Abstract:
Defocus and astigmatism compensation methods and apparatuses for use in an aberration measurement system. The apparatuses including reflectors for altering the optical distance between a pair of lenses passing a wavefront without changing the physical distance between the lenses, thereby compensating for defocus in the wavefront; and cylindrical mirrors for adding and removing curvature from a curved wavefront, thereby compensating for astigmatism in the wavefront. The methods including passing a wavefront having defocus through a first lens on a first path, reflecting the wavefront from the first path to a second path, reflecting the wavefront from the second path to a third path, and passing the wavefront through a second lens as a defocus compensated wavefront; and passing a wavefront through first and second cylindrical lens, and orienting the first and second cylindrical lenses with respect to the wavefront and to one another to compensate for astigmatism in the wavefront.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to optical instruments and, more particularly, to a method and device for defocus and astigmatism compensation in wavefront aberration measurement systems. The present invention is particularly useful, but not exclusively so, for defocus and astigmatism compensation in ophthalmic applications.  
         BACKGROUND OF THE INVENTION  
         [0002]    The human eye is an optical system employing several lens elements to focus light rays representing images onto the retina within the eye. The sharpness of the images produced on the retina is a factor in determining the visual acuity of the eye. Imperfections within the lens and other components and material within the eye, however, may cause the light rays to deviate from the desired path. These deviations, referred to as aberrations, result in blurred images and decreased visual acuity. Hence, methods and apparatuses for measuring aberrations are desirable to aid in the correction of such problems.  
           [0003]    One method of detecting aberrations introduced by the eye involves determining the aberrations of light rays exiting from the eye. A beam of light directed into the eye as a point on the retina is reflected or scattered back out of the eye as a wavefront, with the wavefront containing aberrations introduced by the eye. By determining the propagation direction of discrete portions (i.e., samples) of the wavefront, the aberrations introduced by the eye can be determined and corrected.  
           [0004]    A general illustration of the generation of a wavefront is shown in FIG. 1. FIG. 1 is a schematic view of a wavefront  10  generated by reflecting a laser beam  12  off of the retina  14  of an eye  16 . The laser beam  12  focuses to a small spot  18  on the retina  14 . The retina  14 , acting as a diffuse reflector, reflects the laser beam  12 , resulting in the point source wavefront  10 . Ideally, the wavefront  10  would be represented by a planar wavefront  20 . However, aberrations introduced by the eye  16  as the wavefront  10  passes out of the eye  16  result in an imperfect wavefront, as illustrated by the aberrated wavefront  20 A. The wavefront  10  represents aberrations which lead to defocus, astigmatism, spherical aberrations, coma, and other irregularities. Measuring and correcting these aberrations allow the eye  16  to approach its full potential, i.e., the limits of visual resolution.  
           [0005]    [0005]FIG. 2 is an illustration of a prior art apparatus for measuring the wavefront  10  as illustrated in FIG. 1. By measuring the aberrations, corrective lenses can be produced and/or corrective procedures performed to improve vision. In FIG. 2, a laser  22  generates the laser beam  12  which is routed to the eye  16  by a beam splitter  24 . The laser beam  12  forms a spot  18  on the retina  14  of the eye  16 . The retina  14  reflects the light from the spot  18  to create a point source wavefront  10  which becomes aberrated as it passes through the lens and other components and materials within the eye  16 . The wavefront  10  then passes through a first lens  11  and a second lens  13  to focus the wavefront  10  so that the wavefront  10  is collimated. The wavefront  10  then passes through the beam splitter  24  toward a wavefront sensor  26 . Information detected by the wavefront sensor  26  is then processed by a processor  27  to determine the aberrations of the wavefront  10 .  
