U.S. patent application number 09/769892 was filed with the patent office on 2002-04-25 for method and apparatus for measuring optical aberrations of the human eye.
This patent application is currently assigned to Zyoptics, Inc.. Invention is credited to Graves, J. Elon, Northcott, Malcolm J..
Application Number | 20020047992 09/769892 |
Document ID | / |
Family ID | 25086812 |
Filed Date | 2002-04-25 |
United States Patent
Application |
20020047992 |
Kind Code |
A1 |
Graves, J. Elon ; et
al. |
April 25, 2002 |
Method and apparatus for measuring optical aberrations of the human
eye
Abstract
An apparatus for measuring optical aberrations of the human eye
wherein the person positions his or her eye on an optical axis of
the apparatus and looks at an illuminated target on the optical
axis that is visible to the eye for allowing the eye to focus on
the target and establish a position of the eye. A collimating lens
on the optical axis is movable along the optical axis for adjusting
the apparent optical distance between the eye and the target. A
light source directs a predetermined light beam along the optical
axis into the eye and onto the retina of the eye as a spot of
light. A lens reimages the light scattered from the light spot on
the eye retina into a wavefront curvature sensor that forms two
oppositely defocused images on an image detector, and a computer
processes and analyzes the two defocused images for measuring the
optical aberrations of the eye.
Inventors: |
Graves, J. Elon; (Kailua,
HI) ; Northcott, Malcolm J.; (Kailua, HI) |
Correspondence
Address: |
LYON & LYON LLP
633 WEST FIFTH STREET
SUITE 4700
LOS ANGELES
CA
90071
US
|
Assignee: |
Zyoptics, Inc.
|
Family ID: |
25086812 |
Appl. No.: |
09/769892 |
Filed: |
January 25, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60178416 |
Jan 27, 2000 |
|
|
|
Current U.S.
Class: |
351/212 |
Current CPC
Class: |
A61B 3/103 20130101;
G02B 26/06 20130101 |
Class at
Publication: |
351/212 |
International
Class: |
A61B 003/10 |
Claims
What is claimed:
1. An apparatus for measuring optical aberrations of the human eye,
comprising; a light source for directing a predetermined light beam
along the optical axis into the eye and onto the retina of the eye,
a lens on the optical axis for reimaging the light from the eye
retina of the light from the light beam, a wavefront curvature
sensor for receiving the reimaged light and having means for
forming two oppositely defocused pupil images on an image detector,
and computer means for processing and analyzing the two defocused
images for measuring the optical aberrations of the eye.
2. The apparatus of claim 1, wherein said wavefront curvature
sensor means forming the two oppositely defocused pupil images
forms one negatively defocused pupil image and one positively
defocused pupil image of equal focus distances from an optical
image plane.
3. The apparatus of claim 1, wherein said means for forming two
oppositely defocused pupil images comprises a negatively powered
lens forming one defocused pupil image and a positively powered
lens forming the other defocused pupil image, said negatively
powered lens and positively powered lens having equal focal
lengths.
4. The apparatus of claim 1, wherein said means for forming two
oppositely defocused pupil images comprises a flexible mirror
positioned in the path of the reimaged light and means for causing
the flexible mirror to flex to concave and convex conditions to
reflect the pupil image to said image detector.
5. The apparatus of claim 4, wherein said flexible mirror is
decentered with respect to the reimaged light for forming separate
defocused images on said image detector.
6. The apparatus of claim 1, wherein the apparatus is provided with
means for supporting the head and positioning the eye on the
optical axis.
7. The apparatus of claim 1, wherein the apparatus provided with an
illuminated target on the optical axis that is visible to the
eye.
8. The apparatus of claim 1, wherein the apparatus is provided with
a camera for recording a focused pupil image.
9. The apparatus of claim 1, wherein said light source emits a
light beam that is substantially invisible to the human eye.
10. The apparatus of claim 9, wherein said light beam is
polarized.
11. The apparatus of claim 1, wherein said light source is a laser
diode.
12. The apparatus of claim 1, wherein a polarized beam splitter is
provided on the optical axis for reflecting the light beam into the
eye and allowing the light reflect from the retina to pass to said
lens for reimaging that light.
13. The apparatus of claim 1, further comprising an illuminated
target on the optical axis and visible by the eye for allowing the
eye to focus on said target and for establish a position of the
eye, and a collimating lens on the optical axis between the eye and
said target, said collimating lens being movable along the optical
axis for adjusting the apparent optical distance between the eye
and said target.
14. The apparatus of claim 13, wherein said wavefront curvature
sensor means forming the two oppositely defocused pupil images
forms one negatively defocused pupil image and one positively
defocused pupil image of equal focus distances from an optical
image plane.
15. The apparatus of claim 13, wherein the apparatus s provided
with means for supporting the head and positioning the eye on the
optical axis.
16. The apparatus of claim 13, wherein the apparatus is provided
with a camera for recording a focus pupil image.
17. The apparatus of claim 13, wherein said light source emits a
light beam that is substantially invisible to the human eye.
18. The apparatus of claim 13, wherein a polarized beam splitter is
provided on the optical axis for reflecting the light beam into the
eye and allowing the light reflect from the retina to pass to said
lens for reimaging that light.
19. The apparatus of claim 1, wherein said predetermined light beam
is an annular ring.
20. the apparatus of claim 1, further including a mask in the path
of the light source and having a central obscuration for
eliminating light from the center of said predetermined light
beam.
21. The apparatus of claim 20, wherein said central obscuration is
of a size to prevent light reflected from optical surfaces of the
eye from reaching said wavefront sensor.
22. The apparatus of claim 1, further comprising a deformable
mirror positioned on the optical axis for receiving the reimaged
light reflected from the retina, and means for causing controlled
deformation of said mirror for correcting the measured
aberrations.
