HOW THE KERATOMETER WORKS



To measure the image size, which is directly 
related to the corneal radius, the prisms that 
form the left and top images can be moved 
forward and back.   As the prisms move they 
increase or decrease the separation of the 
deviated image from the central image.  The 
examiner moves the prisms by rotating the 
power drums.  When they are moved to the 
point where the sides of the images touch 
(plus signs or minus signs overlap) the size 
of the image is know and is converted to a 
dioptric power by the markings on the drum.