           [0006]    [0006]FIG. 3 illustrates the focusing of the wavefront  10  to produce a flat wavefront for projection onto the wavefront sensor  26 . If the wavefront  10  contains diverging light, the light rays which make up the wavefront  10  would continue to diverge until they were no longer contained within the system, thereby losing valuable wavefront  10  information. This is especially problematic for an eye  16  having a large degree of defocus. In FIG. 3 the curved wavefront  10 A containing diverging light rays passes through the first lens  11  where it converges to a crossover point  15 , and then through the second lens  13 . When the crossover point  15  occurs at one focal length before the second lens  13 , the resultant wavefront  10 B will be collimated (i.e., flat). For different degrees of defocus, the lenses  11  and  13  can be moved relative to one another in order for the focal point of lens  13  to match the cross-over point  15 . Unfortunately, for an eye  16  having a great deal of defocus, the lenses  11  and  13  may need to be moved a relatively large distance from one another, which may be problematic if space is limited. In addition, the defocus mechanism of FIG. 3 does not correct other eye aberrations such as astigmatism in which light along one axis converges/diverges more rapidly than light along another axis. Since the lenses  11  and  13  converge or diverge light along every axis equally, this arrangement does not compensate for astigmatism.  
           [0007]    Typical wavefront sensors  26  include either an aberroscope  28  (FIG. 4) or a Hartman-Shack lenslet array  30  (FIG. 5), with an imaging device  32 . The aberroscope  28  and the Hartman-Shack lenslet array  30  each produce an array of spots when a wavefront passes through them. The imaging device  32  contains an imaging plane  34  for capturing the spots generated by the aberroscope  28  or the Hartman-Shack Sensor  30 . Generally, the imaging device  32  is a charge coupled device (CCD) camera.  
           [0008]    The wavefront sensor  26  samples the wavefront  10  by passing the wavefront  10  through the aberroscope  28  or the Hartman-Shack sensor  30 , resulting in an array of spots on the imaging plane  34 . Each spot on the imaging plane  34  represents a portion of the wavefront  10 , with smaller portions enabling the aberrations to be determined with greater accuracy. By comparing the array of spots produced on the imaging plane  34  by the wavefront  10  with a reference array of spots corresponding to the wavefront of an ideal eye, the aberrations introduced by the eye  16  can be computed.  
           [0009]    An example of a Hartman-Shack system is described in U.S. Pat. No. 6,095,651 to Williams et al., entitled Method and Apparatus for Improving Vision and the Resolution of Retinal Images, filed on Jul. 2, 1999, is incorporated herein by reference.  
           [0010]    The resolution of the aberrations in such prior art devices, however, is limited by the sub-aperture spacing  36  and the sub-aperture size  38  in an aberroscope sensor (see FIG. 4), and by the lenslet sub-aperture size  40  and focal length in a Hartman-Shack sensor (see FIG. 5). In addition, large aberrations due to excessive defocus or astigmatism may result in foldover. Foldover occurs in an aberroscope sensor, for example, when two or more spots  42 A,  42 B, and  42 C on the imaging plane  34  overlap, thereby leading to confusion between adjacent sub-aperture spots. Similarly, foldover occurs in Hartman-Shack sensors when two or more spots  44 A,  44 B,  44 C, and  44 D on the imaging plane  34  overlap. Typical systems are designed to accommodate a certain amount of defocus and astigmatism, however, these systems are unable to handle defocus and astigmatism of individuals with large astigmatism and/or large defocus.  
           [0011]    Foldover may result from a sub-aperture spacing  36 , sub-aperture size  38 , or lenslet size  40  which is too small, a high degree of aberration (e.g., large defocus and/or astigmatism); or a combination of these conditions. Hence, the sub-aperture spacing  36  and sub-aperture size  38  in the aberroscope sensor (FIG. 4), and the lenslet sub-aperture spacing  40  and focal length in the Hartman-Shack sensor (FIG. 5) must be selected to achieve good spatial resolution while enabling the measurement of large aberrations. Accordingly, the ability to measure a high degree of aberration comes at the expense of spatial resolution and/or dynamic range and vice versa.  
           [0012]    The constraints imposed by the aberroscope and Hartman-Shack approaches limit the effectiveness of these systems for measuring wavefronts having a wide range of aberrations, such as those exhibiting a large degree of defocus and astigmatism. These limitations prevent existing optical systems from achieving their full potential. Accordingly, ophthalmic devices and methods which can measure a wide range of aberrations having of defocus and/or astigmatism with a high degree of accuracy would be useful.  
