Ophthalmic Prisms Research Articles

Measurement Errors Induced by Inaccurate Positioning of Ophthalmic Prisms and a Simple Way to Minimize Them

Purpose: Ophthalmic prisms are routinely used to measure ocular deviation. However, large measurement errors could be induced when a prism is not accurately positioned at its calibration position. We report a simple way to minimize the errors. Methods: We measured powers of prisms and plastic prisms at their calibration position and at angles the prism was rotated away from the calibration position. Glass prisms that have power of 20, 25, 30, 40, 50, and 60 prism diopters (PD) and plastic prisms that has power of 20, 25, 30, 35, 40, 45, 50 PD were used. To minimize the errors, we developed a pair of prism frame so that the prism can always be positioned accurately. Results: We have demonstrated large errors induced by rotating glass prisms and plastic prism away from their calibration position. Larger errors were recorded with glass prisms than with plastic prisms. The measurement errors are significant even with a small angle rotation for a large power glass prism. For instance, a 10-degrees rotation of a 50PD glass prism produces an error of 30PD. However, measurements errors were minimized when the prism frame was used. Conclusion: It suggests that the glass ophthalmic prism should be abandoned and the use of ophthalmic prisms in strabismus measurement should be standardized with assistance of the prism frame.

Experimental Study of Refraction Effects of Nominally Plano Ophthalmic Prisms and Magnifying Lenses.

Nominally plano ophthalmic prisms give autorefraction results similar to those predicted on the basis of how effective powers change with pantoscopic tilt, and magnifying lenses give autorefraction results similar to those predicted on the basis of vergence changes. Without appreciation of the optics involved, these effects might wrongly be considered artifacts. The purpose of this study was to investigate the interactions of autorefractors with lenses and prisms. There were 15 adult participants across three experiments, with a range of ages and refractions. In experiments 1 and 2, participants wore frames containing base-up and base-down nominally plano prisms. In experiment 3, participants wore a lens that produced either 6.3% magnification or 5.9% minification, depending on which surface faced the eye. Autorefracting instruments with different operating principles were used: Shin-Nippon SRW5000 autorefractor, Grand Seiko 5100K autorefractor, Hoya AR-530 autorefractor, a Complete Ophthalmic Analysis System-High Definition wavefront sensor, and Tomey FC-800 autorefractor. A theory on the likely effects of magnifying lenses was presented. For ophthalmic prisms, refractions showed results similar to those predicted on the basis of how effective prism powers change with pantoscopic tilt. As tilt increased, base-up prism gave more positive mean refractions and more negative horizontal/vertical astigmatism and vice versa for base-down prisms. In the presence of 10° tilt, 8Δ base-up prisms and 8Δ base-down prisms had different effects by a mean of 0.36 diopters. Magnifying lenses affected refractions similar to those predicted on the basis of vergence changes, with 6% magnification and minification producing mean changes of -11 and +8%, respectively, in absolute mean refraction. There was no strong evidence that different instruments had different effects. The results have implications for studies in which prisms and lenses are placed in the front eyes, such as accommodation studies using thick lenses close to the eyes to stimulate accommodation rather than by changing object distance.

Theoretical Study of Refraction Effects of Plano Ophthalmic Prisms

Nominally plano prisms can have appreciable refractive errors that exceed the usual prescribing step of 0.25 D, particularly when an eye rotates to view off-axis objects. The purpose of this study was to determine theoretically the refractive power effects of nominally plano-refracting power prisms. Plano prisms with zero refraction were designed for the as-worn condition. A basic method was developed to determine refractive effects in the presence of pantoscopic tilt. A refined method was developed that considers the eye rotating behind the lens, and this and the basic method were compared with accurate raytracing. Plano prisms of 4 and 8 Δ were designed with astigmatic back surfaces to compensate for oblique incidence, and tangential and sagittal image vergence errors were investigated for base-up (BU) and base-down (BD) directions, 0 and -3.33 D object vergences, and pantoscopic tilts up to 10°. Basic and refined results did not differ from accurate results by more than 0.04 and 0.08 D, respectively. Errors for 8 Δ prisms were approximately twice those for 4 Δ prisms. Errors were approximately proportional to tilt. With 10° tilt, the errors ranged between -0.65 D/-0.23 D (8 Δ BD, -3.33 D object vergence) and +0.36 D/+0.15 D (8 Δ BU, 0 D object vergence). Sagittal errors were generally about one third of corresponding tangential errors. In the presence of tilt, BU prisms had positive errors, and BD prisms had similar, but negative, errors for distance objects. At -3.33 D object vergence with tilt, negative errors for BD were greater than positive errors for BU. When the eye rotates to look at objects at different positions, errors can increase beyond those occurring on-axis. When designed for nontilted conditions, but then subjected to tilt or to viewing off-axis objects, plano prisms can have errors exceeding the usual prescribing step of 0.25 D.

