HOW THE KERATOMETER WORKS

The keratometer treats the cornea 
as a convex mirror.  A pattern, 
called mires, of known size are 
reflected off the cornea.  The mire 
pattern used with the keratometer 
is a circle with plus signs on each 
side in the horizontal meridian 
and a line (minus sign) top and 
bottom.  

The mire pattern is formed by a 
plate on the front of the keratometer 
which is opaque except for the 
transparent pattern which is illuminated 
from behind by a mirror at a 45 degree 
angle which reflects the light from the 
bulb below the axis of the instrument.