         SUMMARY OF THE INVENTION  
         [0013]    The present invention provides for a method and apparatus of compensating for defocus and astigmatism in a wavefront for use in an ophthalmic system for measuring eye aberrations. By compensating for at least a portion of defocus and astigmatism, the method and apparatus of the present invention are capable of measuring a wide range of aberrations in a wavefront with a high degree of accuracy.  
           [0014]    In an ophthalmic system for measuring eye aberrations having first and second optical lenses separated by a physical distance for focusing a wavefront, the present invention includes a method of adjusting the optical distance between the two lenses without changing the physical distance between the two lenses. The method of the present invention includes passing a wavefront through a first optical lens in a first optical path, reflecting the wavefront from the first optical path to a second optical path, reflecting the wavefront to a third optical path, and passing the wavefront through a second optical lens. In addition, the method may include reflecting the wavefront to a fourth optical path after being reflected to the third optical path and before being passed through the second optical lens. The reflections allow the optical distance between the first and second optical lenses, and therefore the defocus compensation, to be changed without altering the physical distance between the lenses. Also, the reflections allow incremental changes in certain components to result in larger incremental changes in the optical distance between the lenses, thereby allowing a larger range of defocus compensation to be performed in a smaller physical area.  
           [0015]    Another method of the present invention includes passing a wavefront through a cylindrical lens assembly to remove astigmatism from the wavefront. The method includes passing the wavefront through a first cylindrical lens and a second cylindrical lens, orienting the first cylindrical lens and the second cylindrical lens such that an astigmatism compensation position of the cylindrical lens assembly is in-line with a bisector position of the wavefront, and orienting the first and second cylindrical lenses relative to one another to adjust the astigmatism compensation power of the cylindrical lenses to compensate for astigmatism in the wavefront.  
           [0016]    An apparatus of the present invention for changing the optical distance traveled by a wavefront between a pair of lenses without changing the physical distance between the lenses includes a first reflector positioned to reflect the wavefront received from a first lens along a first optical path to a second optical path, a second reflector positioned to reflect the wavefront from the second optical path to a third optical path, and a third reflector positioned to reflect the wavefront from the third optical path to a fourth optical path which passes through a second optical lens.  
           [0017]    An apparatus of the present invention for compensating for astigmatism includes a first cylindrical lens for introducing a first cylindrical refraction to a wavefront, a second cylindrical lens for introducing a second cylindrical refraction to the wavefront, and a support for rotatably mounting the first and second cylindrical lenses, the first and second cylindrical lenses being rotatable relative to the wavefront and relative to one another, whereby an astigmatism within the wavefront is compensated by adjusting the orientation of the first cylindrical lens and the second cylindrical lens relative to the wavefront and to one another. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a schematic of a wave produced by a laser beam reflected by the retina of an eye;  
         [0019]    [0019]FIG. 2 is a schematic of a prior art apparatus for measuring aberrations introduced by an eye;  
         [0020]    [0020]FIG. 3 is a schematic of a part of a prior art defocus compensation device;  
         [0021]    [0021]FIG. 4 is a schematic of an aberroscope system for use in a prior art apparatus for measuring aberrations;  
         [0022]    [0022]FIG. 5 is a schematic of a Hartman-Shack lenslet array system for use in a prior art apparatus for measuring aberrations;  
         [0023]    [0023]FIG. 6 is a schematic of an apparatus for measuring aberrations in a wavefront introduced by an optical system in accordance with the present invention;  
         [0024]    [0024]FIG. 7 is an illustrative schematic of a defocus compensation device for removing a defocus component from a wavefront for use in the apparatus of FIG. 6 in accordance with the present invention;  
         [0025]    [0025]FIG. 8A is an illustrative depiction of a wavefront free of astigmatism;  
         [0026]    [0026]FIG. 8B is an illustrative depiction of an astigmatic wavefront;  
         [0027]    [0027]FIG. 9 is an illustrative schematic of an astigmatism compensation device for removing astigmatism component from a wavefront for use in the apparatus of FIG. 6 in accordance with the present invention;  
         [0028]    [0028]FIG. 10A is a perspective view of a concave cylindrical lens for use with the present invention; and  
         [0029]    [0029]FIG. 10B is a perspective view of a convex cylindrical lens for use with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    Illustrated in FIG. 6 is a preferred embodiment of a wavefront measuring apparatus  100  for measuring the aberrations of an eye  16  in accordance with the present invention. In a general overview, a beam  12  is generated by a laser  22  and directed by a beam splitter  24  into the eye  16 . The diameter of the beam  12  is small, thereby minimizing the effect of optical components between the laser  22  and the eye  16  on the beam  12 . A wavefront  10  is reflected out of the eye toward a wavefront sensor  26  for measurement of aberrations introduced to the wavefront  10  by the eye  16 .  
