In practice, this means taking a picture of the cornea and analysing its shape in great detail. This is particularly important in the assesment of patients for laser vision correction so that patients with keratoconus can be screened out. Patients with keratoconus (even very mild variants) can develop more rapidly developing thinning of the cornea after laser refractive surgery which leads to progressive deterioration of vision.

The pictures either look at the front surface of the cornea by examining the reflection of white circles of light (topographay) or examine both the front and back surface of the cornea by looking at the reflection of a scanning slit beam of light passed over the cornea (tomography).