PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “Optics and Refraction”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 577 records · Page 32Linked to original sources
PubMed1997

Relationship between cup-disc ratio and optic disc diameter: the Blue Mountains Eye Study.

PURPOSE: To determine the effects of optic disc size on vertical cup-disc ratio in subjects free of glaucoma and other optic nerve disease. METHODS: Data were collected from 3654 people, 49 years of age of older, living in the Blue Mountains, west of Sydney (NSW, Australia). Examinations performed included subjective refraction and Zeiss colour stereo optic disc photographs. Eye and camera magnification effects were corrected. RESULTS: Mean vertical disc diameter was 1.51 mm (95% confidence interval (CI) 1.504-1.516 mm) and the mean vertical cup-disc ratio was 0.43 (95% CI 0.425-0.435). Both parameters were distributed unimodally. The cup-disc ratio was strongly associated with disc diameter. Controlling for other variables, cup-disc ratio increased 0.270 (95% CI 0.250-0.290) per mm increase in disc diameter. CONCLUSIONS: Vertical optic disc diameter and cup-disc ratio are distributed near normally in older Australians. Optic discs with larger vertical diameters have considerably greater vertical cup-disc ratios.

Anthropometry↗
PubMedDate not supplied

Angular measurements of light scattered by turbid chiral media using linear Stokes polarimeter.

The effects of turbid chiral media on light polarization are studied in different directions around the scattering samples using a refined linear Stokes polarimeter, which simplifies the signal analysis, and allows for the detailed investigations of scattered light. Because no moving parts are involved in a measurement at a specific detection direction, the determination accuracy of polarization states is increased. The results show that light depolarization increases with both turbidity and detection angle for low and moderately turbid samples; however, the angular dependence decreases with increasing turbidity. When the turbidity is increased to approximately 100 cm(-1), the depolarization becomes higher in the forward than in the backward direction. Polarization sensitive Monte Carlo simulations are used to verify some experimental observations. The results also demonstrate that surviving linear polarization fraction and overall intensity are more sensitive to the increase of glucose concentration in backward than in the forward direction in highly turbid media, indicating that backward geometry may be preferable for potential glucose detection in a biomedical context. Comparison measurements with optically inactive glycerol suggest that the refractive index matching effect, and not the chiral nature of the solute, dominates the observed optical rotation engendered by glucose in highly turbid media.

Equipment Design↗
PubMed1999

The relationship between optic disc area and open-angle glaucoma: the Baltimore Eye Survey.

PURPOSE: To determine if eyes with larger optic disc area are more likely to have open-angle glaucoma or to have glaucoma at lower intraocular pressure (IOP). METHODS: Data were collected from a population-based sample of adults residing in East Baltimore, consisting of demographic information, ocular examinations, automated and static/kinetic visual field tests, IOP as measured by applanation tonometry, and image analysis of the optic disc. Optic disc area was calculated using refractive error to correct magnification. Open-angle glaucoma was defined by visual field and optic disc criteria. One eye from each of 75 patients with glaucoma was compared to those of 3,518 subjects without glaucoma. RESULTS: Although optic disc area was somewhat larger among patients with glaucoma than control subjects, in a regression model adjusting for age, gender, and race, the significance of this difference had a probability of 0.06. Among patients with glaucoma, disc area was not related to IOP level measured at study examination. CONCLUSION: Disc area is a weak risk factor for open-angle glaucoma. Disc area did not differ between patients with glaucoma who had lower IOP and those who had higher IOP among a group with glaucoma that were identified in a population survey.

Adult↗
PubMed1990

Avian models of experimental myopia: environmental factors in the regulation of eye growth.

During normal development a precise match between the length of the path of light within the eye and the refractive state of the eye's optical elements is required for the retinal image to remain sharply in focus. Recent studies have shown that this match between eye growth and refractive state is partially under the control of environmental factors. Investigations in several species of ground-foraging birds have shown the existence of an optical mechanism that maintains the retinal image of the ground in focus on the upper retina without a change in accommodation, when the bird is standing erect. Moreover, as a young bird grows, the refractive state changes to maintain a sharp focus of the retinal image of the ground. These findings suggest that the quality of the retinal image may be an important regulator of the matching of refractive state to growth of the ocular globe. We have found in young, rapidly growing birds that the application of optical devices that degrade the quality of the retinal image interferes with the normal regulation of eye growth and results in excessive growth and severe myopia. Additional environmental variables that affect eye growth include nutritional deficiencies of calcium and vitamin D and elevated ocular temperature.

