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Related Experiment Videos

How accurately can videokeratographic systems measure surface elevation?

R A Applegate1, R Nuñez, J Buettner

  • 1Department of Ophthalmology, University of Texas Health Science Center at San Antonio, USA.

Optometry and Vision Science : Official Publication of the American Academy of Optometry
|November 1, 1995
PubMed
Summary

The TMS-1 videokeratometer accurately measures spherical and elliptical corneal surface topography. However, for surfaces with abrupt transitions, the TMS-1 elevation file offers more precise data than back-calculation methods.

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Area of Science:

  • Ophthalmology
  • Biomedical Engineering
  • Optical Metrology

Background:

  • Corneal topography assessment is crucial in ophthalmology.
  • Surface topography provides an assumption-free description of the corneal surface.
  • Evaluating the precision of videokeratometric machines for surface topography measurement is essential.

Purpose of the Study:

  • To quantify the accuracy of the TMS-1 Corneal Modeling System.
  • To assess the measurement accuracy for calibrated spherical, elliptical, and bicurve surfaces.

Main Methods:

  • Utilized the TMS-1 videokeratometer to measure calibrated spherical, elliptical, and bicurve surfaces.
  • Obtained surface characteristics by back-calculating from dioptric files or directly from the TMS-1 elevation file.

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  • Quantified accuracy using root mean squared error (RMSE) of measured vs. known surface elevations.

    • Main Results:

    • Back-calculation from dioptric files yielded RMSE ≤ 5 μm for spherical and elliptical surfaces without abrupt transitions.
    • The TMS-1 elevation file provided RMSE ≤ 5 μm for spherical surfaces.
    • RMSE increased with distance from the videokeratometric axis.
    • Abrupt transitions on bicurve surfaces were smoothed by the TMS-1, with back-calculation methods further increasing RMSE.

    Conclusions:

    • Surface elevations can be accurately back-calculated from dioptric files (RMSE ≤ 5 μm) for smooth spheres and ellipses.
    • For surfaces with abrupt transitions, the TMS-1 elevation file provides superior surface profile accuracy.