Apparatus for Automatically Testing Vision by Phase Measurements

This invention relates to vision testing equipment and, more particularly to apparatus for automatically generating eye glass prescriptions to correct faulty vision. In the past, one way to test vision was to have the patient read an eye chart, through a series of graded lenses. Using this technique, an eye glass prescription is determined from the patient's comments as to which lens produces the most clear image. This procedure however, has the undesirable characteristic of being dependent upon the patient's subjective opinion. Further vision correcting by this procedure takes a relatively long time. This is because the lens changing is manually performed, and because a large number of subjective opinions is usually required to be elicited from the patient before the optimal lens prescription can be determined.

In an attempt to overcome the above described and other deficiencies, various other vision testers have been devised. One such tester is disclosed in the U.S. Pat. No. 3,574,450 which was issued to Carol T. White in 1971. That vision tester presents a light stimulus through a graded series of lenses, and it further includes a computer having inputs which are coupled to receive voltage response signals from the patient's brain. In operation, the light stimulus is passed through one of the lenses and the magnitude of the evoked response is measured and remembered by the computer. Subsequently, a differently graded lens is manually selected and the steps are repeated. After a response has been measured through all of the lens, the computer calculates which of the evoked responses have the largest amplitude. Then the lens corresponding to that evoked response is indicated as giving the best visual correction.

Another prior art automatic vision testing device called "Dioptron" has been developed by Coherent Radiation Corporation of Palo Alto, Calif. This device passes a beam of light through a lens into the patient's eye, and includes apparatus for detecting the angle at which the light is reflected from the patient's retina. The angular information is fed to a computer; and based thereon, the computer calculates the lens prescription for the patient.

All of these devices have deficiencies however, which are overcome by the disclosed invention. For example, the apparatus in U.S. Pat. No. 3,574,450 includes a graded series of lenses which are manually selected during the testing process. Accordingly, testing vision with the apparatus disclosed therein is not fully automatic and is relatively slow. Similarly, the Dioptron tester is deficient in that is does not utilize signals evoked in the brain to determine the lens correction. Instead it utilizes signals which are derived from the mechanical characteristics of the eyeball. As such, the results obtained do not take into consideration the effect of the optic nerve, the optic chiasm, the occipital lobe, and other parts of the brain which affect the patient's total vision. Further, the disclosed invention differs from both U.S. Pat. No. 3,574,450 and the Dioptron device in that it utilizes a totally different signal processing scheme to determine a lens prescription. In particular, the disclosed invention processes variations in time lapse that occurs between an input stimulus to the eye and an evoked response in the brain. That is, the disclosed invention processes phase as opposed to magnitude of the evoked response.

Therefore, it is one object of the invention to provide improved apparatus for automatically testing spatial vision.

Still another object of the invention to provide a controller for automatically testing vision based on a variable time delay which occurs between applications of a light stimulus to the eye and an evoked response from the brain.

These and other objectives are accomplished in accordance with the invention by a controller which includes an optical mechanism containing a target, at least one lens, and an eyepiece. This target, lens, and eyepiece are aligned along one optical path. Either the target or lens have selectable positions on this path.

Also included are motorized drive circuits and a light emitting circuit. The motorized driver circuits move the target or lens to the selectable positions along the optical paths in response to predetermined digital control signals. Similarly, the light circuit enables a light stimulus to be directed through the target, lens and eyepiece in response to other predetermined digital control signals.

The controller further includes a digital control circuit. It has outputs coupled to the motorized driver circuit and the light circuit; and it generates the predetermined digital control signals on these outputs. These signals are generated in a sequence such that the lens or target is moved to various positions along the optical path and a light stimulus is enabled at each of these positions.

The digital control circuit also has inputs for receiving electrical signals evoked in the patient's brain by the light stimulus. These evoked signals are used by the digital control circuit to determine a corrective lens prescription. In particular, the digital control circuit measures the time delay between the light stimulus and the evoked response at each of the various positions for the lens and target; and the corrective prescription is based on which of these time delays is the longest.

The novel features believed characteristic of the invention are set forth in the appended claims; the invention itself, however, as well as other features and advantages thereof, will best be understood by reference to the following detailed description of preferred embodiments, when read in conjunction with the accompanying drawings, wherein:

FIG. 1 is a block diagram of apparatus constructed according to the invention for automatically testing vision and calculating corrective lens prescriptions.

FIG. 2 is a block diagram illustrating the components of a motorized drive circuit within the apparatus of FIG. 1.

FIG. 3 is a detailed logic diagram of a portion of the motorized drive circuit of FIG. 2.

FIG. 4 is a detailed block diagram of the light enabling circuit within the apparatus of FIG. 1.

