Quantitative measurements of anterior corneal surface curvature (corneal topography) are made with the Placido ring approach. Patterns on an illuminated target take the form of mires or a grid pattern. Their reflection from the anterior corneal surface tear film, shown in Figure NNN.3-1, is captured with a video camera. Their positions relative to the instrument axis are determined through image analysis and these data are used to calculate anterior corneal curvature distribution.

Corneal curvature calculations are accomplished with three different methods that provide corneal powers. The axial power map, shown in Figure NNN.3-2, is most useful clinically for routine diagnostic use as the method of calculation presents corneal topography maps that match the transitions known for corneal shape-the cornea is relatively steep in its central area, flattening toward the periphery. This figure shows an example where the map is superimposed over the source image based upon the corneal vertex Frame of Reference. The Blending Presentation State SOP Class may be used to specify this superimposed processing.

The instantaneous power map, shown in Figure NNN.3-3, reveals more detail for corneas that have marked changes in curvature as with the transition zone that rings the intended optical zone of a refractive surgical procedure.

The refractive power map, shown in Figure NNN.3-4, uses Snell's Law of refraction to calculate corneal power to reveal, for example, uncompensated spherical aberration.

The height map, shown in Figure NNN.3-5, displays the height of the cornea relative to a sphere or ellipsoid.