Topographical analysis of the cornea: ten caveats in keratorefractive surgery.
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Contact Lenses↗
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Results for “Refractometry”
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Management of astigmatism with toric contact lenses.
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Astigmatism↗
Measurement of the refractive indices of hydrogel materials by interferometry.
The purpose of this study was to measure the refractive indices of hydrophilic hydrogel materials of different equilibrium water content. The need to measure the refractive indices precisely has been an important factor in the design of soft hydrophilic lenses. The inaccuracy of measuring the refractive index of hydrogel materials using an Abbé refractometer led to the use of a Linnik micro-interferometer. The experimental results showed a close agreement to the theoretical data.
Hydrogel, Polyethylene Glycol Dimethacrylate↗
Dehydration of hydrogel lenses during overnight wear.
We studied the extent of dehydration of hydrogel lenses during overnight wear. Seven subjects used a hand refractometer to measure the water content of five different lenses (Hydron zero-6 (nominal water content 38.6%), -0.50 D; Snoflex 50 (52.5%), -0.50 and + 15.00 D, and Hydron Z-67 (67.5%), -0.50 and + 15.00 D) before and after 7 h of both open- and closed-eye wear. No statistically significant difference was observed in dehydration between open and closed eye lens wear. Thick and thin lenses made of the same material were found to dehydrate to an equal extent. Contrary to expectations, the medium water content Snoflex 50 lenses displayed a greater absolute decrease in water content than the higher water content Hydron Z-67 lenses (p less than 0.01). Factors that may influence the extent of dehydration under open- and closed-eye wearing conditions, and the clinical implications of these results, are discussed.
Adult↗
Effect of chromatic dispersion of a lens on visual acuity.
Materials with a high refractive index have a considerable level of chromatic dispersion which, around the periphery of corrective lenses, may affect the wearer's visual acuity. By measuring visual acuity through prisms of increasing chromatic power we have established the relation between chromatic power and visual acuity. The maximum chromatic dispersion of materials useable in ophthalmic optics can be deduced from these results.
Adult↗
Matrix methods for the evaluation of lens systems with radial gradient-index elements.
Matrix methods have been used to examine the paraxial performance of homogeneous systems. This method has been extended to include the presence of gradient lens elements in the optical array. To accomplish this, an inhomogeneous rotation-translation matrix is developed to describe the path of a ray through the lens elements with quadratic radial index gradients. The system matrix elements are then used to predict the paraxial properties of the array. Three examples are examined: the Wood lens, two thin lenses coupled to a radial gradient index rod, and a model of the human eye which contains a radial gradient index crystalline lens.
Aging↗
Refractive index gradient of human lenses.
We report nondestructive measurements and the modeling of the refractive index profiles in human lenses. Results in the equatorial and sagittal planes are compared with destructive measurements of refractive index, modeled data as well as with microdensitometric measurements of protein concentration. These comparisons highlight the differences between current models and measured data on human lenses.
Adolescent↗
In vivo dehydration of disposable (Acuvue) contact lenses.
The in vivo dehydration of the Acuvue disposable contact lens was measured in six subjects using a contact lens refractometer. On average, the lenses dehydrated 6.2 +/- 1.9% after 20 min and 10.2 +/- 3.9% after 6 h of wear. The magnitude of dehydration is greater than has been reported for other hydrogel lens types of similar water content. This information may assist practitioners in the interpretation of clinical signs pertaining to lens behavior and associated patient symptoms.
Adult↗
Effect of bleaching on the width and index of refraction of goldfish rod and cone outer segment fragments.
Data on goldfish rod and cone fragments were obtained before and after a strong bleach by using a Zeiss Jamin-Lebedeff infrared interference microscope and computer image processing techniques. The receptor fragments were fractured in the inner segment and were immersed in goldfish aqueous humor medium. On bleaching we found that: (1) there is a small increase in rod diameter. This effect is of the same magnitude reported earlier in Rana pipiens outer segments (Vision Res 1973; 13:171). (2) There is an inferred decrease in rod refractive index. (3) There is a decrease in cone width. (4) There is a slight increase in inferred cone refractive index. These data are presented.
Animals↗
New options for aniseikonia correction: the use of high index materials.
The increased popularity of refractive surgery, intraocular lenses, and penetrating keratoplasty has resulted in larger numbers of anisometropes with aniseikonia. We have analyzed the effects of the new high index glass and plastic spectacle lens materials on the design of eikonic lenses sometimes used for these patients. Our results demonstrate that high index materials allow thinner and flatter lens designs. Thus, higher levels of aniseikonia correction and improved cosmetic appearance of eikonic spectacle lenses can be achieved. We present shape magnification nomographs that facilitate eikonic lens design with these materials.
Aniseikonia↗
Humidity-conditioned gravimetric method to measure the water content of hydrogel contact lens materials.
A method to determine the humidity-conditioned gravimetric water content of hydrogel contact lens materials has been developed, in which errors due to blotting have been eliminated by conditioning the lens in a series of relative humidity (RH) environments before measuring the water content gravimetrically, and then extrapolating the water content to 100% RH. This method has been used to determine the water contents of representative materials from each of the four FDA lens groups, which were compared with their labeled values, as well as with values obtained from refractive index measurements. The deviation of the water content of soft contact lenses as measured by refractive index from that obtained gravimetrically increased as the water content decreased. The humidity-conditioned gravimetric method to determine water content of hydrophilic contact lenses is being proposed as an International Organization for Standardization (ISO) standard, as an improvement over the gravimetric and refractive index methods.
