HOW THE KERATOMETER WORKS


The reflected pattern is viewed 
through the optics of the instrument 
including the objective (hole in the 
center of the mire pattern) of the 
instrument, a hole in the illuminating 
mirror, condensing lens, a plate with 
4 apertures and the eyepiece.  

The measurement is made using the 
doubling principle.  The reflected image 
is doubled with a prism or prisms.  By 
varying the prism position, the size of the reflected image can be measured since when the two images formed just touch the size is known.   With a steep cornea the reflected image size is smaller than with a flat cornea.  Therefore, more prism power or a different position of the prism is required to get the two images to touch with a steep radius.