U.S. patent application number 12/611438 was filed with the patent office on 2010-02-25 for dynamic range extension techniques for a wavefront sensor including use in ophthalmic measurement.
This patent application is currently assigned to AMO WAVEFRONT SCIENCES, LLC. Invention is credited to Richard James Copland, Daniel R. Neal.
Application Number | 20100045934 12/611438 |
Document ID | / |
Family ID | 33545514 |
Filed Date | 2010-02-25 |
United States Patent
Application |
20100045934 |
Kind Code |
A1 |
Neal; Daniel R. ; et
al. |
February 25, 2010 |
DYNAMIC RANGE EXTENSION TECHNIQUES FOR A WAVEFRONT SENSOR INCLUDING
USE IN OPHTHALMIC MEASUREMENT
Abstract
An ophthalmic error measurement system includes a projecting
optical system delivering light onto a retina of an eye, a
pre-correction system which compensates a light beam to be injected
into the eye for aberrations in the eye, the pre-correction system
being positioned in between the projecting optical system and the
eye, an imaging system which collects light scattered by the
retina, and a detector receiving light returned by the retina from
the imaging system. Use of the pre-correction system allows the
end-to-end aberrations of the ocular system to be analyzed. The use
of a pre-correction system also allows use of a minimized spot size
on the retina, and all of its attendant advantages.
Inventors: |
Neal; Daniel R.; (Tijeras,
NM) ; Copland; Richard James; (Albuquerque,
NM) |
Correspondence
Address: |
ABBOTT MEDICAL OPTICS, INC.
1700 E. ST. ANDREW PLACE
SANTA ANA
CA
92705
US
|
Assignee: |
AMO WAVEFRONT SCIENCES, LLC
ALBUQUERQUE
NM
|
Family ID: |
33545514 |
Appl. No.: |
12/611438 |
Filed: |
November 3, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12474607 |
May 29, 2009 |
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12611438 |
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11829184 |
Jul 27, 2007 |
7553022 |
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12474607 |
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10828550 |
Apr 21, 2004 |
7455407 |
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11829184 |
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10369513 |
Feb 21, 2003 |
6908196 |
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10828550 |
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10419072 |
Apr 21, 2003 |
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10369513 |
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09692483 |
Oct 20, 2000 |
6550917 |
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10419072 |
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60182088 |
Feb 11, 2000 |
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Current U.S.
Class: |
351/211 ;
351/246 |
Current CPC
Class: |
G01J 9/00 20130101; G01M
11/0257 20130101; G01B 11/306 20130101 |
Class at
Publication: |
351/211 ;
351/246 |
International
Class: |
A61B 3/103 20060101
A61B003/103; A61B 3/10 20060101 A61B003/10 |
Claims
1. A system for measuring errors in an eye, comprising: a
projecting optical system configured to focus light onto a retina
of the eye; an imaging system configured to receive light from the
projecting optical system that is scattered by the retina; a
wavefront sensor configured to receive light returned by the retina
from the imaging system; and a target and a target optical system
configured to project the target onto the eye so as to stimulate
near vision accommodation of the eye.
2. The system of claim 1, wherein a relative distance between the
target and the optical system is variable.
3. The system of claim 1, wherein the target is moveable relative
to the target optical system.
4. The system of claim 1, further comprising a light source
disposed behind the target, a brightness of the light source being
electronically adjustable.
5. The system of claim 1, further comprising a pre-correction
system configured to compensate a light beam to be injected into
the eye for aberrations in the eye.
6. The system of claim 3, wherein the pre-correction system is
positioned in between the projecting optical system and the
eye.
7. The system of claim 1, wherein the system is configured to
measure near vision visual acuity.
8. The method of claim 1, wherein the target optical system
comprises a lens.
9. The method of claim 8, wherein the lens is a variable focal
length lens.
