Unlike keratometry instruments, corneal topography instruments produce a detailed map of the shape of the entire corneal surface. Originally, corneal topography was limited to analysis of the anterior corneal surface, with the predominant technique being based on the Placido-disk principle (Fig 8-5). Computerized Placido disk–based topographers assess the reflection of a circular mire of concentric lighted rings (Fig 8-6). More precisely, the distance between the rings is measured, from which the height of the cornea is calculated and commonly displayed as dioptric color maps of the corneal surface (see also BCSC Section 13, Refractive Surgery).
Figure 8-5 A Placido disk.
(Courtesy of Neal H. Atebara, MD. Redrawn by C. H. Wooley.)
Figure 8-6 Computerized Placido disk–based topography of the cornea. A, The ring reflections of the Placido disk–based topographer can be seen on this patient’s cornea. This image is then captured and analyzed. B, Image of distorted mires on a cornea with keratoconus.
(Courtesy of M. Bowes Hamill, MD.)
Technological advances enable the characterization of both the anterior and posterior corneal surfaces. These alternative techniques are actually “tomography”-based (and therefore are also suited for corneal pachymetry) and derive topography information via elevation measurements from cross-sectional images of the cornea obtained photographically. They involve the projection of slit beams of light that scan the cornea and either use a stereo-triangulation method (slit-scanning technique), or a camera that rotates perpendicular with the slit beams, to capture the illuminated corneal cross sections. The latter, known as the Scheimpflug principle or Scheimpflug camera, corrects for the nonplanar shape of the cornea and, thus, enables “distortion-free” imaging.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.