Animal Nutritional Physiological Phenomena↗
PubMed2002

Refractive anomalies and visual activation in functional magnetic resonance imaging (fMRI): a versatile and low-cost MR-compatible device to correct a potential confound.

PURPOSE: To develop a magnetic resonance (MR)-compatible, versatile, easy-to-use, and low-cost device for refractive correction. MATERIALS AND METHODS: We retrospectively evaluated the application and practicability of the refractive correction in 110 subjects who had participated in various functional MR imaging (fMRI) studies with complex visual stimulation paradigms since the introduction of the MR-compatible lens frame at our site. The subjects consisted of 31 patients with Parkinson's disease (age range, 40-85; mean age, 63.2 years) and 79 healthy volunteers (age range, 18-79; mean age, 46.7 years). RESULTS: In volunteers, experimentally induced myopia caused a substantial (>20%) reduction of the blood oxygenation level dependent (BOLD) response to a flickering dartboard. CONCLUSION: Refractive errors (and the resulting optical blur) may play a nonnegligible role as a possible interfering factor in fMRI experiments with visual stimuli. The MR-compatible frame (fitted with appropriate lenses) used in this study affords full refractive correction at reasonable cost and preparation time.

Adult↗
PubMed2002

[Full optical correction after cycloplegia in headache].

PURPOSE: To study the advantages of cycloplegia in the accurate assessment of refractive errors and to evaluate the impact of full optical correction in the course of headache. MATERIAL: and methods: A prospective study was carried out in 164 eyes of 82 patients with headache. Corrected visual acuity was 10/10 in all cases. Refraction was measured before and under cycloplegia by cyclopentolate with a CANON R 50 infrared autorefractometer. Full optical cycloplegic correction based on the value of refraction under cycloplegia was prescribed in all cases. Mean follow-up was 10 months. Statistical analyses were used: Student's t test, Pearson coefficient correlation, and Chi2. RESULTS: Under cycloplegia, hypermetropia was significantly the most frequent ametropia (67.1%, p<10(- 9)). The difference between the spherical equivalent, before and during cycloplegia, was highly significant (p<10(- 5)) at 0.79. Optical correction eliminated headaches in 76.5% of the cases. CONCLUSION: The authors conclude on the role of accommodation in headache onset and the need to continue cycloplegia until the age of 45. The importance of optical correction in alleviating headaches is underlined.

Accommodation, Ocular↗
PubMed2001

Photographic reproduction of out-of-focus and distorted ocular imagery.

The necessary settings and parameters were determined for ordinary camera and lens systems to faithfully reproduce out-of-focus and distorted imagery as it falls upon the retina of the human eye. Theoretic considerations of both geometric and physical optics were used to calculate the "relative blur" and distortion produced by refractive error added to ordinary camera lenses as opposed to refractive error in an arbitrary thick-lens optical system bounded by air and fluid (i.e. the eye). In both the camera and the eye, "relative blur" was determined to be directly proportional to dioptric defocus and to aperture size, and effectively independent of the focal length. Distortion of imagery was also found to be independent of the focal length. Photographs corroborate the theoretic findings. A given amount of relative blur, however, appeared somewhat greater when recorded on photographic film than when appreciated by the human eye. The Stiles-Crawford effect, the chromatic aberration of the eye, and neural processing probably each contribute to this difference. Previous investigators have grossly exaggerated blur and distortion in photographs intended to simulate ocular imagery and have drawn misleading conclusions from their results.

Humans↗
PubMed1990

Birefringent multifocal lenses: theory and application to the correction of refractive error.

