Abstract:
An apparatus and method for measuring wavefront aberrations. The apparatus comprises a reflecting device for reflecting selected portions of the wavefront, an imaging device for capturing information related to the selected portions, and a processor for calculating aberrations of the wavefront from the captured information. The method comprises reflecting selected portions of a wavefront onto the imaging device, capturing information related to the selected portions, and processing the captured information to derive the aberrations.

Description:
Divisional of prior application Ser. No. 09/677,191, filed Oct. 2, 2000, now U.S. Pat. No. 6,616,279. 

   FIELD OF THE INVENTION 
   The present invention relates generally to optical instruments and, more particularly, to a method and apparatus for measuring wavefront aberrations. The present invention is particularly useful, but not exclusively so, for measuring the optical wavefront in ophthalmic applications, e.g., measurement of aberrations of the eye, in corrective devices such as lenses (e.g., contact, spectacle, and intraocular), and for evaluating the ocular aberrations before, during and after refractive surgery to improve vision. 
   BACKGROUND OF THE INVENTION 
   The human eye is an optical system which employs a lens 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 a desired path. These deviations, referred to as aberrations, result in blurred images and decreased visual acuity. Hence, a method and apparatus for measuring aberrations is desirable to aid in the correction of such problems. 
   One method of detecting aberrations introduced by an eye involves determining the aberrations of light rays exiting from within 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. The wavefront represents the direction of light rays exiting from the eye. By determining the propagation direction of individual portions of the wavefront, the aberrations introduced to the light rays passing through parts of the eye such as the cornea can be determined and corrected. In this type of system, increased accuracy in determining the aberrations can be achieved by reducing the size of the regions of the wavefront used to derive the propagation direction. 
   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  20  of an eye  16 . The laser beam  12  focuses to a small spot  14  on the retina  20 . The retina  20 , acting as a diffuse reflector, reflects the laser beam  12 , resulting in a point source wavefront  10 . Ideally, the wavefront  10  from a point source leaving a perfect eye would be represented by a spherical or planar wavefront  22 . However, aberrations introduced by the eye  16  as the wavefront passes out of the eye result in an imperfect wavefront, as illustrated by the wavefront  10 . 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. 
     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 lens 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  25 . The laser beam  12  forms a spot  14  on the retina  20  of the eye  16 . The retina reflects the light from the spot  14  to create a point source wavefront  10  which becomes aberrated as it passes through the lens and other components and material within the eye  16 . The wavefront  10  passes through the beam splitter  25  toward a wavefront sensor  26 . The apparatus described in  FIG. 2  is commonly described as single-pass wavefront measurement system. 
   Typical prior art wavefront sensors  26  include either an aberroscope  30  and an imaging plane  28 , as illustrated in  FIG. 3 , or a Hartmann-Shack sensor  40  and an imaging plane  28 , as illustrated in FIG.  4 . The wavefront sensor  26  samples the wavefront  10  by passing the wavefront  10  through the aberroscope  30  or the Hartmann-Shack sensor  40 , resulting in the wavefront  10  producing an array of spots on an imaging plane  28 . Generally, the imaging plane  28  is a charge coupled device (CCD) camera. By comparing an array of spots produced by a reference wavefront to the array of spots produced by the wavefront  10 , the aberrations introduced by the eye  16  can be computed. 
   Each spot on the imaging plane  28  represents a portion of the wavefront  10 , with smaller portions enabling the aberrations to be determined with greater precision. Thus, the smaller the sub-aperture spacing  32  and the size of the sub-aperture  33  in the aberroscope  30  of  FIG. 3 , and the smaller the lenslet sub-aperture spacing  42  in the Hartmann-Shack sensor  40  of  FIG. 4 , the more accurately the aberrations can be determined. 
   An example of a Hartmann-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, incorporated herein by reference. 
