Search

Purpose: Photorefractive keratectomy (PRK) for hyperopia requires both a steepening of the central cornea and a flattening of the mid-periphery to achieve its effect and is likely to affect the optical aberrations of the eye., Methods: Nine patients underwent PRK to correct between +2.00 and +4.00 D of hyperopia (first eye treated for each patient) using the Summit Technology Apex Plus excimer laser. Anterior corneal aberrations for pupil diameters of 3, 5.5 and 7 mm were estimated from corneal topography data (TMS-1), assuming a uni-index, single surface cornea. Refractive error was assessed using retinoscopy and standard subjective tests., Results: Apart from the intended change in refraction (mean spherical equivalent manifest refraction, +4.60 +/- 1.60 D before surgery and +0.70 +/- 1.60 D at 1 year after surgery), the most significant change was in spherical aberration. Anterior corneal spherical aberration was positive (+1.60 +/- 0.60 D for a 5.5-mm pupil) before surgery and became negative after surgery (-1.80 +/- 1.20 D at 1 year). The change in spherical aberration was related to the achieved change in refractive error., Conclusions: The large change (approximately 3.00 D) in spherical aberration (from positive to negative aberration) has implications for the optical performance of the whole eye, where the effects of lenticular aberration must also be considered.

Background: Photorefractive keratectomy causes marked alteration to anterior corneal topography, and is likely to induce major changes to the optical aberrations of the eye., Methods: Six diopters (D) of myopia correction was attempted on one eye of 50 patients, randomly allocated to one of three different treatments: 5-mm or 6-mm single ablation zone, or a double ablation (multizone; -5.00 D correction over 4.6 mm and -1.00 D over 6 mm). Topographic data was used to estimate corneal aberration coefficients. These were compared for effect of ablation zone, before and 1 year after photorefractive keratectomy. The coefficients were used to derive modulation transfer functions for the anterior corneal surface., Results: Corneal spherical aberrations and coma-like aberrations both increased significantly following photorefractive keratectomy (p < 0.001). The mean spherical aberration coefficient increased from 0.36 +/- 0.11 before, to 0.91 +/- 0.37 after treatment, while the mean coma-like aberration coefficient changed from 0.28 +/- 0.16 before, to 0.60 +/- 0.31 after treatment. Ablation zone form had a significant effect on spherical aberration (p = 0.030), but not for coma (p = 0.96). The spherical aberration coefficient increased least for the 6-mm ablation (by 0.38 +/- 0.17), compared with the 5-mm ablation (0.69 +/- 0.45) and the multizone (0.62 +/- 0.38). Corneal modulation transfer functions were reduced significantly following the photorefractive procedure. The effect was greatest for large pupil diameters and for spatial frequencies between 2 and 15 cycles per degree., Conclusions: Corneal modulation transfer function calculations suggest that a significant loss of visual performance should be anticipated following photorefractive keratectomy, the effect being greatest for large pupil diameters. Results for three ablation zones show that induced aberrations are least for the largest (6 mm) ablation zone.

The purpose of this research was to determine if a clinically significant variation in corneal curvature occurs with the menstrual cycle. The EyeSys Corneal Analysis System (CAS) was used to measure corneal curvature of a test group of three normally menstruating female subjects over three cycles and compared with a control group of three males, for three pseudocycles of 28 days. Measurements were taken on days 1, 4, 8, 15 and 22 of each of three menstrual cycles, with four CAS images taken on each day. Four points (along 45, 135, 225 and 315 degrees) were selected for each of rings 1, 3, 6 and 8, and the average curvature for each of these rings was calculated. The results showed that no detectable temporal effect occurred with the menstrual cycle, thus demonstrating that either cyclic variation in corneal topography was too small to be identified by this system or it did not take place.

Background: The superposition of thin sphero-cylindrical lenses with arbitrary angles between cylindrical axes is sometimes required in a clinical setting. For example, it may be useful in some instances to perform an overrefraction which, added to the spectacle or toric contact lens in place on the eye, yields a final correcting lens., Methods: Computer calculations have been used to create contour graphs allowing a graphical solution to the problem of combining sphero-cylindrical lenses. Although conceptually strightforward, the method has not been presented before., Results: The use of contour graphs to superpose sphero-cylindrical lenses is shown by examples to be accurate and much simpler than calculational methods., Conclusions: Lacking a pre-programmed computer, the method of contour graphs offers a practical method of combining sphero-cylindrical lenses.

