An applanation tonometer with a replaceable prism for contacting a cornea
has emitter and detector ports arranged opposite each other on opposite
sides of a longitudinal axis, with the ports aimed at about 45.degree. to
the longitudinal axis. Light from an emitter then passes through the
emitter port and crosses the prism axis to be incident and reflected from
an emitter-reflecting surface of the prism and become incident on the
applanation surface. Light reflected from the applanation surface is
incident on and reflected from a detector-reflecting surface opposite and
parallel with the emitter-reflecting surface to proceed across the prism
axis to the detector port. Such an arrangement produces reliable
applanation signals and varies the spacial intensity of light reflected
from the applanation surface so that the tonometer can automatically
distinguish between concentric and eccentric contact of the applanation
surface with a cornea.