   The resolution of the aberrations in such prior art devices, however, is limited by the grid size  32  and aperture size  33  in an aberroscope  30  (see FIG.  3 ), and by the lenslet sub-aperture spacing  42  in a Hartmann-Shack sensor  40  (see FIG.  4 ). Due to foldover, reductions to grid size  32  and lenslet sub-aperture spacing  42  are limited. Foldover occurs in an aberroscope sensor  30 , for example, when two or more spots  31 A,  31 B, and  31 C on imaging plane  28  overlap thereby leading to confusion between adjacent sub-aperture spots. Similarly, foldover occurs in Hartmann-Shack sensors  40  when two or more spots  41 A,  41 B,  41 C, and  41 D on imaging plane  28  overlap. Foldover may result from a grid size  32  or lenslet sub-aperture spacing  42  which is too small, a high degree of aberration, or a combination of these conditions. Hence, the grid size  32  or lenslet sub-aperture spacing  42  must be balanced 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 vice versa. 
   The constraints imposed by the aberroscope and Hartmann-Shack approaches limit the effectiveness of these systems for measuring large aberrations with a high degree of spatial resolution. These limitations prevent optical systems with large aberrations from being measured, thereby preventing them from achieving their full potential. Accordingly, ophthalmic devices and methods which can measure a wide range of aberrations with a high degree of spatial resolution would be useful. 
   SUMMARY OF THE INVENTION 
   The present invention discloses an apparatus and method for determining the aberrations of a wavefront with a high degree of accuracy. The apparatus includes a plurality of mirrors for reflecting selected portions of the wavefront, an imaging device for capturing information related to the selected portions, and a processor for controlling the plurality of mirrors and interpreting the captured information to compute the aberrations. The method includes reflecting selected portions of a wavefront onto an imaging device, capturing information related to the selected portions, and processing the captured information to derive the aberrations. The apparatus and method of the present invention are capable of measuring a wide range of aberrations with a high degree of spatial resolution. 
   The wavefront originates as a point source within a focusing optical system (e.g. the eye). The point source is generated by directing a beam of radiation (e.g., a laser) through the focusing optical system and scattering or reflecting the beam. A beam splitter disposed in the path of the laser beam directs the laser beam through the focusing optical system. The focusing optical system has an interior portion functioning as a diffuse reflector for reflecting or scattering the beam. The wavefront resulting from the point source passes through the focusing optical system and the beam splitter to the wavefront sensor of the present invention. The wavefront sensor measures distortions of the wavefront as an estimate of aberrations introduced by the focusing optical system. Aberrations are then computed by a processor coupled to the wavefront sensor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic of a wave produced by a laser beam reflected by the retina of an eye; 
       FIG. 2  is a schematic of a prior art apparatus for measuring aberrations introduced by an eye; 
       FIG. 3  is a schematic of an aberroscope for use in a prior art apparatus for measuring aberrations; 
       FIG. 4  is a schematic of a Hartmann-Shack lenslet array for use in a prior art apparatus for measuring aberrations; 
       FIG. 5  is a schematic of an apparatus for measuring aberrations introduced by an optical system in accordance with the present invention; 
       FIG. 5A  is an illustrative schematic of a reflection device in accordance with the present invention; 
       FIG. 5B  is a cross sectional view of the reflection device of  FIG. 5A  including a wavefront and an imaging device in accordance with the present invention; 
       FIG. 6  is a schematic illustrating the reflection of a portion of a wavefront in accordance with the present invention; 
       FIG. 7  is an perspective view of a portion of a Digital Micromirror Device™ (DMD™); 
       FIG. 8  is a schematic illustrating the reflection of a portion of a wavefront by a single mirror within the DMD™ of  FIG. 7  in accordance with the present invention; and 
       FIG. 9  is a schematic illustrating the reflection and redirection of a portion of a wavefront onto an imaging device in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Illustrated in  FIG. 5  is a preferred embodiment of a wavefront measuring device  100  in accordance with the present invention. In a general overview of the device  100  illustrated in  FIG. 5 , a radiation source  110  generates a beam  112 . The beam  112  passes through an optional beam splitter  114  unaltered. Another beam splitter  116  then redirects the beam  112  toward an optical system  115 , e.g., an eye  118 . The beam  112  enters the eye  118  through the cornea  120  where it is reflected by the retina  124  to produce a point source image wavefront  126  that travels back out of the eye  118 . The wavefront  126  is affected by defects within the eye  118  which cause the aberrations. The affected wavefront  126  passes through the beam splitter  116  toward a reflection device  128 . Individual mirror regions  130  within the reflection device  128  selectively reflect portions of the wavefront  126  toward an imaging device  132 , via a redirecting mirror  134 , which captures information related to the wavefront  126 . A processor  136  is used to control the reflection device  130  and to process the captured information. 
