The Experts below are selected from a list of 90 Experts worldwide ranked by ideXlab platform
James M. Coggins - One of the best experts on this subject based on the ideXlab platform.
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height measurement of astigmatic test surfaces by a keratoscope that uses plane geometry surface reconstruction
American Journal of Ophthalmology, 1996Co-Authors: Nancy K. Tripoli, James M. Coggins, Kenneth L. Cohen, Pritvinath Obla, Douglas E. HolmgrenAbstract:Purpose To assess the accuracy with which the Keratron keratoscope (Optikon 2000, Rome, Italy) measured astigmatic test surfaces by a profile reconstruction algorithm within a plane geometry model and to discriminate between error caused by the model and error caused by other factors. Methods Height was reported by the Keratron for eight surfaces with central astigmatism ranging from 4 to 16 diopters. A three-dimensional ray tracing simulation produced theoretic reflected ring patterns on which the Keratron's reconstruction algorithm was performed. The Keratron's measurements were compared with the surfaces' formulas and the ray-traced simulations. Results With a new mathematical filter for smoothing ring data, now part of the Keratron's software, maximum error was 0.47% of the total height and was usually less than 1% of local power for surfaces with 4 diopters of astigmatism. For surfaces with 16 diopters of astigmatism, maximum error was as high as 2.9% of total height and was usually less than 2.5% of local power. The reconstruction algorithm accounted for 40% and 70% of height error, respectively. Conclusions The efficacy of keratoscopes cannot be assumed from their design theories but must be tested. Although plane geometry surface reconstruction contributed greatly to total height error, total error was so small that it is unlikely to affect clinical use.
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assessment of radial aspheres by the arc step algorithm as implemented by the keratron keratoscope
American Journal of Ophthalmology, 1995Co-Authors: Nancy K. Tripoli, Douglas E. Holmgren, Kenneth L. Cohen, James M. CogginsAbstract:Purpose To assess the accuracy with which the Keratron (Optikon 2000, Rome, Italy) measured rotationally symmetric, radially aspheric test surfaces according to an arc-step profile reconstruction algorithm and to discriminate between error caused by the algorithm and error from other sources. Methods Height, local power, and axial power calculated from radius of curvature centered on the instrument's axis were reported by the Keratron for four surfaces that had radial profiles similar to normal corneas. The Keratron profile reconstruction algorithm was simulated by using ray tracing. Keratron measurements were compared with the surfaces' formulas and the ray-traced simulations. Results The heights reported by the Keratron were within 0.25 μm from the four surfaces at less than 3 mm from the keratoscope axis and generally within 1 μm of the height calculated from the surfaces' formulas. The Keratron's axial powers were within ±0.1 diopter of the simulation of the axial solution between 1 and 4 mm of the axis but were greater central to 1 mm and peripheral to 4 mm. The Keratron's local powers were within −0.25 diopters at less than 4 mm from the axis and peripherally were between +1.75 diopters and −0.75 diopter of power calculated from the surface's instantaneous radii of curvature. Height error because of the arc-step algorithm was less than −0.2 μm. Conclusions The Keratron's arc-step profile reconstruction algorithm contributed to its ability to measure height more accurately than keratoscopes that use spherically biased algorithms and provided measurement of local power.