10. A method of measuring optical characteristics of an eye,
comprising: providing a measurement system including: a projecting
optical system; an imaging system for receiving light scattered by
the retina; a wavefront sensor; and a target system comprising a
target and a target optical system; projecting the target onto the
eye; stimulating near vision accommodation of the eye; directing
light from the light source onto a retina of the eye; receiving in
the imaging system light scattered by the retina; receiving in the
wavefront sensor the light received by the imaging system; and
measuring a near vision visual acuity of the eye.
11. The method of claim 9, wherein stimulating near vision
accommodation includes varying an optical distance between the
target and the eye.
12. The method of claim 9, wherein stimulating near vision
accommodation includes moving the target relative to the target
optical system.
13. The method of claim 9, wherein the target optical system
comprises a lens.
14. The method of claim 13, wherein the lens is a variable focal
length lens.
15. A system for measuring errors in an eye, comprising: a
projecting optical system configured to focus light onto a retina
of the eye; an imaging system configured to receive light from the
projecting optical system that is scattered by the retina; a
wavefront sensor configured to receive light returned by the retina
from the imaging system; a target and a target optical system
configured to project an optical image of the target onto the eye;
wherein an optical distance between the target and the eye is
variable so as to allow measurement of near vision visual
acuity.
16. The system of claim 15, wherein the target is configured to
stimulate near vision accommodation of the eye.
17. The system of claim 15, wherein the target is configured to be
located so as to minimize accommodation of the eye.
18. The system of claim 15, wherein the target optical system
comprises a lens.
19. The system of claim 18, wherein the lens is a variable focal
length lens.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a continuation of pending U.S. patent
application Ser. No. 12/474,607 filed on 29 May 2009, which is a
continuation of U.S. patent application Ser. No. 11/829,184 filed
on 27 Jul. 2007, which issued on 30 Jun. 2009 as U.S. Pat. No.
7,553,022, which is a divisional of U.S. patent application Ser.
No. 10/828,550, filed on 21 Apr. 2004, which issued on 25 Nov. 2008
as U.S. Pat. No. 7,455,407, which is in turn a continuation-in-part
of U.S. patent application Ser. No. 10/369,513, filed on 21 Feb.
2003 and which issued on 21 Jun. 2005 as U.S. Pat. No. 6,908,196,
and is also a continuation-in-part of U.S. patent application Ser.
No. 10/419,072, filed on 21 Apr. 2003 which is in turn a
continuation of U.S. patent application Ser. No. 09/692,483, filed
on 20 Oct. 2000 and which issued on 22 Apr. 2003 as U.S. Pat. No.
6,550,917, which application in turn claims priority under 35
U.S.C. .sctn.119 from U.S. provisional patent application
60/182,088 filed on 11 Feb. 2000, the entire contents of each of
which applications are hereby incorporated by reference in their
entirety for all purposes as if fully set forth herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention is directed to measurement of the
refractive error in the eye, more particularly to methods and
techniques for compiling a topographic mapping of these refractive
errors.
[0004] 2. Description of Related Art
[0005] Measurements of aberrations in an eye are important for
diagnosis of visual defects and assessment of acuity. These
measurements and their accuracy become increasingly important in
light of the growing number of ways, both surgical and
non-surgical, that aberrations can be corrected. These corrections
rely on accurate, precise measurements of the entire ocular system,
allowing successful screening, treatment and follow-up.
Enhancements in the accuracy of ocular measurements may aid in
improving the identification of patients in need of correction and
the performance of the correction itself.
[0006] There are a number of current methods used to measure
performance of the ocular system. The most widely used and well
established are psycho-physical methods, i.e., methods relying on
subjective patient feedback. The oldest of the psycho-physical
methods is the foreopter or trial lens method, which relies on
trial and error to determine the required correction. There are
psycho-physical methods for measuring visual acuity, ocular
modulation transfer function, contrast sensitivity and other
parameters of interest.
[0007] In addition to these subjective methods, there are also
objective methods for assessing the performance of the ocular
system. Such objective methods include corneal topography,
wavefront aberrometry, corneal interferometry, and auto-refraction.
Many of these methods only measure the contribution of specific
elements to the total refractive error. For example, much work has
been directed to measuring the topography of the cornea and
characterizing the corneal layer. However, the corneal shape only
contributes about 30-40% of the total refractive error in most
cases. In order to measure the bulk of the refractive error and to
provide a complete mapping for diagnosis and correction, additional
information and measurements are needed.
