AU2002252345A1 - Replaceable prism for applanation tonometer - Google Patents

Description

REPLACEABLE PRISM FOR APPLANATION TONOMETER

Technical Field

Applanation tonometers.

Background

Prisms for applanation tonometers have proved to be problematic. The art has suggested several prism variations for applanation tonometers, including U.S. Patents 5,070,875, 5,190,042, 5,203,331 , and 6,179,779. 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 prism of the US Patent No. 6,179,779 suggested an input port that directed incoming light to an internally reflecting prism surface that then directed the light to be incident on the applanation surface. Light reflected from the applanation surface was directed by another internally reflecting prism surface to an output port. The signal produced with such a prism was not satisfactorily workable. The light illuminating the internal face of the applanation surface, was not sufficiently uniform over a working area of the applanation surface, and this compromised the accuracy of detected signals purporting to indicate applanation areas. Summary of the Invention

The improved prism for our tonometer solves these problems by reducing the size of the prism and the number of internally reflecting prism surfaces, and by shortening an optical path within the prism to significantly improve prism performance. The improved prism thus goes against the teaching of US Patent No. 6,179,779. It accomplishes this by changing prism geometry and eliminating a pair of internally reflecting prism surfaces. This disposes input and output ports on opposite sides of the applanation surface to form a straight and unreflected light path from the input port to an internal face of the applanation surface. Light reflected from the applanation surface then proceeds through the prism in a straight and unreflected light path directly to the output port. This not only shortens and simplifies the optical path within the prism but greatly improves the prism's optical reliability in delivering to a detector a reflected light signal related to an area of a cornea applanated by the applanation surface.

Such a prism eliminates previously suggested 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 directly through the detector port to the detector. In a preferred embodiment, such a prism can also miniaturize or even eliminate a viewing axis through the prism to the applanation surface.

A light-emitting diode, which is preferred as a source of light for illuminating the tonometer prism, is preferably energizable at different values to illuminate the applanation surface in different ways. For an applanation measurement, the illumination incident on the internal face of the applanation surface is preferably uniformly intense, at least over a working area of the applanation surface. Such even intensity illumination has been found most reliable in producing applanation signals. Energizing such a light-emitting diode more intensely can then illuminate the applanation surface more intensely in a central region and less intensely in a peripheral region. This can be helpful in distinguishing between concentric and eccentric contact of the applanation surface with a cornea of an eye being examined.

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 small and inexpensive, reducing the number of accurate optically reflective prism surfaces, providing a system for requiring and detecting replacement of the prism, and making the tonometer quickly and reliably calibrated to each new prism.

Drawings

Figures 1 -3 are partially schematic plan views of alternative preferred embodiments of prisms according to the invention.

Figure 4 is a partially schematic plan view of a preferred embodiment of a prism and holder components including a replaced prism detector according to the invention.

Figure 5 is a partially schematic side elevational view of the prism of Figure 4 and a portion of the replaced prism detector.

Figure 6 is a partially schematic plan view of a prism and holder components including an alternative replaced prism detector. Figure 7 is a partially schematic elevational view of the prism of Figure 3 showing a portion of the replaced prism detector.

Figure 8 is a partially schematic front elevational view of the prisms of Figures 1 and 3 arranged in a holder.

Figures 9-11 are partially schematic plan views of preferred alternative embodiments of prism and port configurations, combined with emitters and detectors to schematically show light paths.

Detailed Description

Infrared light-emitting diodes that are preferred for illuminating applanation tonometers are not point sources. Also, within the compact confines of an applanation tonometer, it is not practically possible to collimate light from LED sources. The light from these sources thus diverges and spreads through a prism, making it difficult to consistently produce a reliable applanation signal with the minor amount of the total light that reaches a detector.

We have found that variable energization of a light-emitting diode can produce the desired distribution of light intensity incident on the applanation surface for determining concentricity of prism contact with the cornea and for producing a signal corresponding to a size of an applanation area for eye examining purposes. This discovery led to a realization that a tonometer prism can be improved by shortening the optical path, making the prism smaller, reducing the number of optical surfaces, and eliminating internally reflective surfaces other than the applanation area. This was found to significantly improve optical performance, even while reducing the size and expense of the prism. Shortening the optical path was found to reduce the light dispersion, improve the uniformity of the illumination of an internal face of the applanation surface, and to provide the detector with a better light signal more accurately corresponding to a corneal applanation area.

