Evaluating the Reproducibility of Topography Systems on Spherical Surfaces

Comparison of Keratometric and Topographic Cylinder and Axis Measurements on Normal Corneas with Low Astigmatism

“…However, the CMS was less accurate for surfaces steeper than the normal corneas (50 D test ball). Comparison studies of accuracy between different models of videokeratography on calibrated spheres have also been performed by other investigators, with inconsistent results (3,4,6,12,13). The accuracy and precision performance depends on the shape of the measured surface, with decreasing accuracy on rapidly flattening surfaces for both central and peripheral radii of curvature (12).…”

“…There is no known mathematical formula that describes exactly the shape of the normal cornea; therefore the algorithm, whatever it is, gives an approximation of the corneal shape. The algorithms employed by the TMS-1 used in our study assume that the cornea is spherical, but algorithms that work well for spheres may not work adequately for an aspheric surface such as the human cornea (6). The keratometer measurements are also correct only when the surface examined is spherical or toroidal with the mires in the meridian planes of greatest or least curvature.…”

Comparison of Keratometric and Topographic Cylinder and Axis Measurements on Normal Corneas with Low Astigmatism

“…However, the CMS was less accurate for surfaces steeper than the normal corneas (50 D test ball). Comparison studies of accuracy between different models of videokeratography on calibrated spheres have also been performed by other investigators, with inconsistent results (3,4,6,12,13). The accuracy and precision performance depends on the shape of the measured surface, with decreasing accuracy on rapidly flattening surfaces for both central and peripheral radii of curvature (12).…”

“…There is no known mathematical formula that describes exactly the shape of the normal cornea; therefore the algorithm, whatever it is, gives an approximation of the corneal shape. The algorithms employed by the TMS-1 used in our study assume that the cornea is spherical, but algorithms that work well for spheres may not work adequately for an aspheric surface such as the human cornea (6). The keratometer measurements are also correct only when the surface examined is spherical or toroidal with the mires in the meridian planes of greatest or least curvature.…”

Diagnosis and Imaging of Corneal Astigmatism

“…The two systems gave similar results, except for a small advantage using the TMS-1 with abnormal corneas. The TMS-1 was slightly more repeatable in the central 0.6 mm than the EyeSys was, according to Maguire et al (1993). Applegate & Howland (1995) examined the accuracy of the TMS-1 in measuring "true surface topography", using elevation data from the instrument referenced to a plane perpendicular to the videokeratoscope axis, or calculating the elevation by integrating the slope of each sample point.…”

Contact lens fitting following unsuccessful refractive surgery

“…The variation from the theoretical distribution could be due to several factors including the intrinsic inaccuracy of mire‐based topography and keratometry with corneal distortion (Klyce, 2000). Also there may be errors associated with measures of corneal curvature at the (flat) extreme of the range of measurement found following refractive surgery as well as other measurement errors of focus and alignment (Maguire et al , 1993). Poor centration of the lens and an irregular tear pool could result in unpredictable tear lens powers, inducing further changes to the predicted contact lens BVP.…”

Comparison of Keratometric and Topographic Cylinder and Axis Measurements on Normal Corneas with Low Astigmatism

“…However, the CMS was less accurate for surfaces steeper than the normal corneas (50 D test ball). Comparison studies of accuracy between different models of videokeratography on calibrated spheres have also been performed by other investigators, with inconsistent results (3,4,6,12,13). The accuracy and precision performance depends on the shape of the measured surface, with decreasing accuracy on rapidly flattening surfaces for both central and peripheral radii of curvature (12).…”

“…There is no known mathematical formula that describes exactly the shape of the normal cornea; therefore the algorithm, whatever it is, gives an approximation of the corneal shape. The algorithms employed by the TMS-1 used in our study assume that the cornea is spherical, but algorithms that work well for spheres may not work adequately for an aspheric surface such as the human cornea (6). The keratometer measurements are also correct only when the surface examined is spherical or toroidal with the mires in the meridian planes of greatest or least curvature.…”

Diagnosis and Imaging of Corneal Astigmatism

“…The two systems gave similar results, except for a small advantage using the TMS-1 with abnormal corneas. The TMS-1 was slightly more repeatable in the central 0.6 mm than the EyeSys was, according to Maguire et al (1993). Applegate & Howland (1995) examined the accuracy of the TMS-1 in measuring "true surface topography", using elevation data from the instrument referenced to a plane perpendicular to the videokeratoscope axis, or calculating the elevation by integrating the slope of each sample point.…”

Contact lens fitting following unsuccessful refractive surgery

“…The variation from the theoretical distribution could be due to several factors including the intrinsic inaccuracy of mire‐based topography and keratometry with corneal distortion (Klyce, 2000). Also there may be errors associated with measures of corneal curvature at the (flat) extreme of the range of measurement found following refractive surgery as well as other measurement errors of focus and alignment (Maguire et al , 1993). Poor centration of the lens and an irregular tear pool could result in unpredictable tear lens powers, inducing further changes to the predicted contact lens BVP.…”

Comparison of Keratometric and Topographic Cylinder and Axis Measurements on Normal Corneas with Low Astigmatism

“…However, the CMS was less accurate for surfaces steeper than the normal corneas (50 D test ball). Comparison studies of accuracy between different models of videokeratography on calibrated spheres have also been performed by other investigators, with inconsistent results (3,4,6,12,13). The accuracy and precision performance depends on the shape of the measured surface, with decreasing accuracy on rapidly flattening surfaces for both central and peripheral radii of curvature (12).…”

“…There is no known mathematical formula that describes exactly the shape of the normal cornea; therefore the algorithm, whatever it is, gives an approximation of the corneal shape. The algorithms employed by the TMS-1 used in our study assume that the cornea is spherical, but algorithms that work well for spheres may not work adequately for an aspheric surface such as the human cornea (6). The keratometer measurements are also correct only when the surface examined is spherical or toroidal with the mires in the meridian planes of greatest or least curvature.…”

Diagnosis and Imaging of Corneal Astigmatism

“…The two systems gave similar results, except for a small advantage using the TMS-1 with abnormal corneas. The TMS-1 was slightly more repeatable in the central 0.6 mm than the EyeSys was, according to Maguire et al (1993). Applegate & Howland (1995) examined the accuracy of the TMS-1 in measuring "true surface topography", using elevation data from the instrument referenced to a plane perpendicular to the videokeratoscope axis, or calculating the elevation by integrating the slope of each sample point.…”

Contact lens fitting following unsuccessful refractive surgery

“…The variation from the theoretical distribution could be due to several factors including the intrinsic inaccuracy of mire‐based topography and keratometry with corneal distortion (Klyce, 2000). Also there may be errors associated with measures of corneal curvature at the (flat) extreme of the range of measurement found following refractive surgery as well as other measurement errors of focus and alignment (Maguire et al , 1993). Poor centration of the lens and an irregular tear pool could result in unpredictable tear lens powers, inducing further changes to the predicted contact lens BVP.…”