[0008] Another method for determining the refraction of the eye is
auto-refraction, which uses a variety of techniques to
automatically determine the required corrective prescription. These
automated techniques include projecting one or more spots or
patterns onto the retina, automatically adjusting optical elements
in the auto-refractor until the desired response is achieved, and
determining the required correction from this adjustment. However,
auto-refractors are not considered especially reliable. Further,
auto-refractors measure only lower order components of the
aberrations, e.g., focus and astigmatic errors.
[0009] Recently, the eye has started being considered as an optical
system, leading to the application of methods previously used for
other optical systems to the measurement of the eye. These methods
include interferometry and Shack-Hartmann wavefront sensing. These
techniques are of particular interest because they measure the
complete aberrations of the eye. This additional information allows
measurement of non-uniform, asymmetric errors that may be affecting
vision. Further, this information may be linked with any of the
various corrective techniques to provide improved vision. For
example, U.S. Pat. No. 5,777,719 to Williams describes the
application of Shack-Hartmann wavefront sensing and adaptive optics
for correcting ocular aberrations to make a super-resolution
retina-scope. U.S. Pat. No. 5,949,521 to Williams et al. describes
using this information to make better contacts, intra-ocular lenses
and other optical elements
[0010] Wavefront aberrometry measures the full, end-to-end
aberrations through the entire optics of the eye. In these
measurements, a spot is projected onto the retina, and the
resulting returned light is measured with an optical system, thus
obtaining a full, integrated, line-of-sight measurement of the
eye=s aberrations. A key limitation of the instruments used in
these measurements is the total resolution, which is ultimately
limited by the lenslet array of the instrument. However, selection
of the lenslet array is itself limited by several factors, most
importantly the size of the spot projected onto the retina.
[0011] A schematic illustration of the basic elements of a two
dimensional embodiment of a Shack-Hartmann wavefront sensor is
shown in FIG. 2. A portion of an incoming wavefront 110 from the
retina is incident on a two-dimensional lenslet array 112. The
lenslet array 112 dissects the incoming wavefront 110 into a number
of small samples. The smaller the lenslet, the higher the spatial
resolution of the sensor. However, the spot size from small the
lenslet, due to diffraction effects, limits the focal length that
may be used, which in turn leads to lower sensitivity. Thus, these
two parameters must be balanced in accordance with desired
measurement performance.
[0012] Mathematically, the image on the detector plane 114 consists
of a pattern of focal spots 116 with regular spacing d created with
lenslets 112 of focal length f, as shown in FIG. 3. These spots
must be distinct and separate, i.e., they must be readily
identifiable. Thus, the spot size .rho. cannot exceed 1/2 of the
separation of the spots. The spot separation parameter N.sub.FR can
be used to characterize the lenslet array 12 and is given by:
N FR = d .rho. ( 1 ) ##EQU00001##
The relationship between the size of a lens and the focal spot it
creates, where .lamda. is the wavelength of the light, is given
by:
.rho. = 1.22 f .lamda. d ( 2 ) ##EQU00002##
for a round lens or
.rho. = f .lamda. d ( 3 ) ##EQU00003##
for a square lens. Thus, for a square lens, the separation
parameter can be given by:
N FR = d 2 f .lamda. ( 4 ) ##EQU00004##
This is also known as the Fresnel number of the lenslet. To avoid
overlapping focal spots, N.sub.FR>2. In practice, the Fresnel
number must be somewhat greater than two to allow for a certain
dynamic range of the instrument. The dynamic range of a
Shack-Hartmann wavefront sensor can be defined as the limiting
travel of the focal spot such that the edge of the spot just
touches the projected lenslet boundary, given by:
.theta. max = d 2 - .rho. f or ( 5 ) .theta. max = d 2 f - .lamda.
d = [ N FR 2 - 1 ] .lamda. d ( 6 ) ##EQU00005##
Thus, the dynamic range is directly proportional to the separation
parameter and the lenslet size.
