Abstract:
A method of compensating for misalignment between an optical measurement instrument and a model eye includes: receiving a light beam from the model eye at the optical measurement instrument; producing image data, including light spot data for a plurality of light spots, from the received light beam; determining an observed location of a corneal reflex from the model eye within an image representing the image data; and determining an angle of misalignment between an axis normal to the front surface of the model eye and the optical axis of the optical measurement instrument from the observed location of the corneal reflex within the image.

Description:
CROSS-REFERENCES TO RELATED APPLICATION 
       [0001]    This application claims priority to U.S. Provisional Application No. 61/490,139, filed on May 26, 2011, the entirety of which is hereby incorporated by reference. 
     
    
     BACKGROUND AND SUMMARY 
       [0002]    1. Field 
         [0003]    This invention pertains to optical measurement instruments, and in particular a method of verifying the correct operation of an optical measurement instrument by using a model eye, and an optical measurement instrument that employs such a method. 
         [0004]    2. Description 
         [0005]    It is sometimes necessary to be able to verify correct operation and specified performance of an optical measurement instrument such as a wavefront aberrometer in an operational setting. In many instances, this is done by operating the optical measurement instrument to make a measurement of a model eye whose characteristics are known. A common version of a model eye is a solid glass or plastic component with a curved front surface and a flat back surface. The front curve serves the role of a “cornea” for the model eye, and the back surface serves as a “retina” for the model eye. Some model eyes have a limiting aperture that serves as an “iris” for the model eye. The aperture is most commonly located in front of the front surface of the model eye, but it can also be inside the model eye. 
         [0006]    To verify correct operation and specified performance of an optical measurement instrument, typically the optical measurement instrument injects a probe beam into a front surface of the model eye. Light scatters from the back surface of the model eye similarly to the way it does with a human eye, and some of the scattered light travels back out of the front surface of the model eye and into the optical measurement instrument. From the received light, the optical measurement instrument makes one or more measurements of the model eye. Typically, the optical measurement instrument measures the sphere and/or cylinder values of the model eye, and compares these measured values with corresponding predetermined calibration data for the model eye to determine whether or not the optical measurement instrument is operating properly. The values must agree within some tolerance for the optical measurement instrument to be considered in good working order. 
         [0007]    However, the values for sphere and cylinder that an optical measuring instrument measures will vary depending on the angle that an axis normal to the model eye makes with respect to the optical measurement instrument&#39;s optical axis (hereinafter referred to as “the misalignment angle”). The predetermined calibration data assumes a misalignment value of zero degrees. Even when the optical measurement instrument is operating perfectly, when the misalignment angle is not zero degrees then there will be a variance between the measured sphere and cylinder values for the model eye and the predetermined calibration values. This variation in the measured sphere and/or cylinder values that depends on the misalignment angle between the optical measurement instrument and the model eye makes it hard to verify proper operation of the optical measurement instrument. For example, experiments have been performed with an example optical measurement instrument making measurements on a model eye that has a front surface curvature that matches that of a human cornea. With a misalignment angle of only three (3) degrees, the model eye measurements were 0.5 Diopters different from the calibration value for perfect alignment (i.e., zero degree misalignment angle). This variation was far in excess of the maximum tolerable variation of 0.1 Diopters for the example optical measurement instrument. 
         [0008]    It should be noted that the problem described here is unique to measuring model eyes. This misalignment does not occur when measuring a human eye, because the patient directs their gaze straight into the optical measurement instrument to view a fixation target of the optical measurement instrument, thus automatically aligning the human eye with the optical axis of the optical measurement instrument. 
         [0009]    In contrast to this simple method of aligning a human eye by means of a fixation target, it may be difficult and/or time-consuming for an operator through trial-and-error to achieve a degree of alignment between a model eye and the optical measurement instrument&#39;s optical axis that renders insignificant the variation in the measured sphere and/or cylinder. One solution is to constrain the model eye by mechanical means so it points directly toward the optical measurement instrument. However, this approach adds expense to the model eye mount and may not reduce the measurement variation to within a desired tolerance. 
         [0010]    Therefore, it would be desirable to provide a method of verifying proper operation with an optical measurement instrument with a model eye that can address variations in measurements that occur when the model eye is misaligned with respect to the optical measurement instrument. It would also be desirable to provide an optical measurement instrument that can operate with such a method. 
         [0011]    In one aspect of the invention, a method comprises: receiving a light beam from a model eye at an optical measurement instrument having an optical axis; producing image data, including light spot data for a plurality of light spots, from the received light beam; determining an observed location of a corneal reflex from the model eye within an image representing the image data; and determining an angle of misalignment between an axis normal to the front surface of the model eye and the optical axis of the optical measurement instrument from the observed location of the corneal reflex within the image. 
