the following specification , taken in conjunction with the drawings , sets forth the preferred embodiments of the present invention in such a manner that any person skilled in the optical and medical fields can utilize the invention . the embodiments of the invention disclosed herein are the best modes contemplated by the inventor in carrying out his invention in a commercial environment , although it should be understood that various modifications can be accomplished within the parameters of this invention . the human eye is enclosed by three membranes . the uppermost one consists of the transparent cornea , forming the outer bulge , and the opaque sclera enclosing the remainder of the eyeball . the choroid coat contains many nerves and blood vessels and is immediately under the sclera . the innermost membrane of the eye is the retina which lines all the posterior wall of the fundus . behind the cornea is the iris and the crystalline lens . muscles in the eye change the size of the pupil for opening an aperture to the lens and other muscles change the shape of the crystalline lens to permit focusing . two chambers , one anterior to the lens and the other posterior , are filled with transparent material . the first with aqueous humor and the second with the vitreous humor . the aqueous humor has a water consistency similar to blood plasma while the vitreous humor is jelly - like . although the retina includes blood vessels , the major blood supply of the eye is the choroid layer . light rays directed towards an eye can experience reflection from the cornea , the fundus and even the iris . in generating a plethysmogram from the fundus of the eye , it is known that the intensity of the reflected light from the fundus will vary due to a pulsation of blood through the blood vessels distributed throughout the fundus , such as in the choroid layer . additionally , the eye itself is constantly seeking to accommodate ambient light and the eyeball will tremble in minute movements , even when a patient is apparently steadily looking at a point object . this minute trembling of the eyeball can result in irregular variations in the light rays which are reflected from the eye to produce a resultant disturbing noise in the reflected light signal . generally , the eyeball movements are in the order of 30 to 100 hz . referring to fig1 a schematic view of one embodiment of the present invention is disclosed . in fig1 a pickup module 4 is in direct contact with the exterior cornea of the eyeball 2 . the cornea contacting area can be formed from a plastic such as polyacetal that is capable of being repetitively sterilized . the configuration of the opening aperture of the pickup module 4 is complimentary to the exterior configuration of the cornea . a light source 6 , for example a tungsten lamp or the like , is appropriately mounted and energized to direct light through a band pass filter 8 having the capacity of transmitting a wavelength band between 550 to 1000 mμ . as can be appreciated , a plurality of filters could be used to provide the same desired transmission band . a fiber optic light guide or conduit 10 carries the light rays from the light source to the pickup module 4 and transmits them into the eye 2 . a second fiber optical light guide 20 can receive the reflected light from the eye fundus and transmit it for subsequent processing . a condenser lens 12 can be mounted adjacent an end of the light guide 20 for focusing the light rays on a dichroic mirror 14 . a first light sensing device 16 and a second light sensing device 18 , such as photodetectors , are aligned with the dichroic mirror 14 . the dichroic mirror 14 has the capability of transmitting a critical wavelength of approximately 650 to 700 mμ , while the second light sensing device 18 receives near infrared light of wavelengths 700 to 1000 mμ . as can be appreciated , the dichroic mirror 14 can be replaced by a semi - transparent mirror if a pair of interference filters having corresponding transmitting wavelengths are located in front of respectively the first and second light receiving devices 16 and 18 . fig2 is a schematic cross sectional model of a portion of an eye fundus disclosing the retinal layer i , the pigmented epithelium layer ii , and the choroidal layer iii , respectively . a light ray from the visible spectrum , v , for example 550 to 650 mμ , is transmitted through the retinal layer i and reflected back from the pigmented epithelium layer ii . the near infrared light rays , ir , are transmitted through each of the respective layers before they are reflected back . therefore , the visible light , v , which is reflected from the eye fundus and enters the first light sensing device 16 , will carry with it information relating to the circulation of blood in the retinal layer i . the near infrared light , ir , reflected from the eye fundus and sensed by the second light sensing device 18 , will carry information relating to the circulation of blood in the choroidal layer iii . fig3 is a schematic circuit diagram that has been simplified to disclose the broad operation of the electrical circuit of the present invention . basically , light transducers or sensors convert the time varying intensities of light received by each of the light sensing devices 16 and 18 into corresponding voltage signals which can be amplified nd filtered to drive a recorder or display device . each of the light sensing devices 16 and 18 will generate a respective plethysmogram which can be synchronized with conventional techniques . a pen recorder 22 or the like can record synchronously on a piece of paper the pair of plethysmographs respectively of visible and near infrared light rays to permit a synchronized comparison between the pair of plethsymographs . alternatively , an oscilloscope display or the like can be used to display synchronously on a screen the pair of plethysmographs to permit a direct observation and comparison of the synchronized wave forms of these plethysmographs . fig4 discloses actual examples of a clinical examination by means of the present invention . in fig4 ir 1 and v 1 are a pair of synchronized plethysmograms for , respectively , the near infrared light rays and the visible light rays for a first patient , while ir 2 and v 2 are similar synchronized plethysmograms for a second patient . the medical diagnosis was that the first patient was normal while the second patient was suffering from diabetic retinopathy . as can be seen , the plethysmograms ir 1 and v 1 for the first patient have wave forms that are substantially similar , although the amplitudes differ from each other . referring to the second set of plethysmograms , ir 2 and v 2 , the wave form of v 2 is almost 180 degrees out of phase or inverted in comparison with that of ir 2 . this variance in the wave form or phase of the wave of the plethysmogram for the visible ray as compared to the synchronized plethysmogram for the near infrared ray provides an objective diagnosis of the diabetic retinopathy of the eye . other diseases than diabetic retinopathy such as black out or macular hale can be detected by a synchronized comparison of plethysmograms which are created by visible rays and near infrared rays , respectively . fig5 and 6 disclose schematic illustrations of the pickup module of the present invention and the manner in which it can be mounted in order to receive a light signal of a high s / n under stabilized conditions . as is appreciated , it is very difficult to obtain from the eye fundus an optical signal of a high s / n under a stabilized condition since the reflecting power of the eye fundus is relatively low and the illumination of the eye fundus and the picking up of the reflected light therefrom are limited by the narrow aperture of the pupil of the iris . generally , the patient will receive a mydriatic drug to dilate the pupil prior to any light measurement of the eye . the actual dilation of a pupil will vary depending upon the age of the patient , and even upon the color of the patient &# 39 ; s eye . generally , a dilated pupil will have a diameter in the range of 10 mm to 7 mm . the conventional technique of using an eye fundus camera is to illuminate the eye from light transmitted through the peripheral edge of the pupil with the reflected light being picked up through the central area of the pupil . the reason for this particular arrangement is to ensure that the transmission of the most significant light containing the information , that is , the reflected light , will occur through the center of the pupil wherein the optical condition is best and the amount of undesirable reflection can be minimized . the use of an optical fiber to introduce illuminating light into the eye will produce a light pencil emerging from the end of the optical fiber that will be divergent within a considerable angle . therefore , even if the diameter of the light exit is less than that of the pupil dilated by the mydriatic drug , a portion of the illuminating light will extend beyond the pupil and inevitably impinge on the iris with corresponding reflection to create significant intensity of signal noise . attempts to decrease the diameter of the light exit , however , will be unfavorable to the projection of a necessary and sufficient amount of illuminating light . the present invention does not attempt to utilize the conventional use of peripheral illuminating light , but instead mounts the light transmitting fiber optic 10 so that its light exit area 10a is aligned with an axis transverse to and extending approximately through the center of the pupil of the eye . the light receiving fiber optic guide 20 , as shown in fig5 is mounted so that its light receiving or entrance portion 20a is located concentrically around the peripheral area of the light exit 10a . the radius of the periphery of the light entrance 20a is less than the radius of the pupil 2a of the eye when dilated by the mydriatic drug . by adopting this particular arrangement , the restrictive aperture of pupil 2a is then most effectively utilized . the physical dimensions of the combined light exit 10a and light entrance 20a is approximately equal to the area of the pupil 2a in the mydriatic condition . the diameter of the periphery of the light exit 10a is sufficiently less than that of the mydriatic pupil 2a by the corresponding width of the ring - shaped light entrance 2a that surrounds light exit 10a . as a result of this configuration , the light exit 10a is sufficiently small so that all the light emerging from light exit 10a will pass through the pupil 2a even though the emerging light pencil will be divergent . as a result , the noise which would be caused by reflection from the iris is avoided , and the illumination of the eye fundus is increased . additionally , by mounting the light exit 10a and light entrance 20a immediately adjacent the exterior surface of the cornea , reflection from the cornea will not be a significant problem . to eliminate the creation of noise from the minute eye movement , the pickup module 4 includes a pliable exterior tube 4a that surrounds and is spaced from the fiber optic members . the relative dimensions of the end of the tube 4a , light exit 10a , and light entrance 20a are capable of being in contact with the exterior surface of the cornea . the tube 4a is spaced from the fiber optics and that space along with a connecting manifold can be attached to a pump 28 that also can include an output gauge for monitoring the reduction in air pressure to provide a subatmospheric suction for holding the tube 2a against the cornea of the eye . complementing this suction connection of the eye module pickup 4 to the eyeball 2 is a flexible mounting or suspension member disclosed in fig6 . thus , the module pickup 4 can be loosely supported by a spring 26 suspended from a fixed base 24 , thereby preventing the weight of the pickup module 4 from pressing against the cornea and for allowing substantially free movement of the pickup module 4 to follow minute motions of the eyeball 2 . by carefully controlling the relative displacement and spring forces utilized , the module pickup 4 can be rendered substantially weightless relative to the eye while adhering to the cornea of the eye , will not burden the eyeball with any additional pressure , and will become substantially one with the movement of the eyeball . as a result of this arrangement , the light signals that are obtainable for generating the plethysmograms will come from a stabilized eye condition irregardless of any movement of the patient or his eye . as a result of this novel arrangement , any efforts to manually press the pickup module 4 on the patient &# 39 ; s eye will not be necessary and the relative position of the pickup module 4 to that of the pupil will be maintained and thereby will eliminate any potential sources of errors to create a noise in the resultant signal . accordingly , a non - invasive ophthalmic apparatus for measuring the eye is provided that not only facilitates the positioning of the measuring eye module relative to the eye , but further holds the module on the eye with substamospheric pressure . accordingly , an easy measurement and generation of a pair of plethysmograms that can be synchronized for diagnosis is possible without inflicting any substantial discomfort or pain on the patient . as can be appreciated by a person skilled in this field , variations of the present invention can be accomplished and accordingly , the scope of the present invention should be measured solely from the following claims .
the present invention provides an ophthalmic device to diagnose the circulation of blood in an eye fundus by producing a pair of plethysmograms for synchronized comparison . a pair of fiber optic conduits are attached to a coupler that can be sealingly affixed to the exterior surface of the cornea of an eye . the coupler can be attached to the exterior surface of the eye by the application of subatmospheric pressure within the coupler . light is introduced on the axis of the pupil through a central light exit region that is physically small enough relative to a dilated pupil to prevent the reflection of light from the iris . the light reflected from the eye fundus is received in a peripheral light receiving region . the light signals are converted into electrical voltage levels that are ultimately used to produce a visual display of the plethysmograms .