8
3.3 Refractive power map
This map (Figure 3) uses only values from the anterior surface, but it also takes effect “A” (see above)
into account. It calculates corneal power according to Snell’s law of refraction assuming a refractive
index of 1.3375 to convert curvature into refractive power (Figure 2). This is a map that other
Placido topographers also may show because it only considers the anterior surface.
Figure 2: Snell´s law of refraction
Figure 3: Refractive power map of a sphere, r = 8 mm
3 Differences between the various topography maps of Pentacam
®
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