         [0031]    If the wavefront  10  contains a relatively large amount of defocus or astigmatism, portions of the wavefront  10  may not reach the wavefront sensor  26  or may be out of range for measurement by the wavefront sensor  26 . Therefore, the wavefront  10  is passed through a novel defocus compensation device  102  and through a novel astigmatism compensation device  104  to compensate for relatively large defocus and astigmatism, respectively, within the wavefront  10 .  
         [0032]    The defocus compensation device  102  adds a defocus compensation component to the wavefront  10 , and the astigmatism compensation device  104  adds an astigmatism compensation component to the wavefront  10 . Remaining aberrations within the wavefront  10 , after defocus and astigmatism compensation, are then detected by the wavefront sensor  26 . The processor  27  then determines the aberrations of the wavefront  10  based of the information obtained from the wavefront sensor  26 , the defocus compensation component added by the defocus compensation device  102 , and the astigmatism compensation component added by the astigmatism compensation device  104 .  
         [0033]    By compensating for defocus and astigmatism prior to measurement by the wavefront sensor  26 , the wavefront sensor  26  can be configured to detect the remaining aberrations more precisely. In addition, the wavefront measuring apparatus  100  is able to detect a wider range of aberrations since defocus and astigmatism aberrations, which were previously out of the wavefront sensor&#39;s range, are compensated for by the defocus compensation device  102  and the astigmatism compensation device  104  with the compensation components of these devices factored into the determination of the aberrations of the wavefront  10 .  
         [0034]    In the present invention, the generation of the beam  12  and the wavefront  10 , and the determination of aberrations of the wavefront  10  by the processor  27  are known in the art. In addition, modifications to processor  27  to factor in the defocus compensation component and the astigmatism compensation component in determining the aberrations of the wavefront  10  will be readily apparent to those in the art. The defocus compensation and astigmatism compensation of the present invention are now described in more detail.  
         [0035]    Defocus Compensation  
         [0036]    [0036]FIG. 7 illustrates a preferred defocus compensation device  102  in accordance with the present invention. The defocus compensation device  102  includes a first and second lens  120  and  122  to compensate for defocus in a wavefront (a wavefront containing defocus is represented by curved wavefront  10 A) and generate a defocus compensated wavefront (represented by flat wavefront  10 B) for measurement by a wavefront sensor  26  (FIG. 6). The defocus compensation device  102  removes at least a portion of defocus within the wavefront such that the remaining defocus within the wavefront is measurable by the wavefront sensor  26 . The wavefront sensor may then be configured to detect the remaining defocus more precisely. The amount of defocus compensated for by the defocus compensation device  102  and the defocus determined by the wavefront sensor  26  may then be combined by the processor  27  to determine the aberrations of the eye  16  (FIG. 6) due to the total defocus.  
         [0037]    A first lens  120  of the defocus compensation device  102  is a spherical lens for focusing the wavefront  10  (FIG. 6). The wavefront  10  passes through the lens  120  along a first optical path  124 A. The lens  120  focuses the diverging light of the curved wavefront  10 A to a cross-over point  125 .  