         [0012]    In another aspect of the invention, a measurement instrument comprises: one or more light sources configured to illuminate a model eye; a light spot generator configured to receive light from the model eye and to generate a plurality of light spots from the light received from the illuminated object; a detector configured to detect the light spots and for outputting image data, including light spot data for the plurality of light spots; and a processor. The processor is configured to process the image data to determine an alignment between the measurement instrument and the model eye by: determining an observed location of a corneal reflex from the model eye within an image representing the light spot data; and determining an angle of misalignment between an axis normal to the front surface of the model eye and an optical axis of the measurement instrument from the observed location of the corneal reflex within the image. 
         [0013]    In yet another aspect of the invention, a method is provided for determining a misalignment between a measurement instrument and a model eye used to verify correct operation of the measurement instrument, by determining a difference between: (1) an observed location of a corneal reflex in an image produced by the measurement instrument from the model eye, and (2) an expected location of the corneal reflex. 
         [0014]    In still another aspect of the invention, a method comprises: receiving a light beam from a model eye at an optical measurement instrument having an optical axis; producing image data, including light spot data for a plurality of light spots, from the received light beam; determining an observed location of a corneal reflex from the model eye within an image representing the image data; defining an analysis area within an image represented by the image data, wherein the analysis area is centered on the observed location of the corneal reflex; and measuring at least one of a sphere value and a cylinder value for the model eye from a portion of the light spot data corresponding to light spots within the analysis area. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  illustrates an example embodiment of an optical measurement instrument making a measurement with an example embodiment of a model eye to verify correct operation and specified performance of the optical measurement instrument. 
           [0016]      FIG. 2  illustrates one example embodiment of a model eye. 
           [0017]      FIG. 3  illustrates an example of a misalignment between an optical measurement instrument and a model eye used to verify correct operation and specified performance of the optical measurement instrument. 
           [0018]      FIG. 4  illustrates an example of a raw image from a wavefront sensor produced from a model eye. 
           [0019]      FIG. 5  illustrates an example of a raw image from a wavefront sensor and a center of a portion of the image defined by an iris of the model eye. 
           [0020]      FIG. 6  shows a flowchart illustrating one embodiment of a method of verifying proper operation of an optical measurement instrument with a model eye. 
           [0021]      FIG. 7  illustrates an example of a raw image from a wavefront sensor and a portion of the image used for wavefront analysis. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Exemplary embodiments of model eyes and methods for verifying proper operation and performance of optical measurement equipment through use of a model eye will be described in some detail below so as to illustrate various aspects and advantages of these devices and methods. However, it should be understood that the principles involved in these devices and methods can be employed in a variety of other contexts, and therefore the novel devices and method disclosed and claimed here should not be construed as being limited to the example embodiments described below. 
         [0023]      FIG. 1  illustrates an example embodiment of an optical measurement instrument  10  making a measurement with an example embodiment of a model eye  20  to verify correct operation and specified performance of an optical measurement instrument. Here optical measurement instrument  10  may be a wavefront aberrometer. Optical measurement instrument  10  includes, among other elements, a coherent light source (e.g., a laser or SLD)  12 , a beamsplitter  14 , a wavefront sensor  16 , a processor  17  and associated memory, and optionally a display  18 . In some embodiments wavefront sensor  16  may be a Shack-Hartmann wavefront sensor including a lenslet array  16   a  and a pixel array  16   b  (e.g., camera, charge-coupled-device (CCD) or CMOS array). In various embodiments, optical measurement instrument  10  may include a variety of other elements not shown in  FIG. 1 , such as optical elements (e.g., lenses, mirrors, etc.), a fixation target, aperture stops, etc. Processor  17  may execute algorithms to control operations of optical measurement instrument  10 , for example by executing instructions in accordance with program code stored in the associated memory, which may include one or more of volatile memory (e.g., random access memory), nonvolatile memory, Flash memory, a hard disk drive, optical disk drive, etc. Processor  17  may operate according to n operating system, which may be a generic operating system such as WINDOWS®, MACINTOSH® Operating System, UNIX, etc., or may employ a custom operating system. Processor  17  may provide a user interface (e.g., a graphical user interface) for an operator of optical measurement instrument  10  and may display measurement results via display  18 . 
         [0024]    Model eye  20  has a front surface  21 , and a rear or back surface  22 , and an iris  23 . Front surface  21  may be curved to focus light onto rear surface  22  such that front surface  21  acts as a “cornea” for model eye  20 , and rear surface  22  acts as a “retina” for model eye  20 . 