Ophthalmic prisms: Optical measurement errors and a simple device to minimize the errors

Ophthalmic prisms: Optical measurement errors and a simple device to minimize the errors

Distortions to visual field expansion with high-power Fresnel prisms

Prisms are used to expand the visual field of patients with peripheral visual field loss. For instance, patients with homonymous hemianopia (HH) have lost half their visual field on the same side in both eyes: see Figure 1(a). They can compensate for field loss by using spectacle-mounted prisms.1–3 Fitting prisms on only one carrier lens achieves actual field expansion by providing different views to each eye: see Figure 1(b). When conventional ophthalmic prisms are used for vision correction, their weight and size limit the prism power to about 20 prism diopters () and hence also the amount by which the visual field can be expanded. (A prism of power 1 would displace an image by one centimeter at one meter.) An approach to fitting high-power prisms for HH was proposed by Eli Peli: see Figure 1(b).4 The ‘Peli lens’ is now marketed by Chadwick Optical (Souderton, PA).5 High-power Fresnel prism segments are placed across the upper and lower peripheral portion of the lens. Fresnel prisms are split into small segments and are small and lightweight in comparison to conventional prisms, permitting the use of up to 57 . 30). Placing the prisms peripherally avoids central double vision (binocular confusion),1, 4 which was common with previous designs, while providing awareness in the periphery of objects in the blind side without scanning. Eli Peli and I have considered the impact of the direction of Fresnel prism serration (the series of slanted surfaces of Fresnel prisms) and side effects such as spurious reflections and distortions on the patient’s perception.6 Although the existence of prism distortions is well known, their effect on patients has not previously been considered. Prism power varies with angle of incidence, but the power variations within the range of practical gaze shift ( 15) are very Figure 1. (a) Illustration of normal binocular visual field, compared with that of a patient with left homonymous hemianopia. (b) Peli prism glasses with prism base to the left (outward prism serration, OPS). (c) Prism power as a function of gaze direction for eyeward prism serration (EPS) and OPS.

Spherical lenses and prisms lead to postural instability in both dyslexic and non dyslexic adolescents.

There is controversy as to whether dyslexic children present systematic postural deficiency. Clinicians use a combination of ophthalmic prisms and proprioceptive soles to improve postural performances. This study examines the effects of convergent prisms and spherical lenses on posture. Fourteen dyslexics (13–17 years-old) and 11 non dyslexics (13–16 years-old) participated in the study. Quiet stance posturography was performed with the TechnoConcept device while subjects fixated a target at eye-level from a distance of 1_m. Four conditions were run: normal viewing; viewing the target with spherical lenses of −1 diopter (ACCOM1) over each eye; viewing with −3 diopters over each eye (ACCOM3); viewing with a convergent prism of 8 diopters per eye. Relative to normal viewing, the −1 lenses increased the surface of body sway significantly whereas the −3 diopter lenses only resulted in a significant increase of antero-posterior body sway. Thus, adolescents would appear to cope more effectively with stronger conflicts rather than subtle ones. The prism condition resulted in a significant increase in both the surface and the antero-posterior body sway. Importantly, all of these effects were similar for the two groups. Wavelet analysis (time frequency domain) revealed high spectral power of antero-posterior sway for the prism condition in both groups. In the ACCOM3 condition, the spectral power of antero-posterior sway decreased for non dyslexics but increased for dyslexics suggesting that dyslexics encounter more difficulty with accommodation. The cancelling time for medium range frequency (believed to be controlled by the cerebellum), was shorter in dyslexics, suggesting fewer instances of optimal control. We conclude that dyslexics achieve similar postural performances albeit less efficiently. Prisms and lenses destabilize posture for all teenagers. Thus, contrary to adults, adolescents do not seem to use efferent, proprioceptive ocular motor signals to improve their posture, at least not immediately when confronted to convergence accommodation conflict.