         [0038]    A first reflector  126 A reflects the wavefront  10  from the first optical path  124 A to a second optical path  124 B which is different from the first optical path  124 A. In the preferred embodiment, the first reflector  126 A is a surface of a prism  126 . Other reflectors may be used, such as a mirror.  
         [0039]    A second reflector  128  reflects the wavefront  10  to a third optical path  124 C which is different from the first optical path  124 A and the second optical path  124 B. The second reflector  128  is preferably a retroreflector. In a retroreflector, an incoming beam such as the wavefront on the second optical path  124 B will be reflected parallel to itself but in the opposite direction of propagation (e.g., optical path  124 C), regardless of the orientation of the wavefront  10  with respect to the retroreflector. The retroreflector may be a corner cube or other well known retroreflector. An alternative embodiment may include a porro reflector or at least two reflective surfaces. For example, the reflector  128  may include a first reflective surface  128 A, e.g., a mirror, for reflecting the wavefront  10  on the second optical path  124 B along an intermediate optical path toward a second reflective surface  128 B, e.g., another mirror. The second reflective surface  128 B then reflects the wavefront received along the intermediate optical path along the third optical path  124 C. In a preferred embodiment, the second optical path  124 B and the third optical path  124 C are substantially the same physical distance.  
         [0040]    In the illustrated embodiment, the third reflector  126 B reflects the wavefront from the third optical path  124 C to a fourth optical path  124 D. The first optical path  124 A and the fourth optical path  124 D are preferably substantially colinear as shown. Here, the third reflector  126 B is formed as another surface of the prism  126  forming the first reflector  126 A. Alternatively, the reflector  126 A and reflector  126 B need not be surfaces of the same device, e.g., prism  126 , but may be separate reflective surfaces.  
         [0041]    The second lens  122  is positioned along the fourth optical path  124 D through which passes the wavefront  10 . If the focal point of the second lens  122  is the same as the crossover point  125 , a defocus compensated wavefront  10 B will be produced.  
         [0042]    It is contemplated, although not preferred, that the second lens  122  may be positioned along the third optical path  124 C. Since the second lens  122  would be positioned to receive the wavefront along the third optical path  124 C, directly, the third reflector  126 B could be eliminated. In addition, it is further contemplated, although not preferred, that the first lens  120  may be positioned along the second optical path  124 B. Since the first lens  120  would be positioned to allow the wavefront to pass along the second optical path  124 B, the first reflector  126 A could be eliminated.  
         [0043]    It is seen that while the first and second lenses  120  and  122  are separated by a physical distance for focusing a wavefront  10 , the optical distance between the two lenses  120  and  122  is adjusted without changing the physical distance between the two lenses  120  and  122 . This is done by changing the distance between the reflector  128  and the other reflectors  126 A and  126 B within the defocus compensation device  102  along the second and third optical paths  124 B and  124 C. By changing the distance between the reflector  128  and the other reflectors  126 A and  126 B, the optical distance along which a wavefront must travel between the two lenses  120  and  122  is changed without changing the physical distance between the lenses  120  and  122 . Further, due to the reflection by reflector  128 , a incremental changes in the distance between the reflector  128  and the other reflectors  126 A and  126 B results in a change in the optical distance between the lenses  120  and  122  which is twice the incremental change. The optical distance changes by twice the incremental change since changing the distance between the reflector  128  and the first and second reflectors  126 A and  126 B will result in an incremental change in the second optical path  124 B and an incremental change in the third optical path. This permits a greater defocus compensation range for the lenses  120 ,  122  in a limited area.  
         [0044]    The reflector  128  is preferably moveable with respect to the other components in the defocus compensation device  102  (i.e., reflector  126 A reflector  126 B, lens  120 , and lens  122 ) to change the lengths of some of the optical paths. In an alternative embodiment, the second reflector  128  remains stationary while the other components in the defocus compensation device move to change the optical path lengths.  
