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Douglas G Horner - One of the best experts on this subject based on the ideXlab platform.

  • Power Vector analysis of the optical outcome of refractive surgery
    Journal of Cataract and Refractive Surgery, 2001
    Co-Authors: Larry N Thibos, Douglas G Horner
    Abstract:

    Abstract Purpose To demonstrate the Power Vector method of representing and analyzing spherocylindrical refractive errors. Setting School of Optometry, Indiana University, Bloomington, Indiana, USA. Methods Manifest and keratometric refractive errors were expressed as Power Vectors suitable for plotting as points in a 3-dimensional dioptric space. The 3 Cartesian coordinates ( x , y , z ) of each Power Vector correspond to the Powers of 3 lenses that, in combination, fulfill a refractive prescription: a spherical lens of Power M , a Jackson crossed cylinder of Power J 0 with axes at 90 degrees and 180 degrees, and a Jackson crossed cylinder of Power J 45 with axes at 45 degrees and 135 degrees. The Pythagorean length of the Power Vector, B , is a measure of overall blurring strength of a spherocylindrical lens or refractive error. Changes in refractive error due to surgery were computed by the ordinary rules of Vector subtraction. Results Frequency distributions of blur strength ( B ) clearly demonstrate the effectiveness of refractive surgery in reducing the overall blurring effect of uncorrected refractive error. Power Vector analysis also revealed a reduction in the astigmatic component of these refractive errors. Paired comparisons revealed that the change in manifest astigmatism due to surgery was well correlated with the change in keratometric astigmatism. Conclusions Power Vectors aid the visualization of complex changes in refractive error by tracing a trajectory in a uniform dioptric space. The Cartesian components of a Power Vector are mutually independent, which simplifies mathematical and statistical analysis of refractive errors. Power Vectors also provide a natural link to a more comprehensive optical description of ocular refractive imperfections in terms of wavefront aberration functions and their description by Zernike polynomials.

  • myopia progression in adolescent wearers of soft contact lenses and spectacles
    Optometry and Vision Science, 1999
    Co-Authors: Douglas G Horner, P S Soni, Thomas O Salmon, T S Swartz
    Abstract:

    The purpose of this 3-year, randomized clinical trial was to determine the difference in myopia progression in adolescents wearing soft contact lenses over a control group wearing spectacles. A total of 175 adolescents between the ages of 11 and 14 years were randomized into 2 groups, spectacle wearers and soft contact lens wearers. The main result was that the spherical equivalent change between the groups showed no clinical or statistically significant difference. However, when a Power Vector analysis was used, which uses all the refractive error data, a small but statistically significant (F test = 4.24, T2 = 17.35, p < 0.01) difference between the groups was found (i.e., the refractive error of the spectacle wearers had a slight increase in astigmatism). It can be concluded that soft contact lens wear does not lead to additional myopia progression in adolescents.

  • Power Vectors an application of fourier analysis to the description and statistical analysis of refractive error
    Optometry and Vision Science, 1997
    Co-Authors: Larry N Thibos, William Wheeler, Douglas G Horner
    Abstract:

    The description of sphero-cylinder lenses is approached from the viewpoint of Fourier analysis of the Power profile. It is shown that the familiar sine-squared law leads naturally to a Fourier series representation with exactly three Fourier coefficients, representing the natural parameters of a thin lens. The constant term corresponds to the mean spherical equivalent (MSE) Power, whereas the amplitude and phase of the harmonic correspond to the Power and axis of a Jackson cross-cylinder (JCC) lens, respectively. Expressing the Fourier series in rectangular form leads to the representation of an arbitrary sphero-cylinder lens as the sum of a spherical lens and two cross-cylinders, one at axis 0 degree and the other at axis 45 degrees. The Power of these three component lenses may be interpreted as (x,y,z) coordinates of a Vector representation of the Power profile. Advantages of this Power Vector representation of a sphero-cylinder lens for numerical and graphical analysis of optometric data are described for problems involving lens combinations, comparison of different lenses, and the statistical distribution of refractive errors.

Camps, Vicente J. - One of the best experts on this subject based on the ideXlab platform.

