U.S. patent application number 13/053073 was filed with the patent office on 2011-07-21 for characterising eye-related optical systems.
Invention is credited to Klaus Ehrmann, Arthur Ho, Brien Anthony Holden.
Application Number | 20110176113 13/053073 |
Document ID | / |
Family ID | 39787975 |
Filed Date | 2011-07-21 |
United States Patent
Application |
20110176113 |
Kind Code |
A1 |
Ho; Arthur ; et al. |
July 21, 2011 |
Characterising Eye-Related Optical Systems
Abstract
An instrument and method for characterising eye-related optical
systems, including the live human eye (18) involves scanning an
illuminating light beam (22) from a light source and light detector
unit (20) from element to element of a beam deflector array (12) of
elements (14) arranged laterally across the optical axis (16) of
eye (18). At each successive element (14) the illuminating beam
(22) is deflected to form an interrogating beam (24) that is
directed into the eye (18) at a peripheral angle that depends upon
the lateral location of the deflector element. A return beam (23)
is reflected or back-scattered from the cornea (38) and returned
via the same deflector element to the light source and detector
unit (20). This allows the interrogating beams to be scanned
sufficiently rapidly into the eye to greatly reduce the variation
of eye fixation and gaze that accompany other methods of measuring
peripheral refraction or aberration of a natural eye. In addition
to or instead of scanning the illumination beam (22) over each
element (14) of the array (12), all or multiple elements (14) of
the array (12) can be illuminated simultaneously and the multiple
interrogating rays (24) thus generated can be gated by the use of
an LCD aperture plate (26). Alternatively, an LCD aperture plate
(28) can be interposed between a wide illuminating beam (22) and
operated to selectively illuminate the beam deflector.
Inventors: |
Ho; Arthur; (US) ;
Holden; Brien Anthony; (US) ; Ehrmann; Klaus;
(US) |
Family ID: |
39787975 |
Appl. No.: |
13/053073 |
Filed: |
March 21, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12593543 |
Apr 15, 2010 |
7909465 |
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PCT/AU2008/000434 |
Mar 28, 2008 |
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13053073 |
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Current U.S.
Class: |
351/246 |
Current CPC
Class: |
A61B 3/1015 20130101;
A61B 3/0008 20130101; A61B 3/103 20130101; A61B 3/1005
20130101 |
Class at
Publication: |
351/246 |
International
Class: |
A61B 3/10 20060101
A61B003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 28, 2007 |
AU |
2007901634 |
Claims
1. A method of optically characterising an eye-related optical
system having a longitudinal optical axis, the method including the
steps of: generating an interrogating light beam from each element
of an array of discrete beam deflector elements, which array that
extends laterally from the optical axis, directing said light beam
into the eye-related system at an angle relative to the optical
axis which is at least in part determined by the lateral position
of the element within said array, detecting a return beam from the
eye-related system that is generated by said interrogating light
beam and that is returned at said angle via said beam deflector
element to detector means, and comparing said detected returned
beam with said interrogating beam or an image of to determine
aberrations of the eye-related optical system at said angle.
2-21. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field
[0002] This invention relates to methods, instruments and apparatus
for optically characterising eye-related optical systems,
preferably over wide angles of view. Eye-related optical systems
include the natural animal eye, alone or in association with
prosthetic lenses and with or without surgical or other
modification. They also include physical eye models or simulated
eyes with or without modification to simulate optical disorders
and/or corrective measures.
[0003] Optical characterisation typically involves refractometry;
that is, the determination of the optical power of portion or the
entire optical path travelled by an interrogating ray and it may,
for example, include mapping--or spatially resolving--refractive
power over an area or surface of the eye-related system, which is
sometimes referred to as wavefront aberrometry. Optical
characterization may also include determination of the length of
the eye-related system, for example the distance from the anterior
surface of the cornea to the anterior surface of the retina. Other
characteristics of natural eyes, such as the profile and thickness
of the cornea, pupil size and the depth of the anterior chamber,
may also be important for certain surgical procedures (eg, lens
replacement or ablative laser treatment) but are not of prime
concern in this invention.
[0004] Of particular--but not exclusive--interest are methods and
instruments suitable for use by optometrists in determining the
peripheral refraction--and, optionally, the length--of the human
eye for the purpose of prescribing anti-myopia treatment.