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Assessment of the Power and Height of Radial Aspheres Reported by a Computer-assisted Keratoscope
American Journal of Ophthalmology, 1995Co-Authors: Kenneth L. Cohen, Douglas E. Holmgren, Nancy K. Tripoli, James M. CogginsAbstract:Purpose The two purposes of this study were (a) to assess the accuracy with which a keratoscope, the Topographic Modeling System (TMS-1), calculated the heights and powers of rotationally symmetric, radially aspheric test surfaces and (b) to determine whether the TMS-1 used an axial solution for radius of curvature to determine the power of a sphere that would produce the same semichord as would the test surface on a keratograph. Methods The TMS-1 heights and powers were studied for four test surfaces that had radial profiles similar to those of normal corneas. The powers of the surfaces were calculated from the local radius of curvature derived from the surfaces' manufacturing formulas. The heights and powers that would result from an axial solution were calculated in a TMS-1 simulator. TMS-1 data were compared with data from the surfaces' formulas and with data from the simulation. Results The TMS-1 data were almost identical to the heights and powers calculated from the simulated axial solution. The TMS-1 data were similar to the heights and powers calculated from the mathematical formulas from the apex to 2 mm from the apex but differed by up to 85 μm of height and 10 diopters of power in the periphery. Conclusions The TMS-1 appeared to use the axial solution that does not calculate power from local radius of curvature. Clinicians should use caution when inferring corneal shape from power maps based on an axial solution, especially outside the central 2-mm radius of a normal cornea, because such power does not depict corneal curvature.
Kenneth L. Cohen - One of the best experts on this subject based on the ideXlab platform.
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height measurement of astigmatic test surfaces by a keratoscope that uses plane geometry surface reconstruction
American Journal of Ophthalmology, 1996Co-Authors: Nancy K. Tripoli, James M. Coggins, Kenneth L. Cohen, Pritvinath Obla, Douglas E. HolmgrenAbstract:Purpose To assess the accuracy with which the Keratron keratoscope (Optikon 2000, Rome, Italy) measured astigmatic test surfaces by a profile reconstruction algorithm within a plane geometry model and to discriminate between error caused by the model and error caused by other factors. Methods Height was reported by the Keratron for eight surfaces with central astigmatism ranging from 4 to 16 diopters. A three-dimensional ray tracing simulation produced theoretic reflected ring patterns on which the Keratron's reconstruction algorithm was performed. The Keratron's measurements were compared with the surfaces' formulas and the ray-traced simulations. Results With a new mathematical filter for smoothing ring data, now part of the Keratron's software, maximum error was 0.47% of the total height and was usually less than 1% of local power for surfaces with 4 diopters of astigmatism. For surfaces with 16 diopters of astigmatism, maximum error was as high as 2.9% of total height and was usually less than 2.5% of local power. The reconstruction algorithm accounted for 40% and 70% of height error, respectively. Conclusions The efficacy of keratoscopes cannot be assumed from their design theories but must be tested. Although plane geometry surface reconstruction contributed greatly to total height error, total error was so small that it is unlikely to affect clinical use.
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assessment of radial aspheres by the arc step algorithm as implemented by the keratron keratoscope
American Journal of Ophthalmology, 1995Co-Authors: Nancy K. Tripoli, Douglas E. Holmgren, Kenneth L. Cohen, James M. CogginsAbstract:Purpose To assess the accuracy with which the Keratron (Optikon 2000, Rome, Italy) measured rotationally symmetric, radially aspheric test surfaces according to an arc-step profile reconstruction algorithm and to discriminate between error caused by the algorithm and error from other sources. Methods Height, local power, and axial power calculated from radius of curvature centered on the instrument's axis were reported by the Keratron for four surfaces that had radial profiles similar to normal corneas. The Keratron profile reconstruction algorithm was simulated by using ray tracing. Keratron measurements were compared with the surfaces' formulas and the ray-traced simulations. Results The heights reported by the Keratron were within 0.25 μm from the four surfaces at less than 3 mm from the keratoscope axis and generally within 1 μm of the height calculated from the surfaces' formulas. The Keratron's axial powers were within ±0.1 diopter of the simulation of the axial solution between 1 and 4 mm of the axis but were greater central to 1 mm and peripheral to 4 mm. The Keratron's local powers were within −0.25 diopters at less than 4 mm from the axis and peripherally were between +1.75 diopters and −0.75 diopter of power calculated from the surface's instantaneous radii of curvature. Height error because of the arc-step algorithm was less than −0.2 μm. Conclusions The Keratron's arc-step profile reconstruction algorithm contributed to its ability to measure height more accurately than keratoscopes that use spherically biased algorithms and provided measurement of local power.
