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At least 919 records · Page 51Linked to original sources
PubMed1996

Image formation by the crystalline lens and eye of the rainbow trout.

The image of a distant unresolved point (point image or PI) and modulation transfer function (MTF) of the eye and lens of the trout were recorded with high spatial (0.3 micron) and dynamic (4096 grey levels) resolution for various entrance aperture sizes and focal positions in monochromatic light, and in broadband light simulating sunlight absorbed by a retinal cone pigment. The PI is irregular, with streaks, wisps and speckle, as a result of lens structural irregularity and diffraction of light scattered within the lens and cornea. Maximum diameter of a diffraction-limited aperture area of the eye is about 0.3 mm. Axially spaced multiple foci are caused by irregular and discontinuous zonal spherical aberration. Lens substance dispersion causes strong longitudinal chromatic aberration, resulting in a broadband PI with concentric coloured haloes. Incident linearly polarized light is slightly depolarized in the PI. The nature of the image is discussed relative to lens and cornea structure, optical modelling and vision. Human subjective entoptic phenomena analogous to those observed objectively in the trout are described.

Animals↗
PubMed2004

The optics of wavefront sensing.

This article reviews the fundamental principles by which wavefront aberrometers measure ocular aberrations. Three different ways of interpreting aberration measurements are described in terms of wavefront phase, slope, and curvature. Although curvature is the more familiar concept from geometrical optics, the wave optics concept of wavefront phase is more useful for understanding the nature of aberrated retinal images and for prescribing treatments to correct high-order aberrations.

Diagnostic Techniques, Ophthalmological↗
PubMed2006

Progressive addition lenses--measurements and ratings.

BACKGROUND: This study is a followup to a previous study in which the optics of several progressive addition lens (PALs) designs were measured and analyzed. The objective was to provide information about various PAL designs to enable eye care practitioners to select designs based on the particular viewing requirements of the patient. METHODS: The optical properties of 12 lenses of the same power for each of 23 different PAL designs were measured with a Rotlex Class Plus lens analyzer. Lenses were ordered through optical laboratories and specified to be plano with a +2.00 diopters add. Measurements were normalized to plano at the manufacturer-assigned location for the distance power to eliminate laboratory tolerance errors. The magnitude of unwanted astigmatism and the widths and areas of the distance, intermediate, and near viewing zones were calculated from the measured data according to the same criteria used in a previous study. RESULTS: The optical characteristics of the different PAL designs were significantly different from one another. The differences were significant in terms of the sizes and widths of the viewing zones, the amount of unwanted astigmatism, and the minimum fitting height. Ratings of the distance, intermediate, and near viewing areas were calculated for each PAL design based on the widths and sizes of those zones. Ratings for unwanted astigmatism and recommended minimum fitting heights were also determined. Ratings based on combinations of viewing zone ratings are also reported. CONCLUSIONS: The ratings are intended to be used to select a PAL design that matches the particular visual needs of the patient and to evaluate the success and performance of currently worn PALs. Reasoning and task analyses suggest that these differences can be used to select a PAL design to meet the individual visual needs of the patient; clinical trials studies are required to test this hypothesis.

Equipment Design↗
PubMed2004

Monochromatic ocular wavefront aberrations in the awake-behaving cat.

Measurement of wavefront aberrations in human eyes has become a reliable, quantitative way of assessing the optical impact of experimental and corrective ocular manipulations. Wavefront measures have also been performed in several other species, but never in cats, an animal model of choice for many ocular studies. Our goal in this study was to measure wavefront aberrations reliably in live, awake-behaving cats in a manner that is directly comparable to that used in human subjects. Six adult cats (felis cattus) were trained to fixate small targets on a computer screen. A compact Shack-Hartmann wavefront sensor was aligned with each animal's pupil center and line of sight during fixation. Wavefront images were then collected from which the cats' ocular aberrations were measured up to tenth order Zernike polynomials over a 6 mm pupil. Results show that cat and human ocular wave aberrations were very similar. Second order Zernike modes accounted for more than 90% of the total wave aberration. In agreement with our observation that cat ocular optics were comparable with those of humans, the half height width of both the cat and human higher order point spread function was about 0.95 degrees. These results form a solid basis for future wavefront sensing studies aiming to quantify the effects of ocular manipulations in experimental animals.

Animals↗
PubMed2004

Compensation for longitudinal chromatic aberration in the eye of the firefly squid, Watasenia scintillans.