  • Comparative analysis of anterior corneal curvature and astigmatism measurements obtained with three different devices
    'Wiley', 2020
    Co-Authors: Molina-martín Ainhoa, Piñero, David P., Dolores De ,fez Saiz, Caballero, María T., Camps, Vicente J.
    Abstract:

    Background: The objective of this study was to compare the central corneal curvature and astigmatism measurements obtained with three different systems in healthy eyes and to assess the level of interchangeability between them. Methods: This was a comparative study examining 30 healthy eyes of 30 patients (age 15–53 years). A complete eye examination was performed in all cases including analysis of anterior corneal curvature and astigmatism with three devices: the colour‐LED topography system Cassini (i‐Optics) (CAS), the Scheimpflug‐based system Pentacam (Oculus Optikgeräte) (PTC) and the optical biometer IOL‐Master 500 (Carl Zeiss Meditec) (IOLM). Differences between devices in terms of curvature in the flattest (flat K) and steepest meridians (steep K) as well as in the magnitude (AST) and Power Vector components of astigmatism (J0 and J45) were evaluated. The interchangeability between devices was evaluated with the Bland–Altman method. Results: Statistically significant differences between devices were found in steep K and flat K (p 

  • Validation of posterior corneal curvature measurements with color light-emitting diode topography
    'SAGE Publications', 2020
    Co-Authors: Piñero, David P., Dolores De ,fez Saiz, Camps, Vicente J., García Llopis Celia, Caballero, María T.
    Abstract:

    Purpose: To evaluate the intrasession repeatability and validity of posterior corneal curvature and astigmatism measurements provided by a color light-emitting diode reflection topography system in healthy eyes. Methods: A total of 40 healthy eyes of 40 patients (age, 16–66 years) were enrolled. A complete eye examination was performed in all cases including posterior topographic analysis with two systems: the Scheimpflug-based system (Pentacam; Oculus Optikgeräte GmbH, Wetzlar, Germany) and the Cassini system (i-Optics; Ophthec, The Hague, The Netherlands). With this last system, three consecutive measurements were taken to assess the level of intrasession repeatability (within-subject standard deviation, Sw; intraclass correlation coefficient). The Bland & Altman analysis was used to evaluate the interchangeability of both devices. Results: The Sw was ⩽0.06 mm for all posterior corneal radius measurements, with intraclass correlation coefficient of ⩾0.960. The Sw for the magnitude of astigmatism, J0, and J45 were 0.15, 0.04, and 0.04 D, respectively, with intraclass correlation coefficient values of 0.876, 0.897, and 0.840, respectively. Statistically significant differences between devices were found in all parameters evaluated (p ⩽ 0.025). The interchangeability analysis revealed the presence of clinically relevant limits of agreement for the flattest (0.03 to 0.50 mm) and steepest posterior corneal radii (–0.01 to 0.39 mm). In contrast, limits of agreements were not clinically relevant for the magnitude of posterior astigmatism (–0.17 to 0.27 D) and their Power Vector components (–0.11 to 0.15 D). Conclusion: The Cassini system provides consistent measures of posterior corneal curvature and astigmatism in healthy eyes, but only measures of posterior astigmatism can be considered as interchangeable with those provided by the Pentacam.This study was supported by the Ministry of Economy, Industry and Competitiveness of Spain within the program Ramón y Cajal, RYC-2016-20471 (to D.P.P.)

  • A New Approach for the Calculation of Total Corneal Astigmatism Considering the Magnitude and Orientation of Posterior Corneal Astigmatism and Thickness
    Wolters Kluwer Health, 2018
    Co-Authors: Piñero, David P., Dolores De ,fez Saiz, Caballero Caballero, María Teresa, Nicolás Albujer, Juan Miguel, Camps, Vicente J.
    Abstract:

    Purpose: To evaluate a new method of calculation of total corneal astigmatism based on Gaussian optics and the Power design of a spherocylindrical lens (C) in the healthy eye and to compare it with keratometric (K) and Power Vector (PV) methods. Methods: A total of 92 healthy eyes of 92 patients (age, 17–65 years) were enrolled. Corneal astigmatism was calculated in all cases using K, PV, and our new approach C that considers the contribution of corneal thickness. An evaluation of the interchangeability of our new approach with the other 2 methods was performed using Bland–Altman analysis. Results: Statistically significant differences between methods were found in the magnitude of astigmatism (P < 0.001), with the highest values provided by K. These differences in the magnitude of astigmatism were clinically relevant when K and C were compared [limits of agreement (LoA), −0.40 to 0.62 D), but not for the comparison between PV and C (LoA, −0.03 to 0.01 D). Differences in the axis of astigmatism between methods did not reach statistical significance (P = 0.408). However, they were clinically relevant when comparing K and C (LoA, −5.48 to 15.68 degrees) but not for the comparison between PV and C (LoA, −1.68 to 1.42 degrees). Conclusions: The use of our new approach for the calculation of total corneal astigmatism provides astigmatic results comparable to the PV method, which suggests that the effect of pachymetry on total corneal astigmatism is minimal in healthy eyes

