Thomas O. Salmon

Normal-eye Zernike coefficients and root-mean-square wavefront errors

PURPOSE: To compare aberrometry measurements from multiple sites and compute mean Zernike coefficients and root‐mean‐square (RMS) values for the entire data pool to serve as a reference set for normal, healthy adult eyes. SETTING: Northeastern State University, Tahlequah, Oklahoma, USA. METHODS: Data were collected from 10 laboratories that measured higher‐order aberrations (HOAs) in normal, healthy adult eyes using Shack‐Hartmann aberrometry (2560 eyes of 1433 subjects). Signed Zernike coefficients were scaled to pupil diameters of 6.0 mm, 5.0 mm, 4.0 mm, and 3.0 mm and corrected to a common wavelength of 550 nm. The mean signed and absolute Zernike coefficients across data sets were compared. Then, the following were computed: overall mean values for signed and absolute Zernike coefficients; polar Zernike magnitudes and RMS values for coma‐like aberrations (Z3±1 and Z5±1 combined); spherical‐like aberrations (Z40 and Z60 combined); and 3rd‐, 4th‐, 5th‐, and 6th‐order, and higher‐order aberrations (orders 3 to 6). RESULTS: The different data sets showed good agreement for Zernike coefficients values across most higher‐order modes, with greater variability for Z40 and Z3−1. The most prominent modes and their mean absolute values (6.0‐mm pupil) were, respectively, Z3−1 and 0.14 μm, Z40 and 0.13 μm, and Z3−3 and 0.11 μm. The mean total higher‐order RMS was 0.33 μm. CONCLUSIONS: There was a general consensus for the magnitude of HOAs expected in normal adult human eyes. At least 90% of the sample had aberrations less than double the mean values reported here. These values can serve as a set of reference norms.

Measurement of refractive errors in young myopes using the COAS Shack-Hartmann aberrometer.

Purpose. To evaluate the Complete Ophthalmic Analysis System (COAS; WaveFront Science) for accuracy, repeatability, and instrument myopia when measuring myopic refractive errors. Methods. We measured the refractive errors of 20 myopic subjects (+0.25 to −10 D sphere; 0 to −1.75 D cylinder) with a COAS, a phoropter, and a Nidek ARK-2000 autorefractor. Measurements were made for right and left eyes, with and without cycloplegia, and data were analyzed for large and small pupils. We used the phoropter refraction as our estimate of the true refractive error, so accuracy was defined as the difference between phoropter refraction and that of the COAS and autorefractor. Differences and means were computed using power vectors, and accuracy was summarized in terms of mean vector and mean spherocylindrical power errors. To assess repeatability, we computed the mean vector deviation for each of five measurements from the mean power vector and computed a coefficient of repeatability. Instrument myopia was defined as the difference between cycloplegic and noncycloplegic refractions for the same eyes. Results. Without cycloplegia, both the COAS and autorefractor had mean power vector errors of 0.3 to 0.4 D. Cycloplegia improved autorefractor accuracy by 0.1 D, but COAS accuracy remained the same. For large pupils, COAS accuracy was best when Zernike mode Z40 (primary spherical aberration) was included in the computation of sphere power. COAS repeatability was slightly better than autorefraction repeatability. Mean instrument myopia for the COAS was not significantly different from zero. Conclusions. When measuring myopes, COAS accuracy, repeatability, and instrument myopia were similar to those of the autorefractor. Error margins for both were better than the accuracy of subjective refraction. We conclude that in addition to its capability to measure higher-order aberrations, the COAS can be used as a reliable, accurate autorefractor.

Comparison of the eye's wave-front aberration measured psychophysically and with the Shack-Hartmann wave-front sensor.

The Shack-Hartmann wave-front sensor offers many theoretical advantages over other methods for measuring aberrations of the eye; therefore it is essential that its accuracy be thoroughly tested. We assessed the accuracy of a Shack-Hartmann sensor by directly comparing its measured wave-front aberration function with that obtained by the Smirnov psychophysical method for the same eyes. Wave-front profiles measured by the two methods agreed closely in terms of shape and magnitude with rms differences of approximately lambda/2 and approximately lambda/6 (5.6-mm pupil) for two eyes. Primary spherical aberration was dominant in these profiles, and, in one subject, secondary coma was opposite in sign to primary coma, thereby canceling its effect. Discovery of an unusual, subtle wave-front anomaly in one individual further demonstrated the accuracy and sensitivity of the Shack-Hartmann wave-front sensor for measuring the optical quality of the human eye.

Comparison of Elevation, Curvature, and Power Descriptors for Corneal Topographic Mapping

Corneal topography systems sample thousands of surface points and from these data different descriptors are derived to create maps of the cornea. Without visualizing and comparing the maps, it is difficult to appreciate the implications of each descriptor for mapping. We created and compared several maps of an ellipsotoric cornea based on the following surface descriptors: relative elevation, dioptric curvature, and refractive power. Each map presented a different picture of the same cornea. Only elevation maps show true topography but must be calculated relative to an appropriate reference surface to reveal important features. Axial and to a greater degree instantaneous curvature maps bring out optically significant shape asymmetries but misrepresent refractive power away from the apex. Ray tracing maps display optical properties that are not apparent from the elevation or curvature maps, including spherical aberration. Oblique astigmatism can be described using a pair of maps for the sagittal and tangential powers at each surface point. A knowledge of these principles is necessary to interpret color maps of the corneal surface correctly.

Comparison of two artificial tear formulations using aberrometry

Background:  This study investigates the effect of artificial tears of different viscosities on aberrations of healthy eyes.

Accuracy of the EyeSys 2000 in measuring surface elevation of calibrated aspheres

Abstract This article examines the accuracy of the EyeSys 2000 videokeratoscope in measuring aspheric surfaces for calculation of the wavefront aberrations of the anterior cornea. Six rotationally symmetric aspheric surfaces were measured. The surface elevation was computed from the normal measurement files. Both elevation error and relative error were computed from the average of the three maps. The root-mean-square errors for the various surfaces ranged from 1.48 to 6.55 microns, with less error on the oblate and spherical surfaces. The error found was very systematic, increasing monotonically toward the periphery. The article includes a strategy to compensate for the systematic error to meet the required 0.5-micron accuracy needed. An equation was developed that used only the apical radius and shape factor, which improved the accuracy to the required 0.5-micron level.