Patent ID: 7798962

Claim:
An ophthalmic instrument comprising: a fluid pump including a plenum chamber; a fluid discharge tube in communication with the pump for directing a fluid pulse at a cornea of a patient to cause reversible deformation of the cornea from an original state of convexity through a first state of applanation to a state of concavity, and back through a second state of applanation to the state of convexity; an applanation detector providing an applanation signal indicating a time of the first state of applanation and a time of the second state of applanation; a pressure sensor arranged to provide a pressure signal indicating fluid pressure in the plenum chamber as a function of time; and a programmable processor connected to the applanation detector and the pressure sensor, wherein the processor is programmed to evaluate the applanation signal and the pressure signal to provide a first pressure value (P 1 ) coinciding with the first state of applanation and a second pressure value (P 2 ) coinciding with the second state of applanation, and the processor is programmed with a predetermined function of the first pressure value (P 1 ) and the second pressure value (P 2 ) to calculate a corneal resistance factor (CRF) indicative of corneal resistance to deformation, wherein the predetermined function is empirically derived to minimize dependence of the calculated corneal resistance factor (CRF) on intraocular pressure, and wherein the predetermined function includes a static resistance term that depends solely on the first pressure value (P 1 ) and a dynamic resistance term that depends on corneal hysteresis (CH) in the reversible deformation of the cornea, wherein corneal hysteresis (CH) is defined as a difference between the first pressure value (P 1 ) and the second pressure value (P 2 ); and means for reporting the calculated corneal resistance factor (CRF).