Aberrations of the anterior corneal surface can be recorded by an instrument based on the Placido principle, which has a number of advantages over the other instruments previously designed for this purpose. It is also possible to make measurements on the original film to determine by means of a graph the exact radius of curvature in any desired area. The construction of the apparatus is not complicated, its operation is simple, and the results highly reproducible.
Donaldson DD. A New Instrument for Keratography. Arch Ophthalmol. 1972;88(4):425-428. doi:10.1001/archopht.1972.01000030427016