A keratoscope, sometimes known as ''Placido's disk'', is an ophthalmic instrument used to assess the shape of the anterior surface of the

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

. 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 Astigmatism is a type of refractive error due to rotational asymmetry in the eye's refractive power. This results in distorted or blurred vision at any distance. Other symptoms can include eyestrain, headaches, and trouble driving at nig ...

or from a

corneal dystrophy Corneal dystrophy is a group of rare hereditary disorders characterised by bilateral abnormal deposition of substances in the transparent front part of the eye called the cornea. Signs and symptoms Corneal dystrophy may not significantly affect v ...

, the rings will be distorted. A modern development of the concept is found in

corneal topography Corneal topography, also known as photokeratoscopy or videokeratography, is a non-invasive medical imaging technique for mapping the anterior curvature of the cornea, the outer structure of the eye. Since the cornea is normally responsible for so ...

, in which analysis of the reflected image is passed to a computer. The automated instrument can produce colour-coded

contour map A contour line (also isoline, isopleth, or isarithm) of a function of two variables is a curve along which the function has a constant value, so that the curve joins points of equal value. It is a plane section of the three-dimensional graph ...

s of the eye's topography or even three-dimensional visualisations of its surface.

References

Ophthalmology Ophthalmic equipment {{Medical-equipment-stub