Applanation tonometers.
This invention improves on the tonometer of U.S. Pat. No. 5,070,875, the disclosure of which is hereby incorporated by reference. The ""875 patent discloses an applanation tonometer having a replaceable prism; and this invention improves on the prism and its replacement, while otherwise relying on-tonometer components such as described in the ""875 patent.
Prisms for applanation tonometers have proved to be problematic. The art has suggested several prism variations for applanation tonometers, including U.S. Pat. Nos. 5,190,042 and 5,203,331. None of these have yet demonstrated operability sufficient to be successful in the marketplace. The problems involve reliable production of an accurate applanation signal and the difficulty of combining this with a tonometer having a conveniently replaceable prism producing reliable operation at a low cost.
The improved prism for our tonometer solves these problems partly by arranging emitter and detector ports opposite each other on opposite sides of a longitudinal or viewing axis so that axes of the emitter and detector ports are angled about 45xc2x0 to the longitudinal axis. This makes the light-transmitting surfaces of the emitter and detector ports approximately perpendicular to each other. Internal-reflecting prism surfaces for the emitter and detector are also arranged opposite each other on opposite sides of the longitudinal axis and proximate to the applanation surface. This makes the emitter- and detector-reflecting surfaces approximately parallel to each other. A light path proceeding from a source through the emitter port is angled at about 45xc2x0 to the longitudinal axis and directed across the longitudinal axis for incidence on the emitter-reflecting surface. From there, the light is incident on an applanation surface, and a portion of the light reflected from the applanation surface is incident on the detector-reflecting surface. A light path to the detector port for the light reflected from the applanation surface and the detector-reflecting surface proceeds at an angle of about 45xc2x0 to the longitudinal prism axis across the longitudinal axis and is output at the detector port.
A simplification of such a prism eliminates the internally reflecting surfaces for the emitter and detector so that light passes directly from the emitter through the emitter port to be incident on the applanation surface from which the light is reflected through the detector port to the detector. Light entering one of the ports normal to the port interface surface is incident on the applanation surface at approximately 45xc2x0 and is reflected to be approximately normal to the opposite port interface surface. Such a prism can have either an elongated or shortened viewing axis, depending partly on the desired configuration of a holder for the prism.
With a prism of such a configuration receiving light from a light-emitting diode, which is presently preferred as a source of prism illumination, the light incident on the applanation surface can be made to vary in intensity radially from a center of the applanation surface to a perimeter of the applanation surface. A consequence of such a varying light distribution at the applanation surface is that the amount of reflected light reaching the detector upon concentric contact of the applanation surface with a cornea differs distinguishably from an amount of reflected light reaching the detector upon eccentric contact of the applanation surface with a cornea. The readily distinguishable difference in detected light from concentric and eccentric contact with a cornea is preferably used to determine that the prism is properly positioned to proceed with an intraocular pressure measurement sequence.
A prism having desirable light-transmitting characteristics is also preferably replaceable and disposable. This is accomplished by provisions for easy insertion and withdrawal of the prism, making the prism inexpensive, providing a system for requiring and detecting replacement of the prism, and making the tonometer quickly and reliably calibrated to each new prism. We also prefer making the tonometer responsive to input for adjusting an intraocular pressure measurement to accommodate variations in corneal curvature and thickness of a particular eye being examined.