23. The apparatus of claim 1 including a wavefront curvature sensor
and adaptive optics means for correcting the sensed wavefront
curvature produced by the eye.
24. The apparatus of claim 23, further comprising a deformable
curvature mirror positioned on the optical axis for receiving the
reimaged light reflected from the retina, and means for causing
controlled deformation of said mirror for correcting the measured
aberrations.
25. A method of measuring optical aberrations of the human eye,
comprising the steps of; introducing a beam of light into the eye
along an optical axis for forming a spot of light on the retina of
the eye from which scattered light rays progress in a direction
generally along the optical axis, reimaging the light scattered
from the eye retina, optically producing two oppositely defocused
pupil images on an image detector, and processing and analyzing the
two defocused images for measuring the optical aberrations of the
eye.
26. The method of claim 25, further comprising the step of
establishing a position, at least momentarily, of the eye on an
optical axis for eye-viewing along the optical axis before
introducing the beam of light into the eye.
27. The method of claim 25, further comprising the step of
providing an illuminated target visible to the eye along the
optical axis for allowing the eye to focus on the target and remain
substantially stationary during testing.
28. The method of claim 27 further including the step of recording
a focused image of the pupil image substantially simultaneously
with detecting the two defocused pupil images.
29. The method of claim 25, further including the step of recording
a focused image of the pupil image substantially simultaneously
with detecting the two defocused pupil images.
30. The method of claim 25, wherein the step of optically producing
two oppositely defocused pupil images produces equal negatively and
positively defocused images.
31. The method of claim 25, wherein the wavefront curvature is
analyzed from the two oppositely defocused pupil images.
32. The method of claim 25, further including the step of directing
the light scattered from the retina onto the surface of a
deformable mirror and deforming said mirror based on the measured
optical aberrations of the eye for correcting those
aberrations.
33. The method of claim 25, wherein said light beam introduced into
the eye is formed into an annular ring for inhibiting the
reflection of light from the optical surfaces of the eye.
34. The method of claim 25, wherein said light beam introduced into
the eye is strobed.
Description
[0001] This application basis priority on Provisional Patent
Application Serial No. 601178,416, filed Jan. 27, 2000.
FIELD OF THE INVENTION
[0002] The present invention relates to methods and optical
instrumentation for objectively measuring the aberrations of the
human eye and specifically to an instrument capable of measuring
not only the focus (spherical) and astigmatism (cylindrical)
characteristics and aberrations of a person's eye but also all of
the lower and higher order optical aberrations that are derived
from a measured wavefront utilizing a wavefront curvature
sensor.
BACKGROUND OF THE INVENTION
[0003] Measuring the aberrations of an optical system, including a
human eye, is an important part of working with any optical system.
Existing methods of measurement include various interferometric
techniques, the Shack-Hartman wavefront sensor, and various systems
involving the projection of patterns through the optical system.
These systems are typically complex and expensive and most require
access to the focal plane.
[0004] The human eye, although comprised of only a few optical
components, may manifest a wide variety of optical aberrations that
vary from person to person and over time. These aberrations may
result from surface contour shape, lens thickness factors, axial
alignment of refractive surfaces, axial length of the eye, and even
localized refractive index variations. The fact that the human eye
possesses optical aberrations has been known for centuries.
Nevertheless, the measurement and characterization of these
aberrations, primarily the monochromatic aberrations, has remained
a problem and has fostered much research in physiological optics
over the years. Finding the proper prescription, even in modern
times, has been primarily based on the subjective responses to the
viewing of eye charts by the person being tested, whereas recent
advances in corrective methods have emphasized objective
measurement of these aberrations.
[0005] Current methods of optical correction of the human eye to
allow clear vision include spectacles, contact and intraocular
lenses, and refractive surgery. Spectacles, the most commonly used
method, only allow correction of sphere (defocus) and regular or
symmetric cylinder (astigmatism). None of the present methods allow
for correction of other aberrations and thus do not maximize the
optical potential of the visual system, leaving the images
generated to be less than optimal. In addition, not only is the
view out of the eye not optimal but so is the view in. Thus, the
examination of the eye's interior is also limited by these
aberrations, and in some clinical situations, is severely
handicapped.
[0006] Recently, great interest in this area has been kindled by
the development of laser technologies, such as the excimer laser,
whereby the refractive or optical errors of the eye, such as myopia
(nearsightedness), hyperopia (farsightedness) and astigmatism, can
be corrected by laser abelation or sculpting of the cornea. Such
treatment creates a new corneal contour, or curvature, designed
such that the image becomes clearly focused on the retina of the
eye. Many degrees of myopia and hyperopia, with or without
astigmatism, and astigmatism alone can now be corrected by such
laser corneal surgery.
[0007] Although the clinical results of such surgery are good, it
has been postulated, on the basis of experiment, that improved
results could be obtained if the surgery were customized fully to
correct all the optical aberrations of the eye, not just the sphere
and cylinder. Super vision at the diffraction limit set by the
aperture is possible. This could be accomplished by a
computer-directed small spot scanning laser and sophisticated
algorithm that takes into consideration all the aberrations of the
eye. Also, many subject's have irregularly shaped corneas, not
currently treatable. In addition, the asphericity of the modified
cornea is often significantly increased. Clinical studies have
indicated that current autorefractors, when used to determine the
refraction, or optical prescription, of surgically modified eyes
may provide less reliable data in such cases. Even in normal eyes,
their accuracy is such that the information cannot be routinely
relied upon but must be verified by further subjective testing.
[0008] It is apparent that a complete diagnosis and understanding
of the eye's optical function, as the organic optical instrument,
is currently very limited. A full evaluation should provide a
complete description of the optical characteristics and aberrations
in a quantitative format. Only then can there arise the possibility
of correcting the abnormalities.