Peripheral angles of 20-30 degrees with respect to the optic axis
are of particular interest in this respect, with angles up to about
40 degrees also being relevant. Even higher peripheral angles are
of research interest to specialists.
[0005] 2. Description of Related Art
[0006] Several methods and instruments have been used to measure
central refraction errors and aberrations of the eye. Refractive
error is a subset of the total aberration of the eye and
traditionally expressed in terms of sphere and cylinder components
along with the orientation of the cylinder axis. Although it is
possible to extract the refractive error, also called lower order
aberrations, from the measurements of total aberration, the
instruments used in clinical practice are usually dedicated to
either measurement of refractive error or total aberration. While
neither instrument is designed for measurement of peripheral
refraction or aberration, commercial instruments have been modified
to obtain measurements of peripheral refraction for both the
accommodated and unaccommodated eye. The modifications include some
form of off-centre fixation with and head or eye movement being
needed.
[0007] Atchinson [Atchinson D A. "Comparison of peripheral
refractions determined by different instruments". Optom Vis Sci
2003;80:655-60] describes one such comparison of two
auto-refractometers (Canon Autoref R-1 and Shin-Nippon SRW-5000)
and one Hartmann-Shack wavefront aberrometer. With all three
instruments, peripheral refraction or total aberration was measured
by rotating and fixating the eye on a series of fixation targets
along the horizontal meridian of up to 40.degree. nasal and
temporal. All measurements were taken sequentially, with patients
being instructed to fixate on a particular target and then
re-centering the pupil position with the optical axis. Overall, the
three instruments produced similar results, although the Canon
results are more variable. Several similar investigations have been
published using similar methods and instruments and obtaining
similar results. [Gwiazda J, Weber C. Comparison of spherical
equivalent refraction of astigmatism measured with three different
model of auto refractors. Optom Vis Sci 2004;81:56-61, and
Gustafsson J, Terenius E, Buchheister J, Unsbo P. Peripheral
astigmatism in emmetropic eyes. Ophthalmic Physiol Opt
2001;21:393-400.]
[0008] A number of different optical methods have been utilised to
automatically determine the refractive status of the live eye. The
basic principle employed is the projection of an optical pattern or
beam onto the retina and the analysis of the reflected pattern. An
overview of these methods is given in [Atchinson D A. "Recent
advances in measurement of monochomatic aberrations of human eyes".
Clin Exp Optom 2005;88: 1: 5-27]. One of the most commonly used
principles is used in the Shin-Nippon SRW-5000 instrument in which
an infrared ring target is projected onto the retina and the
reflected image is captured with a CCD camera. A lens relay system
moves rapidly, scanning through the focus range and the size of the
images are analysed in multiple meridians to provide the data from
which the aberrations (including refraction) can be derived. Some
of these techniques have the advantage of being `open-field` in
that the subject can look through a glass window and semi silvered
mirror into the distance, thus preventing instrument myopia, but
also allowing fixation at off axis angles. Typically, the angular
fixation range is limited to less than 30.degree. in the horizontal
and about half of that in the vertical meridian. This technique is
not sufficient to fully characterize peripheral aberration with and
without vision correction devices.
[0009] A similar instrument was described and used in a laboratory
setting by Artal et al. [Artal P, Derrington A M, Colombo E.
"Refraction, aliasing, and the absence of motion reversals in
peripheral vision". Vision Res 1995; 35: 939-47.] A point image is
projected onto the retina. The reflected image is observed with a
CCD camera while moving the `focusing block` axially until the best
focus position with smallest circle of confusion was found. To
assess astigmatism, the positions for sharpest horizontal and
vertical profiles were also determined. A fixation target was
placed at comfortable viewing distance in locations for 15.degree.,
20.degree. and 40.degree. retinal eccentricities in the horizontal
meridian.
[0010] Webb et al describe a modified Scheiner system whereby the
subject manipulates the incidence angle of one of the Scheiner
beams until two dots on the retina merge into one. [Webb R H,
Penney C M, Thompson K P. "Measurement of ocular local wavefront
distortion with a spatially resolved refractometer". Appl Opt 1992;
31: 3678-3686.] Although the measurement beam enters the pupil
non-parallel to the optical axis, the angular deviations are very
small and only compensate the paraxial wavefront error of the eye.