The camera eyes of fishes and cephalopods have come forth by convergent evolution. In a variety of vertebrates capable of color vision, longitudinal chromatic aberration (LCA) of the optical system is corrected for by the exactly tuned longitudinal spherical aberration (LSA) of the crystalline lens. The LSA leads to multiple focal lengths, such that several wavelengths can be focused on the retina. We investigated whether that is also the case in the firefly squid (Watasenia scintillans), a cephalopod species that is likely to have color vision. It was found that the lens of W. scintillans is virtually free of LSA and uncorrected for LCA. However, the eye does not suffer from LCA because of a banked retina. Photoreceptors sensitive to short and long wavelengths are located at appropriate distances from the lens, such that they receive well-focused images. Such a design is an excellent solution for the firefly squid because a large area of the retina is monochromatically organized and it allows for double use of the surface area in the dichromatically organized part of the retina. However, it is not a universal solution since compensation for LCA by a banked retina requires that eye size and/or spectral separation between photopigments is small.

Animals↗
PubMed1983

Artifacts introduced by spectacle lenses in the measurement of strabismic deviations.

The peripheral prismatic effects of corrective spectacle lenses introduce an artifact when strabismic deviations are measured. Plus lenses decrease and minus lenses increase the measured deviation. This effect begins to become clinically significant with corrective lenses with powers of more than +/- 5 diopters. We developed a simplified model that predicts the magnitude of this effect. The model agreed with actual ray tracing analysis through commonly used spectacle lenses and also agreed closely with clinical data. The clinical use of this model is facilitated by remembering the artifacts produced by +20, +10, -10, and -20 diopter spectacle lenses and interpolating for intermediate values. Correcting for these artifacts should enhance the predictability of strabismus surgery in patients with significant ametropia.

Eyeglasses↗
PubMedDate not supplied

The glasses twist.

Why would a patient without ocular pathology and with carefully prescribed glasses have "fuzzy" vision in one eye except when the left lens is twisted upward? The answer may be simpler than you think.

Adult↗
PubMedDate not supplied

Astigmatism and the toric intraocular lens and other vertex distance effects.

An intraocular lens with a given cylindrical power will correct a variable amount of cylinder at the spectacle plane. This variability depends not only on the vertex distance but also on the degree of ametropia. The amount of spectacle cylinder corrected will be more for myopes and less for hyperopes. Anterior corneal cylinder (as determined by the keratometer) is a more reliable way of estimating the cylinder needed at the intraocular lens plane.

Astigmatism↗
PubMed1998

Is current eye-care-policy focus almost exclusively on cataract adequate to deal with blindness in India?

BACKGROUND: India's National Programme for Control of Blindness focuses almost exclusively on cataract, based on a national survey done in the 1980s which reported that cataract caused 80% of the blindness in India. No current population-based data on the causes of blindness in India are available. We assessed the rate and causes of blindness in an urban population in southern India. METHODS: We selected 2954 participants by stratified, random, cluster, systematic sampling from Hyderabad city. Eligible participants were interviewed and given a detailed ocular assessment, including visual acuity, refraction, slitlamp biomicroscopy, applanation intraocular pressure, gonioscopy, dilatation, grading of cataract, stereoscopic fundus assessment, and automated-threshold visual fields. FINDINGS: 2522 participants, including 1399 aged 30 years or more, were assessed. 49 participants (all aged > or =30 years) were blind (presenting distance visual acuity <6/60 or central visual field <200 in the better eye). The rate of blindness among those aged 30 years or more, adjusted for age and sex, was 3.08% ([95% CI 1.95-4.21]). Causes included cataract (29.7%), retinal disease (17.1%), corneal disease (15.4%), refractive error (12.5%), glaucoma (12.1%), and optic atrophy (11.0%). 15.7% of the blindness caused by visual-field constriction would have been missed without visual-field examination. Also without visual-field and detailed dilated-fundus assessments, blindness attributed to cataract would have been overestimated by up to 75.8%. If the use of cataract surgery in this urban population was half that found in this study, which simulates the situation in rural India, cataract would have caused 51.8% (39.4-64.2) of blindness, significantly less than the 80% accepted by current policy. INTERPRETATION: Much of the blindness in this Indian population was due to non-cataract causes. The previous national survey did not include detailed dilated-fundus assessment and visual-field examination which could have led to overestimation of cataract as a cause of blindness in India. Policy-makers in India should encourage well-designed population-based epidemiological studies from which to develop a comprehensive long-term policy on blindness in addition to dealing with cataract.

Adult↗