ISSN:1040-5488    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

We compared the effects of dioptric blur on Snellen acuity and grating acuity. Dioptric blur had a strong negative effect on Snellen acuity, consistent with previous studies, but had little effect on grating acuity. Between 1 and 12 D both types of acuity were reduced as a linear function of blur. However, 12 D of blur reduced grating acuity to only 6/24 (20/80), whereas letter acuity was worse than 6/300 (20/1000). We suggest that these differences are due to the presence of “spurious resolution” in which phase-reversed gratings are readily detectable. But the phase reversals so distort the relative positions of linear segments within the letters that the letters become unrecognizable. These results indicate that Snellen letters are more sensitive than gratings to a patient's refractive errors, emphasize the differences between Snellen and grating acuity, and indicate that the minimum angle of resolution (MAR) concept is not applicable to letters.

ISSN:1040-5488    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

We studied the effect of dioptric blur on contrast sensitivity using the Vistech Vision Contrast System Chart. One diopter of blur affected the Vistech gratings by an amount consistent with expectations; sensitivity loss was greatest with higher spatial frequencies. However, a second diopter of blur induced an equal sensitivity loss for all spatial frequencies. A third diopter induced the greatest loss at the lowest frequency and an actual increase in sensitivity at the highest frequencies. This surprising finding is due to an interaction between “spuriously resolved”gratings and the border of the grating disc which resulted in the finest grating discs appearing to contain low frequency borders aligned with the fine gratings. Actually the fine gratings in the disc were invisible. When performing scanning photometry on the Vistech chart, we found that the contrasts of the unblurred gratings with higher contrast levels are 2.7 to 5.4 times higher than those stated by the manufacturer. The presence of this aliasing artifact indicates that the Vistech Vision Contrast System may produce erroneous results when used under conditions of high dioptric blur.

ISSN:1040-5488    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

This is a sequel to other papers dealing with the effect of induced aniseikonia and optical anisophoria on fixation eccentricity during bifixation. To simulate the effect of induced anisophoria, controlled amounts of differential prism were induced in the vertical meridian. At the same time, stimuli were presented at various angles from the subject's straight ahead orientation. As demonstrated in other studies, it was found that a vertical prismatic difference will cause the eyes to fixate slightly eccentrically in the horizontal meridian. The eccentricity was measured with the border enhancement method, described and tested in other studies. In addition, it was found that the fixation deviation is increased further by oblique gaze, the greatest obliquity having the greatest effect. It was also found that whereas short-term adaptation reduces the prism effect somewhat, it has less influence on the effect of oblique gaze. In general, the study demonstrates the potential oculomotor problems inherent in optical anisophoria

ISSN:1040-5488    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

Because of the possibility of transmitting communicable diseases, in particular the HIV virus, it has been recommended that all diagnostic contact lenses, including rigid lenses, be disinfected after each use. Hydrogen peroxide is a recommended disinfection agent, but its effect on rigid lens polymers is relatively unknown. We soaked 50 lenses of 5 different polymers in 3% hydrogen peroxide for 10 min and measured the base curves to determine if any changes occurred. Our results showed no statistically significant change in base curve for the lenses measured, but 22% of the lenses did exhibit a small amount of warpage (mean 0.0382 mm) of the base curve. We were not able to determine what caused these lenses, and not all the lenses, to warp. Under the conditions of this study, hydrogen peroxide does not appear to cause clinically significant parameter changes, but lenses should be checked for warpage before use.

ISSN:1040-5488    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

All forms of ophthalmic correction for presbyopia require compromises in viewing flexibility and visual function. The unique effects on vision of contact lenses used in managing presbyopia are especially intriguing and potentially problematic. Bifocal contact lenses produce unique changes in the nature and quality of the retinal image. In monovision (MV) correction, anisometropia is intentionally created by fitting one eye to see clearly at optical infinity and the other eye to see clearly at the near working distance. Our review of the literature indicates that most visual functions are affected by these departures from the conventional optical correction strategies used on nonpresbyopes. Sensory functions such as contrast sensitivity and stereoacuity are affected most, whereas motor functions such as convergence and accommodation are not noticeably impaired. MV appears to produce a more widely acceptable visual compromise than currently available bifocal contact lenses for most patients.

ISSN:1040-5488    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

Proximal vergence was measured haploscopically in response to a change in physical distance of a large screen of blurred horizontal lines. These responses were compared to proximal vergence measurements made using a clinical method. Perceived distance of the horizontal stripes was estimated according to the perceived size of a projected afterimage. For 18 normal observers the change in vergence measured haploscopically (in prism diopters per meter angle change in test distance) averaged 2.59 (SD=1.44) binocularly and 2.11 (SD=1.22) monocularly. Clinical measures averaged 2.88 (SD=1.57) binocularly and 2.10 (SD=1.27) monocularly. Haploscopic and clinical measures of proximal vergence correlated only when the clinical technique was modified by viewing through pinholes. Taking estimated perceived distance into consideration only partially reduced the differences between proximal vergence measured haploscopically and clinically as well as binocularly and monocularly. Both haploscopic and clinical measures of proximal vergence were sizeable, indicating that this vergence component can contribute significantly to the normal vergence response.

ISSN:1040-5488    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

The elements of the dioptric power matrix have previously been interpreted in terms of power along particular meridians with the off-diagonal elements interpreted as a new form of dioptric power called torsional power. This paper shows that such new concepts are not necessary. Decomposition of the matrix provides an interpretation in terms that are quite familiar. The elements represent three component powers: the diagonal elements are two cylinders and the off-diagonal elements a crossed-cylinder with axes at 45° and 135°. The component crossed-cylinder obviates the need for the concept of torsional power. The concept of meridional power is avoided.

ISSN:1040-5488    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

A retrospective survey of pediatric clinical files from the Kooyong Low Vision Clinic (LVC) showed that the major causes of low vision were congenital or inherited conditions and most children had 6/60 (20/200) or better distance acuities. A classroom evaluation of these children showed that overall, the clinically determined visual acuity corresponded with classroom performance. A high rate of use of prescribed low vision aids was found, and a reading evaluation on a standardized test showed poor performance with respect to reading speed and comprehension, but almost all the children had adequate reading accuracy. Considering the importance of reading in education, greater emphasis on reading evaluations in routine low vision examinations is recommended.

ISSN:1040-5488    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

There are several visual mechanisms for analyzing spatial information additional to the much researched mechanism sensitive to luminance contrast. We describe a Snellen-type acuity test for motion-defined (MD) letters. Acuity for these MD letters collapsed at dot speeds slower than 0.05 deg/s, but acuity for contrast-defined (CD) letters was unaffected by speed over the entire 0 to 0.3 deg/s range used. Acuity was a power function of presentation duration for both kinds of letter, but the exponent was higher for MD than for CD letters. Acuity for MD letters was comparatively unaffected by dot density from 50 to 0.05%, below which it suddenly collapsed to zero. On the other hand, acuity for CD letters progressively fell as dot density was reduced from 50%, and below about 0.5% approximated acuity for MD letters.

ISSN:1040-5488    

出版商:OVID    

年代:1990

数据来源: OVID