No peripheral refraction measurements appear possible with this
system.
[0011] In 2003, Schmid presented results of peripheral axial length
measurements from an instrument developed utilising optical low
coherence reflectometry. [Schmid G F. "Axial and peripheral eye
length measured with optical low coherence reflectometry". J.
Biomed. Opt. 2003 8(4): 655-662. See also, Schmid et al,
"Measurement of eye length and eye shape by optical low coherence
reflectometry". Intnl. Opth. 2001 23(4-6).] A fixation LED was
coupled into the central optical path to keep the eye aligned with
the optical axis of the instrument. A beam steering mirror deflects
the measurement beam horizontally and vertically by up to
15.degree. for off-axis measurements. The measurement principle
requires the incident beam to be aligned perpendicular to the
cornea at the point of intersection. Due to the non-spherical shape
of the cornea, small lateral repositioning of the instrument is
necessary for each new incident angle. This manual process
prohibits rapid measurements across the angular range.
[0012] In U.S. Pat. No. 6,439,720, Graves et al disclose an
instrument for measuring lower and higher order aberrations of the
human eye. The method described is one of several variations of
double pass techniques, whereby a probing light beam illuminates a
spot on the retina and the reflected wavefront is analysed after
exiting the eye. In this patent, a pair of Littrow prisms is used
to split the emerging light ray into two parallel beams which pass
through a moveable collimating lens to generate two slightly
defocused images on a CCD detector. From the two computer analysed
images, the ocular aberrations can be determined. The patent
describes only on axis measurements of lower and higher order
aberrations.
[0013] Wei et al [US patent 20052034] disclose a multifunctional
instrument to measure axial eye length and corneal topography.
Although not directly dedicated to obtain aberration and refraction
results, the instrument features several sub-components also used
in aberrometry and the combination of axial length parameters and
keratometry data allows some estimation of the refractive status.
Again, the instrument is designed for on axis measurements only.
The fixation target can be moved but only along the optical axis to
stimulate different accommodative responses.
[0014] An instrument to measure aberrations of the eye at a
plurality of locations was disclosed by Molebny at al [U.S. Pat.
No. 6,409,345]. The plurality of locations are achieved by parallel
offsetting the probing light beam with respect to the optical axis.
Aberration mapping is thus confined to paraxial scanning to obtain
power maps across the entrance pupil. As with Wei et al (above), a
fixation target is added to align the optical axis and to control
accommodation.
[0015] The techniques outlined above are generally unsatisfactory
because the patient is unable to correctly fixate gaze for the time
needed and because the peripheral angle at which the measurement is
taken is difficult to measure with accuracy or reliability.
Attempting to map peripheral power even over a few spots on the eye
in one sitting is impractical and repeatability between different
sittings is generally poor.
[0016] Neal et al [U.S. Pat. No. 6,634,750] disclose a `tomographic
wavefront analysis system and method of mapping an optical system`
in which interrogating beam(s) are scanned into multiple locations
within the eye and back-scattered light from is detected and
processed into an aberration map or representations of the
three-dimensional structure of the eye using computer automated
tomography. While the difficulty of peripheral gaze fixation is
avoided, the system is highly complex, unsuited for use in normal
optometry practise and appears to be incapable of interrogating an
eye at peripheral angles greater than about 10-15 degrees with
respect to the optic axis. Further, the disclosed technique is only
suited to the use of spot beams and does not envisage or permit the
use of interrogating beams having various cross-sectional shapes,
such as squares, circles, ellipses or rings which assist in auto
focussing/ranging and accelerate wavefront analysis.
[0017] Methods and instruments that are capable of more accurate
peripheral refraction measurements over wide peripheral angles are
needed to provide important inputs for the determination of ocular
shape, eye length or retinal contour. Such inputs are now of
significant interest in the study and treatment of eye pathologies
such as progressive myopia.
