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. Since the cornea is normally responsible for some 70% of the eye's refractive power, its topography is of critical importance in determining the quality of vision and corneal health. The three-dimensional map is therefore a valuable aid to the examining ophthalmologist or optometrist and can assist in the diagnosis and treatment of a number of conditions; in planning cataract surgery and intraocular lens (IOL) implantation (plano or toric IOLs); in planning refractive surgery such as LASIK, and evaluating its results; or in assessing the fit of contact lenses. A development of keratoscopy, corneal topography extends the measurement range from the four points a few millimeters apart that is offered by keratometry to a grid of thousands of points covering the entire cornea. The procedure is carried out in seconds and is painless. The patient is seated facing the device, which is raised to eye level. One design consists of a bowl containing an illuminated pattern, such as a series of concentric rings. Another type uses a mechanically rotated arm bearing a light source. In either type, light is focused on the anterior surface of the patient's cornea and reflected back to a digital camera at the device. The topology of the cornea is revealed by the shape taken by the reflected pattern. A computer provides the necessary analysis, typically determining the position and height of several thousand points across the cornea. The topographical map can be represented in a number of graphical formats, such as a "sagittal map", which color-codes the steepness of curvature according to its dioptric value. The corneal topograph owes its heritage to the Portuguese ophthalmologist Antonio Placido, who, in 1880, viewed a painted disk ("Placido's disk") of alternating black and white rings reflected in the cornea. The rings showed as contour lines projected on the corneal tear film. Javal L., a pioneer in the field in the 1880s incorporated the rings in his ophthalmometer and mounted an eyepiece which magnified the image of the eye. He proposed that the image should be photographed or diagrammatically represented to allow analysis of the image. In 1896, Allvar Gullstrand incorporated the disk in his ophthalmoscope, examining photographs of the cornea via a microscope and was able to manually calculate the curvature by means of a numerical algorithm.