Optics: eye examining – vision testing and correcting – Eye examining or testing instrument – Objective type
Reexamination Certificate
1999-10-21
2001-07-24
Manuel, George (Department: 3737)
Optics: eye examining, vision testing and correcting
Eye examining or testing instrument
Objective type
Reexamination Certificate
active
06264328
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to a wavefront sensor, such as a sensor for wavefront aberrations in the eye, and more particularly to such a sensor which avoids corneal reflection by illuminating the retina along a light path off of the optical axis of the eye. The present invention is further directed to a method of sensing a wavefront using such off-axis illumination.
It is known in the art to detect wavefront aberrations in the human eye for such purposes as intraocular surgery and contact lens fabrication. Such detection is disclosed, e.g., in Liang et al, “Objective measurement of wave aberrations of the human eye with the user of a Hartmann-Shack wave-front sensor,”
Journal of the Optical Society of America
, Vol. 11, No. 7, July, 1994, pp. 1-9. A beam of light from a laser diode or other light source is directed toward the pupil and is incident on the retina. Because the retina is highly absorbing, a beam on the order of four orders of magnitude dimmer than the original beam is reflected by the retina and emerges from the pupil. Typically, the incoming and emergent light follow a common optical path; the incoming light is brought into the common optical path with a beamsplitter.
The emergent beam is applied to a Hartmann-Shack detector to detect the aberrations. Such a detector includes an array of lenslets which break up the light into an array of spots and focus the spots onto a charge-coupled detector or other two-dimensional light detector. Each spot is located to determine its displacement from the position which it would occupy in the absence of wavefront aberrations, and the displacements of the spots allow reconstruction of the wavefront and thus detection of the aberrations.
Improvements to the technique of Liang et al are taught in J. Liang and D. R. Williams, “Aberrations and retinal image quality of the normal human eye,”
Journal of the Optical Society of America
, Vol. 4, No. 11, November, 1997, pp. 2873-2883 and in U.S. Pat. No. 5,777,719 to Williams et al. Williams et al teaches techniques for detecting aberrations and for using the aberrations thus detected for eye surgery and the fabrication of intraocular and contact lenses. Moreover, the techniques of those references, unlike that of the Liang et al 1994 article, lend themselves to automation.
The techniques described above involve illuminating the eye along the eye's optical axis. As a consequence, the light reflected from the retina is mixed with stray reflections which can disrupt measurements. More specifically, the stray reflections show up as spurious bright spots amid the array of spots formed in the Hartmann-Shack sensor.
Such stray reflections have several sources in wavefront sensors. Of particular concern are the reflections from the optical elements between the retina and the beamsplitter. Such elements typically include the optics of the eye and a pair of lenses between the beamsplitter and the eye. Back reflections from surfaces other than the retina are weak relative to the illuminating beam but are bright relative to the weak signal reflected from the retina.
In the eye's optics, the only surface whose back reflection is bright enough to be problematic is the first (outer) surface of the cornea. That reflection is comparable in energy to the reflection from the retina and can therefore be a considerable nuisance for wavefront sensing, particularly if the centroids of the spots in the detector are to be computed automatically.
One known way to remove the corneal reflection, taught in Liang and Williams and in Williams et al, uses a polarizing beamsplitter to remove reflected light from all of the surfaces between the beamsplitter and the retina. Because those surfaces retain the linear polarization of the light incident thereon, both lens reflections and the corneal reflection are eliminated. However, much of the light reflected from the retina is also lost. Only depolarized light reflected from the retina, which accounts for only about thirty percent of the total light reflected from the retina, is available to detect the wavefront aberration. Moreover, the depolarized light contains considerable spatial noise. Still another problem is the intensity nonuniformity introduced into the array of spots by the birefringence of the eye's optics, chiefly the cornea.
Another known way to remove reflections from all optics between the beamsplitter and the eye while increasing the signal from the retina involves the use of a polarizing beamsplitter in combination with a quarter-wavelength (&lgr;/4) plate just in front of the eye. German published patent application No. DE 42 22 395 A1 teaches that technique. That technique allows a much greater part of the light reflected from the retina to reach the detector, thereby improving spot quality, while removing the variation in spot brightness caused by the birefringence of the eye. It also removes back reflection from the lenses. However, the corneal reflection is not removed and is thus just as troublesome as it would be in the absence of polarizing optics.
Another problem with the two techniques just described is the cost of the polarizing beamsplitter and of the &lgr;/4 plate. In cost-sensitive commercial settings, it would be desirable to eliminate that cost.
SUMMARY OF THE INVENTION
In light of the foregoing, it will be readily apparent that a need exists in the art to provide a wavefront sensor in which the corneal reflection does not cause spurious spots on the detector or otherwise degrade the signal derived from the light reflected from the retina. More particularly, there is a need to detect wavefront aberrations in an accurate and cost-effective manner by eliminating the problem of corneal reflection without using polarizing optics.
It is therefore an object of the invention to address those needs.
To achieve the above and other objects, the present invention is directed to a wavefront sensor in which the eye is illuminated off-axis. The light not reflected by the cornea impinges on the retina, and the light reflected by the retina returns through the lens and the cornea. That light is thereby focused into an optical path different from the optical path followed by the corneal reflection. The entire retinal reflection is used, and the corneal reflection can be discarded by use of simple, inexpensive, non-polarizing optics such as a stop.
The beam used to illuminate the eye is relatively narrow, e.g., around 1-1.5 mm in diameter, and intersects the cornea in a small area, thus further reducing the probability that the corneal reflection will take a return path to the detector. In addition, the dioptric range over which the small spot is in focus on the retina can be increased. Typically, a displacement of the illuminating beam from the optical axis of the eye by less than one millimeter completely removes the corneal reflection.
The illuminating beam is preferably introduced into the optical path at the last possible location before the eye, e.g., by placing the beamsplitter right before the eye. Thus, back reflection from the lenses is avoided, as the only element between the beamsplitter and the retina is the cornea.
Even with the beamsplitter placed right before the eye, it is possible to adjust the focus of the illuminating beam and that of the exit beam by the use of the same element. One way to do so is to provide a folded optical path with mirrors mounted on a slide. The mirrors are disposed in the path of the illuminating beam before it reaches the beamsplitter and in the path of the exit beam. Thus, movement of the slide focuses both beams.
The light source can be moved in a direction perpendicular (or, more generally, non-parallel) to the direction of its output as needed to accommodate the eyes of different patients.
The present invention has utility in any procedure involving wavefront sensing of the eye or otherwise involving illumination of the retina. Such procedures include, but are not limited to, autorefraction, design of contact or intraocular lenses, refractive surgery and
Williams David R.
Yoon Geun-Young
Blank Rome Comisky & McCauley LLP
Manuel George
University of Rochester
LandOfFree
Wavefront sensor with off-axis illumination does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Wavefront sensor with off-axis illumination, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Wavefront sensor with off-axis illumination will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2535243