Analysis of the inherent error of the TMS-1 Topographic Modeling System in mapping a radially aspheric surface

Area of Science:

• Ophthalmology
• Optical Engineering
• Metrology

Background:

• Accurate measurement of ocular surface topography is crucial for diagnosing and managing conditions like keratoconus.
• The Topographic Modeling System (TMS) is a common device used for corneal topography, but its accuracy with aspheric surfaces is debated.

Purpose of the Study:

• To investigate the spherical bias of the Topographic Modeling System (TMS) when measuring radially aspheric surfaces.
• To quantify the accuracy of TMS measurements on an ellipsoidal surface under various alignment conditions.

Main Methods:

• Experimental measurement of an ellipsoidal surface using the TMS.
• Comparison of TMS measurements with calculations from an instantaneous radius of curvature formula and a spherically biased algorithm.
• Assessment of measurement accuracy under aligned and misaligned conditions.

Main Results:

• The TMS demonstrated increasing measurement error from the center to the periphery of the ellipsoid.
• Errors exceeding 3 Diopters were observed at 4 mm from the apex for an ellipsoid with 7.5 mm apical radius and 0.5 eccentricity.
• Misalignment errors were found to be less significant than the inherent spherical bias of the TMS.

Conclusions:

• The TMS possesses an inherent spherical bias that prevents accurate measurement of instantaneous radii of curvature on aspheric surfaces.
• The system's spherical bias may lead to underestimation of peripheral curvature and inaccurate reconstruction of corneal topography patterns.