The keratoscope (Placido's disk) permits gross evaluation of the uniformity of the surface of the cornea. This instrument consists of a round disk marked with concentric black and white rings around a central aperture. The examiner holds the disk in his or her hand and looks through the aperture. The mirror images of the rings on the patient's cornea indicate the presence of astigmatism (in which case they appear distorted). However, this inexact evaluation method lacks the precision required for modern applications such as refractive surgery. Therefore, the surface of the cornea is now normally evaluated by computerized corneal topography (videokeratoscopy). In this examination, the contours of the cornea are measured by a computer in the same manner as the keratoscope. The refractive values of specific corneal regions are then represented in a color-coded dioptric map. Bright red, for example, represents a steep curvature with a high refractive power. This technique provides a contour map of the distribution of the refractive values over the entire cornea (Figs. 5.3a and b).
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