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At least 703 records · Page 39Linked to original sources
PubMedDate not supplied

Aberrations and visual performance following standard laser vision correction.

PURPOSE: To relate the change of ocular aberrations with the change of visual performance produced by standard laser in situ keratomileusis (LASIK) for myopia. METHODS: Aberrations and visual performance were measured before and after surgery in 22 eyes. Total aberrations were measured using a laser ray tracing technique. Corneal aberrations were obtained using a commercial videokeratoscope and custom software. Visual performance was evaluated in terms of best spectacle-corrected contrast sensitivity and high-contrast visual acuity. RESULTS: The amount of total and corneal aberrations increased with LASIK in all except two eyes. In general, the total and corneal aberration patterns were well-correlated after LASIK. However, the anterior corneal surface alone did not capture all the information (ie, possible changes induced on the posterior corneal surface or interactions between the different ocular components), which suggests that total aberrations are better predictors of visual performance. The decrease of the modulation transfer function (both in terms of area under the curve or as a function of spatial frequency) with LASIK accounted for most of the decrease in contrast sensitivity. High-resolution visual acuity was not very sensitive to the LASIK-induced changes in image quality. CONCLUSION: Wavefront and total aberration measurements, and in particular a combination of the two techniques, provide useful information for understanding the optical changes induced by standard refractive surgery. Predictions of the change in modulation transfer function derived from wave aberrations matched measurements of change in contrast sensitivity.

Contrast Sensitivity↗
PubMedDate not supplied

Strategic planning in topography-guided ablation of irregular astigmatism after laser refractive surgery.

PURPOSE: To identify an optimal customized ablation strategy in the treatment of eyes with secondary irregular astigmatism. METHODS: Corneal anterior surface elevation maps of 50 eyes with secondary irregular astigmatism after decentered laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK) and 50 virgin eyes were used for customized ablation simulations. Two ablation simulations with targeted postoperative surfaces perpendicular to either the visual or corneal morphological axis were made for each eye. All ablations were programmed for correction of corneal irregularities, including corneal astigmatism. The manifest refractive error was not corrected. Optical diameter was 6.5 mm and total diameter was 7.5 mm. Maximum ablation depths and maximum transition zone gradients were registered and analyzed. RESULTS: In eyes with secondary irregular astigmatism, mean maximum ablation depth was 48.21 +/- 25.96 microm and 26.31+/- 14.08 microm, whereas mean maximum transition zone gradient was 29.07 +/- 25.15 microm and 9.88 +/- 6.41 microm in ablation simulations based on the visual and corneal morphological axes, respectively. The difference between the ablation strategies was highly statistically significant for both parameters (P < .001). In virgin eyes, only a minor difference was noted between the visual and corneal morphological axis ablation simulations (P = .15 for maximum ablation depths and P=.19 for maximum transition zone gradient). CONCLUSIONS: In secondary irregular astigmatism, ablation based on the corneal morphological axis appears to minimize corneal tissue consumption and allows a smoother transition zone.

Adult↗
PubMed2000

[Study on the third-order nonlinear optical effect of fused-cyclic aromatic hydrocabon].

The third-order nonlinear optical susceptibility X(3), nonlinear refractive index n2 and time response property of 1,2,5,6,dibenzathracene (C22H14) and dibenzopentacene (C30H18) in fused-cyclic aromatic hydrocarbon were studied with forward degenerate four-wave mixing (DFWM). The mechanism for their larger nonlinear effect and faster time response was discussed.

English Abstract↗
PubMedDate not supplied

The corneal topography of epikeratophakia.

Epikeratophakia is a surgical technique initially developed for the correction of aphakia, and later adapted for the correction of keratoconus and myopia. The concurrent development of color-coded computerized corneal topography has greatly assisted in the evolution of epikeratophakia. Corneal topography has demonstrated the importance of both accurate centration of the epikeratophakia lenticle over the visual axis and the size of the optical zone on the final refractive results. Corneal topographic changes associated with correction of aphakia are primarily the result of steepening of the anterior radius of curvature of the cornea, in contrast to myopic correction which results in flattening of the anterior cornea. Correction of keratoconus results from a physical compression of the cone thereby flattening both the anterior and posterior radii of curvature. Preoperative topography in patients with keratoconus permits the precise position and extent of both regular and irregular astigmatism mation may assist in designing the optimal lenticle size and amount of graft decentration necessary to achieve the maximal surgical effect. Corneal topographic mapping has also been helpful in the management of postoperative astigmatism following epikeratophakia. Topography identifies the location and extent of both regular and irregular astigmatism which assists in planning of both surgical and nonsurgical intervention. Corneal topographic mapping has also been extremely helpful in the management of postoperative astigmatism following epikeratophakia. Topography identifies the location and extent of both regular and irregular astigmatism which assists in planning of both surgical and nonsurgical intervention.