         [0045]    Astigmatism Compensation  
         [0046]    [0046]FIG. 8A illustrates a wavefront pattern produced by an eye without an astigmatism. The concentric circles indicate that the eye converges light equally along every axis. An eye without an astigmatism has a single correction power (e.g., defocus) for the entire eye, which can be corrected with a lens having a single defocus correction power.  
         [0047]    [0047]FIG. 8B illustrates a wavefront pattern produced by an eye with an astigmatism. The concentric ovals indicate that the eye converges light more rapidly along one axis, e.g., the X axis and less rapidly along another axis, e.g., along the Y axis. In an eye with an astigmatism, the eye has essentially two powers, with an astigmatism power representing the difference between the two powers. For descriptive purposes, the line between the two powers will be referred to as the bisector position  146 . The bisector position  146  lies midway between the two powers of the eye.  
         [0048]    [0048]FIG. 9 depicts a preferred astigmatism compensation device  104  for compensating for astigmatism in a wavefront  10  (FIG. 6). The astigmatism compensation device  104  is used to transform an astigmatic wavefront (represented by the concentric ovals of FIG. 8B) into a wavefront having a uniform power (represented by the concentric circles of FIG. 8A). The astigmatism compensation device  104  includes a cylindrical lens assembly having a first cylindrical lens  140 A and a second cylindrical lens  140 B rotatably mounted on a support  141  for selectively adding and removing curvature from the wavefront. In the illustrated astigmatism compensation device  104 , the cylindrical lens  140 A,  140 B are rotatably mounted on a support  141  by a first rotation motor  142 A and a second rotation motor  142 B, respectively, for orienting the first cylindrical lens  140 A and the second cylindrical lens  140 B relative to the wavefront and to one another. By orienting the cylindrical lenses  140 A,  140 B relative to the wavefront  10  and to one another, the astigmatism within the wavefront  10  can be compensated for by removing curvature from regions having too much curvature (e.g., by diverging light along the axis having too much curvature) and adding curvature to regions having too little curvature (e.g., by converging light along the axis having too little curvature).  
         [0049]    The astigmatism compensation device  104  removes at least a portion of astigmatism within the wavefront such that remaining astigmatism within the wavefront is measurable by the wavefront sensor  26 . The wavefront sensor may then be configured to detect the remaining astigmatism more precisely. The amount of astigmatism compensated for by the astigmatism compensation device  104  and the astigmatism determined by the wavefront sensor  26  may then be combined by the processor  27  to determine the aberrations of the eye  16  (FIG. 6) due to the total astigmatism.  
         [0050]    The first cylindrical lens  140 A, in the illustrated embodiment, is a diverging cylindrical lens. Preferably, the diverging cylindrical lens is a plano-concave cylindrical lens (i.e., flat on one side and curved inward on the other, see FIG. 10A). A plano-concave cylindrical lens diverges light along a curved axis, e.g., X′ (FIG. 10A), thereby adding more divergence, and does not affect light along the other axis, e.g., Y′ (FIG. 10A). The first cylindrical lens  140 A is used to remove curvature from the regions of the wavefront  10  which are more curved, e.g., along the X axis (FIG. 8B). Preferably, the flat surface of the plano-concave cylindrical lens receives the wavefront  10  and the curved surface passes the wavefront  10 .  
         [0051]    The second cylindrical lens  140 A, in the illustrated embodiment, is a plano-convex cylindrical lens (i.e., flat on one side and curved outward on the other, see FIG. 10B). A plano-convex cylindrical lens converges light along one axis, e.g., X″ (FIG. 10B), thereby adding more convergence, and does not affect light along the other axis, e.g., Y″ (FIG. 10B). The plano-convex cylindrical lens causes light which passes through it to converge along the curved axis. The second cylindrical lens  140 B is used to add curvature to the regions of the wavefront  10  which are less curved, e.g., along the Y axis (FIG. 8B). Preferably, the curved surface of the plano-concave cylindrical lens receives the wavefront  10  and the flat surface passes the wavefront  10 .  
