to rotate, it is shown that the theoretical effect on ocular rotation of prismatic contact lenses is practically equal to their prismatic effect in air. The effect for high-powered contact lenses is discussed.

Monocular polyopia in nuclear cataract

Only patients with primary nuclear cataracts perceive multiplied images caused by the lens. All of them see these images in a lambda-shaped arrangement. This is not a diplopia caused by the two focal points of a 'Linse mit doppeltem Brennpunkt' (double-focus lens). The monocular polyopia can be explained by a prismatic effect of the sectors of the lens nucleus.

Dynamic Versus Static Aniseikonia

It is well known that spectacle induced aniseikonia produces not only differences in ocular images, but also imposes constant changes in the oculomotor relationship between the two eyes during conjunctive excursions. Yet, this prismatic effect is often ignored. In this study, the two effects are isolated and compared by means of specially designed stimuli. As expected, it is found that both effects are visually more undesirable than the iseikonic condition. Furthermore, the prismatic effect is significantly more undesirable than the mere presence of ocular image differences. It is recommended that the effect on oculomotor relationships be given more attention in aniseikonia management.

Artifacts Introduced by Spectacle Lenses in the Measurement of Strabismic Deviations

The peripheral prismatic effects of corrective spectacle lenses introduce an artifact when strabismic deviations are measured. Plus lenses decrease and minus lenses increase the measured deviation. This effect begins to become clinically significant with corrective lenses with powers of more than +/- 5 diopters. We developed a simplified model that predicts the magnitude of this effect. The model agreed with actual ray tracing analysis through commonly used spectacle lenses and also agreed closely with clinical data. The clinical use of this model is facilitated by remembering the artifacts produced by +20, +10, -10, and -20 diopter spectacle lenses and interpolating for intermediate values. Correcting for these artifacts should enhance the predictability of strabismus surgery in patients with significant ametropia.

Lens holder for vitrectomies in the posterior segment of the eye

A lens holder for posterior segment vitrectomy is described. Its advantages are: better centering, optional use during surgery, simple handling and optimal exploitation of the prismatic effect for a better view into the periphery.

The Kerala Decentration Meter

Centring of spectacle lenses is much neglected field of ophthalmology. The prismatic effect caused by wrong centring results in a phoria on the eye muscles which in turn causes persistent eyestrain. The theory of visual axis, optical axis and angle alpha is discussed. Using new methods the visual axis and optical axis of 35 subjects were measured. The results were computed for facial asymmetry, parallax error, angle alpha and also decentration for near vision. The results show that decentration is required on account of each of these factors. Considerable correction is needed in the vertical direction, a fact much neglected nowadays; and vertical decentration results in vertical phoria which is more symptomatic than horizontal phorias. Angle Alpha was computed for each of these patients. A new devise called 'The Kerala Decentration Meter' using the pinhole method for measuring the degree of decentration from the datum centre of the frame, and capable of correcting all the factors described above, is shown with diagrams.

Spectacle correction for aphakia. The prismatic effect

Because spectacle lenses are fixed in position relative to the eyes, the visual axes during convergence pass nasal to the optical centers, and create a prismatic effect. The eye rotates less than 15° before the head is turned; therefore for central vision, only an area of the lens about 13 mm wide is used. A reading addition of +6.00 D is probably the maximal power bifocal addition that can be prescribed for comfortable binocular vision in a high plus correction. The distance correction produces a base-out effect of at least 4 Δ for each eye at the reading distance, even though the bifocal segments are adequately decentered.

Prismatic Effects in Bicentric Grinding

Bicentric grinding is widely used to neutralize a vertical differential pris­matic effect at the near visual point. However, the prismatic effects in other portions of the reading field dif­fer from those at the near visual point. In this study, the variations in prismatic effect throughout the entire reading field are demonstrated. It is shown that a slab-off prism is most effective in spherical anisometropic prescriptions and when the differen­tial prismatic effects are symmetrical about the horizontal meridian.

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.

A new lens for closed pars plana vitrectomy.

A new prismatic type of lens enables a more peripheral view of the vitreous cavity. The surface in contact with the cornea is concave. The opposite surface is plano but inclined to produce a prismatic effect of 15 degrees or 25 degrees. A cannula attached to the lens provides corneal irrigation. Although this lens was originally designed for vitrectomy use, it is also useful for diagnostic biomicroscopy of the vitreous and fundus in the office.

The Alteration in Static Perimeter Thresholds Caused by the Prismatic Effect of Ophthalmic Lenses

Contrast sensitivity for target I (3.24/diameter) of the 30 cm radius Goldmann perimeter was measured along the horizontal meridian to 50 degrees eccentricity in 5 subjects. Background luminance was 10.0 cd/m2 and stimulus duration was 130±4 ms. Using this data and the ophthalmic lens prismatic effects to be presented in another paper, the alteration in static perimetric sensitivity caused by spectacle lens prismatic effects was estimated. For spectacle lenses with powers between -6 D and + 10 D, the maximum sensitivity alteration was only 0.30 log units (+10 D, 25 degrees temporal) for a 14 mm vertex distance. As the experiment used conditions producing maximum visual field sensitivity gradients, and hence the maximum alteration in sensitivity caused by prismatic effects, prismatic effects of corrective ophthalmic lenses can be ignored within the central 30 degrees - 40 degrees visual field during static perimetry for lens powers less than ± 10 D.

Thermal transient effects in silicate and phosphate rods pumped in pseudoelliptic close-coupled cavities

This paper presents the experimental results of a study on the thermal effects due to pulsed optical pumping in ED2 Nd glass rods, cladded with samarium glass, and in Q88 Nd phosphate glass laser rods. A pseudo-elliptic close-coupled pumping cavity, very efficient and simple to be made has been used; this entails two types of rod deformations which in turn are responsible of two optical effects: a focusing effect and a prismatic effect. The time evolution of these effects under single shot condition and at low repetition rate (1 pulse every 5 s) is described. At single shot the behaviour is opposite to that previously reported in the literature.