BRIEF SUMMARY OF THE INVENTION
[0018] The present invention involves use of an array of discrete
beam deflector elements that extends laterally from the optical
axis of the eye-related system and from which elements
interrogating beams can be directed into the eye-related system
over a wide range of peripheral angles. These interrogating beams
in turn generate return beams from within the eye-related system
that are transmitted to detector means via the same beam deflector
elements. The interrogating beams can be generated by illuminating
elements of the array with an illuminating beam or beams that may
be scanned over the array. The scanning preferably takes place from
a common point so that all return beams are returned to that source
for detection by a single detector. Refractive error and other
aberrations of the eye-related system can be determined and, if
desired, mapped onto a surface of the system, by comparing the
interrogating beam with the returned beam for each element of the
array. This may be done by comparison of wavefronts, relative
displacement, angle, position or cross-sectional shape. Since the
source, illumination and interrogation beams will have
substantially identical optical properties, it will be convenient
to use the source beam as a proxy for the interrogation beam in
such comparisons. Indeed, it will normally be sufficient to store
data concerning the source beam as a basis for such comparisons.
Reference to comparing the return beam with the interrogating beam
should therefore take this into account.
[0019] The beam deflector array may extend on one side or both of
the optical axis, be straight or curved, cruciform, star, disc,
dish or line shaped. For simplicity and convenience, a line or
linear array (straight or curved) that extends equally on each side
of the axis is preferred as it allows a complete meridian of the
eye to be assessed with one setting of the array. A linear array,
whether extending to one or both sides of the axis, can be rotated
to cover all meridians and polar angles.
[0020] Each deflector element of the array functions to (i) deflect
an illuminating beam from a light source as an interrogation beam
into the eye-related system and to (ii) deflect the reflected or
back-scattered return beam to the detector means. The angle of the
interrogating beam relative to the optical axis is determined both
by the nature of the beam deflector element (eg, fixed or steerable
prisms or mirrors) and its lateral location in the array. More
remote deflector elements normally give rise to larger
interrogation angles. In this way, peripheral angles up to and in
excess of 40 degrees can be readily scanned, with angles of between
20 and 30 degrees normally being adequate for characterising myopic
eyes for corrective therapy. A deflector element may be moveable
from location to location within the array to cover multiple angles
but the added complication and inaccuracies are not likely to make
this worthwhile. Preferably, therefore the beam deflector elements
occupy fixed positions in the array, though it is envisaged that
individual elements may be bitable under processor control to
adjust the angle of their interrogation beams. Preferably, however,
a source light beam is scanned sequentially from a common point
over the deflector elements and each return beam is returned via
each element as it is scanned to the common point.
[0021] It is preferable to generate the interrogation beams one at
a time so that the total intensity of the light entering a human
eye being examined at any instant is minimised. This also enhances
the ability of the detector means to discriminate between returned
beams. However, scanning an illuminating beam from one array
element to the next is not essential as sequential generation of
interrogation beams can be achieved in other ways--by the use of
electronic shutters before and/or behind the array or by use of
moveable beam deflector elements, for example. With any of these, a
rapid sequence of interrogating beams can be generated over a wide
range of interrogation angles, the speed of scan being largely
determined by the rate at which return beams can be detected and
the associated data recorded. Scanning and detection are preferably
conducted automatically by or under the control of a PC or other
digital processor/controller.
[0022] Rapid scanning is highly desirable to allow good fixation of
a live natural eye throughout the procedure, it being preferred
that the entire interrogation and detection sequence take place
over a period of a few seconds. Preferably the subject is asked to
fixate gaze upon an on-axis target and, when fixation is confirmed,
the scan sequence is initiated automatically. Optimally, the
technique allows optical characteristics of the eye-related system
to be computed and mapped substantially in real-time.
[0023] Where scanning is thought to be speed-limiting, a few beam
deflection elements may be illuminated at once to generate multiple
simultaneous return beams that will need to be distinguished for
separate detection. This can be done by using the aforementioned
electronic shutters to chop or pulse-code one or more of the return
beams. Selective polarisation may also be employed to distinguish
the return beams, which can also be implemented by a suitable
electronic shutter serving as a selective polariser.