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Assessment of the Power and Height of Radial Aspheres Reported by a Computer-assisted Keratoscope
American Journal of Ophthalmology, 1995Co-Authors: Kenneth L. Cohen, Douglas E. Holmgren, Nancy K. Tripoli, James M. CogginsAbstract:Purpose The two purposes of this study were (a) to assess the accuracy with which a keratoscope, the Topographic Modeling System (TMS-1), calculated the heights and powers of rotationally symmetric, radially aspheric test surfaces and (b) to determine whether the TMS-1 used an axial solution for radius of curvature to determine the power of a sphere that would produce the same semichord as would the test surface on a keratograph. Methods The TMS-1 heights and powers were studied for four test surfaces that had radial profiles similar to those of normal corneas. The powers of the surfaces were calculated from the local radius of curvature derived from the surfaces' manufacturing formulas. The heights and powers that would result from an axial solution were calculated in a TMS-1 simulator. TMS-1 data were compared with data from the surfaces' formulas and with data from the simulation. Results The TMS-1 data were almost identical to the heights and powers calculated from the simulated axial solution. The TMS-1 data were similar to the heights and powers calculated from the mathematical formulas from the apex to 2 mm from the apex but differed by up to 85 μm of height and 10 diopters of power in the periphery. Conclusions The TMS-1 appeared to use the axial solution that does not calculate power from local radius of curvature. Clinicians should use caution when inferring corneal shape from power maps based on an axial solution, especially outside the central 2-mm radius of a normal cornea, because such power does not depict corneal curvature.
Stephen D. Klyce - One of the best experts on this subject based on the ideXlab platform.
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advances in the analysis of corneal topography
Survey of Ophthalmology, 1991Co-Authors: Steven E Wilson, Stephen D. KlyceAbstract:Recent advances in topographic analysis have provided powerful tools for detecting subtle, but clinically significant, alterations of corneal contour. This article compares keratometry, Keratoscopy, and computer-assisted topographic analysis and provides specific examples of the sensitivity of computer-assisted systems in revealing topographic alterations that were not previously discernable. Quantitative descriptors of corneal topography such as the surface asymmetry index, the surface regularity index, and simulated keratometry value augment the information provided by color-coded topographic maps.
Nancy K. Tripoli - One of the best experts on this subject based on the ideXlab platform.
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height measurement of astigmatic test surfaces by a keratoscope that uses plane geometry surface reconstruction
American Journal of Ophthalmology, 1996Co-Authors: Nancy K. Tripoli, James M. Coggins, Kenneth L. Cohen, Pritvinath Obla, Douglas E. HolmgrenAbstract:Purpose To assess the accuracy with which the Keratron keratoscope (Optikon 2000, Rome, Italy) measured astigmatic test surfaces by a profile reconstruction algorithm within a plane geometry model and to discriminate between error caused by the model and error caused by other factors. Methods Height was reported by the Keratron for eight surfaces with central astigmatism ranging from 4 to 16 diopters. A three-dimensional ray tracing simulation produced theoretic reflected ring patterns on which the Keratron's reconstruction algorithm was performed. The Keratron's measurements were compared with the surfaces' formulas and the ray-traced simulations. Results With a new mathematical filter for smoothing ring data, now part of the Keratron's software, maximum error was 0.47% of the total height and was usually less than 1% of local power for surfaces with 4 diopters of astigmatism. For surfaces with 16 diopters of astigmatism, maximum error was as high as 2.9% of total height and was usually less than 2.5% of local power. The reconstruction algorithm accounted for 40% and 70% of height error, respectively. Conclusions The efficacy of keratoscopes cannot be assumed from their design theories but must be tested. Although plane geometry surface reconstruction contributed greatly to total height error, total error was so small that it is unlikely to affect clinical use.