[0009] Theoretically, light arriving at the eye from a point source
at infinity arrives in the form of a plane (flat) wave, whereas
light from closer objects provide a wave with a convex spherical
shape. This wave, in an ideal eye, would be focused as a discrete
point limited only by diffraction on the retina of the eye.
However, because of the optical aberrations of the eye, a degraded
or blurred image is created on the retina. This concept can be
appreciated in the reverse direction with resultant utility.
[0010] A plane wave, directed into the eye, would form a spot on
the retina. In reverse this spot scatters light which escapes
through the same optical path from which it came in. Because this
light originates from a scattering process the incoming wavefront
information is lost, resulting in a new source which originates
from the back of the eye. This emergent wavefront now processes
only the aberrations of the eye on a single pass. The present
inventors have discovered that the distorted shape of this source,
caused by incoming aberrations, can uniquely be eliminated with the
differential curvature wavefront sensing method. Measurement and
characterization of this wavefront allows one to describe the
aberrations of the eye mathematically. Presently, some of these
concepts are taken advantage of in ground-based telescope systems
that are typically coupled with adaptive optical elements in a
closed loop system. They can rapidly neutralize the wavefront
aberrations induced by atmospheric turbulence and produce images
that are limited only by diffraction and the aperture of the
telescope.
[0011] Unfortunately, current subjective clinical methodology and
instrumentation, such as the phoropter and objective devices such
as autorefractors, do not avail themselves of this understanding
and are based on concepts and techniques that restrict measurements
to defocus and astigmatism only. During the past decade, this
limitation has been appreciated and devices called corneal
topographers, utilizing images reflected by the cornea, have been
developed to obtain more optical information about the eye.
However, they gather optical information about only one surface in
the eye's refractive system and reveal nothing about the system as
a whole.
[0012] A number of investigators have attempted or suggested means
whereby the wavefront, either explicitly or implicitly, was
recognized as an entity to be captured and determined. These
studies were interested primarily in determining the monochromatic
aberrations of the eye rather than the development of
autorefractor-like devices for routine clinical use or methods of
correction.
[0013] A number of approaches have been taken to measure the
monochromatic aberrations of the eye. Some used projecting rays or
patterns of light into the eye and analysis of the images by
subjective or objective means. Initially this work, such as present
by M. S. Smirnov ("Measurement of the Wave Aberration of the Human
Eye", Biophysics, 1961; 6:776-94) was carried out using subjective
sequential subject testing, which was inaccurate and time
consuming. More studies, however, have been performed using a
modification of the principle first presented by Tscherning in
1894. One approach employed a device called the crossed cylinder
aberroscope (Howland B and Howland HC: Subjective measurement of
high-order aberrations of the eye. Science 1976; 193:580-02 and
Howland HC and Howland B: "A subjective method for the measurement
of monochromatic aberrations of the eye", J. Opt Soc. Am 1977;
67(11): 1508-1518). Initially, this device was used in a subjective
fashion whereby a drawing was made of an object by the test subject
and later analyzed mathematically by computer to calculate the
wavefront. This allowed for the mathematical characterization of
the wavefront in mathematical terms, such as a Taylor series
expansion or Zernike polynomials, and it was determined that the
aberrations were dominated by third-order Taylor terms. Later, this
method was converted to an objective approach by Walsh et al.
[0014] In the objective aberroscope method a point source of light
is viewed through a grid placed close to the eye (Walsh et al.,
"Objective Technique for the Determination of Monochromatic
Aberrations of the Human Eye", J. Opt. Soc. Am. A., 1984; Vol.1,
No. 9, pp. 987-992; and Walsh G, Chairman WN: "Measurement of the
axial wavefront aberration of the human eye", Opthal Physiol Opt,
1985,, 5:23-31). This results in an aberrated image of the grid on
the retina that can be photographed and analyzed by ray tracing.
Although multiple points at the grid line intersections can be
captured at the same time, it has been found that diffraction
effects prevent sampling the papillary area at intervals much less
than 1 mm which limit the determination of fine detail (Chairman
WN: "Wavefront Aberration of the Eye: A Review", Optometry and
Vision Science 1991; 68(3): 574-583). Other drawbacks were the lack
of a rapid means of analysis and the faulty assumption that the
aberrations could be characterized by terms of only up to the
fourth order. The former problem has been improved upon with more
computerized versions (Atchinson DA, Collins MJ, Wildsoet CF,
Christensen J. Waterworth MD: "Measurement of monochromatic ocular
aberrations of human eyes as a function of accommodation by the
Howland aberroscope technique", Vision Res 1995; 35(3): 313-23 and
Cox MJ and Walsh G: "Reliability and validity studies of a new
computer-assisted crossed-cylinder aberroscope", Optom Vis Sci
1997; 74(7): 570-80).
[0015] Several investigators attempted other objective methods,
whereby the wavefront was determined from the point-spread function
data using a hybrid optical-digital instrument (Artal P, Santamarfa
J. Bescos J: "Retrieval of wave aberration of human eyes from
actual point-spread-function data", J Opt Soc Am 1988; 5(8);
1201-6).
[0016] Another objective approach was based on a modified Foucault
knife-edge method as a double-pass ophthalmoscopic method and
allowed wavefront aberrations to be inferred from two flash
photographs obtained with the knife-edge oriented in orthogonal
directions. This demonstrated significant irregular components
(Berny F and Slansky S: "Wavefront determination resulting from
Foucault tests applied to the human eye and visual instruments",
Optical Instruments and techniques, Dickson JH (ed), London, Oriel,
1969, 375-85).
[0017] Howland used an approach whereby variations of focus across
the natural pupil by employing a small artificial pupil and a
telescope with an adjustable focus, and related the measured
variations in focus to wave aberrations of the eye (Howland HC and
Buettner J: "Computing high order wave aberration coefficients from
small variations of best focus for small artificial pupils", Vision
Res 1989; 29(8): 979-83).
