Abstract:
Embodiments of the present invention advantageously satisfy the above-identified need in the art, and provide method and apparatus for measuring refractive errors of an eye that improve upon wavefront type refractors using a conventional Hartmann-Shack sensor. Specifically, one embodiment of the present invention is an apparatus for measuring refractive errors of an eye which includes: (a) a source of a probe beam; (b) a first Badal lens system adapted to project the probe beam into a subject&#39;s eye to form an illumination spot on a retina; (c) a second Badal lens system adapted to image the illumination spot onto an image plane substantially conjugate to the retina; and (d) a spatial filter disposed in the image plane adapted to transmit at least a portion of the image.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention pertains to method and apparatus for measuring optical quality of an eye. In particular, the present invention pertains to method and apparatus for measuring refractive errors of an eye based on wavefront measurement. 
     BACKGROUND OF THE INVENTION 
     As is well known, a wavefront type refractor is an optical device for measuring refractive errors of an eye based on wavefront measurement. Such a wavefront type refractor can provide comprehensive measurement of the refractive errors of an eye, including high order refractive errors. In addition, such a wavefront type refractor may provide more accurate measurement of the refractive errors of an eye than a conventional auto-refractor. Advantageously, the wavefront measurements can be used to guide refractive laser surgery to correct detected refractive errors. In addition, such a wavefront type refractor may be used to provide prescriptions for eyeglasses and contact lenses. 
     One implementation of a wavefront type refractor that is well known in the art uses a “Hartmann-Shack” sensor to measure the wavefront of a light beam generated from an illumination spot projected on the retina and passed through the eye&#39;s optics. As is well known, in such a wavefront type refractor, a probe beam from a laser or a superluminescent diode is projected onto the retina through the eye&#39;s optics. Light scattered by the retina passes through the eye&#39;s optics, and emerges through the eye&#39;s pupil. The wavefront of the emerging beam carries refractive information relating to the eye&#39;s optics. For example, if the eye is emmetropic (i.e., the eye&#39;s optics is without refractive error), the wavefront of the emerging beam should be flat. Relay optics relays the wavefront emerging from eye&#39;s pupil onto the Hartmann-Shack sensor. The Hartmann-Shack sensor measures the distortion of the wavefront to determine the refractive errors of the eye due to aberrations of the eye&#39;s optics. 
     As is well known, a Hartmann-Shack sensor comprises a lenslet array and a CCD camera located at the focal plane of the lenslet elements of the array. Whenever a beam of radiation to be measured is projected onto a Hartmann-Shack sensor, the lenslet array breaks the beam into sub-apertures, and forms a pattern of focal spots (the pattern of the focal spots carries the signature of the wavefront of the beam to be measured). The CCD camera records the pattern of focal spots, and a computer analyzes the pattern to reconstruct the wavefront of the beam. 
     As one can readily appreciate from the above, the accuracy of wavefront measurement provided by the above-described wavefront type refractor depends on precise measurement of the positions of the focal spots. Good image quality of the Hartmann-Shack focal spots is thus an essential requirement of such a wavefront type refractor. To resolve the positions of the focal spots precisely, the spots need to be kept to a certain size to cover a predetermined number of pixels in the CCD camera. 
     One problem encountered in using a Hartmann-Shack sensor to fabricate a wavefront type refractor relates to the defocusing power of an eye. In particular, the defocusing power of an eye varies from patient to patient, and this defocusing power variation can significantly change the spot size of the probe beam on the retina. Consequently, the focal spot size on the Hartmann-Shack CCD camera can change significantly. Another problem encountered in using a Hartmann-Shack sensor to fabricate a wavefront type refractor relates to diffused scattering from a retina. In particular, diffused scattering from the retina produces a bright background for the Hartmann-Shack focal spots, and as a result, reduces image contrast. As is well known, diffused scattering form the retina is a result of the layer structure of the fibers of the retina (the layer structure serves as a two-dimensional wave-guide to enhance lateral diffusion of scattered light). 
