Abstract:
A tonometer uses ambient light available in an eye examining room, rather than a dedicated source of light, to examine a characteristic of an eye. A digital camera in the tonometer views an image of the eye as it is engaged by a contactor that applanates or indents the cornea. An electromagnetic mount for the contactor can supply a force pressing the contactor against the eye. While the examiner observes the resulting image a strain gauge can also measure the deformation pressure applied to the eye by the contactor. A microprocessor can then determine a characteristic of the eye from signals supplied by the camera and the strain gauge or the electromagnet force applier.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit under 35 USC §119(e) of subject matter disclosed in Provisional Application No. 60/981,930, filed 23 Oct. 2007, entitled “Tonometer Using Camera and Ambient Light”. 
     
    
     FIELD OF THE INVENTION 
       [0002]    Tonometry of the eye 
       BACKGROUND 
       [0003]    Tonometers measure intraocular pressure (IOP) of an eye. A preferred form of tonometer applanates or indents an area of the cornea and uses a light source and a detecting system to determine the size of the corneal area that is deformed by the contact and the force involved in pressing a contactor against the cornea. The tonometer can then determine IOP from the relationship between the force applied and the size of the corneal area that is deformed. Pertinent examples of such tonometers include U.S. Pat. Nos. 6,179,779, 6,736,778, and 7,153,267 to Falck; Publication No. 20030236470 to Falck; U.S. Pat. No. 5,190,042 to Hock; U.S. Pat. No. 6,083,160 to Lipman; U.S. Pat. No. 5,671,737 to Harosi; and U.S. Pat. No. 6,776,756 to Feldon et al. 
         [0004]    This invention improves on previous tonometers in several ways. These include simplifying optical systems, force measurement, and detection systems, and eliminating the need for a dedicated light source. The goals are a tonometer that is accurate, safe, versatile, robust, and inexpensive. 
       SUMMARY 
       [0005]    The tonometer of this invention uses a digital camera to observe a deformed corneal area so that the camera can determine the size of the deformation from the observed image. We have found that this can be done using ambient light, rather than requiring a dedicated source of illumination. The invention also includes a simple and effective way of mounting a force responsive contactor and of measuring a force used in pressing the contactor against a cornea to secure an IOP measurement. 
     
    
     