[0018] The most recent direction taken in the measurement and
correction of monochromatic aberrations of the eye involves the
technologies of adaptive optics or deformable mirrors and wavefront
sensors. The use of a deformable mirror has been proposed to assist
in the neutralization of the wavefront error to improve the
function of a confocal laser scanning ophthalmoscope for use with
the human eye, and the method was to correct the low order
aberration of astigmatism (Bille U.S. Pat. No. 4,838,679 and
Dreher, Bille, and Weinreb, "Active optical depth resolution
improvement of the laser tomographic scanner", Applied Optics, 1989
Vol. 28, No. 4, pp. 804-808). In neither case, however, was a
specific method to measure the aberrations developed or
disclosed.
[0019] Others used a Hartmann-Shack wavefront sensor, developed and
used in astronomy in conjunction with adaptive optics to neutralize
atmospheric turbulence, to measure the eye's aberrations (Williams
et al. U.S. Pat. No. 5,777,719 and Liang, et al., "Objective
measurement of wave aberrations of the human eye with the use of a
Hartmann-Shack wave-front sensor", J. Opt. Soc. Am. A., July 1994
Vol. 11, No. 7, pp. 1-9). The sensor is an array of multiple
lenslets, constructed of two identical layers of cylinders set 90
degrees apart that act as an array of spherical lenslets. The
reflection of a beam of light incident onto the fovea is imaged by
the lenslet array and analyzed by computer, deriving the wavefront
emergent from the eye. An acknowledged limitation of the system was
that only polynomials up to the fourth degree were used to
represent the wavefront, which is considered inadequate (Williams
U.S. Pat. No. 5,777,719). Bille also proposed the combination of a
wavefront sensor and an adaptive optical element but the details
have never been disclosed (Bille et al.; "Scanning laser tomography
of the living human eye"; Noninvasive Diagnostic Techniques in
Ophthalmology. Masters BR (ed), Springer-Verlag, 1990, pp.
528-47).
[0020] The first detailed description of a device that combined
both a wavefront sensor and an adaptive optical element was
disclosed by Williams, et al. U.S. Pat. No. 5,777,719. The proposed
instrument was primarily a retinal fundus imaging device that used
the Hartmann-Shack wavefront sensor as the basis of obtaining the
wavefront. The wavefront is expressed in terms of Zernike
polynomials, which are then used to appropriately deform a mirror
such that the eye's aberrations could be neutralized or compensated
for to provide high resolution retinal imaging or to provide
supernormal vision while viewing with the assistance of the device.
Disadvantages with this technique are the complexity, construction
and cost of the Hartman-Shack wavefront sensor. Also, depending
upon the magnitude of the aberrations, significant deviations of
the wavefront at certain locations could be erroneously ascribed to
the nearest lenslet whereas the signal arose from a location
further away. In addition, the deformable mirror described is
extremely complex and costly to construct. Although perhaps capable
of determining and neutralizing the wavefront, the design does not
describe how the device could be used as a common tool in clinical
practice to determine the refraction of the eye in an economical
way.
[0021] Another approach, called curvature wavefront sensing, is an
old qualitative technique, which the present inventors have
discovered can be made quantitative, with the aid of new technology
and modern computers, for examining the human eye. It is
inexpensive to implement and can determine a wide range or
aberrations through a large tunable dynamic range. Curvature
wavefront sensing has been employed in closed-loop adaptive optics
systems in astronomy by one group for some years and has been
reported on several publications where the optical principles
involved are described (Roddier F: Curvature Sensing and
Compensation: "A new concept in adaptive optics", Applied Optics,
1988, Vol. 27, pp. 1223-5; Roddier F: "Wavefront Sensing and the
Irradiance Transport Equation. Applied Optics, 1990, Vol. 29(10),
pp. 1402-3; and Roddier C and Roddier F: "New Optical Testing
Methods Developed at the University of Hawaii: Results of
ground-based Telescopes and Hubble Space Telescope". SPIE, 1991,
Vol. 1531, pp.37-3), which publications are incorporated herein by
this reference for explanatory background.
SUMMARY AND OBJECTS OF THE INVENTION
[0022] It is a primary object of the present invention to provide
both a method and an apparatus for accurately and quickly measuring
the optical aberrations of the eye, including focus, astigmatism
and higher order aberrations.
[0023] It is another object of the present invention to provide
such an apparatus with a wavefront curvature sensor with the
capability of measuring the optical aberrations of the eye.
[0024] It is a further object of the present invention to provide
methods and apparatus for obtaining defocused pupil images that
make diffractive effects symmetrical, and thus maximizes the
accuracy of the wavefront sensor and the analysis of the
aberrations of the human eye.
[0025] It is a still further object of the present invention to
provide a method and apparatus for obtaining defocused images at
high speed, thus allowing the information to be used for real time
correction of the optical aberrations by several means.
[0026] Still a further object of the present invention is to
provide a method and apparatus for tuning the dynamic range of the
measurements taken with the wavefront sensor to suit the wavefront
under consideration.
[0027] A still further object of the present invention is to
provide a method and apparatus for minimizing strong images caused
by the reflection from the ocular refracting surfaces of the eye in
the measurement beam.
[0028] It is still another and further object of the present
invention to provide a method and apparatus for measuring the pupil
size and shape at the time of the wavefront measurement.
[0029] Other and more detailed objects and advantages of the
present invention will appear to those skilled in the art from the
following detailed description of the preferred embodiments in
conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a diagrammatic side elevation view of a person
using the apparatus of the present invention;
[0031] FIG. 2 is a diagrammatic plan view of the optical system of
a preferred embodiment of the apparatus of the present
invention;
[0032] FIG. 3 is a simplified optical diagram of a portion of the
system of FIG. 2 for describing the light reflections and
scattering by the human eye using the light source of the apparatus
of this invention;
[0033] FIG. 4 is a diagrammatic view of a modified embodiment of
the optical system illustrated in FIG. 2 with adaptive optics
elements and features; and
[0034] FIG. 5 is a diagrammatic plan view of an alternate form of
wavefront curvature sensor for use in the apparatus of FIG. 2 or
FIG. 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT AND VARIATIONS
[0035] Referring now to FIGS. 1 and 2, the subject person S that is
to have his or her eyes analyzed by the optical apparatus,
generally designated 10, of the present invention positions one eye
against a flexible eye cup 12 for aligning the center of the pupil
of that eye with the optical axis O of the apparatus 10.
