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At least 559 records · Page 31Linked to original sources
PubMed1987

The optics of comparative ophthalmoscopy.

One factor peculiar to the practice of comparative ophthalmoscopy is the very large variation in ocular size of the animals examined, a factor which is ignored in current textbook treatments of the subject. We have computed values of lateral magnification, axial magnification, angular field of view and linear field of view for 19 species of terrestrial vertebrates. The dimensional value of a 1 diopter change in direct ophthalmoscopic focus was also determined. The anterior focal length of the eye in air and the vitreal refractive index were the intrinsic optical parameters of the animal's eye necessary for these calculations. Where these values were not available from the literature, the vitreal refractive index was assumed to be 1.336 and the anterior focal length was estimated as two-thirds of the axial length using a regression equation we derived from data in the literature. The angular field of view in ophthalmoscopy was shown to be invariant in the emmetropic eye and equal to the angular subtense of the ophthalmoscopic beam. The lateral field of view and retinal depth corresponding to a 1 diopter change in direct ophthalmoscopic focus varied directly with the anterior focal length of the eye. The remaining parameters of lateral and axial magnification varied inversely with the anterior focal length of the animal's eyes. These findings provide a basis for evaluating the relative size and significance of ophthalmoscopically viewed features in terrestrial vertebrate eyes.

Animals↗
PubMed1990

Experimental studies of emmetropization in the chick.

The eyes of neonates grow from ametropia (refractive error) toward emmetropia. Whether or not this 'emmetropization' is visually guided is controversial. I describe experiments which demonstrate that in the chick refractive state is used to regulate the growth of the eye's vitreous chamber in order to achieve emmetropia from hyperopia or myopia that is induced by different visual deprivations. I discuss several studies that begin to examine the neural pathways that might be involved in the control of eye growth. Optic nerve section was used to examine the level of visual processing necessary for the control of eye growth. Eyes in which the optic nerve has been cut can still grow in the appropriate direction to correct induced hyperopia or myopia. Nevertheless, an intact optic nerve is necessary for normal refractions to be achieved; eyes with optic nerve section overshoot control levels and reverse the sign of the initial refractive error. These findings suggest that eye growth in chicks is controlled by an intraocular mechanism and possibly by a brain-mediated mechanism as well. The hypothesis that ocular accommodation is integral to the control of eye growth was also tested. Complete recovery from induced refractive errors was achieved even when accommodation had been abolished by lesions of the Edinger-Westphal nucleus. The elimination of accommodation did not prevent the ability of chick eyes to compensate for the defocus of spectacle lenses. These results suggest that accommodation is not necessary for the control of eye growth.

Accommodation, Ocular↗
PubMed1997

[Congenital nystagmus with sensory disorder: reaction to the caloric and optokinetic stimulation].

The purpose of this study was to estimate the response on vestibular caloric and optokinetic stimulation in patients with congenital nystagmus and serious congenital anomalies of visual system. 58 patients, aged 11-20 (21 female, 37 male) underwent this stimulation with ENG registration. The patients had visual acuity from 0.04 to 0.4, caused by visual system diseases e.g. congenital cataract, optic atrophy, hypoplasia maculae, high refractive error, albinism ect. The correct response was observed with 15.5% of the patients on vestibular caloric stimulation. There was no statistically significant relationship between visual acuity or visual system disease and the correct response. 28% of the patients responded correctly on optokinetic stimulation. There was statistically significant relationship between visual system anomaly and correct OKN: 80% of this reaction was in the group of optic atrophy, 54% in the group of high refractive error. Statistically significant relationship was no observed between visual acuiy and correct OKN. High level of the incorrect response on the caloric and optokinetic stimulation in the persons with congenital nystagmus and congenital disorder of visual system limits using of this tests for diagnostic purpose.

Caloric Tests↗
PubMed2005

Measurement of the optical parameters of purple membrane and plant light-harvesting complex films with optical waveguide lightmode spectroscopy.