[0013] A particularly useful arrangement for a Shack-Hartmann
wavefront sensor ocular measuring system places the lenslet array
in an image relay optical system at a plane conjugate to the pupil
or corneal surface. In this configuration, the spot size on the
detector of the wavefront sensor is given by:
.rho. 2 = 1 M f L f e .rho. 1 ( 7 ) ##EQU00006##
where M is the magnification of the imaging optics, f.sub.L is the
focal length of the lenslet array, f.sub.e is the focal length of
the eye and .rho..sub.1 is the spot size on the retina.
[0014] Comparing Equations (5) and (7), it is evident that the
dynamic range of the wavefront sensor is limited by the size of the
spot .rho..sub.1 projected on the retina. For a practical system,
the dynamic range must be able to resolve errors in the optical
systems. Thus, the dynamic range is a key limited parameter of the
entire system design. In previous implementations of the
Shack-Hartmann wavefront sensor used for ocular measurement, the
dynamic range has been increased by increasing the size of each
lenslet. However, the eye itself can have significant aberrations.
Thus, any beam projected into the eye will become aberrated,
spreading the focal spot and increasing the spot size .rho..sub.1
on the retina.
[0015] Various techniques have been implemented to address this
problem. A small diameter beam has been used so that the total
wavefront error is minimized across the injected beam. Another
proposed solution projects the light into the eye at the focal
point of a long focal length lens, operating as a field lens so
that the size of the focal spot is not affected by the eye
aberrations. In practice, for both of these cases, the beam is
still somewhat large and is increased in size by the aberrations of
the ocular system.
[0016] Another limitation on the dynamic range of the system is the
sampling size. With a large spot on the retina, the sample size of
the wavefront sensor must be increased to allow even a minimal
dynamic range to be realized. For ocular systems with strong
aberrations, such as found in people with large astigmatism or for
those having undergone LASIK, the aberrations over each lenslet are
sufficient to degrade the lenslet focal spot. Thus, the system is
limited not just by focal spot overlap, but by the fact that the
focal spots themselves fade out or are difficult to track. Using a
small sample size does not allow sufficient light to be gathered,
since the light is scattered by the retina into a large number of
focal spots. Due to safety considerations, the input power may not
be increased to compensate for this scattering.
SUMMARY OF THE INVENTION
[0017] The present invention is therefore directed to measurement
of refractive errors of an eye that substantially overcomes one or
more of the problems due to the limitations and disadvantages of
measurements of the related art.
[0018] It is an object of the present invention to measure the
end-to-end aberrations of the eye with sufficient accuracy and
dynamic range in a practical manner.
[0019] It is a further object of the present invention to project a
light beam into an ocular system so as to minimize the size of the
focal spot on the retina.
[0020] It is another object of the present invention to use this
smaller focal spot to allow much greater sampling density of the
ocular system, thereby enhancing the accuracy and dynamic
range.
[0021] It is yet another object of the present invention to make a
practical, low cost system, available for use in a clinical
setting.
[0022] At least one of the above and other objects may be realized
by providing a system for measuring errors in an eye including a
projecting optical system which delivers light onto a retina of the
eye, a pre-correction system which compensates a light beam to be
injected into the eye for aberrations in the eye, the
pre-correction system being positioned in between the projecting
optical system and the eye, an imaging system which collect light
scattered by the retina, and a detector receiving light from the
retina collected by the imaging system.
[0023] The detector may be a Shack-Hartmann wavefront sensor, a
shearing interferometer, a Moire deflectometer, or other passive
phase measurement systems. The pre-correction system may include a
telescope having at least one movable lens, fixed lenses inserted
at an intermediate image plane, adaptive optical elements, and/or a
cylindrical telescope. The pre-correction system may correct for
focus and/or astigmatism errors in the eye. The telescope may be
arranged so that a fixed lens of the telescope is one focal length
away from the eye. Components used in the pre-correction system may
also be used in the imaging system.
