BASIC CONCEPTS

       Placido’s Disc

Since keratometers and 
ophthalmometers only measure 
from two points close to the 
central axis of the cornea, they 
tell us little about the overall 
shape of the cornea.  To obtain 
more information about the shape 
of the cornea other techniques 
had to be devised.

The earliest technique was the Placido’s disc, a concentric pattern of rings with a sighting hole in the center.  The instrument is held before the eye and the rings reflected from the cornea.  The pattern is subjectively evaluated for the regularity of the pattern.  Steeper portions of the cornea result in the rings being closer together then flatter areas.  A toric surface results in an oval pattern.  Distorted areas of the corneal surface result in distortion of the image.  This instrument can give the clinician a good qualitative picture of the corneal shape.