[0024] Because an interrogating beam will encounter multiple
interfaces between materials of different optical characteristics
as it travels into the eye-related system, the respective returned
beam will be composed of a set of component returned beams. The
component returned beam which is generally of most interest is that
returned from the retina (the rear-most interface of the
eye-related system) because this provides the longest beam path in
the eye. Fortunately, the component beams returned from the cornea
and retina are also usually the most intense and/or distinct. While
component beams returned from other surfaces within the eye-related
system are more difficult to detect and distinguish from one
another, the technique of the present invention allows for such
component returned beams to be selected for analysis. Selection and
comparison of return beam components associated with both the
cornea and the retina will allow the length of the eye to be
determined using interferometric methods, eye length being of
critical interest for the monitoring of myopia progression.
[0025] Interferometric measurement of eye length may be combined
with the mapping of refractive aberrations of an eye with
particular advantage where a scanning illuminating beam is
generated from a source beam at a single or common point, as by the
use of a moving mirror scanner. This allows each return beam with
its retina and cornea components to be returned to a common
location where aberrations and cornea-retina distance can be
determined for every return beam. The common location is the source
beam prior to the scanning point where the return beams can be
coupled into a detector beam path and an interferometer beam path
using beam-splitters. To measure retina-cornea distance, a
reference beam (part of the source beam) is also coupled into the
interferometer beam path so that it can interfere with the return
beam components in a manner that can be detected, interference
being created by changing the length of the interferometer beam
path in such a way that the length of the reference beam relative
to the return beam in that path is changed. This change in length
can be effected by moving a mirror and monitoring for interference,
the distance the mirror moves being related to the retina-cornea
distance, though not identical. To achieve interference in this way
the reference beam (and therefore the source beam) is preferably of
low coherence, substantially monochromatic and preferably in the
near infra-red.
[0026] From one aspect, the invention is concerned with a method
for optically characterising an eye-related optical system
involving, illuminating a beam deflector element in an array that
extends laterally from the optical axis to generate an
interrogating light beam that is directed into the eye-related
system at a predetermined angle relative, detecting a reflected or
back-scattered return beam from the eye-related system that is
returned via the illuminated beam deflector element, and comparing
the returned beam with the interrogating, illuminating or source
beam to determine wavefront differences indicative of aberrations
of the eye-related optical system at the predetermined angle.
[0027] From another aspect, the invention is concerned with an
instrument based on the above method that includes an array of beam
deflector elements extending from the optical axis, means for
illuminating the array to generate the interrogation beams and
means for detecting the returned beams and comparing them with the
undistorted interrogation, illuminating or source beam, all of
which can be assumed to be free of aberrations or at least to have
calibrated aberrations. Typically, the source beam--and therefore
the illuminating beams, the interrogating beams and the return
beam--will be of narrow spectral width within the visible or
infra-red region, near infra-red being preferred as noted
above.
[0028] From another aspect, the method of the invention may employ
a laterally extending array of beam deflector elements to generate
interrogating beams that are directed sequentially into a subject
eye at peripheral angles, using the deflector elements of the array
to direct light returned from the eye from each interrogating beam
to common detector means as a sequence of returned beams,
differencing the wavefronts of each interrogating beam and its
respective returned beam to determine the refractive power of the
eye along the path of that interrogating beam and return beam
within the eye.
[0029] The terms `in front of` and `behind`, and `forwards` and
`rearwards` are used to indicate relative disposition with respect
to the eye-related system. Thus, the array of beam deflector
elements (where employed) will be located in front of the
eye-related system, the interrogation beam will travel rearwards
from the array into the eye-related system and the returned beams
will travel forwards to the array.
DETAILED DESCRIPTION OF EXAMPLES
[0030] Having portrayed the nature of the present invention,
particular examples will now be described with reference to the
accompanying drawings. However, those skilled in the art will
appreciate that many variations and modifications can be made to
the examples without departing from the scope of the invention as
outlined above.
DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0031] In the accompanying drawings:
[0032] FIG. 1 is a basic diagrammatic plan of the basic optical
layout of the first example of an instrument formed in accordance
with the present invention in which the deflector means comprises
and array of prisms.
[0033] FIG. 2 illustrates one possible modification to the example
of FIG. 1 wherein the prisms comprising the deflector array are
tilted and arranged in an arc.
[0034] FIG. 3 is an enlarged and more detailed diagrammatic side
elevation of the optical layout of the instrument of FIG. 1.