[0024] The pre-correction system may include a feedback loop which
determines an appropriate pre-correction to be supplied by the
pre-correction system. The feedback loop may include a detector
receiving light returned from the retina, a processor comparing
detected light with a desired feature of the light and adjusting at
least one parameter of the pre-correction system in accordance with
the comparison. The feedback loop may further include a return
optical system for gathering the light from the retina. The return
optical system may include the pre-correction system. The desired
feature may be a minimized spot size on the retina.
[0025] The system may include an aperture that limits the angular
dynamic range of the system. The system may further include a
polarizing beam splitter between the eye and the wavefront sensor.
The system may include an aligner that determines an appropriate
eye alignment of the system. The projecting optical system may
provide light to the eye at an angle to a central axis of the eye.
The system may include an additional optical system between the
detector and the eye. The system may include a power monitor which
monitors power of the light beam being injected in the eye. The
system may include an eye position detection system including a
target projected on the eye, a position detector sensing the eye,
and an adjustment system which adjusts a position of the system
relative to the eye until the eye is in focus on the detector.
[0026] These and other objects of the present invention will become
more readily apparent from the detailed description given
hereinafter. However, it should be understood that the detailed
description and specific examples, while indicating the preferred
embodiments of the invention, are given by way of illustration
only, since various changes and modifications within the spirit and
scope of the invention will become apparent to those skilled in the
art from this detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The foregoing and other objects, aspects and advantages will
be described with reference to the drawings, in which:
[0028] FIG. 1 is a schematic top view of the measurement system of
the present invention;
[0029] FIG. 2 is a schematic side view of the basic components of a
Shack-Hartmann wavefront sensor;
[0030] FIG. 3 schematically illustrates the relationship between
the size of the lens, its focal length and the spot size;
[0031] FIGS. 4A-4C schematically illustrate the spot size for
different configurations;
[0032] FIGS. 5A-5B schematically illustrate off-axis injection of
the light into the eye and the blocking of the reflected light from
entering the wavefront sensor;
[0033] FIG. 6 is a schematic illustration of a configuration of the
present invention using a fixed telescope and an adjustable
telescope;
[0034] FIG. 7 is a schematic illustration of a configuration of the
present invention using a variable lens;
[0035] FIG. 8 is a schematic illustration of a cylindrical
telescope for use with the present invention; and
[0036] FIG. 9 is a schematic illustration of a configuration of the
present invention using a corrective lens.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] As noted above, the key to designing a practical ocular
wavefront sensor system is how the light is injected into the eye.
Since ocular refractive errors can be large, e.g., up to 20
diopters, the degradation of the injected beam can be significant.
Further, it is difficult to design a wavefront sensor that has
sufficient range to directly measure an extremely large refractive
error. In accordance with the present invention, the spot projected
on the ocular system is predistorted in a manner that compensates
for the eye's fundamental aberrations. This allows the spot
returned to the wavefront sensor to be well formed and minimally
affected by the refractive errors. The small size of the spot
allows small lenslets to be used while maintaining sufficient
dynamic range to measure even large, high order aberrations. Since
the light is tightly focused on the retina, the light is only
scattered from a small region. When this small region is imaged
onto the focal plane of the wavefront sensor, the light is
concentrated onto a small group of pixels. Thus, even though the
reflected light must be divided among a larger number of lenslets,
each focal spot is brighter than in the conventional methods.
Further, the greater sampling density leads to smaller wavefront
aberrations across the aperture of each lenslet.
[0038] A system for such error measurement employing
pre-compensation is shown in FIG. 1. The ocular wavefront
measurement system shown therein generally includes a projection
system for projecting light into the eye, a system for
pre-correcting the injected light for ocular aberrations, a system
for collecting light, a system for determining the pre-correction,
and a system for measuring the collected light.
