Validation of corneal topographic and aberrometric measurements obtained by color light-emitting diode reflection topography in healthy eyes.

PURPOSE: To evaluate the intrasession repeatability of anterior corneal topographic and aberrometric measurements provided by a color-LED topographer as well as their interchangeability with those provided by a Scheimpflug-based system in healthy eyes. METHODS: Thirty-five healthy eyes of 35 patients (age, 16-66 years) were enrolled. A complete eye examination was performed in all cases including a complete corneal analysis with the Scheimpflug-based system Pentacam (Oculus Optikgeräte) (one measurement) and the Cassini system (i-Optics) (three consecutive measurements). Intrasession repeatability of the Cassini measurements was assessed with the within-subject standard deviation (Sw) and the intraclass correlation coefficient (ICC). The Bland-Altman analysis was used to evaluate the agreement between both devices. RESULTS: Mean Sw for keratometric readings was 0.02 mm (ICC ≥ 0.992), ranging between 0.16 and 0.05 D (ICC 0.930-0.978) for anterior and total astigmatic measurements. Mean Sw for asphericity and corneal diameter were 0.06 (ICC 0.926) and 0.03 mm (IC 0.997), respectively. Aberrometric parameters showed ICCs ≥ 0.816, except for Z42 (ICC 0.741) and Z44 (ICC 0.544). When comparing devices, statistically significant differences were found for most of topographic and aberrometric data (p ≤ 0.044). Likewise, ranges of agreement between devices were clinically relevant (keratometry > 0.06 mm; total astigmatic components > 0.69 D; asphericity 0.35; second-, third-, and fourth-order Zernike terms, more than 0.20, 0.13, and 0.01 µm, respectively). CONCLUSIONS: Consistent anterior corneal topographic, total corneal astigmatic, and aberrometric measurements are obtained with color-LED topography in healthy eyes, which are not interchangeable with those provided by the Scheimpflug-based topography.

Can Applications Designed to Evaluate Visual Function Be Used in Different iPads?

SIGNIFICANCE: Apple devices could be suitable for vision tests, provided that the test has been correctly adapted to the device, after considering the spatial and colorimetric characterization of the screen. PURPOSE: The majority of vision applications has not been developed by vision or colorimetry experts and suffers from conceptual and design errors that may lead average users to an erroneous assessment of their visual capabilities. The reliability of vision tests depends on the accurate generation of the necessary visual stimuli in a particular device. Our aim was to ascertain whether a given color test, designed for a colorimetrically characterized device, might be used in another similar device. METHODS: We evaluated color reproduction errors in three iPad tablets of different models with Retina screens, using their individual color characterization models and the model derived for another device. RESULTS: Our results showed, even with this small sample, the high degree of error caused when disregarding the fact that the colorimetric design valid for a given device may not be correct when displayed in another. CONCLUSIONS: The distortion of the chromatic content may lead to subjects with vision defects to pass as normal or vice versa, compromising diagnosis reliability.

Algorithm for correcting the keratometric estimation error in normal eyes..

PURPOSE: To obtain an accurate algorithm for calculating the keratometric index that minimizes the errors in the calculation of corneal power assuming only a single corneal surface in the range of corneal curvatures of the normal population. METHODS: Corneal power was calculated by using the classical keratometric index and also by using the Gaussian equation. Differences between types of calculation of corneal power were determined and modeled by regression analysis. RESULTS: We proposed two options for the selection of the most appropriate keratometric index (n(k)) value for each specific case. First was the use of specific linear equations (depending on the ratio of the anterior to the posterior curvature, k ratio) according to the value of the central radius of curvature of the anterior corneal surface (r(1c)) in 0.1 mm steps and the theoretical eye model considered. The second was the use of a general simplified equation only requiring r(1c) (Gullstrand eye model, n(k) = -0.0064286r(1c) + 1.37688; Le Grand eye model, n(k) = -0.0063804r(1c) + 1.37806). CONCLUSIONS: The generalization of the keratometric index (n(k)) value is not an appropriate approximation for the estimation of the corneal power and it can lead to significant errors. We proposed a new algorithm depending on r(1c), with a maximal associated error in the calculation of the corneal power of 0.5 D and without requiring knowledge of the posterior corneal curvature.

Validation of posterior corneal curvature measurements with color light-emitting diode topography.

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.

Axial apodization in 4Pi-confocal microscopy by annular binary filters.

We present a novel technique for considerably decreasing the sidelobe height of the axial point-spread function of one-photon 4Pi-confocal microscopes. By means of a numerical example, in which the ratio between the excitation and the fluorescence wavelengths was set to epsilon = lambdaexc/lambdadet = 0.8, we show that simply inserting a pair of properly designed two-ring binary masks in the illumination set allows the height of the axial sidelobes to be reduced from 20% to 5% of the height of the central peak. This allows one to receive the full benefit of the strong narrowness of the central lobe provided by the 4Pi-confocal technique.