Purple membrane (bacteriorhodopsin) and plant light-harvesting complexes (LHCII) were dried on the optical waveguide sensor with varying thicknesses in a wide range (from 20 to several hundreds of nanometers) and the optical parameters were studied with optical waveguide lightmode spectroscopy. It was found that applying the approximate 4-layer mode equations for the measured effective refractive indices resulted in unacceptable results for the optical parameters: with increasing thickness the refractive index decreased monotonously from 1.5 to 1.1. Therefore an inverse waveguide numerical method was developed and used to obtain reliable results from the experiments. The inverse method yielded an approximately constant (1.53) refractive index independently of the thickness for the purple membrane and LHCII films. Light-induced changes in the optical parameters of the purple membrane and LHCII films were also studied. For purple membrane films the most significant effect is the change in refractive index and absorption. For LHCII films prolonged illumination induced irreversible structural changes, most probably of thermo-optic origin.

Biosensing Techniques↗
PubMed2001

[Correlations between ocular biometry and post-surgical refraction].

PURPOSE: To show the importance of ocular biometry in establishing the power of intraocular lens. MATERIAL AND METHOD: We studied 212 eyes operated in the University Eye Hospital from Cluj-Napoca by planned extracapsular extraction with intraocular lens implantation. Before operation there was performed ocular biometry, using SRK II formulas for calculation of intraocular lens. At 6 weeks after operation, we checked the ocular refraction by dioptronometry and we established the optical correction of the patients. RESULTS: The standard error was statistically significantly in all cases. In 84.19% of cases it was performed visual acuity with optimal correction over 0.7. CONCLUSION: The ocular biometry represents significant factor in obtaining a very good visual acuity after cataract operation.

Adolescent↗
PubMedDate not supplied

Refractive index of the human corneal epithelium and stroma.

BACKGROUND: The refractive index of the cornea must be determined to optically perfect keratorefractive procedures. There are very few empirical measurements of the human corneal refractive index described in the literature. Throughout its depth, the cornea demonstrates regional variations in physiological properties such as swelling/de-swelling characteristics. These properties suggest there may be a difference in the refractive index between the anterior and posterior corneal surfaces. METHODS: The refractive index of the human corneal epithelium of 10 eyes was measured, in vivo, using a modified hand-held refractometer. The refractive indices of the anterior and posterior surfaces of the bare stroma of fresh human corneas were measured using a bench model Abbe refractometer. RESULTS: The mean refractive index of the epithelium, stromal anterior and posterior surfaces were 1.401 (SD +/- 0.005), 1.380 (SD +/- 0.005), and 1.373 (SD +/- 0.001) respectively. CONCLUSIONS: The refractive index of the cornea is not uniform. The calculated dioptric power of the corneal epithelium is approximately -1.40 diopters (D). The varying refractive index does not significantly affect the total dioptric power of the cornea. The varying refractive index of the cornea has the potential to significantly contribute to the overall optical performance of the eye in relation to refractive surgery. The results should be incorporated into mathematical models, comparing and contrasting the optical performance of the eye before and after surgery.

Adult↗
PubMed1991

Achromatizing the human eye.

Ocular chromatic dispersion manifests itself as wavelength-dependent image planes, image sizes, and image positions, and it has been suggested that ocular chromatic aberration is the most important of the eye's optical aberrations. Most attempts to correct for the eye's chromatic aberration (achromatize the human eye) have concentrated on correcting the wavelength-dependent image planes or chromatic difference of refractive error (CDRx). There are two optical techniques that correct for CDRx (special achromatizing lenses and multiple channel display systems) by making the ocular image planes of all wavelengths coincident. A different approach simply avoids the effects of ocular CDRx by using small pupils which effectively make all images diffraction-limited irrespective of wavelength-dependent differences in image planes. Theoretical and experimental evidence shows that achromatizing lenses provide an accurate correction for CDRx. In spite of the pre-eminence of chromatic aberrations, and the effectiveness of the corrections, no obvious improvements in vision accompany correction. We show that loss of retinal image quality due to CDRx may be subthreshold (less than ocular depth of focus). We also show that achromatizing methods can introduce their own chromatic aberrations that can easily exceed those present in the uncorrected eye. The precise location of the eye with respect to the achromatizing device determines the amount of these additional aberrations. Therefore, in order to achromatize the eye effectively, careful control of eye position is essential.