[0039] The projection system shown in FIG. 1 includes a light
source 12, e.g., a laser, a laser diode, LED, or a
super-luminescent diode, supplied to an optical fiber 14. For
safety reasons, the light source is preferably a pulsed light
source, is limited to a small power, is outside the normal visual
detection range, e.g. infrared, and/or is directly collimated with
an appropriate lens. The optical fiber may be a polarization
maintaining fiber. The light leaving the optical fiber 14 is
provided to a collimating lens 16. The use of an optical fiber 14
to deliver light from the light source 12 simplifies the
collimating lens 16, since the fiber exit mode acts as a
diffraction-limited point source. The collimating lens 16 is
preferably rigidly mounted to the fiber 14. The collimated beam is
then truncated to a desired size by an aperture 18. If needed, a
polarizer 20 may be provided for polarizing the collimated beam. A
polarizing beam splitter 22 directs the light from the projection
system to the rest of the ocular measuring system.
[0040] Alternatively, the light source 12 may be provided alone,
i.e., without the use of the fiber 14. The light from the light
source 12 itself is then collimated by a collimating lens. While
light sources used for ophthalmic measurement typically have a high
degree of astigmatism, by using only a portion of the beam, e.g.,
10-25%, typically from the center of the beam, the wavefront error
over the beam is small enough that the beam size is substantially
stable over the distance traversed in the ophthalmic measurement
system. In other words, even though the beam is still astigmatic,
the beam shape does not change while traversing the ophthalmic
measurement system due to this astigmatism, so the astigmatism does
not influence the measurement. The light may be polarized as
required.
[0041] The light from the projection system is reflected by the
polarizing beam splitter 22 and directed to a pre-compensation
system, shown in FIG. 1 as a telescope 30. The telescope 30
includes lenses 32, 34 with an aperture 36 in between. The
telescope 30 may be adjusted by moving the lenses relative to one
another. This adjustment is to provide the desired pre-correction
for the injected beam by adding defocus that just compensates for
the spherical equivalent defocus of the ocular system being
measured. The light from the telescope is directed by a beam
splitter 38 to an ocular system 40 under measurement. The injected
beam is focused by the ocular system 40 to a focal spot 42 on the
retina of the ocular system 40. Light from this focal spot 42 is
scattered or reflected by the retina.
[0042] The returned light is collected by the cornea and lens of
the ocular system 40 and is approximately collimated. The beam
splitter 38 directs the beam from the ocular system back to the
telescope 30. The same position of the lenses 32, 34 of the
telescope 30 corrects for the defocus aberrations of the ocular
system 40 so that light arrives at a wavefront sensor 50 collimated
to within the dynamic range of the sensor. The aperture 36 blocks
any rays outside the angular dynamic range of the wavefront sensor
50 so that no mixing or measurement confusion occurs. When the
wavefront sensor 50 is a Shack-Hartmann sensor, the focal spots
cannot collide, interfere or cause confusion with adjacent focal
spots. The light from the telescope now passes through the
polarizing beam splitter 22, since the interaction with the retina
will rotate the polarization of the light from the input
polarization. The wavefront sensor 50 may be a Shack-Hartmann
wavefront sensor, a shearing interferometer, a Moire deflectometer
or any other passive phase measurement sensor. When the wavefront
sensor 50 is a Shack-Hartmann wavefront sensor, the wavefront
sensor 50 includes the elements shown in FIG. 2.
[0043] The proper position of the lenses 32, 34 of the telescope 30
may be determined in a number of ways. In a preferred embodiment,
an additional sensor 60 is used with a beam splitter 62 and a
focusing lens 64 to create an image of the light incident upon the
retina. The proper position of the lenses 32, 34 in the telescope
30 is determined by minimizing the spot size 42 on the back of the
retina, performed by comparing the spot sizes from different
positions of the lenses 32, 34 in the telescope 30. If the ocular
system 40 is arranged to be one focal length of the objective lens
34 away from the lens 34, then the telescope 30 will be insensitive
to changes in magnification or other errors. The wavefront sensor
50 should be arranged to be at the conjugate image plane to the
ocular system 40. Preferably, the wavefront sensor 50, the retinal
imaging sensor 60, the projection optics 16, 18, 20, the polarizing
beam splitter 22, the beam splitter 62, and the focusing lens 64
are mounted on a platform 70 which is mounted in a moving stage 72.
