(Placido’s disc) n. an instrument for detecting abnormal curvature of the cornea. It consists of a black disc, about 20 cm in diameter, marked with concentric white rings. The examiner looks through a small lens in the centre at the reflection of the rings in the patient’s cornea. A normal cornea will reflect regular concentric images of the rings; a cornea that is abnormally curved (for example in *keratoconus) or scarred reflects distorted rings. Modern keratoscopes can print out a contour map of the corneal surface.
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