Preferably, the head of the subject person S is adjusted to obtain
that alignment and then supported in that position by any
convenient means, such as an adjustable chin support 14. For
example, a shaped pad 16 may be supported by a pair of pivotally
connected links 18 and 20 that can be manipulated and then locked
in position when the subject person S has his or her eye E in
appropriate alignment with the optical axis O, which alignment can
be determined by the operator of the apparatus in a manner
described below. For clarity of illustration, the eye cup 12 is
omitted from the schematic plan view FIG. 2.
[0036] Referring more particularly to FIG. 2, the optical apparatus
10 is provided with a target 22 that is preferably back-lighted by
an illuminator 24 to be visible by the subject person S having an
eye E looking into the apparatus. The target 22 may be of any shape
or representation that encourages the subject person S to focus his
or her eye E on an exact point for inhibiting movement of the eye E
during the measuring and analyzing steps performed by the apparatus
10, such as a target with gun sight type crosshairs, concentric
circles, a pattern, a scene or the like. The light from target 22
is reflected by the partial mirror or beam splitter 26 along
optical axis portion O' through a collimating lens 28, a mask 30
and another partial mirror or beam splitter 32 to a polarizing beam
splitter 34 which reflects the light from the target 22 into the
eye E of the subject person S along the main portion of the optical
axis O. The collimating lens 28 preferably is adjustably movable
along the optical axis O' , as shown by the double arrow 36 to
adjust the apparent image distance of the accommodation target 22
from the eye E for optimizing the ability of the eye E to focus on
the target 22. Thus, the person S views with the eye E an
incoherently back-illuminated accommodation target 22 during use of
the optical apparatus 10 for establishing a fixed location,
orientation and focus adjustment of the human eye E to the maximum
extent possible without physically contacting the eye E during
operation of the optical apparatus 10.
[0037] A probe beam of light originating from any appropriate light
source 40, such as a laser diode, or any other bright polarized or
non-polarized point source of light, is directed along the optical
axis O' through the beam splitter 26 where it combines with the
light from accommodation target 22 and then through the collimating
lens 28, mask 30 and beam splitter 32 to reflect from the beam
splitter 34 along the main optical axis O into the eye E.
Preferably, the light from light source 40 is of a wavelength that
is readily detected by the type of detectors, described more fully
below, used in the optical apparatus 10, such as silicon detectors,
but of a wavelength that is safe and relatively invisible to the
retina R of the eye E. Red light is particularly suited for this
apparatus because the red light is not absorbed by the eye retina,
as is blue and yellow light. The light is scattered by the retina R
and emerges through the pupil P.
[0038] Referring now to FIG. 3, which is a simplified optical
diagram of a portion of the system of FIG. 2, the light beam from
light source 40 (not shown) is directed along optical axis O' and
passes through mask 30 to polarizing beam splitter 34 where it is
reflected toward eye E along optical axis O. The mask 30 is
comprised of a circular pupil stop opening having an opaque central
obscuration 30a and a surrounding circulars opening 30b to thereby
form a donut-shaped annular ring of light beam 31 from source 40.
The mask 30 may be in the form of a glass plate with the masking
material applied thereto. The annular light beam 31 enters the eye
E and is focused on the retina R as a small spot of light that is
then scattered before emerging from the eye E as a circular
(non-annular) light beam along optical axis O. Since the retina
scatters the light, the emerging light is not polarized and
therefore passes through the polarized beam splitter 34 and then
along optical axis O" to lens 42. However, not all of the light
returning from the eye E comes from the retina R and can cause
problems in measuring an accurate wavefront. Some of the light
projected toward the eye E is reflected by specular reflection from
the eye's optical surfaces, but predominantly this light is
reflected from the anterior crystalline surface ACS. Being a
concave surface this light is reflected back out the eye in a
converging beam whose focal point is located on the iris. This is a
design feature of the eye which assures that all the light
reflected from this surface will exit the eye and not be scatter
off the back of the iris. Since the annular light beam 31 impinges
only on an annular portion of the anterior crystalline surface ACS,
the reflected light is an annular beam of light rays ALR that
expands after passing through the pupil P, with no light in the
center, whereby the light rays ALR can not reach lens 42. As this
annular beam ALR exits the eye it is expanding at the focal ratio
at or near f/1. The central obstruction 30a of the mask is adjusted
to a f/which is faster than that of the wavefront sensor optics.
This arrange effectively removes this reflected light from the
system. The small amount of light reflecting off other surfaces in
the eye which are not removed by the method described above is
partially removed with polarization. Light returning along the
optical axis by reflection, at near normal incidence maintains its
polarization and, therefore, is directed back to the source 40 by
the beam splitter 34 and not into lens 42 and the wavefront sensor
50.
[0039] The portion of light from the annular light beam 31 that
passes through the eye lens L is focused on the retina as a small
spot. Adjustments to lens 36 are made by the patient to sharpen the
image of the target 22 whereby the light spot on the retina is
small and focused. The light rays from that light spot are
scattered by the retina R so the light is non-polarized and
therefore half of such light passes through the polarized beam
splitter 34 to the collimating lens 42 and then to the wavefront
sensor 50. The target 22 and light source 40 are conjugate. The
manner in which that light is analyzed by the wavefront sensor 50
will be described below.