         [0052]    The rotation motors  142 A,  142 B are operably associated with the cylindrical lens  140 A,  140 B, respectively, for rotating its respective cylindrical lens  140 A,  140 B about an optical axis  144  of the wavefront  10  (FIG. 6). Suitable rotation motors for use with the present invention are readily available, with the selection of an appropriate rotation motor and its connection to a cylindrical lens  140 A and  140 B being apparent to those skilled in the art.  
         [0053]    By rotating the cylindrical lenses  140 A,  140 B with respect to the wavefront  10 , an astigmatism compensation position of the astigmatism compensation device  104  can be aligned with the bisector position  146  (FIG. 8B) of the wavefront. The astigmatism compensation position is the position midway between the flat axis of the first cylindrical lens, e.g., Y′, and the flat axis of the second cylindrical lens, e.g., Y″.  
         [0054]    The astigmatism compensation power is set by rotating the cylindrical lenses  140 A,  140 B with respect to one another. The astigmatism compensation power is greatest when the flat axes of the cylindrical lenses  140 A,  140 B are perpendicular to one another and least when the flat axes of the cylindrical lenses  140 A,  140 B are parallel to one another. If the cylindrical lens  140 A,  140 B have matched powers of opposite sign, the cylindrical lenses  140 A,  140 B will have no affect on the wavefront  10  when the flat axes of the cylindrical lenses are parallel.  
         [0055]    In use, the astigmatism compensation device  104  of the illustrated embodiment receives the wavefront  10  along an optical axis  144 . The wavefront  10  passes through the first cylindrical lens  140 A and the second cylindrical lens  140 B. Initially, both of the flat axes of the cylindrical lenses  140 A,  140 B are aligned with the bisector position  146  of the wavefront by their respective rotation motors  142 A,  142 B. The flat axis are aligned with one another so as not to add any astigmatism compensation to the wavefront  10 . The motors  142 A,  142 B then rotate the flat axis of the cylindrical lenses  140 A,  140 B an equal amount in opposite directions from the bisector position  146  to add astigmatism compensation to the wavefront  10 . The astigmatism compensation position and the astigmatism compensation power will be factored into the determination of aberrations of the wavefront  10  by the processor  27  of the wavefront compensation device  100 .  
         [0056]    As an illustrative example, if the bisector position  146  is at 45 degrees (FIG. 8B), the flat axes of the cylindrical lenses  140 A,  140 B (i.e, Y′ in the plano-concave lens  140 A depicted in FIG. 10A and Y″ in the plano-convex lens  140 B depicted in FIG. 10B) would be initially set at 45 degrees and, then, the first cylindrical lens  140 A would be rotated to 60 degrees and the second cylindrical lens  140 B would be rotated to 30 degrees to add astigmatism compensation. To add the maximum astigmatism compensation in the present example, the first cylindrical lens  140 A would be rotated to 90 degrees and the second cylindrical lens  140 B would be rotated to 0 degrees so that the flat axes of the first and second cylindrical lenses  140 A and  140 B would be perpendicular to one another.  
         [0057]    For illustrative purposes, the present invention has been described in terms of measuring wavefront aberrations introduced by a human eye. However, it will be readily apparent to those skilled in the art that the present invention can be used to measure aberrations created by other optical systems, e.g. eyeglasses, telescopes, binoculars, monoculars, contact lenses, non-human eyes, or combination of these systems.  
         [0058]    Having thus described a few particular embodiments of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. For example, in alternative embodiments: the first cylindrical lens  140 A is a converging lens and the second cylindrical lens  140 B is a diverging lens; the flat surfaces of the plano-concave/convex lenses are facing one another; additional lens are used to fine tune the astigmatism compensation; the lenses are oriented relative to one another first and, then, the lenses are oriented relative to the wavefront  10 ; and the lenses are oriented relative to themselves and relative to one another substantially simultaneously. Such alterations, modifications and improvements as are made obvious by this disclosure are intended to be part of this description though not expressly stated herein, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only, and not limiting. The invention is limited only as defined in the following claims and equivalents thereto.