Without any extra internally reflecting surfaces, an emitter directs incoming light through an input port and directly onto an applanation surface, and light reflecting internally of the prism from an applanation surface proceeds directly to a detector via an output port. Prisms 70, 80, and 90 of Figures 1 , 2, and 3 respectively show these improvements. These prisms are illustrated with schematically associated emitters 21 and detectors 22 so that a light path from emitter 21 to detector 22 can be seen reflecting directly from applanation surface 12, without any intermediate bounces from other internally reflecting surfaces of the prism. Such an arrangement shortens and reduces the overall size of the prism and reduces the number of optical quality reflecting surfaces, which in turn reduces the cost of the prism.

The arrangement of Figures 1 -3 leaves applanation surface 12 centered on optical axis 11 and disposes ports 15 and 16 closer to applanation surface 12. Light transmitting port interface surfaces 15a and 16a are each disposed at about 45° to optical axis 11 and about 45° to applanation surface 12. This makes interface surfaces 15a and 16a approximately perpendicular to each other so that light normally entering emitter port 15 from emitter 21 is internally incident on applanation surface 12 at about 45°, and light internally reflected from applanation surface 12 proceeds directly to detector port 16 via port interface surface 16a transmitting to detector 22. Illuminating light proceeds from input port 15a in an unreflected straight path to an internal face of applanation surface 12, and light reflected from the internal face of applanation surface 12 proceeds in a straight and unreflected path to output port 16a.

Prism embodiments 70, 80, and 90 differ from each other in the length of holder portion or viewing arm 13. This is shown at full length in prism 70, considerably shortened in prism 80, and eliminated in prism 90. Viewing window 63 is formed transverse to optical axis 11 opposite applanation surface 12 regardless of the length or absence of viewing arm 13. Prisms 80 and 90, with short and nonexistent viewing arms respectively, can be dropped or lowered into a holder from above, rather than pushed laterally into a holder. Since any version of a prism according to the invention is replaced after use, all variations of prisms are preferably molded of plastic material. Reducing the prism size and the number of optical surfaces required tends to reduce the expense of molding prisms so that prism replacement is affordably inexpensive.

Prism holder 65 is also preferably molded of resin material. Different shapes of prisms and different forms of replaced prism detectors can affect the configuration and components of a prism holder 65, as shown in Figures 4 and 8. Prism 80 preferably drops or is lowered downward into holder 65 so that its port interface surfaces 15a and 16a confront emitter 21 and detector 22 respectively. Since ports 15 and 16 and their interface surfaces 15a and 16a are symmetrically congruent, each port can confront either emitter 21 or detector 22. Co-operation with a replaced prism detector may require dropping prism 80 into holder 65 in a predetermined orientation, though.

Holder 65 can have different configurations depending on the shape of prisms to be inserted and on the direction of insertion and removal of a prism. Otherwise, holder 65 carries emitter 21 and detector 22 in a position to confront port surfaces of a prism and also preferably includes a replaced prism detector, and any related components involved in changing a state of a detectable element on a prism so as to require prism replacement after completion of an eye examination.

The replaced prism detector shown in Figures 4 and 5 involves a fusible link of thin conductive material 67 that engages contacts 68 and 69 when prism 80 is seated in holder 65. A weak current directed between contacts 68 and 69 can determine that fusible link 67 is intact on a prism, and a stronger current directed between contact 68 and 69 can open the circuit formed by fusible link 67 in the same way that a fuse is blown. When a fresh or previously unused prism 80 is inserted into holder 65, a low amperage current directed between contact 68 and 69 determines that prism 80 has not previously been used. A completed circuit through conductivity intact fusible link 67 supplies information to a microprocessor enabling the tonometer to proceed with the measurements required for determining the intraocular pressure of a pair of eyes. When such an examination is completed, a larger amperage current is passed between contact 68 and 69 to fuse or burn out fusible link 67 and render it no longer conductive. A tonometer microprocessor will then be programmed to refuse any further measurements until prism 80 is replaced. Presence of new or previously unused prism 80 with an intact fusible link 67 can then be detected by a small current passing between contacts 68 and 69 so that another set of intraocular pressure measurements can proceed. The embodiment of Figure 4 also schematically illustrates a lens 66 arranged on a viewing axis of prism 80 to facilitate viewing by an operator when a tonometer is portable. The lens for portable use allows the user to see the front of the prism and helps with alignment on the eye.