[0040] Although it is not absolutely essential to the herein
disclosed apparatus and method for measuring the optical
aberrations of the human eye, it is desirable and the aberration
measurement accuracy can be improved by recording the geometry of
the pupil P of the human eye E at the exact moment that the
defocused pupil images are being recorded by the wavefront sensor
50. This is because not all human eye pupils P are exactly round
and any deviation from round can introduce errors in the wavefront
calculations because such calculations initially are based on the
assumption that the pupil is round and are not modified unless a
different shape is determined. In addition, unless immobilized with
drugs, the average pupil is constantly changing size with time, its
exact size at the moment the defocused images are recorded is
needed to improve the measurement accuracy. Further, by dilating
the eye E, the maximum amount of eye lens L can be analyzed. In the
apparatus 10 the pupil geometry is obtained and recorded with a
suitable camera 46. The iris can be illuminated by any suitable
light source which will return light along the optical axis O to
beam splitter 34 where the light is reflected along optical axis
portion O' to the beam splitter 32 where a portion of the light is
reflected to and through a lens 44 and then to a suitable camera 46
which records the exact shape of the pupil P of this eye E of the
subject person S. By providing the camera 46 with a monitor, the
sharpness of the pupil image may be checked by the operator and
appropriate adjustments made, such as by adjusting lens 46 or
moving person's head. The capturing of the pupil image is
synchronized by the computer 62, described below, with the
capturing of the images by the wavefront sensor 50 to thereby
enhance the measurements of the aberrations.
[0041] The wavefront sensor 50 of the optical apparatus 10 may be
of any convenient type that provides an accurate measurement and
record of the defocused pupil images in either visible or invisible
light rays scattered from the retina R of eye E along the axes O O'
to the wavefront sensor 50. It is preferred that the wavefront
sensor is of the type that obtains two defocused or extra-focal
images of the pupil P, such as disclosed in the copending U.S. Pat.
application Ser. No. 09/579,786, filed May 26, 2000 by the
inventors hereof and entitled "Method And Apparatus For Wavefront
Sensing", which is incorporated herein by this reference. To avoid
any inaccuracies that may result from changes in the human eye E
over a few moments time, it is preferred that the two defocused
images be obtained simultaneously. One of the embodiments disclosed
in our aforesaid copending patent application that simultaneously
obtains such defocused images, whereby it is well suited for the
optical apparatus 10, is shown in FIG. 2 for illustrative purposes
but the present invention is not limited to that embodiment of the
wavefront sensor.
[0042] Any light rays scattered from the retina R of the eye E that
exit the eye, carry information about any aberrations that exist in
the crystalline lens and cornea of the eye, which in turn produce
variations in the wavefront that are sensed by the wavefront sensor
50 that measures those aberrations to then permit, for example, the
creation of corrective eyeglass lenses, contact lenses, inner
ocular lenses or laser corrective surgery. The light rays from the
eye E that pass through the polarized beam splitter 34 are received
by the lens 42 and focused at two separate locations on an object
image plane OIP through a pair of Littrow prisms 52 and 54 that are
mounted back to back in the Koster configuration in the wavefront
sensor 50. A partial mirror or beam splitter 55 is provided at the
abutting surfaces of the two Littrow prisms 52, 54 which may be in
the form of a thin glass plate beam splitter or a coating on one of
the abutting surfaces of the prism to split the incoming light rays
evenly (50-50), that is, to cause one-half of the light to pass
through the beam splitter 55 to the prism 52 and cause the other
one-half of the light to reflect from the beam splitter 55 back
into the prism 54. The light passing through the beam splitter 55
reflects off the interior surface 52a of prism 52 to a negatively
powered analyzer lens 58 located in the object image plane OIP. The
light reflected off the beam splitter 55 then reflects off of the
inside of surface 54a of prism 54 to a positively powered analyzer
lens 56 located in the object image plane OIP. Thus, the Littrow
prisms 52, 54 of the wavefront sensor 50 create two optical axes
along which the light from the same light source simultaneously
passes through the analyzer lenses 56 and 58 to a collimating lens
60 and then to a pupil image plane PIP on a detector D. Preferably,
the Littrow prisms 52 and 54 are slightly titled with respect to
each other from their apexes A or, as an alternative, the two
Littrow prisms 52, 54 are tilted together about the apex A to
create an angle .theta. between the optical axis O" and surface 54a
of slightly less than or more than 90.degree. whereby, in either
case, the optical axes of the two light beams emerging from
analyzer lenses 56, 58 are not precisely parallel and therefore two
pupil images la and lb are separately formed on the detector D. The
spacing of the collimating lens 60 and detector D from the object
image plane OIP are such that without the analyzer lenses 56 and 58
the pupil images la and lb will be in focus on the pupil image
plane PIP of detector D but, if not, the position of lens 60 may be
adjusted. However, the analyzer lenses 56 and 58 create two
defocused pupil images on the detector and, preferably, the optical
powers of the negative analyzer lens 58 and positive analyzer lens
56 are equal but opposite whereby the pupil images la and lb are
equally but oppositely defocused and therefore the analysis of
those images is simplified. The optical powers of the analyzer
lenses 56, 58 may be varied for increasing or decreasing the
sensitivity of the wavefront sensor 50.
[0043] The detector D of the wavefront sensor 50 may be of any
convenient type that is particularly suited for this application of
sensing the pupil image of the pupil P of a human eye E. For
example, the detector D may be a conventional detector of the type
found in a video camera, a custom format of charge couple devices
(CCD), an array of PIN diodes, an array of lenslets focusing the
light onto an array of optical fibers, photon detectors, etc. that
will provide images and/or data relative to the light intensity
throughout the defocused pupil images.