Purpose: To study age-related changes in the refractive index distribution of the human ocular lens., Methods: Biometric data collected on 48 eyes in subjects ranging in age from 19 to 31 years and 48 eyes in subjects ranging in age from 49 to 61 years allowed estimation of a single parameter related to the refractive index distribution of the crystalline lens. The authors selected a gradient index model of the lens characterized by a fixed index at the lens center, a somewhat lower fixed index at the surface, and a continuum of index values between center and surface depending on a single parameter, beta. This parameter was evaluated for each of the two age groups., Results: The distributions of the gradient index parameter beta for the two age groups were found to be statistically well separated. On average, the older group was found to have an index gradient that was flatter near the lens center and steeper near the surface, implying a lower refractive power of about 2 D for representative lens surface curvatures., Conclusions: It has been observed that surface curvatures and thicknesses of the ocular lens increase with age, whereas other ocular dimensions apparently do not change, implying a trend toward myopia. This trend has not been observed. The authors' results are consistent with and strongly in support of the hypothesis that subtle index changes in the aging lens compensate to a large extent for changes in surface curvatures.

Recent developments in measuring corneal topography make possible an improved understanding of corneal optics. It is shown, using a wavefront analysis, that optical quantities of interest follow from knowledge of sagittal depths of a cornea, and that sagittal depths are provided by or readily found from the output of commercially available instruments. The errors for normal corneas are expected to be quite small based on measurements on calibration spheres and ellipsoids. As an example the optical consequences of a peripheral asymmetry are estimated for a particular normal cornea.

Purpose: The authors describe a method for estimating the spheric aberration (SA) of the human crystalline lens in vivo., Methods: This method was based on a comparison of the measured change in contrast sensitivity (delta log (CSF)) of the eye and the calculated change in modulation transfer function of the cornea with undilated and dilated pupils. The method assumes that the total change in visual efficiency (delta log (CSF)) in these two states is caused entirely by the SA of the eye combined with the Stiles-Crawford effect., Results: Data are reported for 20 subjects (age range, 20-56 yr). SA for the whole eye was found to be positive and in the range 0-0.83 D (0-0.80 D using Stiles-Crawford apodization). The SA of the crystalline lens was generally negative and in the range 0 to -0.81 D (0 to -0.78 D using apodization)., Conclusions: The method gives results consistent with previous reports in the literature. Despite potential limitations, the technique offers a new method for estimating the SA of the human crystalline lens in vivo.

A method is proposed which may prove useful as an aid in the design of non-spherical surfaces with desired multifocal properties. More specifically, if an axially symmetrical surface is described by a polynomial then the coefficients of this polynomial can be found such that optical path lengths between given object and image points are nearly constant over desired portions of the surface. Three examples are given. One of the examples, a 'trifocal' surface, is compared to an aberration-free two-segment concentric bifocal contact lens.

Many questions remain concerning the sources of intraocular light scatter. One approach, inversion of the empirical disability glare function, implies that refractive-index fluctuations extend over distances of tens of micrometers. Randomly placed inhomogeneities of these sizes, such as protein aggregates, are ruled out. It is argued that refractive-index fluctuations associated with the lens fiber lattice are capable of contributing significantly to intraocular scatter and that they provide the necessary large spatial extent.

It has been observed that, after radial keratotomy (RK), uncorrected residual hyperopes may have better visual acuity than expected and that some presbyopes, corrected only for distance, have adequate near-vision. We offer a possible explanation for these observations by showing that the increased spherical aberration after RK may produce a second focus 1.5 diopters or more in front of the principal focus of the eye.