         [0025]      FIG. 2  illustrates in greater detail one example embodiment of model eye  20 . Model eye  20  includes a model eye holder or mount  25 , and an opaque structure  22  having an aperture  24  therethrough disposed in front of the front surface  21  and which may act as an “iris” for model eye  20 . Beneficially, mount  35  may be adjustable by an operator in x, y, z directions, and may be tiltable and rotatable. 
         [0026]    To verify that optical measurement instrument  10  is performing correctly, coherent light source  12  generates a probe light beam  3  which is injected into front surface  21  of model eye  20 . Light scatters from rear surface  22  of model eye  20  and some of the scattered light travels back out of front surface  21  and into optical measurement instrument  10  as a return beam  5 . Return light beam  5  is provided to wavefront sensor  16  which can operate with processor  17  to make one or more measurements of one or more characteristics of model eye  20 , for example a sphere and/or cylinder value for model eye  20 . The measured value(s) can be compared with known or previously measured calibration value(s) of sphere and/or cylinder of model eye  20  to allow a determination to be made as to whether optical measurement instrument  10  is operating correctly and/or within its specified performance tolerances. 
         [0027]    As noted above, in practice it can be difficult to precisely align optical measurement instrument  10  and model eye  20 . 
         [0028]      FIG. 3  illustrates an example of a misalignment between optical measurement instrument  10  and model eye  20 . As shown in  FIG. 2 , optical measurement instrument  10  has an optical axis  19  corresponding to the axis upon which probe light beam  3  is directed toward model eye  20 .  FIG. 3  also illustrates an axis  29  normal to the front surface  21  or “cornea” of model eye.  FIG. 3  also illustrates a misalignment angle θ between optical axis  19  and normal axis  29 . For clarity of illustration, misalignment angle θ is somewhat exaggerated in  FIG. 3  compared to a typical example where misalignment angle θ might be more in the range of about three degrees or so. 
         [0029]      FIG. 4  illustrates an example of a raw image  400  from a wavefront sensor  16  of optical measurement instrument  10  produced from the light of return light beam  5  received from model eye  20 . In particular, raw image  400  may be generated in a case where wavefront sensor  16  is a Shack-Hartmann wavefront sensor. For example, lenslet array  16   a  of wavefront sensor  16  may receive the return light beam  5  and in response thereto may produce image  400  on detector array  16   b , including a plurality of light spots  410 . Detector array  16   b  outputs image data corresponding to the image, including light spot data for the plurality of light spots  430 , and processor  17  processes the image data to determine one or more characteristics of model eye  20 . 
         [0030]    As explained above, during the process for verifying that optical measurement instrument  10  is performing correctly, the characteristics of model eye  20  measured by are varied because of the misalignment angle θ, and this variance may be greater than an allowable tolerance for optical measurement instrument  10 . 
         [0031]    To address this problem, optical measurement instrument  10  may execute an algorithm using the single bright spot near the center of image  400 , known as the corneal reflex  420 , to determine whether optical instrument  10  is operating properly even when there is a significant misalignment angle θ between normal axis  29  of model eye  20  and optical axis  19  of optical measurement instrument  10 . Corneal reflex  420  is produced from the reflection of the probe light beam  3  as it enters model eye  20 . Corneal reflex  420  is also known in the art as the Purkinje I reflection. Other Purkinje reflections also come from the posterior cornea, anterior lens and posterior lens surfaces, and these are referred to as the Purkinje II, III and IV reflections, respectively. Purkinje analysis can be used to calculate anatomical structures of an eye such as lens curvatures and tilts. 
         [0032]    Several embodiments will be described below. 
         [0033]    In a first embodiment, optical measurement instrument  10  (e.g., processor  17  of optical measurement instrument  10 ) may execute an algorithm that uses the observed location of corneal reflex  420  to determine misalignment angle θ and to compensate the measured characteristics (e.g., sphere and/or cylinder) of model eye  20  for variances caused by the misalignment angle θ. 
         [0034]    Corneal reflex  420  is located where the normal axis  29  to the surface of “cornea”  21  of model eye  20  is aligned with optical axis  19  of optical measurement instrument  10 . When aperture  24  defined by iris  23  is located centrally around normal axis  19 , and with model eye  20  aligned with optical measurement instrument  10 , then corneal reflex  21  should appear in the center of the portion of image  400  corresponding to aperture  24  defined by iris  23 . This can be considered to be the expected location of the corneal reflex  430 . 
         [0035]      FIG. 5  illustrates a boundary  430 , e.g. a circular boundary, circumscribing a portion of image  400  corresponding to aperture  24  defined by iris  23  of model eye  20 . Processor  17  may determine boundary  430  from intensity values for the light spot data corresponding to light spots  410 . Using the determined boundary  430 , then optical measurement instrument  10  (e.g., processor  17  of optical measurement instrument  10 ) may determine an expected location  440  of the corneal reflex as the center of boundary  430 . 