  • Intrasession repeatability of refractive and ocular aberrometric measurements obtained using a multidiagnostic device in healthy eyes
    Dove Press, 2017
    Co-Authors: Piñero, David P., López Navarro Alberto, Cabezos Juan Inmaculada, Dolores De ,fez Saiz, Caballero Caballero, María Teresa, Camps, Vicente J.
    Abstract:

    Purpose: To evaluate the intrasession repeatability of refractive and ocular aberrometric measurements obtained using a new multidiagnostic device in healthy eyes. Patients and methods: A total of 107 eyes of 107 patients, age ranging from 23 to 65 years, were enrolled in this study. A complete eye examination was performed in all eyes, including an ocular examination using the VX120 system. Three consecutive measurements were obtained using this device to assess the intrasession repeatability of different refractive and ocular aberrometric parameters. The within-subject standard deviation (Sw), intrasubject precision (1.96×Sw), and intraclass correlation coefficient (ICC) were calculated. Results: Sw and intrasubject precision for refractive data were below 0.12 and 0.20 D, respectively, in all cases. The ICC ranged from 0.947 for the J45 Power Vector component to 0.997 for the sphere. Concerning aberrometric measurements Sw and intrasubject precision values were below 0.05 µm and 0.10 µm, respectively. Likewise, the ICC ranged from 0.805 for the quadrafoil root mean square to 0.954 for the primary spherical aberration. Poor correlations were found between most of the refractive parameters and their Sw (–0.033≤r≤0.053, p≥0.064). Moderate and significant positive correlations were found between the magnitude of the aberrometric parameters evaluated and their Sw (r≥0.446, p

G D Marques - One of the best experts on this subject based on the ideXlab platform.

  • sensorless direct slip position estimator of a dfim based on the air gap pq Vector sensitivity study
    IEEE Transactions on Industrial Electronics, 2013
    Co-Authors: G D Marques, Duarte M Sousa
    Abstract:

    This paper presents a new sensorless method for the direct estimation of the slip position of the wound-rotor induction machine. The main purpose of this study is to implement the control of the doubly fed induction machine (DFIM). The method computes directly the slip position as the difference between the position of the air gap Power Vector (qe, pe) and the position of the measured rotor current. The method does not need any information about the stator or rotor fluxes, and so, it is only indirectly dependent of the flux dynamics. Because the slip position is directly obtained, it is not necessary the knowledge of the stator flux to implement field orientation. The method is sensitive to errors in the stator inductance parameter. A general analytic expression for the estimation error is presented. The sensitivity becomes important when the rotor current is small. To overcome the sensitivity problem, an open-loop on-line adjustment is proposed and tested with acceptable results. Experimental results confirm that the method is appropriate for the Vector control of the DFIM.

  • air gap Power Vector based sensorless method for dfig control without flux estimator
    IEEE Transactions on Industrial Electronics, 2011
    Co-Authors: G D Marques, Diana Sousa
    Abstract:

    A sensorless method for the direct estimation of the slip position of the wound-rotor induction machine is introduced, discussed, and experimentally validated in this paper. The slip position is necessary for the implementation of control by flux orientation. The method proposed is based on the phase comparison of an estimated air-gap Power Vector and the measured rotor currents in a common reference frame. The proposed method can be implemented in the rotor or in the field reference frames with a hysteresis or proportional-integral controller. The method is sensitive to the stator no-load active and reactive Powers, but this dependence is not very important. Simulation and experimental results show that the method is appropriate for the Vector control of the doubly fed induction machine. Unlike other methods, this method gives acceptable results when the load is small. In addition, it does not need a flux estimator for the implementation of flux orientation.

Piñero, David P. - One of the best experts on this subject based on the ideXlab platform.

