摘要:

For a sample of college observers, visual performance was assessed with three types of measures: (1) contrast sensitivity for a range of spatial frequency gratings with 4-Hz flicker, (2) contrast sensitivity for a range of stationary wallchart gratings, and (3) dynamic visual acuity (DVA) for a range of target velocities. Within each of these three performance measures, mean results were generally consistent with previous reports. However, across-test comparisons revealed little commonality of processes between either of the contrast sensitivity scores and the dynamic acuity scores. These findings are discussed in terms of the separate contributions to overall visual performance made by the increasingly popular contrast sensitivity function (CSF) and DVA.

ISSN:1040-5488    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Stereopsis is said to be disabled when the third dimension (3-D) cannot be perceived during everyday life but can be when viewing conditions are appropriately arranged. To discover a test suitable for detecting disabled stereopsis, I measured stereoacuity, stereolatency, and the subjective difference in depth perception between monocular and binocular vision of a group of 41 optometry students. Only 2 students showed no sign of this disability. A predictive value for each test was calculated from the results of 14 students whose disability was confirmed by another means. Stereoacuity had the worst predictive value (43%) and stereolatency the best (100%). The prevalence of disabled stereopsis in this group was no less than 34% and might have been as great as 95%.

ISSN:1040-5488    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

An instrument is described which tests for the use of stereopsis by measuring how long it takes for depth perception to begin (its latency) after a transition from monocular to binocular vision. Starting at 1 s, the duration of binocular vision is reduced progressively toward a limit of 16 ms (or increased to a limit of 4 s) until the latency of stereopsis is found. The preliminary period of monocular vision acts as a probe for suppression of either eye and allows time for accommodation and vergence to stabilize before binocular vision begins. Typical results are presented.

ISSN:1040-5488    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

The oxygen transmissibility of a contact lens is defined as its oxygen permeability (Dk) divided by its thickness (L). Transmissibility can be obtained from separate measurements of Dk and L, or from a single measurement of oxygen flux through the lens (as for example by the polarographic method). Dk/L of hydrogel contact lenses with optical power measured by the polarographic method was compared with Dk/L calculated from separate measurements of Dk and L. Polarographic sensors of different cathode diameters were used to show the effect of the area over which the flux is measured on the observed Dk/L. Dk/L from oxygen flux measured by the polarographic sensor was found to be a function of the optical power of the lens and cathode diameter. Dk/L calculated from separately determined Dk and L for optically powered lenses was found to be a function of Dk and the choice of L (central or average).

ISSN:1040-5488    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

A computerized image capture and analysis system capable of 256-level intensity resolution was used to digitize and store images as a 512 by 512 pixel array. The system was used to record a standard squarewave grating (0.80 lines/mm) located 1 m from the camera. Subsequently, an ophthalmic lens was interposed between the camera and the grating. Nonzero lens powers induced magnification and thus altered the spatial frequency of the grating image. Subtractive-type Moiré analysis was performed to compare the intensity distributions of the magnified and the unmagnified (reference) gratings. The Moiré patterns created using digital, pixel-by-pixel subtraction of the two images showed graphically the power distributions of the lenses studied. These patterns varied noticeably between different brands of progressive-addition lenses (PAL's) of the same nominal power. Using only commercially available components, this method of analysis was both fast and repeatable, and may be of value to manufacturers and large-volume finishers of progressive-addition lenses.

ISSN:1040-5488    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

In this study we assessed success rates over 12 months with a hydrogel diffractive bifocal contact lens. A total of 108 presbyopes were fitted with lenses, and after 12 months 46% were still wearing the lenses. Subjects with previous presbyopic lens-wearing experience achieved greater success (58% of those fitted) than neophyte lens wearers (33% of those fitted, and 17% of all neophyte presbyopes initially expressing an interest in contact lenses before screening). More neophytes failed for nonvision-related reasons than experienced subjects (31% vs. 12% of those fitted). Consequently, when nonvision-related failures were excluded from the calculation of success rates, 59% of those fitted with lenses (49% of neophytes and 66% of experienced subjects) were still wearing the lenses at 12 months. Most discontinuations from lens wear occurred in the first month of the study, the major reason for discontinuation being poor near vision (blur and/or ghosting; 47% of discontinuations.)

ISSN:1040-5488    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Discoloration of hydrogel contact lenses is frequently encountered in clinical practice and often precipitates lens replacement. This investigation sought to determine if lens discoloration was associated with a compromise in low or high contrast, high illumination visual acuity. This association would lead to a more scientific rationale for lens replacement. Amber lens discoloration was achieved using human-worn 55% water hydrophilic contact lenses (Methafilcon-A) heated in a 1% potassium sorbate, low pH saline solution. A spectrum of 6 discolored lenses was obtained with a peak absorbance at 320 nm. Fifteen subjects, between the ages of 22 and 38 years, were tested wearing each of the discolored lenses. None of the differences in high or low contrast acuity between our discolored and nondiscolored lenses were found to be statistically significant using a paired t-test at the 99% significance level.

ISSN:1040-5488    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

The flexure of spherical rigid lenses (various materials) and a soft lens was measured using automated over-keratometry on 6 adapted rigid lens wearers (12 eyes) whose corneal toricity ranged from 1.37 to 3.87 D. The results showed: (1) that there was no significant difference in flexure between polymethyl methacrylate (PMMA), silicone acrylate, and the fluorosilicone acrylate lenses (whose Dks ranged from 0 to 115). However, Advent (fluoropolymer) did flex significantly more than the other rigid lenses, and significantly less than the soft lens (Bausch & Lomb U4) and (2) that lens flexure of the rigid lenses did not alter over a 2-h period. We also measured high and low contrast visual acuity (HCVA and LCVA), and the results from subjects wearing Advent and the soft lens were significantly worse than with the other rigid lenses. Finally, the results of this study showed no correlation between rigid lens flexure and permeability and between rigid lens flexure and visual acuity when Advent was excluded from the linear regression analysis.

ISSN:1040-5488    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Refractive status can be represented as a point plotted on a three-dimensional graph. As the refractive status changes a curve is traced out on the graph. Trajectories of changing refractive status are illustrated in a number of representative ideal cases. The three-dimensional graphs are shown by means of stereo-pairs. Trajectories are constructed from clinical data and interpreted with reference to the ideal examples. They show changing refractive status with age, after cataract extraction, and after radial keratotomy. The trajectories may prove to be a useful tool for both clinician and researcher. They show overall trends and allow prediction of future changes. Certain types of trajectories could prove to be characteristic of certain conditions and thus be useful for diagnosis. Trajectories allow monitoring of change after intervention.

ISSN:1040-5488    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

We investigated the role of various stimulus parameters that influence motor fusion responses for peripheral as compared to central fusional stimuli. Results from the initial experiment indicated that a central stimulus equal in size to a peripheral fusion stimulus dominated the response independent of the amount of retinal eccentricity of the peripheral target. A second experiment indicated that the central stimulus dominated even when the peripheral stimulus was larger. However, when the peripheral stimulus was changed in shape so that it became an annulus surrounding the central stimulus, the peripheral stimulus was always stronger. In another phase of the experiment, we found that the extent to which a peripheral stimulus surrounded the central one determined which controlled the response. We concluded that the surrounding perceptual gestalt produced by the peripheral stimulus was the most significant variable determining the relative strengths of central and peripheral fusion stimuli. Clinical implications are discussed.

ISSN:1040-5488    

出版商:OVID    

年代:1992

数据来源: OVID