[0044] The two defocused pupil images la and lb (or the shapes and
light intensities for some types of detectors D) are communicated
to a computer 62 and, simultaneously, the focused pupil image
recorded by camera 46 is communicated to the computer 62, which
images may be displayed selectively on the video monitor 64 for
observation by the operator. The two defocused images la and lb are
processed by data reduction software in the computer 62 to derive
the wavefront and provide data appropriate for the measurement of
the aberrations of the human eye E. Here, it is preferred to use a
CCD or similarly imaging detector for detecting the light
intensities. After the normal gradient of the wavefront at the
boundaries of the pupil are found, the difference in intensities
between the two extra-focal or defocused pupil images la and lb is
proportional to the curvature of the wavefront at points inside the
pupil image. The wavefront is derived or recovered by solving the
Poisson equation with respect to intensities with Dirichlet's
boundary conditions relative to shape. An iterative data reduction
algorithm or other non-linear fitting technique may be employed to
compensate for non-linearity in the measurements. The wavefront
obtained may be expressed in orthogonal functions, such as Zernike
polynomials, the terms of which provide values of the optical
aberrations of the eye, including focus, astigmatism and higher
order terms. The full complex wavefront that may be obtained from
the data developed by the wavefront sensor includes the wavefront
phase and amplitude. The wavefront phase provides complete
information relative to what is conventionally thought of as the
optical aberrations and the wavefront amplitude provides
information relative to inclusion opacities or other optical
throughput problems in the subject's eye.
[0045] In the normal operation of the optical apparatus 10, the
operator is involved in both the setup procedure and the
post-examination evaluation at a series of measurements to assess
the data quality. Preferably, the probe beam from light source 40
is strobed to effectively act as a shutter to produce separate and
distinct images at the camera 46 and detector D during the eye
examination. Reliable and very fast shutter speeds are capable with
laser diode technology. The light source 40 and accommodation
target 22 are conjugate for the proper set up, when the operator
has properly aligned the eye E and head of the subject S in the
optical apparatus 10, as shown in FIGS. 1 and 2, and the subject
person is properly focusing and fixating on the accommodation
target 22, the patient sees the sharpest image of the target and
this creates the smallest image of the light source on the retina
for improving the analysis. The smaller spot on the retina the
better. Small wavefront detail structures and wavefront sensor
sensitivity are both related to the size of this image. Another
setup task is to assure that there is a sharp image of the pupil on
camera 46. If it is fuzzy, then the patient's head must be moved
either forward or backwards to bring it into focus. An alternate
method would be to move lens 44 and simultaneously move lens 60. A
sharp image of the pupil on the detector when the analyzer lenses
56 and 58 are removed is very important. The probe beam is briefly
flashed to freeze any motion for evaluating the set up and
collecting test data. Each measurement cycle of an exposure and the
computation takes only a few seconds thereby making it very easy to
take a series of measurements. Post inspection of both the raw and
processed data is useful to determine the quality of the
measurement. Problems which can occur fall into a number of
categories, such as, (1) did the patient move during the procedure,
(2) did the patient blink, (3) did the patient stop looking at the
desired place on the target, (4) was the pupil in good focus, etc.
Inspection of the pupil image lets the operator know about items 1,
2 and 4 but not item 3. Inspection of the defocused images lets the
operator know about item 3 and to a lesser degree items 1 and 2.
The operator may check the quality of the data or, as an
alternative, the computer 62 may be programmed to check the data
quality or to automatically make this determination. The subject
person may wear spectacles or contact lenses during this
examination procedure, resulting in the measurement of the residual
aberrations after normal optical correction. By doing so, only a
small adjustment of lens 44 and lens 60 is then needed to account
for the apparent shift in pupil distance relative to the optical
elements in the apparatus 10.
[0046] Referring now to FIG. 4, a simplified optical diagram of a
modified optical apparatus 110 is diagrammatically illustrated for
describing the manner in which adaptive optics may be used in the
present invention of a method and apparatus for measuring the
optical aberrations of the human eye. Some of the components of the
optical apparatus 10 illustrated in FIG. 2 have been omitted from
FIG. 4 for simplicity and others have been relocated for
accommodating the adaptive optics aspect of the system. The
components of optical apparatus 110 in FIG. 4 that are identical to
the components of optical apparatus 10 in FIG. 2 will be identified
by the same numeral in the one hundred series, such as light source
140 in FIG. 4 being the same as light source 40 in FIG. 2, and
therefore a detailed description of those components will not be
repeated here with respect to FIG. 4. Again, a controlled light
beam emitted from light source 140, such as a laser diode, is
projected along an optical axis O' through a collimating lens 128
to a polarized beam splitter 134 where the light is reflected along
optical axis O into the human eye E where some of the light is
reflected by a specular reflection from the eye's optical surfaces
and some of the light passes through the eye lens L and pupil P to
the retina R where it is focused as a spot of light and scattered
back through the pupil P and eye lens L along optical axis O. The
scattered light from the retina R passes through the polarizing
beam splitter 134 along optical axis O" to a lens 142 which focuses
the spot of light from the retina on an optical image plane OIP.
The light rays continue uncorrected to a collimating lens 160 from
which the light rays are directed to a deformable mirror M
positioned at a small angle to the collimated light. The light is
reflected from deformable mirror M along another optical axis
portion O'" through a lens 70 to a beam splitter 72. Some of the
light passes through beam splitter 72 and is focused on a detector
D' while the balance of the light is reflected from the beam
splitter 72 into a wavefront sensor 150. The detector D' is
connected to a computer 74 which in turn is connected to a video
monitor 76 on which the retina image is sensed by the detector D'
may be displayed. As with the optical apparatus 10 of FIG. 2, the
wavefront sensor 150 is connected to a computer 162 which processes
and analyzes the two defocused pupil images provided by the
scattered light from the spot of light on the retina R of the eye
E. The computers 162 and 74 may be combined.
[0047] In the adaptive optics system of FIG. 3, the computer 162 is
operatively connected to the deformable mirror M, which may be of
any convenient type that is capable of controlled deformation for
correcting the curvature of the wavefront. A preferred form of such
a deformable mirror M is disclosed in the concurrently filed
copending U.S. patent application Ser. No. ______, filed ______,
2001 by the inventors hereof and entitled "Deformable Curvature
Mirror", which is incorporated herein by this reference. As
disclosed more fully in that copending application, a plurality of
wires (not shown) are electrically connected to a like plurality of
electrode segments (not shown) on the back of the deformable mirror
M and by separately applying and controlling a high voltage applied
to each wire by the computer 162, the curvature of the reflective
front surface of the deformable mirror M may be controlled.