   The radiation source  110  is a device capable of generating a focused beam of photons, and is preferably a laser. Alternative radiation sources  110  include a laser diode, super-luminescent diode, or essentially any suitable radiation device. Additionally, the radiation source  110  may include a spacial filter for correcting noise associated with the radiation source  110 . 
   The optional beam splitter  114  is a device capable of selectively passing and directing beams within the wavefront measuring device  100 . In the preferred embodiment, the optional beam splitter  114  is configured to pass light generated by the radiation source  110  and to reflect light from the fixation target  117 . This configuration allows light from the fixation target  117  to be placed in the same path as light from the radiation source  110  that is heading toward the eye  118 . The fixation target  117  is an optional component which provides a focusing point for the person whose eye  118  is being scanned, thereby controlling eye movements and accommodation (focusing). The optional beam splitter  114  can be removed if the fixation target  117  is not used. Preferably, the optional beam splitter  114  is a polarizing beam splitter which selectively passes or reflects light based on the polarization of the light. 
   The other beam splitter  116  is also capable of selectively passing and directing beams. The beam splitter  116  is configured to reflect the beam  112  and light from the fixation target  117  toward the optical system  115 , e.g., the eye  118 , and to pass the light projecting from the optical system  115  unaltered. Preferably, the beam splitter  116  is also a polarizing beam splitter as discussed above. 
   The illustrated optical system  115  is the eye  118 . Alternatively, the optical system may include a reflective surface and a contact lens or eyeglass, an eye and a contact lens or eyeglass, a telescope, a microscope, or other type of optical system. Here, the beam  112  from the radiation source  110  is kept much smaller than the diffraction limited pupil aperture (approx. 2 mm) in order to form a spot  122  on the retina  124 . A focusing lens may also be used in the path of the beam  112  to account for defocus and/or astigmatism of the eye. The retina  124 , acting as a diffuse reflector, effectively becomes the source for light leaving the eye  118 , thereby creating the wavefront  126 . As the light is reflected off of the retina  124 , aberrations due to imperfections within the eye are introduced. Since the beam  112  is small, aberration producing imperfections within the eye  118  have little effect as the beam enters the eye  118 . Therefore, the aberrations are introduced to the light primarily upon exiting the eye  118 , essentially making this a single pass aberration measurement system. Single pass measurement systems are desirable since double pass measurement systems effectively count aberrations twice, e.g., aberrations are introduced to light entering the eye  118 , and introduced again as the light leaves the eye  118 . 
   One or more optical devices, such as lenses  125 , are positioned between the eye  118  and the reflection device  128 . The lenses  125  transfer the point source image wavefront  126  between the eye  118  and the reflection device  128  such that the propagation directions of the waves which make up the wavefront  126  are preserved as they are passed from the eye  118  to the reflection device  128 . Optical devices such as the lenses  125  used in the present invention are well known to those in the art. 
   The reflection device  128  has a plurality of mirrors  129  which form or can be grouped to form mirror regions  130  (see FIGS.  5  and  5 A). Each mirror region  130  is capable of reflecting a portion of the wavefront  126  for measurement of that portion independent of the other portions (see FIG.  5 B). Preferably, each mirror region  130  may be oriented in at least two positions. In a first position  133  (FIG.  5 B), a mirror region  130  will reflect a portion  140  of the wavefront  126  incident on the mirror region  130  in a direction to be received by the imaging device  132  and, in a second position  135 , the mirror regions  130  will reflect the portions of the wavefront  126  in a direction away from the imaging device  132 . 
   Each mirror region  130  may be formed of a single mirror  129 , or multiple mirrors  129  which are preferably adjacent to one another as illustrated in FIG.  5 A. For example, if the reflection device  128  includes an array of 1000 mirrors by 1000 mirrors, each mirror region  130  may include a single mirror  129 , an array of 3 mirrors by 3 mirrors as illustrated in  FIG. 5A , an array of 100 mirrors by 100 mirrors, or any other suitable grouping. While the present embodiment contemplates that each mirror region  130  would have the same configuration of mirrors, such in not believed necessary. 
