Biomedical subjects

Thomas O Salmon

Publications and source records attributed to Thomas O Salmon.

4 recordsLinked to original sources

PubMed2006

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 (Z(3)(+/-1) and Z(5)(+/-1) combined); spherical-like aberrations (Z(4)(0) and Z(6)(0) 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 Z(4)(0) and Z(3)(-1). The most prominent modes and their mean absolute values (6.0-mm pupil) were, respectively, Z(3)(-1) and 0.14 microm, Z(4)(0) and 0.13 microm, and Z(3)(-3) and 0.11 microm. The mean total higher-order RMS was 0.33 microm. 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.

Adult↗

PubMed2005

Evaluation of a clinical aberrometer for lower-order accuracy and repeatability, higher-order repeatability, and instrument myopia.

BACKGROUND: Refractive surgery has stimulated the development of aberrometers, which are instruments that measure higher-order aberrations. The purpose of this study was to test one clinical aberrometer, the Complete Ophthalmic Analysis System (COAS), for its accuracy, repeatability, and instrument myopia for measuring sphere and astigmatism and its repeatability for measuring higher-order aberrations. METHODS: Aberrations of 56 normal eyes (28 subjects) were measured with and without cycloplegia using a COAS, a conventional autorefractor and by subjective refraction. We evaluated lower-order accuracy (sphere and astigmatism) of the COAS and autorefractor by comparing that data with that of subjective refraction. We also tested COAS lower- and higher-order repeatability for 5 measurements taken in less than 1 minute. We evaluated instrument myopia by comparing cycloplegic and noncycloplegic measurements of the same eye. Data were analyzed for a 5.0-mm-diameter pupil. RESULTS: Mean COAS spherical error was between -0.1 and +0.4 diopters (D), depending on cycloplegia and the kind of sphere power computation selected. Cylinder power errors were less than 0.1 D. COAS repeatability coefficients were better than 0.25 D, and instrument myopia was less than 0.4 D. These were comparable with those of autorefraction. Higher-order repeatability was sufficient to allow reliable measurement of normal third-order aberrations and spherical aberration. CONCLUSIONS: Accuracy, repeatability, and instrument myopia of the COAS are similar to those of a conventional autorefractor. Accuracy and repeatability are also similar to those of subjective refraction. Like an autorefractor, the COAS provides instantaneous, objective measurements of sphere and astigmatism, but it also measures higher-order aberrations. We found that it is capable of reliably measuring problematic higher-order aberrations and is therefore a valuable asset for modern clinical eye care.

Adult↗

PubMed2003

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 Z4(0) (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.

Adolescent↗

PubMed2002

Videokeratoscope-line-of-sight misalignment and its effect on measurements of corneal and internal ocular aberrations.

A growing number of research laboratories are using the new technologies of videokeratoscopy and Shack-Hartmann aberrometry, in combination, to study the optical structure of the human eye. A potential source of error arises, however, because the two instruments are designed to measure the human eye along different reference axes. The Shack-Hartmann aberrometer is usually aligned coaxially with the line of sight, but videokeratoscopes usually are not. Thus far, corneal optics research has not adequately addressed the problem of videokeratoscope-line-of-sight misalignment and its effect on the computation of corneal and internal ocular aberrations. We measured corneal, ocular, and internal aberrations for three normal human eyes, developed a method to compensate for videokeratoscope-line-of-sight misalignment, and analyzed the importance of compensating for the misalignment. Our results show that when the value of angle lambda (the angle between the line of sight and the pupillary axis) is larger than 2 degrees-3 degrees, the misalignment, if ignored, can lead to incorrect estimates of corneal and internal aberrations as well as the corneal/internal aberration balance.

Cornea↗