The replaced prism detector of Figures 6 and 7 involves a coating 71 applied preferably to a non-optical surface of prism 80. A pair of light transmitters 72 and a light detector 73 are arranged in a holder of prism 80 to pass light through a region of prism 80 that includes coating 71. Coating 71 is made to change state in response to actinic radiation so that it has one state when prism 80 is unused and another state after prism 80 has been used and actinic radiation has modified coating 71. For example, coating 71 , in an initial state can be made to transmit low intensity light between elements 72 and 73. Coating 71 is then subjected to actinic radiation, either during an intraocular pressure measurement, or by means of a more intense or different transmission between elements 72 and 73 so as to change state and no longer transmit low intensity radiation between elements 72 and 73. Use of prism 80 is thus made detectable so that the microprocessor can refuse further use of prism 80 and not perform another measurement until a fresh prism 80 is replaced with a coating 71 in an initial state.

Prisms 100, 110, and 120, of Figures 9-11 , illustrate preferred alternative possibilities departing somewhat from the prism and port geometries shown in Figures 1 -3, but serving similar functions. These prisms, like the ones illustrated in Figures 1 -3, direct light from an emitter 21 entering the prism through an input port so that the light proceeds in a straight and unreflected path directly to an internal face applanation surface 12 then proceeds in a straight and unreflected path directly to an output port that transmits outgoing light to detector 22.

Prism 100 is substantially triangular with applanation surface 12 forming a triangle base, and an angle opposite the triangular base being approximately 90°. This disposes an input port surface 101 to obliquely confront applanation surface 12 at about a 45° angle. Light from emitter 21 preferably enters the prism perpendicular to input port 101 and proceeds in an unreflected path to an internal face of applanation surface 12. Light reflected from the internal face of applanation surface 12 proceeds directly in an unreflected path to output port 102, which is also arranged to obliquely confront applanation surface 12 at about a 45° angle. Light reflected from applanation surface 12 then transmits through output port 102 through detector 22.

Prism 110 of Figure 10 is similar to prism 100 except that input port 1 1 1 and output port 112 are separated by a small amount to allow room for viewing applanation surface 12 along viewing axis 1 1. Such viewing is preferably enhanced by presence of a molded lens 115 formed on viewing axis 11.

Prism 120 of Figure 1 1 illustrates the possibility of establishing an angle between emitter 21 and its input port 121 and between detector 22 and its output port 122. This causes some refraction as light from emitter 21 enters input port 121 , and again when light reflected from applanation surface 12 leaves output port 122 and proceeds to emitter 22. The refraction angle involved and the angle established between emitter 21 , detector 22, and their respective ports 121 and 122 are then engineered so that the light path within prism 120 is incident on applanation surface 12 at about a 45° angle and reflects from applanation surface 12 to proceed toward detector 22 in an unreflected path.

Claims (11)

We Claim:

1 . A tonometer prism having an applanation surface, an input port arranged to transmit incoming light into the prism, and an output port arranged to transmit outgoing light from the prism, the tonometer prism comprising: a. the input port being arranged to direct the incoming light from the input port in an unreflected straight path through the prism from the input port to an internal face of the applanation surface; and b. light internally reflected from the internal face of the applanation surface proceeding from the applanation surface in an unreflected straight path through the prism to the output port.

2. The tonometer prism of claim 1 wherein the input and output ports are symmetrically congruent with respect to the applanation surface and each obliquely confronts the applanation surface.

3. The tonometer prism of claims 1 or 2 wherein the input and output ports are respectively normal to the light paths within the prism.

4. The tonometer prism of any of the claims 1 -3 wherein axes of the input and output ports intersect in a central region of the applanation surface.

5. The tonometer prism of any of the claims 1 -4 wherein the input port is a plane surface arranged at about a 45° angle to the applanation surface on one side of the applanation surface and the output port is a plane surface arranged at about a 45° angle to the applanation surface on an opposite side of an applanation surface.

6. The tonometer prism of any of the claims 1 -5 including an applanation surface viewing port arranged between the input and output ports.

7. The tonometer prism of any of the claims 1 -6 wherein the prism is a single-use prism.

8. The tonometer prism of any of the claims 1-7 wherein the light from the input port substantially uniformly illuminates the internal face of the applanation surface.

9. The tonometer prism of any of the claims 1 -8 wherein substantially all of the light internally reflected from the internal face of the applanation surface proceeds to the output port.

1 0. The tonometer prism of any of the claims 1 -9 wherein the prism is substantially triangular.

1 1 . The tonometer prism of any of the claims 1 -9 wherein the prism is substantially hexagonal.