     FIG. 6  illustrates the reflection of a portion  140  of the wavefront  126  by a mirror region  130  within a reflection device  128  toward an imaging device  132  to determine an aberration. Here the mirror region  130  has a single mirror  129 . When a mirror  129  such as mirror  131  is in the first position  133  (see FIG.  5 B), the wavefront portion  140  is directed toward an imaging plane  142  of the imaging device  132  as a reflected wavefront portion  144 . The other mirrors  129  such as the mirror  137  in the second position  135  (see  FIG. 5B ) reflect the portion of the wavefront  126  incident thereon away from imaging plane  142 , such as to area  139 . 
   To capture the entire wavefront  126 , each of the mirrors  129  or group of mirrors  130 , are in turn positioned to reflect the respective portion of the wavefront incident thereon towards the imaging device  132 , and then repositioned to reflect away as another mirror  129  is positioned to reflect towards the imaging device  132 . Of course if a mirror region  130  has more than one mirror  129 , then preferably, all mirrors  129  of each mirror region  130  are positioned as a unit. 
   Aberrations within the wavefront portion  140  displace the reflected wavefront portion  144  from an aberration free path  146  by an amount proportional to the local slope of the wavefront portion  140  corresponding to the mirror  131 . Given the displacement  145  between the location of reflected wavefront portion  144  and aberration free path  146  incident on imaging plane  142  and the distance from the wavefront portion  140  to the imaging plane  142 , the propagation direction of the wavefront portion  140  can be computed using a known method such as an inverse tangential function, i.e., the ratio of the length of the side opposite the angle of the wavefront portion  140  to the length of the side adjacent to the angle. The aberrations of the wavefront portion  140  can then be calculated using known methods. 
   In the preferred embodiment, each mirror region  130  is individually oriented to direct a corresponding portion of the wavefront  126  toward the imaging device  132  where information related to that portion is captured by the imaging device  132 . Alternatively, more than one of the mirror regions  130  may be oriented to direct respective portions of the wavefront  126  toward the imaging device  132  substantially simultaneously. If more than one of the mirror regions  130  direct simultaneously respective portions of the wavefront  126  toward the imaging device  132 , such mirror regions  130  should be separated by another region of mirrors which reflect away from the imaging device  132  to prevent foldover between the imaged regions. For example, referring to  FIG. 5A , if two mirror regions  130 A and  130 C are oriented substantially simultaneously to direct respective portions of the wavefront  126  toward the imaging device  132 , the two mirror regions  130 A and  130 C will be separated by one or more mirror regions  130  such as a third mirror region  130 B which will be oriented to reflect a respective portion of the wavefront  126  away from the imaging device  132 . By varying the size of the mirror regions  130 , and the number of mirror regions  130  that simultaneously direct portions of the wavefront  126  toward the imaging device  132 , the speed required to capture all of the wavefront  126  and the spatial resolution of the system can be adjusted. 
   One preferable reflection device  128  is a Digital Micromirror Device™ (DMD™). It will be apparent to those in the art that other types of reflecting devices may be used in accordance with the present invention. DMDs™ are described in U.S. Pat. No. 5,096,279 to Hornbeck et al., entitled “Spatial Light Modulator and Method,” and in U.S. Pat. No. 4,954,789 to Sampsell, entitled “Spatial Light Modulator,” both of which are incorporated herein by reference. 
     FIG. 7  depicts a portion of a Digital Micromirror Device™ (DMD™)  150 . A DMD™ includes an array of hundreds or thousands of tiny tiltable mirrors  129 , each of which is capable of reflecting a portion of the wavefront  126 .  FIG. 7  depicts two individual mirrors  129  within the DMD™  150 . To permit the mirrors to tilt, each mirror  129  is attached to one or more hinges  152  mounted on support posts, and spaced by means of a fluidic (air or liquid) gap over underlying control circuitry on a CMOS substrate  154 . The control circuitry provides electrostatic forces, which cause each mirror  129  to selectively tilt. In operation, data is loaded to memory cells of the DMD™  150  and, in accordance with this data, individual mirrors  129  are tilted so as to either reflect light towards or away from the imaging device  132  via the redirecting mirror  134  as seen in FIG.  5 . Suitable DMD™ devices include SXGA and SVGA DMD™ devices available from Texas Instruments. 