     As one can readily appreciate from the above, a need exists in the art for method and apparatus for measuring refractive errors of an eye that improve upon wavefront type refractors using a conventional Hartmann-Shack sensor. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention advantageously satisfy the above-identified need in the art, and provide method and apparatus for measuring refractive errors of an eye that improve upon wavefront type refractors using a conventional Hartmann-Shack sensor. 
     Specifically, one embodiment of the present invention is an apparatus for measuring refractive errors of an eye which comprises: (a) a source of a probe beam; (b) a first Badal lens system adapted to project the probe beam into a subject&#39;s eye to form an illumination spot on a retina; (c) a second Badal lens system adapted to image the illumination spot onto an image plane substantially conjugate to the retina; and (d) a spatial filter disposed in the image plane adapted to transmit at least a portion of the image. Advantageously, such an embodiment provides: (a) more accurate wavefront measurement when using a Hartmann-Shack sensor by improving a Hartmann-Shack image; (b) a probe beam spot size that is substantially independent of the defocusing power of the eye; and (c) that a minimal of diffused scattering from the retina falls onto the Hartmann-Shack image of a Hartmann-Shack sensor. 
    
    
     BRIEF DESCRIPTION OF THE FIGURE 
     FIG. 1 shows a block diagram of a wavefront refractor that is fabricated in accordance with one embodiment of the present invention; and 
     FIG. 2 shows a block diagram of a Badal optics apparatus in the prior art that is used to fabricate a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a block diagram of wavefront refractor  100  that is fabricated in accordance with the present invention. As shown in FIG. 1, wavefront refractor  100  comprises probe beam assembly  10 , polarizing beamsplitter  20 , relay optics assembly  40 , and Hartmann-Shack sensor  50 . 
     As shown in FIG. 1, probe beam assembly  10  comprises radiation source  12  which outputs a beam of radiation, which beam of radiation is applied as input to fiber  13 . The beam of radiation is typically radiation that is not detected by a patient such as, for example and without limitation, infrared or near infrared radiation. A beam of radiation output from fiber  13  passes through collimation optical system  14  (collimation lens system  14  may comprise one or more lenses) and polarizer  17  to output linearly polarized radiation (polarizer  17  can be fabricated in accordance with a number of methods that are well known to those of ordinary skill in the art). The linearly polarized beam of radiation passes through Badal lens system  15  (a Badal configuration is described in detail below), is redirected by turning mirror  16 , and, as projected beam  11 , impinges upon polarizing beamsplitter  20 . Although only one lens is illustrated, those of ordinary skill in the art will readily understand that more than one lens is more typical of such Badal lens systems. Polarizing beamsplitter  20  directs projected beam  11  to impinge upon eye  30  to form illumination spot  32  on retina  31 . Advantageously, in accordance with this embodiment of the present invention, use of Badal lens system  15 , causes illumination spot  32  on retina  31  to be rendered with a spot size that is independent of the defocusing power of eye  30 . As a result, the spot size of illumination spot  32  on retina  31  can be predetermined, and is substantially independent of the defocusing power of eye  30 . 
     It is desirable to utilize a super-luminescent diode to fabricate radiation source  12  due to its high brightness and short coherence length. A desirable wavelength of the super-luminescent diode is in the near infrared spectrum range. However, other radiation sources may be used such as, for example and without limitation, a laser or a light emitting diode. In addition it is preferred that fiber  13  be a single mode fiber to enable good beam quality and fine collimation. Polarizer  17  is set to select a polarization defined by polarizing beamsplitter  20 . 
     In accordance with this embodiment of the present invention, Badal lens system  15  is located one focal length away from pupil plane P of eye  30 . As a result, probe beam  11  is focused onto pupil plane P. For an example wherein: (a) fiber  13  has a fiber core of 5 microns; (b) collimating lens  14  has a focal length of 15 mm; and (c) Badal lens system  15  has a focal length of 200 mm, the spot size on pupil plane P is about 65 microns. 