The Use of Fresnel and Ophthalmic Prisms with Persons with Hemianopic Visual Field Loss

The Use of Fresnel and Ophthalmic Prisms with Persons with Hemianopic Visual Field Loss

Changes in fixation in the presence of prism monitored with a confocal scanning laser ophthalmoscope

Background:The main optical effect of an ophthalmic prism is to deviate uniformly the entire field of view seen through the prism, resulting in an equal eye movement, if fixation on the same object is to be maintained. Conversely, it has been suggested that, in the case of low vision due to central scotoma, the eye changes its eccentric viewing behaviour when a prism is introduced, that is, the eye remains stationary with a change of retinal image location. Method:A new method is described in which retinal image position was recorded with a confocal scanning laser ophthalmoscope in four experimental conditions. Results:Results showed that the normally‐sighted eye rotates rapidly to compensate for the introduction of a prism. Conclusion:Future studies are required involving subjects with central scotoma but it is likely that many subjects with low vision will also refixate behind a prism and that the prescribing and wearing of bilateral prism in cases of central vision loss will have limited effectiveness in the demonstration or training of eccentric viewing.

Vertical gaze angle: absolute height-in-scene information for the programming of prehension.

One possible source of information regarding the distance of a fixated target is provided by the height of the object within the visual scene. It is accepted that this cue can provide ordinal information, but generally it has been assumed that the nervous system cannot extract "absolute" information from height-in-scene. In order to use height-in-scene, the nervous system would need to be sensitive to ocular position with respect to the head and to head orientation with respect to the shoulders (i.e. vertical gaze angle or VGA). We used a perturbation technique to establish whether the nervous system uses vertical gaze angle as a distance cue. Vertical gaze angle was perturbed using ophthalmic prisms with the base oriented either up or down. In experiment 1, participants were required to carry out an open-loop pointing task whilst wearing: (1) no prisms; (2) a base-up prism; or (3) a base-down prism. In experiment 2, the participants reached to grasp an object under closed-loop viewing conditions whilst wearing: (1) no prisms; (2) a base-up prism; or (3) a base-down prism. Experiment 1 and 2 provided clear evidence that the human nervous system uses vertical gaze angle as a distance cue. It was found that the weighting attached to VGA decreased with increasing target distance. The weighting attached to VGA was also affected by the discrepancy between the height of the target, as specified by all other distance cues, and the height indicated by the initial estimate of the position of the supporting surface. We conclude by considering the use of height-in-scene information in the perception of surface slant and highlight some of the complexities that must be involved in the computation of environmental layout.

Residual binocular interactions in the striate cortex of monkeys reared with abnormal binocular vision.

We investigated the nature of residual binocular interactions in the striate cortex (V1) of monkey models for the two most common causes of visual dysfunction in young children, specifically anisometropia and strabismus. Infant rhesus monkeys were raised wearing either anisometropic spectacle lenses that optically defocused one eye or ophthalmic prisms that optically produced diplopia and binocular confusion. Earlier psychophysical investigations had demonstrated that all subjects exhibited permanent binocular vision deficits and, in some cases, amblyopia. When the monkeys were adults, the responses of individual V1 neurons were studied with the use of microelectrode recording techniques while the animals were anesthetized and paralyzed. The manner in which the signals from the two eyes were combined in individual cells was investigated by dichoptically stimulating both eyes simultaneously with drifting sine wave gratings. In both lens- and prism-reared monkeys, fewer neurons had balanced ocular dominances and greater numbers of neurons were excited by only one eye. However, many neurons that appeared to be monocular exhibited clear binocular interactions during dichoptic stimulation. For the surviving binocular neurons, the maximum binocular response amplitudes were lower than normal; fewer neurons, particularly complex cells, were sensitive to relative interocular spatial phase disparities; and the remaining disparity-sensitive neurons exhibited lower degrees of binocular interaction. In prism-reared monkeys, an unusually high proportion of complex cells exhibited binocular suppression during dichoptic stimulation. Binocular contrast summation experiments showed that for both cooperative and antagonistic binocular interactions, contrast signals from the two eyes were combined by individual neurons in a normal linear fashion in both lens- and prism-reared monkeys. The observed binocular deficits appear to reflect a reduction in functional inputs from one eye and/or spatial imprecision in the monocular receptive fields rather than an aberrant form of binocular interaction. In the prism-reared monkeys, the predominance of suppression suggests that inhibitory connections were, however, less susceptible to diplopia and confusion than excitatory connections. Overall, there were many parallels between V1 physiology in our monkey models and the residual vision of humans with anisometropia or strabismus.

