23 June 1994 Frequency modulation as an alternative for local phase in 3D corneal topography
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Proceedings Volume 2126, Ophthalmic Technologies IV; (1994) https://doi.org/10.1117/12.178553
Event: OE/LASE '94, 1994, Los Angeles, CA, United States
Abstract
There is little time for acquiring coherent data in corneal topography. Especially in height contouring, only a single flash exposure excludes movement artifacts. Starting with a single flash image, from a projected low frequency line grating on a fluorescein stained cornea, there are basically two ways of analyzing the acquired data. First by locating the local phase of the grating which is not very accurate and second, a straight forward frequency demodulation on the complete recorded image. The frequency demodulation had one great advantage, all points were both horizontally and vertically correlated. One major problem was the sensitivity to noise in the amplitude of the recorded image. This problem was solved by limiting the maximum allowable slope in the measured object to +/- 60 degree(s) and using steep digital filters in both spatial and frequency domains. With these improvements we could reconstruct the corneal height image with a lateral resolution of approximately 30 $umm and an axial resolution of +/- 3 micrometers . The accurate information of the entire corneal surface that became available, gave us reasons to doubt the reliability of some commonly used parameters; e.g., the radius of curvature assumes one optical axis but the calculated vectors from the corneal surface do not point to one axis. Because of the nonspherical shape, there probably is a focal volume instead of a focal point.
© (1994) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Bertho A. Th. Stultiens, Franciscus H. M. Jongsma, "Frequency modulation as an alternative for local phase in 3D corneal topography", Proc. SPIE 2126, Ophthalmic Technologies IV, (23 June 1994); doi: 10.1117/12.178553; https://doi.org/10.1117/12.178553
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