         [0036]    From the observed location of corneal reflex optical measurement instrument  10  (e.g., processor  17  of optical measurement instrument  10 ) 
         [0037]    Once misalignment angle θ is known, then optical measurement instrument  10  (e.g., processor  17  of optical measurement instrument  10 ) may compensate the measured characteristics (e.g., the sphere and cylinder values) for model eye  20  for the misalignment angle θ to produced compensated values. The compensation to be applied to the measured values can be determined from correction values stored in a memory in optical measurement instrument  10  that were previously determined from experimental measurements, or can be determined by performing a ray tracing algorithm using the misalignment angle θ. Finally, optical measurement instrument  10  (e.g., processor  17  of optical measurement instrument  10 ) compares the measured values to predetermined calibration values for model eye  20  to determine whether or not optical measurement instrument  10  is operating properly and within specifications. The predetermined calibration values may be determined theoretically, or experimentally during a qualification process for optical measurement instrument  10  by aligning the normal axis  29  and the optical axis  19  of optical measurement instrument  10  to within a predetermined tolerance, generating calibration image data while the normal axis  29  and the optical axis  19  are aligned within the predetermined tolerance, and processing the calibration image data to extract the calibration values. 
         [0038]      FIG. 6  shows a flowchart illustrating one embodiment of a method of verifying proper operation of an optical measurement instrument with a model eye. 
         [0039]    In a step  610 , an optical measurement instrument receives a light beam from a model eye. 
         [0040]    In a step  620 , the optical measurement instrument, and particularly a wavefront sensor of the optical measurement instrument, produces image data, including light spot data for a plurality of light spots, from the received light beam. 
         [0041]    In a step  630 , the optical measurement instrument, and particularly a processor of the optical measurement instrument, calculates or measures one or more calibrated characteristics of the model eye (e.g., a sphere value and/or a cylinder value of the model eye) from the light spot data. 
         [0042]    In a step  640 , the optical measurement instrument, and particularly the processor of the optical measurement instrument, determines an expected location of a corneal reflex from the model eye within an image representing the image data. 
         [0043]    In a step  650 , the optical measurement instrument, and particularly the processor of the optical measurement instrument, determines the observed location of the corneal reflex from the model eye within the image representing the image data. 
         [0044]    In a step  660 , the optical measurement instrument, and particularly the processor of the optical measurement instrument, compares the observed location of the corneal reflex to the expected location of the corneal reflex. 
         [0045]    In a step  670 , the optical measurement instrument, and particularly the processor of the optical measurement instrument, determines an angle of misalignment between an axis normal to the model eye and the optical axis of the optical measurement instrument from the observed location of the corneal reflex within the image and the expected location of the corneal reflex within the image. 
         [0046]    In a step  680 , the optical measurement instrument, and particularly the processor of the optical measurement instrument, compensates the measured sphere value and/or measured cylinder value for the misalignment between the optical measurement instrument and the model eye. 
         [0047]    In a step  690 , the optical measurement instrument, and particularly the processor of the optical measurement instrument, compares the compensated sphere and/or cylinder value(s) to calibration value(s) that have previously been established for the model eye (e.g., in an initial qualification process for the optical measurement instrument), and which may be stored in memory associated with the processor, to determine whether the optical measurement instrument is working properly and according to its specifications. 
         [0048]    In another embodiment, which can be described with respect to  FIG. 7 , processor  17  positions a wavefront analysis region  750  symmetrically around the location of corneal reflex  430 , and performs its measurements of the characteristics (e.g., sphere and/or cylinder) of model eye  20  using only the portion of the image data that lies within wavefront analysis region  750 . In this case, the diameter of wavefront analysis region  750  is less than the full diameter of the aperture  24  of model eye  20  defined by iris  23 . For example, in an example case where aperture  24  of model eye  20  is 7 mm, then wavefront analysis region  750  may be a circular region having a diameter of 4 mm centered on the observed location of corneal reflex  430 . 
         [0049]    In yet another embodiment, optical measurement instrument  10  provides an indication via display  18  to the instrument operator of the misalignment angle θ, and the operator adjusts a mechanical mount  25  to adjust the misalignment angle θ to be within some allowed tolerance. 
         [0050]    While preferred embodiments are disclosed herein, many variations are possible which remain within the concept and scope of the invention. Such variations would become clear to one of ordinary skill in the art after inspection of the specification, drawings and claims herein. The invention therefore is not to be restricted except within the spirit and scope of the appended claims.