     FIG. 8  depicts in detail the reflection of the wavefront portion  140  ( FIG. 6 ) by a mirror  129  of a DMD™. The individual mirror  129  has three positions (i.e., −10°, 0°, +10°). In the +10° position, representing the first position  133  of  FIG. 5B , the wavefront portion  140  is directed toward the imaging plane  142 . In the 0° and −10° positions, either representing the second position  135  of  FIG. 5B , the wavefront portion  140  is directed away from the imaging plane  142 . Preferably, the imaging plane  142  includes a plurality of cells  143  capable of detecting energy from the wavefront portion  140 . Although each mirror of a DMD™ has three positions, only two are needed in the present invention. 
   In the illustrated embodiment, the wavefront portions  140  are directed toward the imaging device  132  via a redirecting mirror  134 . The redirecting mirror  134  is optically positioned (not necessarily physically positioned) between the reflection device  128  and the imaging device  132  to reflect the wavefront portions  140  from the mirror regions  130  to the imaging device  132 . This facilitates the placement of the imaging device  132  in relation to the plurality of mirrors  128 . Alternatively, the wavefront portions could pass directly from the reflection device  128  to the imaging device  132 , thereby eliminating the need for the redirecting mirror  134 . 
     FIG. 9  depicts in detail the operation of redirecting mirror  134  as seen in FIG.  5 . In  FIG. 9 , the reflection of a wavefront portion  144  is isolated from the entire wavefront  126  by the mirror region  130  within the reflection device  128 . The reflection of the wavefront portion  144  is reflected off of a redirecting mirror  134  onto the imaging plane  142  of the imaging device  132 . The unmeasured portions  147  of the wavefront  126  are directed away from the imaging plane  142 . The redirecting mirror  134  facilitates the placement of the imaging device  132  in relation to the reflection device  128  by adding flexibility. The flexibility is due to the ability to position the imaging device  132  in a location other than in the direct line of sight the reflection device  128 . 
   The imaging device  132  is capable of precisely detecting the location of energy incident to an imaging plane  133 . Preferably, the imaging device  132  is a charge coupled device (CCD) camera. A charge coupled camera is a device capable of converting energy incident to an imaging plane  133  into a digital representation. Charge coupled devices are well known and a suitable device for use with the present invention would be readily apparent to those skilled in the art. 
   The processor  136  controls the orientation of the mirror regions  130 . In addition, the processor  136  receives information from the imaging device  132  and analyzes the information to compute the aberrations. The information may be stored in a storage register prior to processing by processor  136  or may be processed immediately. In the preferred embodiment, the processor  136  orients the individual mirror regions  130  (all the mirrors  129  of the mirror region  130 ) to reflect towards the imaging device  128  at different times for computing the aberrations of the wavefront  126 . In an alternative embodiment, the processor  136  substantially simultaneously orients two or more mirror regions toward the imaging device  132  to compute the aberrations of the wavefront  126 . In this alternative embodiment, the individual mirror regions  130  are separated by a buffer mirror region reflecting away from the imaging device  132  to prevent foldover between portions of the wavefront  126  corresponding to the individual mirror regions  130  as previously discussed. It is apparent to those skilled in the art that the control of the plurality of mirrors  128 , the receipt of information from the imaging device  132 , and the processing of information may be performed by a single processor or divided among a plurality of processors. 
   In accordance with an embodiment of the present invention, the aberration correction device  138  is coupled to the processor  136 . Alternatively, information calculated by the processor  136  may be stored on a hard drive, diskette, server, compact disc, digital versatile disc, or essentially any device capable of storing information. The stored information is then passed to an aberration correction device  138 . The aberration correction device  138  includes a known lens grinder, contact lens manufacturing system, surgical laser system, or other optical system correction device. In a surgical laser system, a laser can be optically positioned relative to the beam splitter  116  to direct a laser cutting beam toward the cornea  120  of the eye  118 , in a manner well known in the art, for the purpose of performing ophthalmic surgery. 
   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. 
   Having thus described a few particular embodiments of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. 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.