       DRAWINGS 
         [0006]      FIG. 1  is a partially schematic, cross-sectional view of a preferred embodiment of the inventive tonometer. 
           [0007]      FIG. 2  is a view of a contactor showing a deformed corneal area. 
           [0008]      FIG. 3  is a view of the contactor of  FIGS. 1 and 2  from the eye being examined. 
           [0009]      FIG. 4  is a schematic, side view of another form of tonometer provided with a beam splitter for use on a slit lamp microscope. 
           [0010]      FIG. 5  is a schematic view similar to the view of  FIG. 1  showing a manual operation system for pressing a contactor against a cornea. 
           [0011]      FIG. 6  is a schematic view of another form of tonometer that requires replacement of used contactors. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Tonometer  10 , as schematically illustrated in  FIG. 1 , includes a tube or other structure  11  that is movable as indicated by the double-headed arrow, to press a contacting surface or window  15  against a cornea of an eye  35 . The movement required is about 1 mm, but is not necessarily limited to that amount. A preferably digital camera  20  is positioned to observe the size of a corneal area that is applanated or indented by contactor surface  15 . A linear bearing  12  supports tube  11  for the required axial movement, and a resilient element  13  holds tube  11  and accommodates the axial movement while supplying a small resistance to the movement. Element  13  can advantageously be formed as an audio speaker diaphragm, which is readily available and well understood. Other resilient elements and supports for contactor  15  are also possible. 
         [0013]    The embodiment of  FIG. 1  includes a force generating system using an annular coil  16  secured to diaphragm or resilient element  13 , and a fixed annular magnet  17  surrounding coil  16 . A current applied to coil  16  can then move tube  11  to press contactor  15  against a cornea, and stopping the current to coil  16  can allow tube  11  to retract from a cornea under the resilient influence of element  13 . 
         [0014]    Tonometer  10  uses ambient illumination such as is generally available in places where eyes are examined. We have found that ambient light in an examining room is adequate to provide camera  20  with a view of the size of a deformed area  25  of a cornea against which surface  15  is pressed. This simplifies the construction of tonometer  10  by eliminating the need for a dedicated light source. In effect, camera  20  observes an eye as contactor  15  approaches. Then when surface  15  contacts a cornea, a small deformation area  25  occurs, as shown in  FIG. 2 . This area  25  can be enlarged by pressing contactor  15  with increasing force against the cornea. 
         [0015]    The deformation of a cornea by contactor  15  can cause applanation or indentation of the cornea. Either of these slightly decreases the volume of the eye and raises the eye pressure. The applanated or indented area of the cornea is observable as an image viewed by camera  20 , which can see from the image the extent of the applanation or indentation. A schematically shown lens  21  can facilitate the viewing by camera  20 . 
         [0016]    A signal from camera  20  can determine the size of applanated or deformed area  25  in various geometrical ways. These can be based on the fact that some of the pixels in camera  20  receive significantly reduced illumination in the observed image area  25 , so that the difference between well illuminated pixels and reduced illumination pixels can be exploited. Diameters of the deformed area  25  can be used to calculate the size of area  25 , and counting the illuminated or unilluminated pixels can also produce a deformed area determination. 
         [0017]    When coil  16  and magnet  17  are used to apply force to press contactor  15  against a cornea and enlarge an affected area  25 , then the current supplied to coil  16  can also produce a measure of the force applied in pressing contactor  15  against the cornea. The force applied as evidenced by the current to coil  16  and the size of the area affected, as evidenced by an image signal from camera  20 , can then indicate IOP. This is preferably done with microprocessor  50  which can operate coil  16 , collect signals from camera  20 , coil  16 , and strain gauge  28 , and produce an output  51  indicating a characteristic of the eye being examined. Such a characteristic can include intraocular pressure, ocular pulse pressure, ocular blood flow, and tonography. 
         [0018]    Another way of determining the force applied in pressing contactor  15  against a cornea is by use of strain gauge  28  as shown in  FIGS. 1 ,  3  and  5 . This can be positioned to sense movement of tube  11  or movement of contactor  15  against the resilient bias of element  13 . Alternatively, the amount of current applied to coil  16  can also measure force applied to contactor  15 , and the two different force measurements can be used corroboratively: one being the force derived from the current applied to coil  16  to move diaphragm  13 , and the other being movement detected by strain gauge  28 . Each of these can represent force applied to applanator window  15 . 
         [0019]    Contactor  15  is preferably molded of resin material that is thin, clear, and flat in a central surface area  15 . Window  15  and the other elements of movable tube  11  are preferably made compact and lightweight to simplify the support and movement operations and improve measurement accuracy. Shapes other than tubes and flat windows can also work. 
         [0020]    It is generally preferred for tonometers that an element contacting the cornea be disposable to prevent transfer of microorganisms or prions from one eye to another. For this purpose, contactor  15  is preferably required to be replaced after examining a pair of eyes. This can be done by using strain gauge  26 , which is deflected when contactor  15  is pressed into an operating position. A flexible region  27  of contactor  15  moves strain gauge  26  as contactor  15  is mounted on tonometer  10 . The flexible portion  27  of contactor  15  is preferably configured so that strain gauge  26  can distinguish between a used or previously mounted contactor and a new or not previously mounted contactor. There are many ways that this can be done, and these include forming contactor  15  with a flexible tab  27  that engages a strain gauge  26  either from direct axial pressure, or from rotational movement that may be required to seat contactor  15  in place. Tonometer  10  can be made inoperable until a fresh contactor  15  is properly positioned, and strain gauge  26  can determine this and also distinguish between a used contactor that is reinserted and an unused contactor inserted for the first time. 
         [0021]    The embodiment of  FIG. 6  illustrates an alternative possibility. Its contactor  15  is thimble shaped with a side wall formed to include a flexible tab  27 . Contactor  15  preferably has a snap fit onto the end of tube  11 , which is mounted on diaphragm  13 . Snapping contactor  15  onto the end of tube  11  requires that strain gauge  26  measure the required flexure of element  27 . If contactor  15  was previously mounted on the tonometer, element  27  will flex more easily than if contactor  15  is mounted for the first time on tube  11 . This allows strain gauge  26  to distinguish between a previously used contactor and a previously unused contactor. 
         [0022]    Since many tonometers are mounted on slit lamp microscopes where they enable an operator to view the eye and the affected corneal area during an examination, tonometer  10  can also accomplish this. As schematically shown in  FIG. 4 , a beam splitter  30  can direct light to a camera  20  positioned along side an optical viewing axis  31 . An operator looking through beam splitter  30 , along optical access  31 , can observe an eye  35  being examined, while camera  20  can also observe via the beam splitter  30  the size of an area deformed by contactor  15 . 
         [0023]    Tonometer  40 , as schematically shown in  FIG. 5 , eliminates coil  16  and magnet  17  and relies on manual force to press contactor  15  against a cornea. A resilient support  13 , such as an audio speaker diaphragm, resiliently holds tube  11  and allows for its movement as indicated by the double headed arrow. The amount of force manually applied is preferably monitored by strain gauge  28 , which can measure the displacement of tube  11  and window  15 . Linear bearing  12  supports tube  11  for this motion. A manually forced contactor  15 , such as illustrated in  FIG. 5 , can also be provided with a beam splitter  30  to move camera  20  off a viewing axis along which an operator can observe. Also, a manually pressed contactor can be used in either a portable or slit lamp mounted tonometer. Microprocessor  50  can receive signals from camera  20  and strain gauge  28  and can then calculate a characteristic of the eye being examined and provide the calculation to an operator via output  51 . 
         [0024]    Another difference in the tonometer of  FIG. 6  is that permanent magnet  17  is formed as part of tube  11 . Coil  16  and magnet  17  are preferably part of a miniature audio speaker, which is compact, inexpensive, and readily available. The embodiment of  FIG. 6  also eliminates any strain gauge measuring movement of contactor  15 . Such a strain gauge  28 , as shown in  FIGS. 1 and 5  is necessary if the contactor pressing force is applied manually, but is an optional possibility when contactor pressing force is applied by coil  16  and magnet  17 .