Adult↗
PubMed1976

The early history of optometry in Maryland.

The progress of optometry in Maryland began with the passage of the first optometry law in 1914. Following its passage by the Maryland Legislature, the Maryland Optometric Association embarked on a plan to develop the profession by sponsoring (1) educational programs, (2) the gradual elimination of store-front practices, (3) the adoption of a code of ethics, and (4) establishing free eye clinics throughout the state. Optometry's steady climb toward professional status also resulted in the introduction of new optical instruments and techniques of refraction. The American Academy of Optometry and the educational courses of the Optometric Extension Program are credited for the impetus to professionalize optometry.

History, 20th Century↗
PubMed1993

[Cataract surgery using phacoemulsification].

Clinical results are reported in a consecutive series of 111 patients undergoing cataract surgery by phacoemulsification. On the first day after surgery, 69% of all cases had a visual acuity of 20/40 or better without any spectacle correction. Four months later this percentage had increased to 79%. Compared with a series of patients, operated by conventional extracapsular extraction, the corneal astigmatism was found to be significantly reduced in the phacoemulsification series (p < 0.001). The decrease in surgically induced astigmatism together with tailoring of the power of the intraocular lens (IOL) accounted for the good unaided visual acuity in the phacoemulsification group. Phacoemulsification is concluded to offer good optical control of the postoperative refraction in cataract patients.

Adult↗
PubMed1995

Correlation between presbyopia, age and number of births of mothers in the Kumasi area of Ghana.

There is an assumption in Ghana that motherhood has an effect on presbyopia. This study thus attempts to correlate the relationship between presbyopia, age and number of births of mothers in the Kumasi area of Ghana. The data were compiled from records of patients who attended routine eye examinations and refraction tests at the Morny Optical Centre, Kumasi, during the last quarter of 1993. Presbyopia in males and females, and those of mothers and childless female adults for particular age groups were compared. Correlations were found between age and reading additions for both sexes, and presbyopia occurs at a younger age than published. This preliminary study does not indicate any inter-dependence of births on female presbyopia. Further studies with select data may be required to confirm the above findings.

Adolescent↗
PubMed2003

Relationship between refractive error and monochromatic aberrations of the eye.

PURPOSE: To examine the relationship between ametropia and optical aberrations in a population of 200 normal human eyes with refractive errors spanning the range from +5.00 to -10.00 D. METHODS: Using a reduced-eye model of ametropia, we tested the hypothesis that the optical system of the eye is uncorrelated with the degree of ametropia. These predictions were evaluated experimentally with a Shack-Hartmann aberrometer that measured the monochromatic aberrations across the central 6 mm of the dilated pupil in well-corrected, cyclopleged eyes. RESULTS: Optical theory predicted, and control experiments on a model eye verified, that Shack-Hartmann measurements of spherical aberration will vary with axial elongation of the eye even if the dioptric components of the eye are fixed. Contrary to these predictions, spherical aberration was not significantly different from emmetropic eyes. Root mean square of third-order aberrations, fourth-order aberrations, and total higher aberrations (third to 10th) in myopic and hyperopic eyes were also uncorrelated with refractive error. Astigmatic eyes tended to have larger total higher-order aberrations than nonastigmatic eyes. CONCLUSIONS: We conclude that a reduced-eye model of myopia assuming fixed optical parameters and variable axial length is not tenable.

Accommodation, Ocular↗
PubMed2003

Wavelength-scanning interferometry of a transparent parallel plate with refractive-index dispersion.

Testing for flatness of an optical parallel plate in a Fizeau interferometer suffers from problems caused by multiple-beam interference noise. Each internal-reflection component can be separated from the signal by its modulation frequency in a wavelength-scanned interferometer; however, the frequency depends on the thickness and the refractive-index dispersion of the test plate and on the nonlinearity of the scanning source. With a new 19-sample wavelength-scanning algorithm we demonstrate the elimination of the reflection noise, the effect of the dispersion up to the second order of the reflectance of the test plate, and as the nonlinearity of the source. The algorithm permits large tolerance in the air-gap distance, thus making it somewhat independent of the thickness of the test plate. The minimum residual reflection noise with this algorithm for testing a glass plate is approximately lambda/600. Experimental results show that the front surface of the test plate was measured within 1 nm rms of its true shape over a 230-mm-diameter aperture.

Journal Article↗
PubMed2004

Three-dimensional photonic crystal of negative refraction achieved by interference lithography.

A three-dimensional bicontinuous photonic crystal of a bcc lattice with all-angle negative refraction that can be achieved at optical frequencies by interference lithography is proposed. A numerical simulation of the focusing imaging of a slab of this crystal was performed to verify the property of all-angle negative refraction. The dependence of the negative-refraction frequency range on the threshold of photoresist development and on the dielectric constant is also discussed.