Lateral Incomitance in Exotropia: Fact or Artifact?/Lateral Incomitance in Exotropia: Fact or Artifact? Discussion

The prevalence of significant lateral incomitance in patients with nonparetic exotropia is reported to be 22%. We speculated that measurement artifact may be the cause for some cases of apparent lateral incomitance. We measured the effective power of plastic ophthalmic prisms using a helium-neon laser in the frontal plane position and at 10 degrees, 20 degrees, and 30 degrees of rotation from the frontal plane. The rotated prisms represented the situation in which a neutralizing prism rotates with the head during measurement of lateral gaze positions. For prisms of 35 prism diopters or more, even 10 degrees of rotation produced significant artifactual incomitance. For smaller prisms, 20 degrees or more of rotation was necessary to induce significant lateral incomitance. We prospectively measured 40 consecutive patients with exotropia. Only three patients (9%) had true incomitance greater than 5 delta, and only one had incomitance in both directions of gaze. Significant lateral incomitance could be induced in every patient examined by improperly positioning the neutralizing prism. Because the detection of lateral incomitance causes most strabismus surgeons to reduce the amount of surgery they perform, special care is necessary when measuring deviations in lateral gazes.

Ophthalmic Prisms: Deviant Behavior at Near

Unpredictability in strabismus surgery results may be due in part to errors in the measurement of strabismic deviations. Whenever measuring strabismic deviations with a fixation target at near, the distance from the eye to the prism must be taken into account to measure the true deviation. For example, when a prism is held 4 cm from the cornea of an eye deviating 50 prism diopters from a fixation target at 33 cm, the measured deviation is 62 prism diopters. The required prism power needed to neutralize a deviation with fixation at near will be increased, the further the prism is held from the cornea. This may lead to surgical overcorrections if the surgery is based on the near measurement. The deviation neutralized by a prism held in any angular position, the effect of prism measurements at near, and the measurement of deviations over Fresnel prisms can be calculated and the appropriate corrections made.

Ophthalmic Prisms: Measurement Errors and How to Minimize Them

Variable results of strabismus surgery may be due in part to errors in prism measurement. The amount of deviation neutralized by an ophthalmic prism is variable depending on how the prism is held. For example, a 40Δ glass prism with the posterior face held in the frontal plane gives only 32Δ of effect. Glass prisms are calibrated for use in the Prentice position. Plastic prisms are calibrated for use in the frontal plane position. Surprisingly large errors in prism measurement are produced when adding a small prism to a large prism. For example, adding a 5Δ glass prism to a 40Δ glass prism gives not 45Δ of effect, but 59Δ. This error can be minimized but not eliminated by holding one prism in front of each eye. The error can also be calculated so that the appropriate correction can be made.

Behavioral studies on the effect of abnormal early visual experience in monkeys: Temporal modulation sensitivity

Temporal modulation sensitivity functions were investigated by behavioral methods in two monkeys reared with normal visual experience and 12 monkeys reared with abnormal early visual experience. Experimental treatments were initiated when the animals were approximately one month of age. Two monkeys were each treated with one of the following procedures: (1) long-term monocular lid suture. (2) short-term monocular lid suture. (3) surgically induced esotropia. (4) surgically induced exotropia. (5) optical dissociation of binocular vision with ophthalmic prisms (6) chronic monocular cycloplegia. The temporal modulation sensitivity functions for uniform field flicker for both eyes of the control subjects and for the untreated eyes of the experimental subjects were similar to functions for humans measured on the same apparatus using the same behavioral procedures. All eight of the monkeys preconditioned by lid suture or surgically induced strabismus showed reduced sensitivity at all temporal frequencies with the difference between the experimental and control eyes being larger for the high than low temporal frequencies. The monkeys reared with optical dissociation of binocular vision or chronic monocular cycloplegia showed equal temporal modulation sensitivities of the two eyes, but failed to show binocular summation. It was concluded from these studies that abnormal early visual experience in monkeys results in deficiencies in the processing of both spatial and temporal information, but the differences between the treated and untreated eyes were usually greater in the spatial domain than the temporal domain.