FIG. 5 is a set of curves illustrating signals at various points within the apparatus of FIG. 1.

FIG. 6 is a detailed circuit diagram of an evoked response input amplifier within the apparatus of FIG. 1.

FIG. 7 is a flow diagram illustrating various sequences directed by a digital control circuit within the apparatus of FIG. 1.

FIG. 8 is a logic diagram of a portion of the output circuit within the apparatus of FIG. 1.

Referring now to FIG. 1, there is illustrated a block diagram of vision testing apparatus constructed according to the invention. Included therein as one portion of this apparatus is as optical lens system 10. This lens system is comprised of a target 11, at least one lens 12, and an eye piece 13 respectively aligned along one optical path 14. In general, target 11 and/or lens 12 are movable along path 14 in a manner that is suitable for testing either spherical correction, cylindrical correction, axial correction, or any combination thereof. Preferably, lens 12 include a spherical lens 12a, a plano convex lens 12b, and a plano concave lens 12c as illustrated.

Basically, spherical correction is determined by moving target 11 along path 14; cylindrical correction is determined by moving lens 12c along path 14; and axial correction is determined by rotating lens 12b and 12c about path 14. Also included within lens system 10 is a strobe light 15 which provides a light stimulus to be directed through target 11 and lens 12 into eye piece 13. In operation, target 11 and lens 12 are iteratively positioned along path 14; and light 15 provides a light stimulus at each of these positions.

A motorized drive circuit 20 couples via leads 21 to lens system 10; and similarly, a light drive circuit 30 couples via leads 31 to the lens system. Drive circuit 20 sequentially moves target 11 and lens 12 to various selectable positions along path 14 in response to digital control signals on leads 41. These signals are generated by a digital control circuit 40. Similarly circuit 30 causes light to be emitted from strobe light 15 and directed through target 11 and 12 at each of the various positions in response to other digital control signals on leads 41 for control circuit 40.

In operation, control circuit 40 moves target 11 and lens 12 to one predetermined position by sending appropriate control signals to drive circuit 20. Subsequently, control circuit 40 causes multiple strobes of light to be passed through the positioned target and lens. In response to these strobes of light, electrical signals are evoked in the patients brain. These signals are detected by the electrodes 42 on the patients scalp; and are fed to control circuit 40 via leads 43.

There, the phase relation between the control signals that cause the light stimulus, and the evoked response is measured and stored. This measurement is made several times (10 or 20 times) for each particular position of target 11 and lens 12. These measurements are then averaged by control circuit 40 and stored for further computation.

Subsequently, control circuit 40 sends control signals to drive circuit 20 which move target 11 and lens 12 to a new position. Then control circuit 40 causes light stimulus to be emitted by circuit 30, and repeats the averaging sequence as described above. After the averaged evoked response for each of a plurality of lens and target positions has been stored, control circuit 40 analyzes the data in a manner to be described in conjunction with FIG. 5. Based on this analysis circuit 40 manifests the patients lens prescription by signals on leads 44 to an output device 45. In one preferred embodiment, output device 45 is a digital display. There, digits are selectively illuminated to display numbers indicating the spherical correction and cylindrical correction in diopters, and the axial correction in degrees.

An operator panel 46 also couples to controller 40 via leads 47. Panel 46 provides a means for manually initiating the above described functions. Suitably, panel 46 includes a plurality of switches that enable an operator to start the test, stop the test, and power the apparatus on and off.

Further details of drive circuit 20 and light drive circuit 30 will now be described in conjunction with FIG. 2. As therein illustrated, circuit 20 includes a plurality of address decoders 22a-22c. Leads 41 coupled to inputs on these decoders, which operate to detect predetermined combinations of digital signals on their inputs and generate output signals in response thereto.

Drive circuit 30 also includes pulse generators 23a-23c having inputs that are respectively coupled to the outputs of address decoders 22a-22c. The pulses produced by these generators are received by stepping motors 24a-24c. Motor 24a has a shaft 25a which couples to and moves target 11. Similarly, motor 24b has a shaft 25b which couples to and moves cylindrical lens 12c; while motor 24c has a shaft 25c which couples to and moves lens 12c and 12b.

In operation, target 11 is moved along path 14 in response to predetermined logic signals which are detected by address decoder 22a. This decoder activates generator 23a which send pulses of a predetermined width to motor 24a. In response to these pulses, motor 24a moves target 11 by a predetermined distance. Lens 12c is moved along path 14; and lens 12b and 12c are rotated about path 14 by similar sequences initiated by decoders 22b and 22c.