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assessment of radial aspheres by the arc step algorithm as implemented by the keratron keratoscope
American Journal of Ophthalmology, 1995Co-Authors: Nancy K. Tripoli, Douglas E. Holmgren, Kenneth L. Cohen, James M. CogginsAbstract:Purpose To assess the accuracy with which the Keratron (Optikon 2000, Rome, Italy) measured rotationally symmetric, radially aspheric test surfaces according to an arc-step profile reconstruction algorithm and to discriminate between error caused by the algorithm and error from other sources. Methods Height, local power, and axial power calculated from radius of curvature centered on the instrument's axis were reported by the Keratron for four surfaces that had radial profiles similar to normal corneas. The Keratron profile reconstruction algorithm was simulated by using ray tracing. Keratron measurements were compared with the surfaces' formulas and the ray-traced simulations. Results The heights reported by the Keratron were within 0.25 μm from the four surfaces at less than 3 mm from the keratoscope axis and generally within 1 μm of the height calculated from the surfaces' formulas. The Keratron's axial powers were within ±0.1 diopter of the simulation of the axial solution between 1 and 4 mm of the axis but were greater central to 1 mm and peripheral to 4 mm. The Keratron's local powers were within −0.25 diopters at less than 4 mm from the axis and peripherally were between +1.75 diopters and −0.75 diopter of power calculated from the surface's instantaneous radii of curvature. Height error because of the arc-step algorithm was less than −0.2 μm. Conclusions The Keratron's arc-step profile reconstruction algorithm contributed to its ability to measure height more accurately than keratoscopes that use spherically biased algorithms and provided measurement of local power.
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Assessment of the Power and Height of Radial Aspheres Reported by a Computer-assisted Keratoscope
American Journal of Ophthalmology, 1995Co-Authors: Kenneth L. Cohen, Douglas E. Holmgren, Nancy K. Tripoli, James M. CogginsAbstract:Purpose The two purposes of this study were (a) to assess the accuracy with which a keratoscope, the Topographic Modeling System (TMS-1), calculated the heights and powers of rotationally symmetric, radially aspheric test surfaces and (b) to determine whether the TMS-1 used an axial solution for radius of curvature to determine the power of a sphere that would produce the same semichord as would the test surface on a keratograph. Methods The TMS-1 heights and powers were studied for four test surfaces that had radial profiles similar to those of normal corneas. The powers of the surfaces were calculated from the local radius of curvature derived from the surfaces' manufacturing formulas. The heights and powers that would result from an axial solution were calculated in a TMS-1 simulator. TMS-1 data were compared with data from the surfaces' formulas and with data from the simulation. Results The TMS-1 data were almost identical to the heights and powers calculated from the simulated axial solution. The TMS-1 data were similar to the heights and powers calculated from the mathematical formulas from the apex to 2 mm from the apex but differed by up to 85 μm of height and 10 diopters of power in the periphery. Conclusions The TMS-1 appeared to use the axial solution that does not calculate power from local radius of curvature. Clinicians should use caution when inferring corneal shape from power maps based on an axial solution, especially outside the central 2-mm radius of a normal cornea, because such power does not depict corneal curvature.
Douglas E. Holmgren - One of the best experts on this subject based on the ideXlab platform.