[0048] In using the adaptive optics system of FIG. 4, the
deformable mirror M is initially set at a completely flat condition
and the light scattered from the spot of light on the retina R
proceeds through the apparatus in the manner described above to
form an image on the detector D' that may be viewed on the monitor
76 as the uncorrected retina image lr. Simultaneously, the
uncorrected pupil image is supplied by beam splitter 72 to the
wavefront sensor 150 wherein two defocused pupil images are formed
on a detector D and supplied to the computer 162. Upon processing
and analyzing the two defocused images, the computer 162 provides
separate, appropriate voltages to each of the wires attached to
mirror M to cause appropriate deformations in the reflective
surface of the deformable mirror M for correcting the aberrations
in the eye E as measured by the wavefront sensor 150. As a result
of such corrective deformation of the deformable mirror M, the
image reaching the detector D' and displayed on video monitor 76 as
the retina image lr is corrected and the operator of the optical
apparatus 110 may visually confirm that the corrections are
adequate and complete. Again, as with the optical apparatus 10
illustrated in FIG. 2 and described above, the data developed by
the wavefront sensor 150 and processed by computer 162 may be used
for creating corrective spectacles, contact lenses, innerocular
lenses, or performing corrective eye surgery. The results thereof
may be checked on the apparatus 110 by again testing the eye E with
the corrective lens or procedure in place and the deformable mirror
M repositioned to a flat condition, whereupon the retina image lr
should be as optically perfect as possible and the wavefront sensor
150 with computer 162 should indicate that no other correction is
required. If the corrected vision is not essentially perfect or of
the level desired, such remaining imperfections will be observed on
the monitor 76 and recorded by the wavefront sensor 150 and
analyzed by the computer 162, whereby further corrections may be
made.
[0049] Referring now to FIG. 5, an alternative form of wavefront
sensor is illustrated for further demonstrating that the method and
apparatus of the present invention is not limited to the specific
form of wavefront sensor shown in FIG. 2 and described with respect
thereto. As with the embodiment of FIG. 2, the light reflected from
the retina R of eye E proceeds along optical axis O through
polarized beam splitter 34 (which items are not shown in FIG. 5) to
and through the lens 42 on optical axis O". The light beam is
focused by lens 42 on an optical image plane OIP at a location
slightly off-center of a flexible membrane mirror 82 from which the
pupil image is reflected to and through a collimating lens 84 to an
image detector D". When the flexible membrane mirror 82 is
maintained in a flat condition, the pupil image If formed on the
detector D" is focused. The flexible membrane mirror 82 and
actuator (not shown) may be any device that is capable of being
switched between positive and negative optical power, preferably by
rapid switching, and examples of such devices are a piezo-electric
bimorph mirror and an acoustically driven pellicle mirror. In any
event, the mirror mount 86 is provided with means for causing
flexing of the membrane mirror 82, which means may comprise
acoustical, hydraulic, electrical, magnetic or other forms of
actuators that will create a change in the curvature of the face of
the mirror 82 from flat to either concave (as shown) or convex. By
creating a curvature in the face of mirror 82, the pupil image lp
on detector D" becomes defocused. By selecting two equally and
oppositely defocused images ld and ld' to be recorded by the
computer 62, the wavefront curvature can be processed and analyzed
in the same manner as described above with respect to the
embodiment of FIG. 2. Preferably, the defocused images are equally
defocused positively and negatively by deforming the membrane
mirror 82 convexly and concavely, respectively, in the same amount,
which is controlled by the computer 62, with the images being
integrated over precisely equal (but opposite) flexing of the
mirror. By decentering the image focused on the mirror 82, the
focused image lf appears in the center and the two defocused images
appear on opposite sides thereof. For this embodiment of a
wavefront sensor, the detector D" may use a CCD detector but for
speed of operation, a more coarsely spatially-sampled detector may
be used, such as a discrete array of PIN diodes or a customized
small format CCD.
[0050] In the wavefront sensor 80 of FIG. 5, if the light beam is
not reflected from the center of the flexible membrane mirror 82,
some time dependent tilt is introduced when the membrane vibrates.
If this is combined with a strobing light source synchronized with
the membrane, several images can be recorded on a single detector
with out blurring. If the light source is turned on very briefly
when the membrane is concave, then flat and then convex, three
images (as shown) will appear on the detector D" after one cycle of
the membrane. Several cycles may be used for increasing the
intensity of the images. Optically the membrane is similar to the
wavefront sensor 50 where the membrane takes the place of the
analyzer lens 56 and 58. This effect may be used in the case where
the flexible mirror is an acoustically driven membrane whereby the
two separate defocused images and potentially in-focus images may
be recorded on a single imaging device whose frame rate is much
slower than the membrane vibration frequency. Also, any method of
sequentially tilting the wavefront mirror to the image on the
membrane mirror would also serve to separate the defocused images
on the detector D", such as by using an adaptive optics mirror like
deformable mirror M shown in FIG. 4.
[0051] From the foregoing description of preferred embodiments and
alternatives of the present invention of an optical apparatus for
measuring the optical aberrations of the human eye, it will readily
appear to those skilled in the art that various other embodiments,
modifications and alternatives may be used without departing from
the present invention as claimed below.
* * * * *