摘要:

Few studies have been done on the absorption and release of chemical vapors by high water content hydrogel lenses. In this study we investigated the absorption of ammonia vapors by this type of contact lens (CL). Ten high water content hydrogel lenses were exposed to vapor generated in a cuvette by 1 drop of aqueous ammonia. The contact lens was transferred to a 0.9% saline solution containing a colorometric indicator for ammonia and the amount of ammonia released into the saline was measured. We found that significant quantities of ammonia were absorbed into the lenses and the relation between average concentration of ammonia vapor and amount absorbed per contact lens was nearly linear from 50 parts per million (ppm) to 250 ppm ammonia vapor. For concentrations greater than 250 ppm there appears to be a saturation effect. Our findings indicate that high water contact lenses will absorb ammonia and release it into a solution similar to tears.

ISSN:1040-5488    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

Although it has been shown that photochromic glass industrial lenses do not provide adequate occupational impact protection, they are frequently requested by outdoor workers wanting sunglasses. The recent introduction of Transitions Plus (TP), a photochromic plastic lens, may meet the needs of these workers. The impact resistance of 60 TP lenses of dress thickness was studied using a ballistic test. Lenses were edged and mounted in metal industrial frames. The completed spectacles were placed on a headform for ballistic testing using a 6.5 mm (1/4 in) steel ball propelled from an airgun. The lenses were divided into test groups that received: (1) a single impact at the geometrical center by a ball traveling at 18 m/s (59 ft/s) and (2) 50 consecutive impacts at 18 m/s, or (3) a single impact at 46.5 m/s (152 ft/s). All lenses passed the single impact test at 18 m/s. Four lenses broke under repeated impact. All lenses subjected to the high-speed impact test failed, either breaking or being dislodged from the frame. These findings show that dress thickness TP lenses provide impact protection from small blunt missiles traveling at moderate speed, and that they are durable under repeated low energy impact. However, thicker lenses must be used when the wearer has a high risk of ocular exposure to small high-speed missiles in the workplace. It can be concluded that TP lenses may be used in occupational eyewear for many outdoor workers who want photochromic sunglass lenses, provided that other needs for vision and occupational safety are also satisfied.

ISSN:1040-5488    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

Amblyopia and optic atrophy are two very different causes of unilateral long-standing visual impairment. Yet, in some patients the differential diagnosis is not always manifest and standard clinical tests may fail to provide accurate information. We tested the efficacy of a nonstandard clinical test [flash visual evoked potentials (VEP's)] and quantitative ultivariate statistical techniques as aids in the assessment of this differential. Thirty-three patients were separated into four groups (normal, anisometropic amblyopia, strabismic amblyopia, and unilateral optic atrophy). Nonpatterned flash VEP's were obtained using several different temporal frequency rates. Patients with optic atrophy had significantly reduced VEP's in the affected eye at all temporal frequencies. Strabismic amblyopes, but not anisometropic amblyopes, often showed upranormal responses in the affected eye at the higher temporal frequencies. Finally, by using discriminant analysis (DA) we were able to classify correctly almost 70% of the patients, well above chance level of 25%. This DA provided very good sensitivity and specificity. We have shown that the use of flash VEP's and of multivariate statistical techniques may provide an effective way to diagnose amblyopia differentially from optic atrophy.

ISSN:1040-5488    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

Background. Patients with dense ocular media disorders retain the ability to project or point to an intense source of light. Using this response capability and high luminance points of light as stimuli, Vernier judgments (a hyperacuity test) can be made by these patients, even without the presence of a “window” through a leucoma, cataract, or bleed. Without coaching, these individuals are able to locate the centers of the individual degraded point images if the individual light sources are adequately separated (i.e., if sufficient “gaps” exist between the individual stimuli), and they can spatially align the degraded images. Advanced cataracts are the main cause of blindness in the developing world, and this is a treatable condition. In these nations, only a modest proportion of affected patients receive surgery, and only 5% or less of these individuals obtain treatment in two eyes. There are incredibly large and rapidly growing backlogs of advanced cataract patients requiring care (many millions). Because of the 20 to 30% failure rates that occur after treatment (all causes) in many developing world settings, a test performed before surgery, which offers a meaningful estimate of postsurgical visual outcome, can be valuable. Using the principle defined above, we seek to determine before surgery those individuals who will derive most benefit from cataract removal, and which of two cataractous eyes has the better postsurgical visual prognosis. Experimental. In Berkeley, we performed a series of preliminary studies on a Vernier acuity test before initiating a clinical study in a developing world setting. These studies were conducted upon young adult normal subjects wearing their usual vision corrections, with and without induced refractive errors, and/or with or without simulated dense nuclear cataracts. We sought (1) to determine the number of repeat trials necessary for reliable outcomes; (2) to compare a two-point and a three-point Vernier acuity display; (3) to determine the shape of the measured response function at large gap separations between test points; (4) to define optimal test distance and stimulus size; (5) to assess the effect(s) of a broad range of uncorrected refractive errors upon outcomes; and (6) to consider means to minimize refraction-based errors by using a pinhole, a refractive correction, and/or selective spatial filtering. We compared responses obtained using the current CRT/VDT-based, computer- driven (Berkeley) instrument with a new precision optical/mechanical computer-driven (India) instrumerit. The India instrument is needed to determine design parameters for a next stage simpler, cheaper, more rugged field instrument(s). A preliminary protocol, the HyperAcuity Study Protocol (HASP), was formulated for later use in the clinical study and for initial later trial in India