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height measurement of astigmatic test surfaces by a keratoscope that uses plane geometry surface reconstruction
American Journal of Ophthalmology, 1996Co-Authors: Nancy K. Tripoli, James M. Coggins, Kenneth L. Cohen, Pritvinath Obla, Douglas E. HolmgrenAbstract:Purpose To assess the accuracy with which the Keratron keratoscope (Optikon 2000, Rome, Italy) measured astigmatic test surfaces by a profile reconstruction algorithm within a plane geometry model and to discriminate between error caused by the model and error caused by other factors. Methods Height was reported by the Keratron for eight surfaces with central astigmatism ranging from 4 to 16 diopters. A three-dimensional ray tracing simulation produced theoretic reflected ring patterns on which the Keratron's reconstruction algorithm was performed. The Keratron's measurements were compared with the surfaces' formulas and the ray-traced simulations. Results With a new mathematical filter for smoothing ring data, now part of the Keratron's software, maximum error was 0.47% of the total height and was usually less than 1% of local power for surfaces with 4 diopters of astigmatism. For surfaces with 16 diopters of astigmatism, maximum error was as high as 2.9% of total height and was usually less than 2.5% of local power. The reconstruction algorithm accounted for 40% and 70% of height error, respectively. Conclusions The efficacy of keratoscopes cannot be assumed from their design theories but must be tested. Although plane geometry surface reconstruction contributed greatly to total height error, total error was so small that it is unlikely to affect clinical use.
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assessment of radial aspheres by the arc step algorithm as implemented by the keratron keratoscope
American Journal of Ophthalmology, 1995Co-Authors: Nancy K. Tripoli, Douglas E. Holmgren, Kenneth L. Cohen, James M. CogginsAbstract:Purpose To assess the accuracy with which the Keratron (Optikon 2000, Rome, Italy) measured rotationally symmetric, radially aspheric test surfaces according to an arc-step profile reconstruction algorithm and to discriminate between error caused by the algorithm and error from other sources. Methods Height, local power, and axial power calculated from radius of curvature centered on the instrument's axis were reported by the Keratron for four surfaces that had radial profiles similar to normal corneas. The Keratron profile reconstruction algorithm was simulated by using ray tracing. Keratron measurements were compared with the surfaces' formulas and the ray-traced simulations. Results The heights reported by the Keratron were within 0.25 μm from the four surfaces at less than 3 mm from the keratoscope axis and generally within 1 μm of the height calculated from the surfaces' formulas. The Keratron's axial powers were within ±0.1 diopter of the simulation of the axial solution between 1 and 4 mm of the axis but were greater central to 1 mm and peripheral to 4 mm. The Keratron's local powers were within −0.25 diopters at less than 4 mm from the axis and peripherally were between +1.75 diopters and −0.75 diopter of power calculated from the surface's instantaneous radii of curvature. Height error because of the arc-step algorithm was less than −0.2 μm. Conclusions The Keratron's arc-step profile reconstruction algorithm contributed to its ability to measure height more accurately than keratoscopes that use spherically biased algorithms and provided measurement of local power.
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Assessment of the Power and Height of Radial Aspheres Reported by a Computer-assisted Keratoscope
American Journal of Ophthalmology, 1995Co-Authors: Kenneth L. Cohen, Douglas E. Holmgren, Nancy K. Tripoli, James M. CogginsAbstract:Purpose The two purposes of this study were (a) to assess the accuracy with which a keratoscope, the Topographic Modeling System (TMS-1), calculated the heights and powers of rotationally symmetric, radially aspheric test surfaces and (b) to determine whether the TMS-1 used an axial solution for radius of curvature to determine the power of a sphere that would produce the same semichord as would the test surface on a keratograph. Methods The TMS-1 heights and powers were studied for four test surfaces that had radial profiles similar to those of normal corneas. The powers of the surfaces were calculated from the local radius of curvature derived from the surfaces' manufacturing formulas. The heights and powers that would result from an axial solution were calculated in a TMS-1 simulator. TMS-1 data were compared with data from the surfaces' formulas and with data from the simulation. Results The TMS-1 data were almost identical to the heights and powers calculated from the simulated axial solution. The TMS-1 data were similar to the heights and powers calculated from the mathematical formulas from the apex to 2 mm from the apex but differed by up to 85 μm of height and 10 diopters of power in the periphery. Conclusions The TMS-1 appeared to use the axial solution that does not calculate power from local radius of curvature. Clinicians should use caution when inferring corneal shape from power maps based on an axial solution, especially outside the central 2-mm radius of a normal cornea, because such power does not depict corneal curvature.
