Comparison between Placido Rings and modified hartmann test as a method to measure cornea surface

Abstract

This study analyses main curvatures and their methods of measurement. These methods are divided in three groups regarding the following optic principles: (1) Specular Reflection: it includes the system of Placido rings, Twyman-Green interferometry, and Moiré deflectometry; (2) Diffuse reflection that includes Moiré stripes, Fourier Profilometry and the Raster-stereography method, and (3) dispersion of light which includes: slit lamp and Orbscan II. This study also mentions the modified Hartmann method which is projected on the anterior surface of the cornea through an ellipsoidal screen with a distribution of luminous points strategically located so when they are projected in the cornea it generates the traditional Hartmann test mask. When the modified Hartmann mask is projected on the cornea it is shown that in some patients the center of the mask does not match the pupillary center, making evident that the visual axis does not match the optical axis of the eye, this measure known as Kappa angle.