Height Measurement of Astigmatic Test Surfaces by a Keratoscope That Uses Plane Geometry Surface Reconstruction

Corneal topography with Scheimpflug imaging and videokeratography: Comparative study of normal eyes

“…These instruments provide detailed topographical information from thousands of points across the anterior corneal surface. Studies of the accuracy and repeatability of modern Placido disk-based videokeratoscopes have generally found these instruments to be highly accurate and repeatable for measuring spherical, aspheric, and astigmatic inanimate test objects [5][6][7] and highly repeatable for measurements of human corneas. 8 A relatively recently introduced instrument for measuring corneal topography is the Pentacam HR system (Oculus Inc.).…”

Forward ray tracing for image projection prediction and surface reconstruction in the evaluation of corneal topography systems

“…The forward ray tracing model described above can now be used to illustrate the skew ray error problem encountered in Placido-based corneal topography [9,10,19,23]. Figure 2 describes forward ray traced simulated Placido reflection pattern of a 6 Diopter toric surface ( Fig.…”

“…Because the surface reconstruction in Placido-based system is limited to meridional ray tracing, taking the Red circles instead of the Blue squares as actual input for the surface reconstruction produces the so called "skew ray error". Although the skew ray error problem has been known for quite some time, motivation to correct this problem was initially not strong because the effect of this problem does not have a large impact when evaluating corneal curvature, astigmatism and asphericity, which are standard parameters used in the clinical practice [9]. However, due to recent developments in ophthalmic optics, particularly in customized laser refractive surgery procedures, the importance of being able to measure other higher order aberrations such as coma, trefoil, quadrafoil and spherical aberration has become apparent.…”

“…These algorithms were further improved throughout the years [6][7][8]. A major issue that was addressed is the skew ray error in the surface reconstruction of Placido-based systems [8][9][10].…”

“…Therefore, errors in the corneal height reconstruction occur and are aptly called skew ray errors. This problem was first mentioned by Tripoli [9] and methods were thought of to solve this [10][11][12]. One approach was to stick to the Placido-based system and develop appropriate surface reconstruction algorithms that would correct for skew-ray errors.…”