Drive circuit 30 also includes shaft encoders 26a-26c which respectively couple to motors 24a-24c. These encoders have output 27a-27c which are coupled back to digital controller 40 via leads 48. In operation, the encoders generate logic signals on their respective outputs indicating the position of the shaft of the motor to which they are coupled. Thus, encoders 26a-26c provide a means by which controller 40 may sense the result of an operation which it initiated by control signals on leads 41.

Next, referring to FIG. 3, a detailed logic diagram of components 22a-27a of FIG. 2 will be described. Components 22b-27b, and components 22c-27c are similar in structure, and thus a detailed drawing of them is not separately included. As illustrated in FIG. 3, decoder 22a consists of a single semiconductor chip 50. Chip 50 is a one-out-of-eight decoder, such as a T.sup.2 L 74136 as an example. Leads 41 coupled to inputs of chip 50; and the chip operates to decode various binary combinations of the input signals.

One of the decode outputs from chip 50 couples to the triggering input of a one-shot circuit 51; while another output couples to the triggering input of a second one-shot circuit 52. Suitably, these circuits consist of a T.sup.2 L 74123. Circuits 51 and 52 respectively have an RC timing circuit 53 and 54 coupled thereto. The actual values of the resistor and capacitor in these timing circuits determine the width of the output pulse from the one-shots.

The outputs of one-shots 51 and 52 connect to the control inputs of solid state relay circuits 55 and 56. In operation, relay 55 couples a 110 volt AC source at 57 to one terminal 58 of motor 24a; while relay 56 couples the 110 volt AC source to a second terminal 59 of the motor. Application of the 110 volts to terminal 58 causes motor 24a to move target 11 further away from lens 12c; while application of the 110 volts to terminal 59 causes motor 24a to move target 11 closer to lens 12c. The exact amount of movement of the lens is directly proportional to the length of time which the 110 volts is applied to the terminals; and this in turn is directly proportional to the width of the pulse from one-shots 51 and 52. A variety of stepping motors will suitably respond to the pulses from relays 55 and 56. In one embodiment, a 1/500 horsepower, shaded pole motor was used.

Next, with reference to FIG. 4, the details of light drive circuit 30 will be described. Basically, circuit 30 is comprised of an address decoder 61, a strobe drive circuit 62, and a feedback circuit 63. Address decoder 61 has inputs coupled to receive logical control signals from leads 41; and it operates to detect predetermined logical signals on those leads. Output signals from decoder 61 activate driver circuit 62, which in turn activate strobe light 15. Feedback circuit 63 generates digital signals on an output 65 which couples to leads 48 for detection by controller 40. This structure allows controller 40 to receive timing signals indicating a precise reference point at which the patient is subjected to the light stimulus.

The light stimulus emitted by strobe light 15 passes into the patient's retina and causes responses in the patient's brain which are detected by electrodes 42. More specifically, signals from the retina of each eye are transmitted down the optic nerve to the optic chiasma. There electrical signals from the right field of vision pass to the left half of the brain; and electrical signals from the left field of vision pass to the right half of the brain. From there the electrical signals pass through the thalamus and lateral geniculate to the striate area of the occipital lobe. As signals pass along the above described paths, various processing and functions are performed thereon by the brain. In the present invention, this activity is monitored by electrode 42. Preferably, this electrode is placed on the exterior of the scalp about one centimeter to the right or to the left above the inion. A second electrode preferably is also used as a reference; and it is placed on the ear over the temporal lobe.

FIG. 5 is a set of curves illustrating various evoked response signals which are detected by electrode 42, as well as feedback timing signals on leads 48. The former are illustrated as S42, and the latter as S48. Typically, signal S48 is a rectangular shaped pulse having a width of approximately 20 milliseconds. These pulses are repeated at some convenient repetition rate, such as one pulse every 240 milliseconds.

The present invention operates on the principal that clearness of vision is indicated by the time delays between the reference signal S48 and the occurrence of the evoked response S42. Clearest vision is indicated by a relatively long time delay; whereas blurred vision is indicated by a relative short time delay. This effect is explained by the theory that the brain requires more time to process signals along the optic track when those signals represent clearly perceived images, than when those signals represent blurred images.

To determine the time delay of an evoked response, controller 40 samples signal S42 and obtains samples 71. A sampling rate of 20 milliseconds per sample is suitable. In addition, controller 40 takes multiple readings for each particular setting of target 11 and lens 12. Ten or twenty such readings have been found suitable.

As indicated in FIG. 5, each particular evoked response falls easily within approximately 240 milliseconds. Thus twenty such evoked responses fall within 4.6 seconds, which is quite fast for the eye test. Samples from the twenty evoked responses are then averaged by controller 20; and then the average time delay is calculated. This average time delay is stored for future reference and comparison with other similarly calculated time delays obtained at different target and lens settings.

