Biomedical subjects

Charles E Campbell

Publications and source records attributed to Charles E Campbell.

9 recordsLinked to original sources

PubMed2006

Analysis of wavefront-guided corrections to see if they fully correct ocular aberrations.

In a recent paper [J. Opt. Soc. Am. A22, 1473 (2005)], the authors concluded, as a result of analysis using an optical model of the eye, that wavefront-guided ablative correction of the refractive errors of the eye does not efficiently remove those errors. An analytical method is given that analyzes the effect of a wavefront-guided ablative treatment of the aberrations of the eye using only the measured wavefront error of the eye and a measured or assumed anterior corneal surface shape and shows that in fact a wavefront-guided ablative treatment does remove all clinically significant error of the eye if only the anterior surface of the cornea is physically altered as planned as a result of treatment. The use of a variation of the method to analyze the effect of treatment when other changes occur as a result of treatment is also given.

Computer Simulation↗

PubMed2006

Generalized Coddington equations found via an operator method.

Generalized Coddington equations allow the optical properties of an arbitrarily oriented incoming astigmatic ray bundle to be found following refraction by an arbitrary surface. Generalized Coddington equations are developed using the abstract concept of vergence and refraction operators. After suitable incoming vergence and refraction operators have been formed, these operators are re-expressed in a common coordinate system via similarity transformations created from the series of space rotations necessary to align the coordinate systems. The transformed operators are then added together to produce the vergence operator of the refracted ray bundle. When properly applied, these generalized Coddington equations may be used with complex wavefronts and complex refracting surfaces if local surface curvature properties are known for both where the two intersect. The generalized Coddington equations are given in matrix form so that they may be easily implemented.

Journal Article↗

PubMed2005

Variable power phase lenses.

A method is given to construct a phase lens capable of creating an optical aberration of variable power that is described by a single Zernike polynomial function whose meridional index is 1 or greater. The phase lens is created from two identical phase elements, each creating a single Zernike aberration, that can be rotated with respect to each other, thereby increasing the aberration effect from zero to twice the value of either. This is possible because these aberrations are vectorlike. Results are given from the testing of an example that was manufactured and designed to produce coma (Zernike term Z(3,1)).

Journal Article↗

PubMed2003

Systematic underablation in laser in situ keratomileusis: ablation pattern identified by advanced topographical analysis.

Topographical analysis based on the differential geometry of surfaces-curvature topography-was developed and applied to a patient after laser in situ keratomileusis. The patient had a minimal residual refractive error and normal best corrected visual acuity but had multiple visual aberrations, including ghosting and glare, unless the pupils were maximally constricted. The corneal loci responsible for the aberrations were difficult or impossible to identify on axial topographies but were readily identified with curvature topography. The patient's ablations appeared to be miniature versions of the intended ablation profiles, with small areas of emmetropic central cornea surrounded by annuli of rapidly increasing keratometric power; that is, systematic underablation. This may explain why some patients have visual aberrations with pupil diameters smaller than the programmed optical zones.

Adult↗

PubMed2003

A new method for describing the aberrations of the eye using Zernike polynomials.

The standard Zernike polynomial functions are reformulated in a way so that the number of functions (or terms) needed to describe an arbitrary wavefront surface to a given Zernike radial order is reduced by a factor of approximately two, and the terms are described in a fashion quite similar to that used to describe common sphero-cylindrical errors of the eye. A wavefront is represented using these terms by assigning a pair of values, a magnitude and an axis, to all terms that are radially symmetric so that the individual aberrations are presented in a way similar to the way common astigmatism is currently given in terms of cylinder power and axis. The root mean square of these magnitudes gives the root mean square wavefront error just as does the root mean square of the standard Zernike coefficients. Formulas are given to convert standard Zernike coefficients to the magnitude and axis values.

Humans↗

PubMed2003

Matrix method to find a new set of Zernike coefficients from an original set when the aperture radius is changed.