Caprolactam↗
Luminance of projection focimeter target with oblique viewing.
In order to test the capacity of an optical projection focimeter for simultaneous use by several observers gathered around the instrument, the luminance of the projected target was measured for 3 projection focimeters (model Axil from Essilor, model LP2 from Nikon, and model LM-P5 from Topcon) when the photometer axis was perpendicular to the instrument screen and at oblique angles up to 75 degrees in 5 degrees increments. As expected, the luminance of the target decreased as obliquity of viewing increased; however, the rate of reduction is characteristic of each focimeter model. The repeatability of dioptric measurements with the Axil focimeter was assessed in the usual viewing position when the luminance of the target was reduced by optical filters (neutral density varying from 0.2 to 1 in 0.2 steps). The repeatablity remains within +/- 0.05 D for optical densities < or = 0.8. Our results show that a useful luminance is maintained over an observation angle that extends to 45 degrees. Depending on the model of focimeter, several observers may simultaneously observe and judge the focusing of the instrument.
Humans↗
Oblique central refraction in spherocylindrical corrections with both faceform and pantoscopic tilt.
Thin lens equations, accurate to third order, are presented for the effective spherocylindrical parameters for oblique central refraction (OCR) through spherocylindrical lenses with both faceform and pantoscopic tilt. The equations can also be used to find the parameters of a lens that compensates for the tilts. Accuracy of the equations was checked by exact ray trace results.
Eyeglasses↗
An apparent pH-induced effect on extended wear hydrogel lens water content.
Numerous studies have examined the separate issues of precorneal tearfilm pH and hydrogel lens water content. None have examined the two issues in an associated, clinical fashion. Earlier research by this author has shown that stabilized hydrogel lens anterior surface pH correlates with established values for the precorneal tearfilm pH. This technique was therefore paired with two different methods of hydrogel lens water content assessment to determine if there was a predictable tear pH-related effect on soft lens hydration. Sixty-five volunteer subjects were fitted with either 38% (polymacon) or 58% (etafilcon A) water content hydrogel lenses on an extended-wear basis. The in situ hydrogel lens anterior surface pH was measured with a flat-surfaced, self-referenced pH electrode 5 min after initial fitting, and on subsequent extended wear follow-up examinations. On each follow-up, associated lens water content was determined by gravimetric and/or refractive analysis, yielding a total of 517 paired data points over a 33 month period. While the anterior lens surface pH increased in a nonlinear fashion with immediate duration of extended lens wear [p < 0.001 analysis of variance (ANOVA) by number of days extended lens wear], the water content decreased in a nonlinear fashion under the same conditions (p < 0.001 by ANOVA). Correlation of these two nonlinear functions yielded a negatively sloped linear regression (r = 0.99 and 0.97) for each material. The slopes of the resultant linear functions were significantly different (p < 0.01) by t-test; the higher water content material exhibited a greater sensitivity to pH. The routine labeling of pH-induced water content changes by hydrogel lens manufacturers would provide an objective model for clinical lens behavior, and assist in lens type and parameter selection when fitting extended-wear soft lenses. Additionally, as other lens materials are characterized, it may be possible to model the "ideal" hydrogel material in terms of apparent pH-dependent or-independent behavior.
Contact Lenses, Extended-Wear↗
On-eye power characteristics of soft contact lenses.
BACKGROUND: The power of a soft contact lens on the eye is a function of its off-eye power, the manner in which the lens flexes on the eye, lens hydration changes, and the corneal topography. METHODS: In the present study, we used a high illumination keratometer, which allowed us to obtain front and back surface keratometric readings when the lenses were in position on the eye. The on-eye power of the lens could then be calculated from these readings, with the assumption that the center thickness and refractive index of the lens corresponded to those in vitro. RESULTS: The estimates of the on-eye powers agreed closely with the results indicated by over-refraction. Moreover, comparison of in vitro with in vivo power estimates indicated that the positive lenses lost power on-eye, whereas the negative ones maintained their power. CONCLUSIONS: The present study confirms the results of earlier workers, who suggested that soft lenses drape to fit the cornea. Our findings appeared to be in agreement with the predictions of most of the models developed in the past.
Contact Lenses, Hydrophilic↗
On-eye power characteristics of soft contact lenses.
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Contact Lenses, Hydrophilic↗
Subjective refraction techniques in the frame of the three-dimensional dioptric space.
A novel heuristic approach to the well-known representation of the dioptric power in a three-dimensional space is presented. It is shown how this theoretical framework is ideal for discussing the principles of several subjective refraction methods. In particular, this formalism is used to justify the stenopaic slit refraction, the Barnes subjective refraction technique, and the Jackson cross-cylinder procedure. In view of this analysis, some modifications to the traditional procedures are proposed.
Models, Theoretical↗
Oblique central refraction in tilted spherocylindrical lenses.
Spectacle corrections worn in a frame with a faceform tilt should be used with effective spherocylindrical parameters. For the case of oblique central refraction, Keating presented a procedure to determine these parameters in a third-order approximation. The central equation of his approach, the effective dioptric power matrix, is partly the result of an educated guess. Based on the equations of wavefront tracing, an analytical derivation of his equation, slightly modified however, is given in this paper.
Eyeglasses↗