Light passing through birefringent materials experiences two indices of refraction, and lenses made of or incorporating birefringent materials will therefore exhibit two focal points. The optics of birefringence are summarized, and their application to lenses is described. Prototype lenses based on these principles were produced, and the bifocal effect was observed. Multifocal lenses based on these principles have numerous uses, including contact and intraocular lenses.

Contact Lenses↗
PubMed1995

Shifts in tonic accommodation after near work are related to refractive errors in children.

A link between changes in tonic accommodation (TA) produced by sustained near work and the development of adult-onset myopia has been suggested in studies of young adults. Measures of TA before and after near work have been lacking in children of school age, which is the most susceptible period for the development and progression of juvenile-onset myopia. In the present study accommodation was measured in 87 children, aged 7 to 16 years, before and after 15 minutes of video game playing. All children were refracted before testing and wore optical correction during measures of accommodation with a Canon R1 autorefractor. Most children showed initial values of TA (far focus minus dark focus) between 0.0 and 1.0 D, with a mean of 0.68 D. Grouped by refractive status, the myopic children initially showed 0.30 D of TA, while the emmetropic children showed 0.75 D and the hyperopic children showed 0.94 D. After playing the video game, TA of the myopes increased by 1.15 D, compared to smaller increases for the emmetropes (0.68 D) and hyperopes (0.24 D). Comparable values have been obtained from young adults. These results indicate that the smallest initial values of TA and the largest inward shifts in TA are found during the period of acquisition and progression of myopia, regardless of age.

Accommodation, Ocular↗
PubMed1993

Oblique central refraction in spherocylindrical lenses tilted around an off-axis meridian.

Thin lens equations accurate to third order are presented for the effective spherocylindrical parameters for oblique central refraction through spherocylindrical lenses that are tilted around an off-axis meridian. This situation occurs in either pantoscopic or faceform tilt for spectacle corrections for oblique astigmats. The thin lens equations appear to be good approximations for oblique central refraction through actual spectacle lenses.

Eyeglasses↗
PubMed2004

Group index of the human cornea at 1.3-microm wavelength obtained in vitro by optical coherence domain reflectometry.

The group index of the cornea, rather than the phase refractive index, is required for thickness calculations with optical coherence tomography. Recent advances with high-speed optical coherence tomography at 1.3 microm make index measurement at this wavelength of great interest. Group indices of three human corneas from an eye bank were measured in vitro with optical coherence domain reflectometry. Measurements were made in a calibrated cuvette filled with a preservation medium to maintain proper corneal hydration. Group indices were calculated from the optical path lengths measured. The corneal group index was 1.389 +/- 0.004 (average +/- standard deviation). The average group index of a balanced salt solution, an approximation to aqueous humor, was 1.343 +/- 0.001.

Chondroitin Sulfates↗
PubMed1991

Reappraisal of the ratio of disc to macula/disc diameter in optic nerve hypoplasia.

The ratio of disc to macula/disc diameter is characteristically increased in eyes with optic nerve hypoplasia. We present the largest reported series of patients with a definitive diagnosis of optic nerve hypoplasia for whom this ratio has been determined. All measurements were made by an independent masked observer. Our results are in accordance with previous reports. A ratio of 2.94 provides a one-tailed upper population limit of 95%. An attempt has been made to correlate optic disc size and visual acuity. In 75% of bilateral cases the eye with the relatively smaller optic disc was found to have a better Snellen visual acuity than the fellow eye. This suggests that additional pathogenic mechanism(s) may have determined the eventual visual outcome in such eyes. Such mechanisms include macular hypoplasia, high refractive error, refractive amblyopia, central scotoma, and optic atrophy.

Adolescent↗
PubMed2006

Glaucoma in children: are we making progress?