ISSN:1040-5488    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

Background. Our purpose is to develop a clinical test capable of assessing vision through dense cataracts (and other advanced media opacities) before surgery. Because such anomalies are endemic in the developing world, it was desirable to conduct a clinical study of a prototype India instrument and HyperAcuity Study Protocol (HASP) in such a setting. This investigation, preliminary to a planned clinical study, was conducted at the Aravind (Free) Eye Hospital, Madurai, India. Experimental. The preliminary HASP protocol, developed in Berkeley, was adapted to existing clinical practices in Madurai. Included was a new test of visual acuity, the Gap “VA” test, developed as part of the revised protocol. The rational for this test, methods used, and sample data are presented. A Vernier alignment test (one of the hyperacuities) is used to assess vision through dense ocular media disorders. The task of the patient is to align vertically three high luminance, discrete visual stimuli. The revised HASP protocol and adapted India instrument were tested upon patients with advanced cataracts. The cataract grading system used in these studies (provided by Drs. N. V. Projna and G. Rohini) is defined. Patients were tested before and shortly after surgery. This investigation allowed us to refine the test protocol and instrument design preparatory for a clinical study of HASP. A much simpler/cheaper version of the India instrument will be used in the planned clinical study. Additional experiments are scheduled in order to address issues which require resolution before initiation of next-stage testing. In part, in these papers, we seek to help others understand some problems encountered when conducting research in a developing world environment, even a very good one!

ISSN:1040-5488    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

Lectin receptors of the rabbit corneal epithelium were investigated ultrastructurally using gold-conjugated lectins. Corneal epithelium with an intact mucous layer was readily labeled with wheat-germ agglutinin (WGA), whereas after mechanical or chemical removal of the mucus, labeling was minimal, suggesting that the lectin receptors were located mainly in the mucus. Corneal epithelium subjected to slight superficial injury was also labeled with WGA. In this case, however, the gold particles were in close contact with the cell membrane of injured and/or newly exposed cells. In the more injured cells, gold particles were seen in the cytoplasm as well. Labeling with WGA indicates the presence of sialyl residues, known to be attachment sites for Pseudomonas aeruginosa. The results suggest a protective role of mucus against infection. The association of WGA lectin receptors with the plasma membrane of compromised corneal epithelial cells may help to explain the mechanism of bacterial invasion of the cornea, e.g., in overnight wear of contact lenses with insufficient oxygen transmissibility.

ISSN:1040-5488    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

The influence of color signals on stereopsis has been studied using figural-stimuli stereograms with variations introduced according to the opponent chromatic channels (red-green and yellow-blue), derived from Boynton's color-vision model. We used wallpaper stereograms, which enable the rank-order disparity ranges of the chromatic and luminance signals to be compared with the rank-order disparity range of proximity, a particular spatial configuration of the stereogram in which there are no variations in chromaticity and/or luminance. The results indicate that both chromatic signals contribute to stereopsis as does the luminance signal, contradicting the model of Hubel and Livingstone. The results also show there are no clear dependencies upon the kind of signal processed, as luminance and chromatic variations are processed with the same efficiency.

ISSN:1040-5488    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

The prevailing view of accommodation is that the eye changes focus to maximize luminance contrast by trial and error. Negative feedback is considered essential in this view because luminance contrast provides no directional information. Fincham proposed an alternate view in which longitudinal (axial) chromatic aberration (LCA) provides a directional stimulus for accommodation. For spatial frequencies above approximately 0.5 cpd contrast of the retinal image is different for long, middle, and short spectral waveband components of the image. We varied the amount of LCA in small steps (0.25 D) to determine how much LCA is needed to enhance or impair the response. An infrared optometer monitored accommodation continuously while subjects viewed a yellow/ black square-wave grating (3.5 cpd) in a Badal stimulus system. The yellow/black grating was produced by superimposing red (600 nm) and green (520 nm) gratings, and LCA was increased, decreased, neutralized, and reversed by repositioning the red grating component along the axis of the optical system. Target vergence was modulated sinusoidally (0.2 Hz) over a 1 D range (1.5 to 2.5 D) and gain and phase-lag of the accommodation response were determined by Fourier analysis. Subjects accommodated well as long as a normal amount of LCA was present—0.5 D in the correct direction enhanced accommodative gain, and 0.25 D in the reverse direction markedly inhibited the response. We conclude that the contrast of the retinal image in different spectral wavebands specifies focus of the eye, and provides a powerful directional stimulus for reflex accommodation.

ISSN:1040-5488    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

Colorimetric analyses were performed in both normal trichromatic and dichromatic color spaces to determine whether several light sources were suitable illuminants for the Farnsworth-Munsell Panel D-15 (D-15) color vision test. Results for fluorescent lamps showed that lamps with a correlated color temperature (CCT) of 7500°K and a general color rendering index (GCRI) of at least 90 are acceptable substitutes for Illuminant C. Predictions for filtered tungsten light indicated that lights with a color temperature near 5000°K are unsuitable because of nonuniformities in the glass daylight filter transmittance. Conclusions based on these analyses are conservative because, with exception of the GCRI, color adaptation effects were not taken into account.

ISSN:1040-5488    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

We developed a system of coaxial video cameras that records monocular eye position and scene, and superimposes these images using a digital video mixer. We mounted miniature video cameras above and below a cube beam-splitting prism in the spectacle plane. An infrared emitting diode was imaged in the cornea to locate eye position. The technique was accurate to about 0.5° within 15° of primary gaze; however, we see its main advantages as being its low cost and simple design that, for some applications, does not require complex computer analysis and data manipulation. With improved camera optics, it has the potential for helmet mounting and use remote from a recording console. We used the instrument to monitor a reader's eye position when using low vision devices, and see applications of the technique in the field of visual ergonomics and sports vision.

ISSN:1040-5488    

出版商:OVID    

年代:1995

数据来源: OVID