This allows the relative position of the telescope lenses 32, 34 to
be varied while fixing the position of the remaining elements on
the platform 70. The use of the optical fiber 14 allows the light
source to be mounted off the platform 70, minimizing the mass of
the elements moved by the translation stage 72. A processor 68 may
be included to control movement of the translation stage 72 and to
allow data processing, analysis and/or display.
[0044] As an additional safety measure, a small portion of the beam
incident on beam splitter 38 is transmitted to a lens 44 which
focuses the light onto a power monitor 46. The output of this power
monitor 46 may be used to shut down the system if the power exceeds
the safety limits of the system or to alter the power supplied to
the light source 12 to reduce the power output by the light source
in a known manner.
[0045] To measure the proper eye position relative to the measuring
system, an additional detector 80 is included. Imaging optics 82
are designed such that the iris or cornea will be in focus for only
a narrow region of space. A mirror 84 may be used to direct light
onto the iris detector 80. The position of the system relative to
the eye is adjusted until the iris or cornea is detected. The
detection may be indicated to a user on an indicator 86.
Preferably, this detection is used just during patient alignment
and only uses a small percentage, e.g., less than 10% of the
light.
[0046] To insure that the patient is viewing the correct line of
sight, a target 90 is made visible through a beam splitter 94. The
target 90 is imaged at infinity through a lens 92. The target
position may be varied by moving the target relative to the lens 92
to present targets that are either in focus or slightly
out-of-focus to minimize patient accommodation. Movement of the
target 90 closer to the lens 92 stimulates near vision
accommodation, allowing measurement of near vision visual acuity or
the target may be arranged with the image past infinity to measure
distance vision. The patient merely attempts to focus on the
target. A light source behind the target is electronically
controlled to adjust the target brightness and the position of the
target is also electronically adjustable.
[0047] Thus, the telescope 30 is used to pre-compensate the
injected light and to compensate for the returned wavefront to
minimize the total wavefront error incident on the wavefront
sensor. In the related art, telescopes have been used to relay
image the light onto the wavefront sensor and to compensate for
strong spherical and cylindrical aberrations, but the light was
injected separately. This separate handling is due to strong back
reflections that occur even for lenses having anti-reflection
coatings thereon. Since the returned light from the retina may be
very weak, even a small reflection from the lenses can quickly
dominate the measurement and saturate the wavefront sensor 50.
There are several ways of dealing with the problem. First, as shown
in FIG. 1, polarized light and a polarizing beam splitter in
conjunction with a quarter-wave plate may be used. Off axis
parabolas or other curved mirrors may be used to direct the light
to the telescope. The light may be injected off axis, so that any
reflected light from the cornea is filtered out by the apertures of
the system, as shown in FIGS. 5A and 5B. FIG. 5B illustrates how
the light reflected by the cornea of the eye 40 is blocked by the
aperture 36 from entering the wavefront sensor and influencing the
measurement. The use of one or more of these schemes is sufficient
to allow pre-compensation of the injected beam in accordance with
the present invention without introducing unwanted reflections.
[0048] As an alternative, a second telescope may be used in
conjunction with the first telescope to increase the dynamic range
by providing an alternative location for the filtering aperture.
Thus, one telescope can be completely fixed, while the other has a
degree of freedom allowing movement until the lenses of the two
telescopes are in contact. Such a configuration is shown in FIG. 6,
in which a fixed telescope 51 with lenses 52, 54 and aperture 56,
is used to supply light to the wavefront sensor 50. This is in
conjunction with the elements discussed above regarding FIG. 1. For
simplicity, only the essential elements of the light delivery
system 14, the collimating lens 16, the polarizing beam splitter
22, the adjustable telescope 30, and the eye 40, have been
shown.