Color Perception↗
PubMed1997

[Measuring device for determining monochromatic aberration of the human eye].

BACKGROUND: After refractive or cataract surgery, ocular optical errors can occur that are not correctable with spherical or astigmatic lenses and are probably responsible for the fact that in many cases the best possible (retinal) acuity is not achieved in spite of an optimum refraction. Assessment of these aberrations in the clinical routine is an important first step towards documentation and correction of these errors with modern photorefractive methods. PRINCIPLE OF MEASURING: Ocular optical errors are assessed from the viewpoint of the wave property of light as an aberration of the real wavefront of a central retinal image point from the ideal spherical from (wavefront aberration). The measurement is based on the principle of the Tscherning aberroscope. RESULTS: A test setup is presented consisting of a laser system, a CCD fundus camera and a PC. A bundle of parallel equidistant rays is refracted by means of a lens in front of the eye producing an equivalent pattern of light spots on the retina. This pattern is more or less distorted according to the ocular aberrations. The deviations of all spots from their ideal (equidistant) positions are measured by means of the PC and from these values the wavefront aberration is computed in the form of Zernike and Taylor polynomials. The first results of emmetropic eyes are presented. CONCLUSIONS: The method allows sufficiently accurate assessment of the ocular wavefront aberration and might be on principle suitable for clinical use, provided that some technical improvements are installed.

Adult↗
PubMed1992

A method of surgical correction of myopia by fashioning a new optically active epithelial surface: keratoepitheliomileusis.

A new form of refractive keratoplasty for the correction of myopia was developed using a new approach to the problem. The midperipheral annular area of the recipient cornea was deepithelialized, and Bowman's layer was removed using a special bore coated with diamond crumbs. A cryolathed annular graft was placed on this prepared midperipheral area of the recipient cornea concentrically with its center. As a result of subsequent reepithelialization inside the transplanted ring, a regular optically active surface was formed. The refractive effect gradually appeared eight to ten weeks after surgery This new optically active surface acted as a lens, consisting of epithelial cells on top of the host cornea. The method required neither cutting of the stroma nor any manipulation in the optical zone of the cornea. A new term "keratoepitheliomileusis" was coined for this new procedure. Eleven patients (12 eyes) with high myopia ranging from 7.5 to 22.0D underwent keratoepitheliomileusis. The refractive and visual results found after four to 16 months of follow-up are presented. Emmetropic refraction occurred in three patients. In the other nine eyes, the deviation from emmetropia ranged from 0.75 to 2.5D. In all patients, uncorrected visual acuity was improved considerably (up to 0.3 to 1.OD) without loss in their best-corrected vision.

Adult↗
PubMedDate not supplied

In vivo birefringence and thickness measurements of the human retinal nerve fiber layer using polarization-sensitive optical coherence tomography.

Glaucoma causes damage of the nerve fiber layer, which may cause loss of retinal birefringence. Therefore, PS-OCT is a potentially useful technique for the early detection of glaucoma. We built a fiber-based PS-OCT setup that produces real-time images of the human retina in vivo, coregistered with retinal video images of the location of PS-OCT scans. Preliminary measurements of a healthy volunteer show that the double-pass phase retardation per unit of depth of the RNFL is not constant and varies with location, with values between 0.18 and 0.37 deg/microm. A trend in the preliminary measurements shows that the nerve fiber layer located inferior and superior to the optic nerve head is more birefringent than the thinner layer of nerve fiber tissue in the temporal and nasal regions.

Adult↗
PubMed1978

Visual acuity: the influence of refraction and diffraction and the use of interference fringes.

