1) Medical instrument 
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 symme....
<instrument> An instrument marked with lines or circles by means of which the corneal reflex can be observed. ... Synonym: Placido da Costa's disk. ... Origin: kerato-+ G. Skopeo, to examine ... (05 Mar 2000) ... 
(ker´ә-to-skōp″) a device consisting of alternate black and white concentric circles and used for examining corneal curvature.
Type: Term Pronunciation: ker′ă-tō-skōp′ Definitions: 1. An instrument marked with lines or circles by means of which the corneal reflex can be observed. Synonyms: Placido da Costa disc
[n] - medical instrument to examine the cornea in order to detect irregularities in its anterior surface
noun medical instrument to examine the cornea in order to detect irregularities in its anterior surface
an instrument, as Placido's disk, for determining the symmetry of the curvature of the cornea.