[0049] Compensation of astigmatism of the ocular system and of the
injected beam may be achieved in the following ways. The telescope
30 may be a cylindrical lens telescope or a pair of positive and
negative lenses. Such a cylindrical lens configuration is shown in
FIG. 8, in which a pair of cylindrical lenses 132, 134 is used in
place of lenses 32, 34. The spacing s between the lenses may be
adjusted to increase or decrease power of the telescope. The angle
of the pair 120, 122 is adjusted relative to the axis of the
transmission path. This complicates the instrument, but provides
for a better beam projected into the eye, requiring a wavefront
sensor of only limited dynamic range, since both spherical and
cylindrical aberrations would be subtracted from the wavefront, and
only higher order terms would remain.
[0050] Alternatively, a high dynamic range wavefront sensor can be
used. Since, in accordance with the present invention, only a small
beam is injected into the eye, which will only pick up only a small
wavefront aberration across its aperture, the focal spot on the eye
will still be quite small, even with some astigmatism. Thus,
cylindrical compensation is usually not needed. While some
distortion will take place, it will be limited in size and an
adequately small spot will still be realized. A high dynamic range
wavefront sensor corresponds to the use of a smaller focal length
for the wavefront sensor lenslet array, as set forth in Equations
(3) and (7). While the use of only spherical lenses will result in
a loss of accuracy, the larger number of measurements afforded by
the smaller lenslet array will sufficiently compensate for this
degradation.
[0051] An alternative to using the telescope with a movable lens,
as shown in FIG. 1, for correcting base aberrations of the eye in
the injected and reflected wavefront includes placing a corrective
lens in front of the eye. If this lens is not a contact lens, it
cannot be placed at the actual pupil plane of the eye, as shown in
FIG. 9, in which a corrective lens 35 is placed adjacent to the eye
40. Thus, there will always be some magnification introduced by the
combination of the refractive error of the eye and the correcting
lens. Since it is difficult to set or know the vertex distance of
the corrective lens, this magnification would be poorly known at
best, and introduce error into the entire measurement.
[0052] Another alternative includes using fixed or variable lenses.
Ideally, these lenses are placed at an optical plane that is
conjugate to the surface of the eye. Since it is also desirable for
the wavefront sensor to be at this plane, a second telescope will
need to be used in series with the first telescope. Further, since
all of the lenses are fixed, some means will be needed for changing
the various pre-corrector lenses in a known manner to achieve the
proper result. A lens 37 in FIG. 7 may be from a trial lens kit,
such as is commonly used for measuring a patient's manifest
refraction, but is limited to the prescription accuracy.
Alternatively, the lens 37 in FIG. 7 may be a variable focal length
lens, e.g., adaptive optics, liquid crystal displays, deformable
mirrors. The focal lengths of these elements may be controlled
electronically, e.g., by the processor 68 shown in FIG. 1, rather
than by movement. Either of these configurations is shown in FIG.
7, in which the lens 37 may be a trial lens or a variable focal
length lens. The applicability of these configurations and the
telescope configuration is shown in FIG. 4A-4C, in which the size
of the spot in a myopic eye alone is shown in FIG. 4A, the size of
the spot size with correction with a lens 37 is shown in FIG. 4B
and the spot size with the adjustable telescope 30 is shown in FIG.
4C. As can be seen, both configurations in FIGS. 4B and 4C result
in the desired small spot size of the present invention.
[0053] By pre-compensating for aberrations of the eye in the
injected beam in accordance with the present invention, a small
focal spot can be created on the retina. This small focal spot will
concentrate light more, allowing the light to be divided into a
larger number of focal spots. This, in turn, allows higher spatial
resolution and the use of lower injected light power. Higher
spatial resolution means that the assumption that each lenslet
measures only tilt is valid over a much larger range. Higher
spatial resolution also leads to greater dynamic range and
accuracy. Higher dynamic range means that measurement of even high
order terms of aberration can be accomplished accurately, without
significant degradation of the measurement.
[0054] While the present invention is described herein with
reference to illustrative embodiments for particular applications,
it should be understood that the present invention is not limited
thereto. Those having ordinary skill in the art and access to the
teachings provided herein will recognize additional modifications,
applications, and embodiments within the scope thereof and
additional fields in which the invention would be of significant
utility without undue experimentation. Thus, the scope of the
invention should be determined by the appended claims and their
legal equivalents, rather than by the examples given.
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