BASIC CONCEPTS

In order to obtain quantitative data that 
would be more clinically useful, 
photokeratoscopes were developed.  
These instruments have a modified 
Placido’s disc, usually  with an ellipsoidal 
shape, which were reflected from the 
cornea.  The image of the pattern was then 
photographed, usually with a Polaroid 
camera.  The picture was analyzed by 
measuring the separation of the rings with 
a projection system (comparator).  By 
knowing the separation of the rings and 
proper calibration the corneal curvature 
could be calculated.  This procedure was 
fairly long and tedious.  With some of the 
instruments this was done in the 
practitioner’s office while with others the 
photos were sent to a laboratory for 
analysis.