A matrix method is developed that allows a new set of Zernike coefficients that describe a surface or wave front appropriate for a new aperture size to be found from an original set of Zernike coefficients that describe the same surface or wave front but use a different aperture size. The new set of coefficients, arranged as elements of a vector, is formed by multiplying the original set of coefficients, also arranged as elements of a vector, by a conversion matrix formed from powers of the ratio of the new to the original aperture and elements of a matrix that forms the weighting coefficients of the radial Zernike polynomial functions. In developing the method, a new matrix method for expressing Zernike polynomial functions is introduced and used. An algorithm is given for creating the conversion matrix along with computer code to implement the algorithm.

Journal Article↗

PubMed2003

System for the design, manufacture, and testing of custom lenses with known amounts of high-order aberrations.

Now that excimer laser systems can be programmed to correct complex aberrations of the eye on the basis of wave-front measurements, a method is needed to test the accuracy of the system from measurement through treatment. A closed-loop test method was developed to ensure that treatment plans generated by a wavefront measuring system were accurately transferred to and executed by the excimer laser. A surface was analytically defined, and a Shack-Hartmann-based wave-front system was used to formulate a treatment plan, which was downloaded to an excimer laser system. A plastic lens was ablated by the laser and then returned to the wave-front device, where it was measured and compared with the analytically defined wave-front surface. The two surfaces agreed up to 6th-order Zernike terms, validating the accuracy of the system.

Equipment Design↗

PubMedDate not supplied

Improving visual function diagnostic metrics with the use of higher-order aberration information from the eye.

PURPOSE: This paper reviews the currently used visual function diagnostic metrics, acuity, refractive error, and contrast sensitivity, and suggests ways to create new metrics using the information that has recently become available due to advances in measuring the higher-order aberrations of the eye. Particularly, emphasis is placed on finding metrics that address certain aspects of vision rather than on general metrics. METHODS: Two metrics based on the modulation transfer function are introduced, the Visual Quality Factor (VQF), which is a value based on the modulation transfer function between the spatial frequencies of 3 and 12 cycles per degree (c/deg) giving a measure of the overall degradation of visual quality due to aberrations, and the Subjective Sharpness Factor (SSF), which is a value based on the modulation transfer function between the spatial frequencies of 15 and 40 c/deg, giving a measure of the decrease in perceived image "sharpness" due to aberrations. Two metrics based on the point spread function are suggested, Point Spread Quality (PSQ), which is a measure of the "compactness" of the point spread, and Multiplicity Factor, which is a measure of the multiplicity of perceived images by measuring the number of discrete peaks in the point spread function. RESULTS: The VQF and SSF values for both monochromatic and polychromatic cases are analyzed with regard to the number of acuity letters lost using data from a published study. CONCLUSIONS: The SSF is found to be the best overall correlate with acuity performance if the degradation of the letter stimuli is not excessive.

Diagnostic Techniques, Ophthalmological↗

PubMedDate not supplied

A test eye for wavefront eye refractors.

PURPOSE: A multi-site study was conducted to test feasibility of a modified automatic refractor style test eye as a test device for wavefront refractors of various types and to determine whether a) they could be measured and b) when measurements could be made, to see if they were similar. This study did not attempt to assess which instrument most accurately measures the aberrations of the test eye or human eye. METHODS: Three automatic refractor style test eyes were modified for use as test devices for wavefront refractors. One had a simple spherical front surface, and two had additional aberrations added. The test eyes and holder were circulated to 11 test sites where attempts were made to measure them with eight different wave-front refractor systems. RESULTS: Eight (100%) of the eight wavefront refractor systems tested successfully measured the test eyes. The systems did not give similar results for the same test eye. In some cases, coma was reported where none was present. Differences in reported defocus values reflect different approaches for compensating for the dispersion of the eye. A corneal topography system could measure and recognize the aberrations of the test eyes as well as the wavefront refractor systems tested. Interferometry, on the other hand, did not prove to be a successful method to assess the surface of the test eyes. CONCLUSIONS: The test eye design may be used as a test device for wavefront refractor systems. This type of test eye can detect systematic differences between various wavefront refractors and can serve as a useful calibration and comparison tool.

Diagnostic Techniques, Ophthalmological↗