     In such a Badal configuration, the spot size of illumination spot  32  on retina  31  is about 300 microns for a normal eye length of 22 mm. This spot size is substantially independent of the defocusing power of eye  30 , while it is proportional to eye length. 
     As shown in FIG. 1, light scattered from illumination spot  32  passes through the eye&#39;s optics (including eye lens  34  and cornea  35 ), and emerges as outgoing beam  33 . The wavefront of outgoing beam  33  carries aberration information relating to the eye&#39;s optics. Polarizing beamsplitter  20  only passes a depolarized portion of outgoing beam  33  (i.e., polarizing beamsplitter  20  rejects reflections from, among other things, eye lens  34 , cornea  35 , and retina  31 ). 
     As shown in FIG. 1, relay optics assembly  40  comprises Badal lens system  41  and lens system  42 , which Badal lens system  41  and lens system  42  may each comprise one or more lenses. Relay optics assembly  40  relays the wavefront at pupil plane P to a conjugate plane P′. As further shown in FIG. 1, Badal lens system  41  images illumination spot  32  as image spot  48  on plane R′ inside relay optics assembly  40  (plane R′ is a focal plane of Badal lens system  41 , and is conjugated to retina  31 ). In accordance with this embodiment of the present invention, the spot size of the image of illumination spot  32  at plane R′ is substantially independent of the defocusing power of eye  30 . Further, in accordance with this embodiment of the present invention, spatial filter  44  has an aperture substantially equal to the size of the image spot, and is positioned at focal plane R′ to reject trace light caused by diffused scattering on retina  31 . 
     The position of image plane R′ varies as a function of the defocusing power of eye  30 . However, as is well known to those of ordinary skill in the art, the position of image plane R′ can be determined using an optometer, which optometer may be used as an accessory alignment device for apparatus  100 . Then, in accordance with a further embodiment of the present invention, a driving mechanism (not shown) can move spatial filter  44  to overlap image plane R′. The driving mechanism for moving spatial filter  44  may be fabricated in accordance with any one of a number of methods that are well known to those of ordinary skill in the art. For example and without limitation, spatial filter  44  may be moved by a linear motor or a motorized drive screw. 
     In accordance with this further embodiment of the present invention, spatial filter  44  has an aperture substantially equal to the size of image spot  48 . Thus, when movable spatial filter  44  is located at image plane R′, maximum transmission of outgoing beam  33  (scattered from illumination spot  32 ) occurs, and trace radiation from diffused scattering around illumination spot  32  is rejected. Advantageously, in accordance with this embodiment of the present invention, the use of spatial filter  44  significantly improves the contrast of an image obtained on Hartmann-Shack sensor  50 , and as a result, the detection of focal spots  52  (to be described in detail below) can be more precise. 
     As further shown in FIG. 1, Hartmann-Shack wavefront sensor  50  comprises lenslet array  51  and CCD camera  53 . Lenslet array  51  is located at plane P′ and CCD camera  53  is located at the focal plane of the lenslet elements of lenslet array  51 . Wavefront sensor  50  detects the wavefront of outgoing beam  33  when lenslet array  51  forms a pattern of focal spots  52  on CCD camera  53 . 
     In accordance with this embodiment of the present invention, the output from CCD camera  53  is applied as input to an analyzer (not shown), for example, a personal computer. The analyzer then determines the x, y, z position of a centroid of focal spots in accordance with any one or a number of methods that are well known to those of ordinary skill in the art. Then, the slope of each beam segment is determined using the coordinates of the centroids to determine the slope of a portion of the beam passing through each of the elements of lenslet array  50 . Next, the analyzer uses any one of a number of methods that are well known to those of ordinary skill in the art to use the slopes of the beam segments to reconstruct the wavefront of beam  33  at plane P′. For example, in one such embodiment, the analyzer fits the slopes of the beam segments to a set of Zernike polynomials to reconstruct the wavefront of beam  33  at plane P′ in accordance with the teaching of an article entitled “Objective measurement of wave aberrations of the human eye with the use of a Hartmann-Shack wave-front sensor” by J. Liang et al.,  J. Opt. Soc. Am. A , Vol. 11, No. 7, July 1994, pp. 1949-1957 (the “Liang article”), which Liang article is incorporated by reference herein. The wavefront of beam  33  is then reconstructed at plane P via a scale factor determined by the relay optics. A comprehensive review of the Hartmann-Shack wavefront sensor, and wavefront reconstruction is found in U.S. Pat. No. 5,777,719. 