In summary, interference acuity provides an exceedingly useful technique for separating optical from retinal causes for poor vision. Opacities can cause gross disturbances of the fringe pattern, so failure in this test may or may not be caused by poor macular function. For patients who would be operated on only if indications of existing potential for vision could be obtained, this test is invaluable. We had several patients who would have been excluded from consideration for a cataract extraction but who were not because we were able to show good potential for acuity; postoperatively, they achieved acuities as predicted. The test succeeded in evaluating macular function in instances when the usual methods of testing for vision potential, such as two-point discrimination, electroretinography, and ultrasound, provided only crude estimates of retinal function. Although the apparatus required is simple-a small laser, a few optical components, and a patient chair-I am not sure how long it will take before ophthalmologists will have it available for office use. But I am optimistic about the possibilities offered by the widespread use of what I consider to be a promising new technique for evaluating macular function behind opacities of the ocular media.

Cataract↗
PubMed2006

Refractive index measurement for biomaterial samples by total internal reflection.

The refractive index of biological tissue is a fundamental parameter in applications of optical diagnosis and laser treatments. In the present work, the refractive indices and thermo-optic coefficients of some basic biomaterials, such as blood plasma, haemoglobin solution and lipid membrane, were studied by the method of total internal reflection at the wavelengths of 532 and 632.8 nm that are the most frequently used laser wavelengths in the biomedical field. The effects of the sample concentration and the temperature on refractive index were measured, and empirical relationships were summarized, accompanied by a theoretical explanation based on molecular polarization theory. The results provide some fundamental data for the refractive indices of these biomaterials under variant conditions, and also demonstrate that the total internal reflection method is a feasible and reliable way to measure the refractive indices of biological samples.

Biocompatible Materials↗
PubMed2006

Laser trapping of low refractive index colloids in a nematic liquid crystal.

We describe and analyze laser trapping of small colloidal particles in a nematic liquid crystal, where the index of refraction of colloids is smaller compared to the indices of the liquid crystal. Two mechanisms are identified that are responsible for this anomalous trapping: (i) below the optical Fréedericksz transition, the trapping is due to the anisotropic dielectric interaction of the polarized light with the inhomogeneous director field around the colloid, (ii) above the optical Fréedericksz transition, the optical trapping is accompanied by the elasticity-mediated interaction between the optically distorted region of a liquid crystal and the colloid. In the majority of the experiments, the trapping above the Fréedericksz transition is highly anisotropic. Qualitative agreement is found with a numerical analysis, considering the nematic director elastic distortion, dielectric director-light field coupling and optical repulsion due to low refraction index colloid in high index surroundings.

Journal Article↗
PubMed2006

Optical processing with photochromic switches.

The absorbance, fluorescence, and refractive index of a photochromic material can be modulated under the influence of optical stimulations. The reversible modification of these macroscopic properties is a result of photoinduced transformations at the molecular level. These processes can be exploited to mediate the interplay of optical signals and offer the opportunity to design and implement photonic devices for optical processing based on molecular components.

Journal Article↗
PubMed2003

On confocal Raman spectroscopy of semicrystalline polymers: the effect of optical scattering.

A series of semicrystalline polymers has been prepared through morphological control. Each of these has an identical refractive index but a different, well-defined, scattering behavior. From existing geometrical optical theories of confocal Raman spectroscopy, these materials should behave identically. Initially, the extent of scattering in each system was assessed quantitatively, from the near-infrared through the visible wavelength range, by UV/visible spectroscopy. The effect of optical scattering on the variation of intensity of the Raman scattered radiation with subsurface position was then examined in all four materials; the effect of surface roughness was also considered in the highest clarity system. Where surface effects are removed through careful sample preparation and the materials are interrogated using identical optical systems to mitigate against the impact of refractive index mismatch and other optical effects, the Raman response is strongly affected by the scattering characteristics of each material. A simple empirical relationship has been determined that adequately described all our specimens.

Journal Article↗