Preferably, before the signals detected by electrodes 42 are sampled, they are amplified. This is because they generally are no larger than 30 microvolts. FIG. 6 is a diagram of a circuit suitable for amplifying signals S42. This circuit has a gain of approximately 1.0.times.10.sup.5. Basically, the circuit is comprised of operational amplifiers 81 and 82. Low pass filters 83 and 84 respectively connect to the inputs on amplifiers 81 and 82, and they have a cutoff frequency at approximately 30 HZ. Feedback circuits 85 and 86 respectively connect to operational amplifiers 81 and 82 to determine the gain of each of these amplifiers. Resistance and capacitances of the values indicated in FIG. 6 may be utilized to achieve the above described gain and frequency characteristics.

In FIG. 7, there is illustrated a flow diagram describing a series of steps which digital controller 40 performs during a vision test. At the start of the test, controller 40 moves target 11 to a +15 diopter position along path 14; moves cylindrical lens 12c and 12b to a zero power position; and rotates cylindrical lens 12c and 12b to a zero degree setting about path 14. These steps are performed by motorized drive circuits 20 and 30 in response to control signals on leads 41 as was previously described.

Subsequently, logical controller 40 activates light drive circuit 30 and measures the response evoked from the patient on electrodes 42. Ten responses are evoked; and the average of these responses is calculated by logical controller 40. Then target 11 is moved by a 1/4 diopter; and ten new responses are taken and averaged at the new target setting. These steps are repeated until target 11 has moved to a -10 diopter position along paths 14.

Next, control circuit 40 operates on each of the averaged responses to determine the target position at which the longest delay in the evoked response occurred. After making this calculation, the target is returned to that position and the long response time is verified. Upon verification, control circuit 40 has all of the information necessary to calculate a prescription for spherical lens correction.

The remaining steps performed by controller 40 are made to determine the required cylindrical correction and axial correction. To this end, controller 40 takes averaged readings with the cylindrical power of lens 12b and 12c being changed in steps of 1/4 diopter, and the axial orientation of lens 12b and 12c being rotated to 0.degree., 45.degree., 90.degree., and 135.degree.. At each cylindrical power setting, the averaged evoked responses are compared with corresponding previous responses to determine which gave the longest delay. If any increase occurs, the cylindrical power is again changed by a -1/4 diopter and the testing continues. Conversely, if no increase occurs, testing ceases; and controller 40 displays the lens prescription corresponding to the longest delay via output circuit 45.

Each of the above described steps may be carried out by special purpose hardwired logic circuitry; or alternatively, may be carried out by a general purpose computer. Using the latter, it is only necessary that the processor have the capability to transmit and receive messages from external circuits such as 20, 30, and 42; and have computing capability sufficient to perform the above described decision making and averaging. In one preferred embodiment, control circuit 40 is a PDP 11/40 computer. A listing of a program in RT-11 compiler language for this computer which performs the above tasks is given in Table 1.

Details of one preferred output circuit 45 is illustrated in FIG. 8. Specifically, FIG. 8a is a detailed logic diagram illustrating circuit interconnections while FIG. 8b is a pictorial diagram illustrating the output circuit as viewed by the operator. In this circuit, numeric display chips 91-93 and individual light emitting diodes 94-95 display the calculated lens prescription. Specifically, chips 91-93 display the numerical amount of spherical correction required; while diodes 94 and 95 display whether near-sighted or far-sighted correction is required. Output circuit 45 preferably also includes second and third circuits similar to those illustrated in FIG. 8A. These respectively display cylindrical correction and axial correction. TI-TI/311 chips may suitably be used as chips 91-93.

Various embodiments of the invention have now been described in detail. Further, many changes and modifications can be made to the above described details without departing from the nature and spirit of the invention. For example, the strobe light 15 may be replaced by a light of constant luminance with the visual stimulus being provided by a target 11 which periodically changes its position. For example, target 11 may include a checkboard pattern of black and white squares which alternately change position. Utilizing this pattern shifting, any effect on the evoked response of the intensity of the strobe light is minimized. ##SPC1##

As another modification, the sequence of steps by which logical controller 40 automatically performs the vision test is not limited to that described in conjunction with FIG. 7. For example, logical controller 40 may perform specialized sequences depending upon whether the patient already has a lens prescription, or whether the patient has some particular eye disease such as glaucoma. Such special testing sequences could be stored in read only memory (ROM) chips includes within controller 40. Therefore, since many changes and modifications can be made to the above details without departing from the nature and spirit of the invention, it is to be understood that the invention is to be limited only as set forth in the appended claims: