A corneal model for slit‐scanning elevation topography

Abstract

Computerised videokeratography is increasing in sophistication and accuracy. The Orbscan II Videokeratographer combines the advantages of Placido derived and slit-scanning derived topography, to acquire shape information on the posterior surface of the cornea. The purpose of this paper is to construct a model of slit-scanning elevation topography and highlight potential sources of variation in this methodology. Ray tracing calculations were performed on a defined, theoretical, spherical model to obtain a pachymetric measurement. These calculations were tested by comparing the pachymetry measurement derived from the model, over a refractive index shift from 1.376 to 1.400 with the pachymetry measurement obtained by Orbscan II, of a human eye, with an identical alteration in refractive index. The two methods of measurement differed by only 0.157 microm, suggesting that the model is a good representation of slit-scanning elevation topography, and that refractive index variations have only a minimal effect on Orbscan II pachymetry measurement.

Posterior corneal surface changes after hyperopic laser in situ keratomileusis

“…In addition, the difference map obtained by Orbscan may be insufficient for evaluating the posterior corneal surface change after keratorefractive surgery because the measurement of the posterior cornea could be affected by various factors, such as the anterior cornea. 11 In each side, moreover, correlation of the attempted correction with posterior corneal topographic change was significant; the larger the attempted correction, the larger the backward shift of the posterior cornea. This relation between the 2 could support our hypothesis.…”

Orbscan computerized topography: Attributes, applications, and limitations

“…A framework to spatially localize or ''triangulate'' in 3 dimensions the exact position of each data point using a slit-scanning method has been described. 11,12 This demonstrates that triangulation of the anterior surface of the cornea is an exact technique, limited by the constraints of hardware and operator use only. Subsequent software processing detects the edges of the reflected ring mires of the Placido disk, which allows curvature reconstruction of the anterior surface of the cornea.…”

“…The triangulation of internal ocular surfaces is not as precise as that of the anterior surface because in addition to hardware and operator acquisition tolerances, the precision of the anterior surface maps will obviously exert an influence. 12 The entire processing procedure typically takes 30 seconds or less, and a total of more than 30 anterior segment topographic maps can subsequently be created.…”

Comparison of Posterior Corneal Imaging Before and After LASIK Using Dual Rotating Scheimpflug and Scanning Slit-Beam Corneal Tomography Systems

“…18 The system digitally recreates the posterior cornea, anterior iris, and lens from the collected data using triangulation of previously generated anterior elevation and curvature using a proprietary algorithm. 11,19…”