A keratoscope, sometimes known as Placido's disk, is an ophthalmic

Ophthalmology

Ophthalmology is the branch of medicine that deals with the anatomy, physiology and diseases of the eye. An ophthalmologist is a specialist in medical and surgical eye problems...

 instrument for assessing the shape of the anterior surface of the cornea

Cornea

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

. A series of concentric rings are 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

Astigmatism (eye)

Astigmatism is an optical defect in which vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina. This may be due to an irregular or toric curvature of the cornea or lens. There are two types of astigmatism: regular and...

 or from a corneal dystrophy, the rings will be distorted.

A modern development of the concept is found in corneal topography

Corneal topography

Corneal topography, also known as photokeratoscopy or videokeratography, is a non-invasive medical imaging technique for mapping the surface curvature of the cornea, the outer structure of the eye...

, in which analysis of the reflected image is passed to a computer. The automated instrument can produce colour-coded contour maps of the eye's topology or even three-dimensional visualisations of its surface.