A hand-held non contact tonometer includes a piston having a transparent
plane parallel window normal to a measurement axis of the tonometer along
which the piston travels to discharge a fluid pulse without interfering
with optical alignment functions of the instrument. In a first embodiment
intended for home use by a patient, an alignment guidance system utilizing
a concave mirror and a target source in a plane containing the center of
curvature of the concave mirror present an alignment image to the patient
for guiding self-alignment. An infra-red light source at the focal point
of the concave mirror irradiates the concave mirror so that a collimated
beam is reflected along the measurement axis and focused by an objective
lens. When the focal point of the objective lens coincides with the center
of curvature of the cornea, the infra-red light is retro-reflected through
the system and diverted orthogonally from measurement axis to a masked
detector to passively monitor alignment and trigger release of the piston.
A second embodiment for office use by a general medical practitioner
includes an eyepiece in place of the concave mirror for allowing operator
observation along the measurement axis during alignment.
