The Skew Ray Ambiguity in the Analysis of Videokeratoscopic Data

Iskander, D. Robert, Davis, Brett A., & Collins, Michael J. (2007) The Skew Ray Ambiguity in the Analysis of Videokeratoscopic Data. Optometry and Vision Science, 84(5), pp. 435-442.

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Abstract Purpose. Skew ray ambiguity is present in most videokeratoscopic measurements when azimuthal components of the corneal curvature are not taken into account. There have been some reported studies based on theoretical predictions and measured test surfaces suggesting that skew ray ambiguity is significant for highly deformed corneas or decentered corneal measurements. However, the effect of skew ray ambiguity in ray tracing through videokeratoscopic data has not been studied in depth.

Methods. We have evaluated the significance of the skew ray ambiguity and its effect on the analyzed corneal optics. This has been achieved by devising a procedure in which we compared the corneal wavefront aberrations estimated from 3D ray tracing with those determined from 2D (meridional based) estimates of the refractive power. The latter was possible due to recently developed concept of refractive Zernike power polynomials which links the refractive power domain with that of the wavefront. Simulated corneal surfaces as well as data from a range of corneas (from two different Placido disk-based videokeratoscopes) were used to find the limit at which the difference in estimated corneal wavefronts (or the corresponding refractive powers) would have clinical significance (e.g., equivalent to 0.125 D or more).

Results. The inclusion/exclusion of the skew ray in the analyses showed some differences in the results. However, the proposed procedure showed clinically significant differences only for highly deformed corneas and only for large corneal diameters.

Conclusions. For the overwhelming majority of surfaces, the skew ray ambiguity is not a clinically significant issue in the analysis of the videokeratoscopic data indicating that the meridional processing such as that encountered in calculation of the refractive power maps is adequate.

Impact and interest:

11 citations in Scopus
11 citations in Web of Science®
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ID Code: 10864
Item Type: Journal Article
Refereed: Yes
Additional Information: The contents of this journal can be freely accessed online via the journal's web page (see hypertext link) 12 months after publication.
Keywords: ray tracing, corneal wavefront aberration, refractive power, OAVJ
DOI: 10.1097/OPX.0b013e3180590654
ISSN: 1538-9235
Subjects: Australian and New Zealand Standard Research Classification > MEDICAL AND HEALTH SCIENCES (110000) > OPTOMETRY AND OPHTHALMOLOGY (111300)
Divisions: Current > Research Centres > Centre for Health Research
Current > QUT Faculties and Divisions > Faculty of Health
Current > Institutes > Institute of Health and Biomedical Innovation
Copyright Owner: Copyright 2007 Lippincott Williams & Wilkins
Deposited On: 22 Nov 2007 00:00
Last Modified: 29 Feb 2012 13:36

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