Behavioral studies on the effect of abnormal early visual experience in monkeys: Spatial modulation sensitivity

Spatial modulation sensitivity functions have been investigated by behavioral methods in two monkeys reared with normal visual experience and 12 monkeys reared with abnormal, early visual experience. Experimental treatments were initiated when the animals were approximately one month of age. Two monkeys were each treated with one of the following procedures: (1) long-term monocular lid suture. (2) short-term monocular lid suture. (3) surgically induced esotropia. (4) surgically induced exotropia. (5) optical dissociation of binocular vision with ophthalmic prisms (6) chronic monocular cycloplegia. The results of the studies showed a severe loss of contrast sensitivity of the treated eyes compared to the control eyes for monkeys reared with monocular lid suture or surgically induced esotropia. Surgically induced exotropia resulted in a moderate reduction in sensitivity of the deviated eye while optical dissociation resulted in a mild reduction in sensitivity of one eye compared to the other. One of the two monkeys reared for seven months with chronic monocular cyclopegia had a relative reduction in contrast sensitivity of the treated eye, but the other monkey had equal sensitivities in the two eyes. However, binocular summation experiments showed that even though the relative difference between the monocular sensitivities was small or absent for the monkeys reared with optical dissociation or chronic monocular cycloplegia, none of them demonstrated binocular vision in these experiments.

Binocularity in Kittens Reared with Optically Induced Squint

The effects of conflicting visual images were studied without the confounding influences of oculomotor abnormalities: strabismus was simulated by rearing kittens with ophthalmic prisms before the eyes. After the animals had matured, the response properties of neurons in the visual cortex were studied. The proportion of binocularly excited neurons decreased; however, the extent of the ocular dominance alterations was related to the amount and direction of the prism-induced deviation.

Prism distortion and accommodative change

The literature concerning oculomotor changes during adaptation to prism distortion has dealt mainly with the question of eye movements. The present study examined accommodation during exposure to distortion by ophthalmic prisms. Ss were asked to fixate four targets of equal visual angle at four distances. Accommodation was measured at each distance by means of a Laser-Badal optometer. Repeated measures for the same target distances were obtained prior to during and after exposure to binocular prisms. The results indicated that prisms induce underaccommodation at each of the target distances, although the total range of predistortion and distortion accommodation was restricted. Significant recovery from the effects of induced underaccommodation was observed at near target distances. No significant aftereffects of wearing the prisms was observed. In a second experiment, where target background contrast was increased, consistent underaccommodation was again observed and recover was observed at the near target distance. The range of accommodative change was also considerably improved. It was concluded that the angular magnification of ophthalmic prisms induced underaccommodation. It was suggested that partial recovery from the prism effect may be related to alterations in vergence.

Optical Requirements for Personal Eye Protectors

A brief discussion of some of the physical properties of ophthalmic materials is given together with a general introduction to the optics of ophthalmic lenses including the concepts of surface power, effectivity and lens power. The common errors of refraction and the methods of correction are outlined. Ophthalmic prisms and prismatic effects are discussed and the results of calculations of the prismatic effects of afocal lenses are considered with reference to Australian Standard 27–1967. Some of the more common optical problems associated with ophthalmic lenses are given, with particular attention given to the problems that may arise with afocal lenses.

Adaptation to prisms: change in internally registered eye-position.

It was shown that subjects who inspected immobile parts of their own body through ophthalmic prisms subsequently pointed incorrectly towards visual targets with both hands, i.e. they pointed as if signals derived from the system responsible for positioning the eye had been changed by a constant. It was further shown by a direct method that the subjects' appreciation of eye‐position had indeed undergone a change as a result of such inspection, and that it was of an order great enough to explain the observed changes in pointing behaviour.

Mechanisms of prism adaptation in normal monkeys

Ophthalmic prisms placed over the eyes of monkeys displace the retinal image and induce systematic misreaching. The rate at which accurate reaching is recovered during the prism exposure depends upon the type of information S obtains. It is most rapid when S can see his reaching errors and apparently displaced body and moves his head while wearing the prisms. Significant adaptation, although less marked, also occurs if S is permitted merely to move his head. It does not occur if the head is immobilized during the prism exposure.