Journal Article↗
PubMed1988

Optic disk morphometry in high myopia.

The optic nerve head in highly myopic eyes is distinctly different from normal optic disks. We performed magnification-corrected morphometry of photographs of 51 optic nerve heads in highly myopic eyes (myopic refraction of more than -8.00 diopters). Mean refraction was -15.49 +/- 5.76 diopters (range, -8.00 to 28.00 diopters), mean age 63.0 +/- 12.1 years (range, 27-87 years). The disks were significantly (P less than 0.000001; Wilcoxon-Mann-Whitney test) larger and more ovally configurated than 457 unselected normal optic nerve heads with a myopic refraction of less than -8.00 diopters. Refraction, size of the disk, and area of the parapapillary region with chorioretinal atrophy were significantly (P less than 0.00001) correlated with each other. The parapapillary vessel diameter was independent from the disk size. Highly myopic disks can be regarded as secondary acquired macrodisks, the size of which is correlated with refraction and possibly age. They should be differentiated from secondary, acquired macrodisks in congenital glaucoma and from primary macrodisks. As in normal eyes, the parapapillary vessel caliber can be used to estimate the optic disk size in relative and approximately absolute units.

Aged↗
PubMed2004

The average eye.

For statistical and other purposes one needs to be able to determine an average eye. An average of refractive errors is readily calculated as an average of dioptric power matrices. A refractive error, however, is not so much a property of the eye as a property of the compensating lens in front of the eye. As such, it ignores other aspects of the optical character of the eye. This paper discusses the difficulties of finding a suitable average that fully accounts for the first-order optics of a set of optical systems. It proposes an average based on ray transferences and logarithms and exponentials of matrices. Application to eyes in particular is discussed.

Eye↗
PubMed2005

The lens of the eye as a focusing device and its response to stress.

The continued peripheral growth of the lens, resulting in the concentration of older tissue toward the center, has the important optical consequence of producing a lens of variable refractive index. An approach consisting of the projection of fine laser beams through excised lenses in physiological solution has been used for in vitro study of lens optical quality. By varying the separation of the incident beams and/or the wavelength characteristics of the laser used, lens refractive properties and relative transparency may be examined. In the review provided, these optical properties are correlated to lens suture anatomy, lens mitochondrial morphology and function and the function of lens heat shock proteins. In addition, lens spherical aberration is evaluated as a function of accommodation. This work can be highlighted as follows: Mammalian lens suture morphology has a direct impact on lens optical function and, while suture structure of mammalian and avian lenses are very different, they both show an age-related deterioration in morphology and focusing ability. The distribution and appearance of mitochondria of the lens epithelium and superficial fiber cells are similar in all vertebrates. Lens mitochondrial integrity is correlated to lens focusing ability, suggesting a correlation between lens optical properties and lens metabolic function. The induction of cold cataract measured optically in cultured mammalian lenses is enhanced by thermal (heat) shock and this effect is prevented by inhibiting heat shock protein production. Finally, lens accommodative function can be studied by measuring lens refractive change using a physiological model involving an intact accommodative apparatus.

Animals↗
PubMed2001

[Fundamentals of wavefront-guided refractive corneal surgery].

The clinical introduction of aberration-sensing and wavefront-guided refractive surgery opens new ways in diagnostic and therapy of high order optical aberrations of the human eye. The main application is the optimization of current treatment methods such as LASIK or PRK. The aim of wavefront-guided treatments is to omit the deterioration of the optical performance under mesopic conditions of the eye by the refractive treatment and, if possible, to increase contrast sensitivity by correcting the higher order aberrations. First clinical trails on wavefront-guided corneal laser surgery demonstrate the possibility of improving the optical quality of the eye by LASIK or PRK and to correct irregular astigmatism.

Contrast Sensitivity↗
PubMed1987

Intraocular lens power calculations. A practical evaluation in normal subjects at the Wilmer Institute.

The practical value of preoperative intraocular lens power calculations in "normal" eyes with less than 4.50 diopters (D) of myopia or hyperopia was evaluated in a consecutive series of 520 eyes that underwent cataract extraction and lens implantation by four experienced surgeons at The Wilmer Ophthalmological Institute. The observed postoperative results with the lenses selected by the surgeons were compared with results calculated for the measurement-predicted emmetropic power lens and for a standard 20-D lens. The need for postoperative spectacle correction of residual refractive errors was comparable for all three choices of lens. Only two eyes (0.4%) would have developed greater than 4.00 D of refractive error with either the "implanted" or "predicted-emmetropic" lenses, as would 11 eyes (2.1%) with the "standard" 20-D lens. The surgeons' deviation from the calculated emmetropic lens did not reduce postoperative refractive error.

Aged↗