     Finally, the refractive errors of the eye are calculated by the analyzer in accordance with any one of a number of methods that are well known to those of ordinary skill in the art using the reconstructed wavefront. For example, one such method is disclosed in a publication of Frey et al. on Jun. 3, 1999, WO 99/27334 entitled “Objective Measurement and Correction of Optical Systems Using Wavefront Analysis” wherein distortions of the wavefront are taken as an estimate of the aberrations, which publication is incorporated by reference herein (see also the Liang article). 
     The use of Hartmann-Shack sensor  50  for wavefront measurement is well known in the art. However, the image quality of focal spots  52  on CCD camera  53  remains as an issue in obtaining accurate measurement of eye aberration. For example, diffused scattering from illumination spot  32  may produce a bright background on focal spots  52 . Such a bright background may reduce the signal to noise ratio of focal spots  52 , and thereby, make it difficult to obtain precise measurement of the position of focal spots  52 . Advantageously in accordance with this embodiment of the present invention, and as described above, Badal lens system  15  provides illumination spot on retina  31  with a spot size that is independent of the defocusing power of eye  30 . In addition, Badal lens system  41  provides an image spot at plane R′ having a spot size that is independent of the defocusing power of eye  30 . This, together with spatial filter  44 , enables one to obtain a Hartmann-Shack image with reduced background from diffuse scattering. As a result, improved accuracy can be achieved for wavefront measurement. 
     FIG. 2 shows a block diagram of prior art Badal optics configuration  200  which produces image  36  on retina  31  having an image size that is independent of the defocusing power of eye  30 . In one embodiment, Badal optics configuration  200  comprises Badal lens system  46  located one focal length away form pupil plane P of eye  30 . As shown in FIG. 2, Badal lens  46  forms an image plane R′ of retina  31 . The distance of image plane R′ from Badal lens  46  depends on the focal power of eye  30 . However, target  47  located on image plane R′ appears the same size, independent of the focal power of eye  30 . Badal optics configurations are commonly used in optometry and a detailed description of the Badal optics configuration can be found in U.S. Pat. No. 5,208,619, which patent is incorporated by reference herein. Note that the Badal configuration shown in FIG. 1 in relay optics assembly  40  (where illumination spot  32  is imaged by Badal lens system  41  to form an image spot  48  on image plane R′ conjugated to retina  31 ) is a reverse arrangement of an optometer (for example, as shown in FIG. 2) where target  47 , located at image plane R′, is imaged onto retina  31 . 
     Those of ordinary skill in the prior art should readily appreciate that the combination of the two Badal optics configurations shown in FIG. 1 produces an image spot, image spot  48 , having a spot size that is substantially independent of eye length of eye  30 . 
     In one example of a wavefront refractor fabricated in accordance with an embodiment of the present invention, the focal length of Badal lens  41  is approximately 100 mm. This focal length is about five (5) times as long as an eye length. As a result, image spot  48  on focal plane R′ is about five (5) times as large as illumination spot  32 . Thus, for an illumination spot having an approximate spot size of 300 microns, spatial filter  44  should have an aperture of about 1.5 mm. 
     Those skilled in the art will recognize that the foregoing description has been presented for the sake of illustration and description only. As such, it is not intended to be exhaustive or to limit the invention to the precise form disclosed. For example, those of ordinary skill in the art should readily appreciate that Badal lens system  15  may be combined with collimating lens system  14  to produce a similar focal spot on pupil plane P. Further, although embodiments of the present invention has been described in conjunction with a Hartmann-Shack sensor, the present invention is not limited by this. In fact, it is within the scope and spirit of the present invention that other wavefront sensors made be used as well.