A technique is described to determine the change in image contrast as a result of the spherical aberration induced by the radial keratotomy procedure. The hypothesis that the loss in contrast sensitivity of the RK eye is due to the change in spherical aberration postsurgically, is found to be acceptable for some patients. However, this is not a sufficient explanation in all cases. This conclusion may be due to assumptions inherent in the technique derived for calculating spherical aberration or to other factors of importance in the post-RK eye.

It seems quite certain that the visual entoptic phenomenon called Haidinger's brushes is a consequence of dichroism of the macular pigment; this dichroism is usually ascribed to orientation of the pigment molecules. An alternative explanation of the dichroism, an explanation that requires no orientation of pigment molecules, investigated and shown to be feasible. Specifically, it is shown that form dichroism that is due to the structure of the Henle fiber layer can account for Haidinger's brushes if the macular pigment is almost entirely confined to the Henle layer, if individual fibers have an index about 5% higher than that of the interfiber medium, and if the fibers form a tightly packed array.

A Beer-Lambert analysis of fluorophotometric scans of ocular lenses has been carried out. It is shown that an assumption of a single flourophor allows us to calculate the distribution of this fluorophor along the central lens axis. For the great majority of lenses in our study, covering a wide range of ages, our results are in essential agreement with published results on excised lenses. For a small subset of lenses in the study the assumption of a single fluorophor is found to be unsatisfactory. In these cases a reasonable symmetry assumption allows us to estimate the distribution of an additional agent which removes light from the entering or exiting beams without contributing to the fluorescence. The fact that this agent is restricted to the lens nucleus and that it occurs in lenses in which nuclear changes are clinically evident suggests that it is an expression of nuclear cataract.

The crystalline lens makes a major contribution to intraocular light scatter, yet the responsible structural features have not been identified. A hypothesis is presented that implicates the fiber architecture of the ocular lens in small-angle scatter, this being the only part contributing to the associated reduction in vision. The main evidence supporting this hypothesis is a calculation of light scatter by a model lattice of fibers simulating the fiber lattice of the ocular lens. The results agree well with the angular dependence of measured intraocular scatter and, for reasonable parametric values, agree also with the measured magnitude.

The low-temperature 800-nm band absorption and circular dichroism spectra of the bacteriochlorophyll (Bchl) a-protein from Prosthecochloris aestuarii strain 2K are analyzed theoretically. These spectra show considerable structure that is attributed primarily to resonance (exciton) interactions among the lowest singlet transitions of the Bchl a molecules contained in each protein. We calculate these spectra from the known arrangement of the Bchl molecules in the protein. With the conventional assignment of the lowest singlet transition of Bchl a as Q(y) (y-polarized), agreement of calculated spectra with experiment is poor. All of our attempts, based on this conventional assignment, to improve the theoretical fits to absorption and circular dichroism spectra simultaneously are unsuccessful. However, by making the simple but unconventional assumption that the lowest singlet transition in each of the Bchl a molecules in each protein is x-polarized rather than y-polarized, we find good agreement between calculated and observed spectra. If these results are not fortuitous, they indicate that there is a systematic error in the protein structural model, that the conventional assignments of Bchl a transitions are incorrect, or that the protein environment provides a sufficiently strong perturbation to rotate the lowest singlet transition moment direction by approximately 90 degrees , presumably by changing the order of certain of the Bchl a orbitals.

A general expression for the form birefringence of a medium of parallel cylinders of arbitrary sizes and separations has been developed. It is necessary to assume only that the refractive index fluctuations through the medium are sufficiently small. This expression has been evaluated for a model of retinal nerve fiber layers. The birefringence of these layers has considerable potential as a diagnostic tool since it is expected to be sensitive to structural changes associated with retinal pathologies. Although accurate measurements of this birefringence have not yet been carried out they appear to be feasible or may soon be so.

The optical density for the noncataractous crystalline lens is written as a sum of two terms, each with a specific dependence on wavelength. The first term, proportional to 1/lambda 2, represents all light-scattering processes in the lens. The second term, assumed significant only for lambda less than or equal to 500 nm, accounts for absorption by lens pigments. By analyzing transmittance data on lenses of subjects aged 21 to 63 years, a spectrum for light absorption by lens pigment is derived and it is shown to be essentially the same for all of the lenses.