The hand held Placido Disc is the easiest and most practical device for assessing corneal topography. It comprises a circular disc with a series of alternating black and white concentric rings with a central viewing lens. The patient’s cornea is viewed through the lens with the rings facing the patient. The rings are illuminated using a light situated behind and adjacent to the patient’s head. The bright images of the rings visible to the observer through the central lens can be examined over an area of 4 to 6 mm in diameter. Any eccentricities in these ring images is caused by irregularities in the topography of the cornea itself.