Keratoscope
（重定向自Keratoscopy）
A keratoscope, sometimes known as Placido's disk, is an ophthalmic instrument used to assess the shape of the anterior surface of the cornea. A series of concentric rings is projected onto the cornea and their reflection viewed by the examiner through a small hole in the centre of the disk. A regular-shaped cornea should show equally spaced symmetric reflections. If the patient is suffering from astigmatism or from a corneal dystrophy, the rings will be distorted.