BACKGROUND: Glaucoma in children presents difficult clinical challenges. Even when appropriately treated, blindness can occur. DESIGN: Retrospective interventional case series and literature review. METHODS: All clinical records of children seen by the author with a diagnosis of glaucoma established before 16 years of age were reviewed from 1977 to 2003. Glaucoma was classified as primary infantile, aphakic, syndrome-related, and secondary. The best-corrected visual acuity, refractive error, configuration of the optic nerve cup, and perimetry were recorded. The intraocular pressure (IOP) for each visit was recorded. IOP measurements of 19 mm Hg or less were considered "good." The percentage of "good" readings was calculated for each eye. Representative visual acuities, refractive errors, IOP, disk configuration, and perimetry were recorded at 6, 12, 18, and 24 years of age for each patient. The admitting ophthalmologic diagnosis for each child at the Western Pennsylvania School for Blind Children was recorded from 1887 to 2003. RESULTS: One hundred twenty-six children (204 eyes) were studied: infantile glaucoma, 52 eyes; aphakic glaucoma, 40 eyes; syndrome associated, 69 eyes; and secondary glaucoma, 43 eyes. The mean follow-up was 11.6 years (1 to 30 years). Overall, 60 (29.4%) of 204 eyes had a 6/12 (20/40) or better corrected visual acuity at the most recent visit. The percentage with this acuity remained stable throughout the follow-up period. Eyes with infantile glaucoma had the best acuity, and 40% had 6/12 (20/40) or better. Amblyopia was common and responded to treatment. Eyes with aphakic glaucoma had the worst acuity with only 10% achieving 6/12 or better. These eyes had a bimodal onset of glaucoma; eyes with an early onset had an angle closure configuration and eyes with a delayed onset had an open angle. Early cataract removal and microcornea were risk factors for glaucoma. If the IOP was maintained at 19 mm Hg or less (good) on 80% of the determinations over time, the optic nerve cup compared with the diameter of the optic nerve (C/D ratios) were stable. Eight patients had multiple, good quality, visual fields performed over 3 to 15 years. If the patients had "good" IOP on 70% of the measurements, the visual fields remained stable. A historical perspective of glaucoma control was gained by looking at the admitting diagnosis at the Western Pennsylvania School for Blind Children. From 1910 to 1970, an average of 9.2 children blind due to glaucoma were admitted each decade. From 1971 to 2003, there were only three children with glaucoma admitted over 30 years. CONCLUSION: Removal of congenital cataracts should be delayed until 3 to 4 weeks of age. Consideration should be given for using 19 mm Hg or less to measure the success of glaucoma treatment in children. Treatment of amblyopia is as important as IOP control in children. Imaging technology such as optical coherence tomography and measurement of central corneal thickness may play an important future role in the assessment of children with suspected or known glaucoma.

Child↗
PubMedDate not supplied

Corneal power correction factor for photorefractive keratectomy.

BACKGROUND: Studies of corneal power changes resulting from photorefractive keratectomy generally rely on keratometer or videokeratograph measurements. These instruments convert corneal radius of curvatures values to optical powers by means of the single refracting surface formula, which incorporates an index of refraction value of 1.3375. This index approximates that of the tears but not the 1.376 index of the corneal epithelium or stroma. A hypothetical optical model was used to determine the most appropriate index to be chosen with respect to corneal power calculations relative to photorefractive keratectomy. METHODS: The contribution of each refractive element in the tear lens-corneal surface to the total power of the eye was calculated in order to identify which index of refraction was most appropriate for the corneal power calculation. RESULTS: The outer tear surface has significant optical power but the tear layer as a whole has nearly zero power due to the offsetting negative power of the posterior test surface. There is no significant difference in the effective power of light leaving the corneal anterior surface when considered with or without the tear layer. Photorefractive keratectomy changes the epithelium and anterior surface of the corneal stroma, but does not affect the posterior stroma or other ocular media. Hence the refractive index for the corneal epithelium or stroma of 1.376 should be used in converting radius to optical power values. The error in assuming a corneal index of 1.3375 is a constant proportion equal to 11.4% of the corneal power reading. CONCLUSIONS: Photorefractive keratectomy presents a situation in which the actual corneal refractive index of 1.376 should be used for correct corneal radius to power conversions. This may be accomplished by changing the index value in the instrument algorithm for keratometry and videokeratography to 1.376 or by adding a correction factor of either 11.4% of the regular reading to its value or multiplying by the factor 1.114. In other applications of keratometry or videokeratography, the index 1.3375 may be more appropriate.

Cornea↗