  • Comparative analysis of anterior corneal curvature and astigmatism measurements obtained with three different devices
    'Wiley', 2020
    Co-Authors: Molina-martín Ainhoa, Piñero, David P., Dolores De ,fez Saiz, Caballero, María T., Camps, Vicente J.
    Abstract:

    Background: The objective of this study was to compare the central corneal curvature and astigmatism measurements obtained with three different systems in healthy eyes and to assess the level of interchangeability between them. Methods: This was a comparative study examining 30 healthy eyes of 30 patients (age 15–53 years). A complete eye examination was performed in all cases including analysis of anterior corneal curvature and astigmatism with three devices: the colour‐LED topography system Cassini (i‐Optics) (CAS), the Scheimpflug‐based system Pentacam (Oculus Optikgeräte) (PTC) and the optical biometer IOL‐Master 500 (Carl Zeiss Meditec) (IOLM). Differences between devices in terms of curvature in the flattest (flat K) and steepest meridians (steep K) as well as in the magnitude (AST) and Power Vector components of astigmatism (J0 and J45) were evaluated. The interchangeability between devices was evaluated with the Bland–Altman method. Results: Statistically significant differences between devices were found in steep K and flat K (p 

  • Validation of posterior corneal curvature measurements with color light-emitting diode topography
    'SAGE Publications', 2020
    Co-Authors: Piñero, David P., Dolores De ,fez Saiz, Camps, Vicente J., García Llopis Celia, Caballero, María T.
    Abstract:

    Purpose: To evaluate the intrasession repeatability and validity of posterior corneal curvature and astigmatism measurements provided by a color light-emitting diode reflection topography system in healthy eyes. Methods: A total of 40 healthy eyes of 40 patients (age, 16–66 years) were enrolled. A complete eye examination was performed in all cases including posterior topographic analysis with two systems: the Scheimpflug-based system (Pentacam; Oculus Optikgeräte GmbH, Wetzlar, Germany) and the Cassini system (i-Optics; Ophthec, The Hague, The Netherlands). With this last system, three consecutive measurements were taken to assess the level of intrasession repeatability (within-subject standard deviation, Sw; intraclass correlation coefficient). The Bland & Altman analysis was used to evaluate the interchangeability of both devices. Results: The Sw was ⩽0.06 mm for all posterior corneal radius measurements, with intraclass correlation coefficient of ⩾0.960. The Sw for the magnitude of astigmatism, J0, and J45 were 0.15, 0.04, and 0.04 D, respectively, with intraclass correlation coefficient values of 0.876, 0.897, and 0.840, respectively. Statistically significant differences between devices were found in all parameters evaluated (p ⩽ 0.025). The interchangeability analysis revealed the presence of clinically relevant limits of agreement for the flattest (0.03 to 0.50 mm) and steepest posterior corneal radii (–0.01 to 0.39 mm). In contrast, limits of agreements were not clinically relevant for the magnitude of posterior astigmatism (–0.17 to 0.27 D) and their Power Vector components (–0.11 to 0.15 D). Conclusion: The Cassini system provides consistent measures of posterior corneal curvature and astigmatism in healthy eyes, but only measures of posterior astigmatism can be considered as interchangeable with those provided by the Pentacam.This study was supported by the Ministry of Economy, Industry and Competitiveness of Spain within the program Ramón y Cajal, RYC-2016-20471 (to D.P.P.)

  • A New Approach for the Calculation of Total Corneal Astigmatism Considering the Magnitude and Orientation of Posterior Corneal Astigmatism and Thickness
    Wolters Kluwer Health, 2018
    Co-Authors: Piñero, David P., Dolores De ,fez Saiz, Caballero Caballero, María Teresa, Nicolás Albujer, Juan Miguel, Camps, Vicente J.
    Abstract:

    Purpose: To evaluate a new method of calculation of total corneal astigmatism based on Gaussian optics and the Power design of a spherocylindrical lens (C) in the healthy eye and to compare it with keratometric (K) and Power Vector (PV) methods. Methods: A total of 92 healthy eyes of 92 patients (age, 17–65 years) were enrolled. Corneal astigmatism was calculated in all cases using K, PV, and our new approach C that considers the contribution of corneal thickness. An evaluation of the interchangeability of our new approach with the other 2 methods was performed using Bland–Altman analysis. Results: Statistically significant differences between methods were found in the magnitude of astigmatism (P < 0.001), with the highest values provided by K. These differences in the magnitude of astigmatism were clinically relevant when K and C were compared [limits of agreement (LoA), −0.40 to 0.62 D), but not for the comparison between PV and C (LoA, −0.03 to 0.01 D). Differences in the axis of astigmatism between methods did not reach statistical significance (P = 0.408). However, they were clinically relevant when comparing K and C (LoA, −5.48 to 15.68 degrees) but not for the comparison between PV and C (LoA, −1.68 to 1.42 degrees). Conclusions: The use of our new approach for the calculation of total corneal astigmatism provides astigmatic results comparable to the PV method, which suggests that the effect of pachymetry on total corneal astigmatism is minimal in healthy eyes

  • Intrasession repeatability of refractive and ocular aberrometric measurements obtained using a multidiagnostic device in healthy eyes
    Dove Press, 2017
    Co-Authors: Piñero, David P., López Navarro Alberto, Cabezos Juan Inmaculada, Dolores De ,fez Saiz, Caballero Caballero, María Teresa, Camps, Vicente J.
    Abstract:

    Purpose: To evaluate the intrasession repeatability of refractive and ocular aberrometric measurements obtained using a new multidiagnostic device in healthy eyes. Patients and methods: A total of 107 eyes of 107 patients, age ranging from 23 to 65 years, were enrolled in this study. A complete eye examination was performed in all eyes, including an ocular examination using the VX120 system. Three consecutive measurements were obtained using this device to assess the intrasession repeatability of different refractive and ocular aberrometric parameters. The within-subject standard deviation (Sw), intrasubject precision (1.96×Sw), and intraclass correlation coefficient (ICC) were calculated. Results: Sw and intrasubject precision for refractive data were below 0.12 and 0.20 D, respectively, in all cases. The ICC ranged from 0.947 for the J45 Power Vector component to 0.997 for the sphere. Concerning aberrometric measurements Sw and intrasubject precision values were below 0.05 µm and 0.10 µm, respectively. Likewise, the ICC ranged from 0.805 for the quadrafoil root mean square to 0.954 for the primary spherical aberration. Poor correlations were found between most of the refractive parameters and their Sw (–0.033≤r≤0.053, p≥0.064). Moderate and significant positive correlations were found between the magnitude of the aberrometric parameters evaluated and their Sw (r≥0.446, p

Jornhon Liu - One of the best experts on this subject based on the ideXlab platform.

  • Power Vector analysis of refractive corneal and internal astigmatism in an elderly chinese population the shihpai eye study
    Investigative Ophthalmology & Visual Science, 2011
    Co-Authors: Yuchi Liu, Pesus Chou, Robert Wojciechowski, Peiyu Lin, Catherine Juiling Liu, Shihjen Chen, Jornhon Liu
    Abstract:

    PURPOSE: To investigate age-related trends in refractive, corneal, and internal astigmatism and to assess the association between internal astigmatism and lens opacity in an elderly Chinese population. METHODS: A population-based study was conducted among 1360 inhabitants aged 65 years and older in Taipei, Taiwan. Participants underwent measurements of refraction, corneal dioptric Power, and slit lamp biomicroscopy with lens grading. A total of 2084 eyes were included in Power Vector analyses of Cartesian astigmatism (J(0)) and oblique astigmatism (J(45)) components of refractive, corneal, and internal astigmatism. RESULTS: The crude prevalence of refractive astigmatism (defined as ≥0.75 diopters) was 73.0% based on the right eyes and 76.4% based on the left eyes. The Vector values in both refractive J(0) and corneal J(0) tended to be more negative with increasing age (P < 0.001), indicating the trend toward against-the-rule (ATR) astigmatism. Corneal J(0) alone accounted for 54% of the variability in refractive J(0). Refractive J(45) increased with age in the right eyes (P < 0.001) and decreased slightly with age in the left eyes (P = 0.012). Cortical opacity was associated with internal J(0) (P = 0.025), but the association was weak. CONCLUSIONS: Astigmatism affects approximately three quarters of the Chinese population aged 65 years and older in Taiwan. With increasing age, the prevalence of astigmatism increases, and refractive and corneal astigmatism shift toward ATR. Continuous corneal changes appear to be responsible for the age trend in refractive astigmatism. The severity of lens opacity plays only a minor role in the change of internal astigmatism.