There are times when a patient's cornea is steeper than 52.00 D. (6.49 mm) or flatter than 36.00 D. and off the keratometer scale. It may still be possible to make an accurate measurement of the cornea.
In the case of a steeper than
52.00 D. cornea, a +1.25 D.
spectacle trial lens can be
held up in front of the
objective of the keratometer
and then take the reading the
same as you would without the
lens in place.
The dioptric value read from
the drum must be converted to
the actual value. For example, if
the drum dioptric value is 45.00 D.,
the actual value is 52.37 D. The manual that comes with the keratometer gives such a table as do most contact lens textbooks.