[0035] FIG. 4 is an enlarged partial detail of FIG. 3 showing some
additional features or refinements.
[0036] FIG. 5 is diagrammatic side elevation of an instrument that
comprises the second example with a different optical configuration
for beam steering from that of the first example of FIGS. 1-4.
[0037] FIG. 6 is a diagrammatic side elevation of the instrument of
FIG. 3 including means for measuring the length of the eye and/or
the distance between elements thereof.
[0038] The basic layout of the instrument 10 of the first example
is shown diagrammatically in FIG. 1. The array 12 of deflector
elements 14, in this case is a linear row that extends
symmetrically and laterally on either side of the optical axis 16
of the eye-related optical system 18 under investigation. It will
be assumed that system 18 is the eye of a patient with or without
the addition of prosthetic lenses or other modifications. An
illuminating light source, controlling processor and return-beam
detector are indicated by a single undifferentiated unit 20
arranged on axis 16, which is described in more detail with
reference to FIG. 3. Unit 20 directs illuminating beams, indicated
by arrow heads 22, to array elements 14 to generate a corresponding
set of interrogating beams, indicated by arrow heads 24, that are
directed into eye-system 18 at different peripheral angles relative
to axis 16. A return beam, indicated by arrow heads 23, is
generated by each interrogating beam 24 and is directed back to
unit 20 via the respective element 14 for detection. It is
convenient for illuminating beams 22 to be directed in sequence
from one element 14 to the next to thereby sequentially generate
the interrogating beams 24 and return beams 23.
[0039] In this example, a central illuminating beam, a
corresponding central interrogation beam and a corresponding
central return beam are indicated by arrow heads 25, 27 and 29.
Also in this example, each deflector element is a prism (except
central element 14c) that has an apex angle such that each
interrogation beam 24 is directed into eye 18 and each return beam
23 is directed to unit 20. Central element 14c is effectively a
null element that does not deflect the illuminating beam; it may be
a parallel-sided plain glass as shown, but that is not even
necessary. Also in this example, array 12 is substantially linear
so that interrogating beams 24 and 27 are substantially co-planar
allowing one meridian--the horizontal in this example--of system 18
to be investigated. Non-horizontal meridians of the system can be
investigated by simply rotating the instrument 10 about optic axis
16 relative to eye 18.
[0040] The transmission of interrogating beams 24 and 27 one at a
time into eye 18, and the generation of a corresponding sequence of
return beams 23 and 29, can be effected in a variety of ways. First
(as will be described below), unit 20 may include a beam scanner
that directs a single narrow illuminating beam from one element 14
to another. Second, multiple elements 14 can be illuminated at one
time and interrogating beams 24 and 27 can be gated to effect
scanning of eye 18 and the generation of a sequence of return beams
23 and 25, This can be done by, for example, inserting an
electronically controllable LCD shutter 26 between array 12 and eye
18 and using it as scanning means by which interrogating beams 24
from prisms 14 are admitted into eye 18 one at a time. Third, a
similar shutter 28 may be inserted between array 12 and unit 20 to
gate illuminating beams 22 and 25 to illuminate one or more
elements 14 at a time. Thus, it is not essential for unit 20 to
include scanning means and it is possible to distribute the
scanning function between scanner means in unit 20 and shutters
such as indicated at 26 and/or 28
[0041] In this way, successive interrogation/return beam pairs
diverge/converge at successively larger/smaller angles with respect
to axis 16 as they pass into and out of eye 18. Sequential scanning
from one angle to the next adjacent will probably be most
convenient but many other scan sequences may be used to minimise
biases that might arise due to fixed sequential scanning. While
illumination of more than one beam deflector element 14 at a time
can easily be achieved by use of a scanner in unit 20, it is then
necessary to distinguish the multiple simultaneous return beams
that will result. This can be done by using shutter 26 or 28 as a
beam-chopper or selective polariser to differentially encode each
return beam that needs to be distinguished from another at the
detector.
[0042] A second variant of the instrument of FIG. 1 is illustrated
in FIG. 2 and this instrument is indicated at 10a. In this variant
the prisms 14a that comprise deflector elements of array 12 are
arranged in a curve or arc (rather than being coplanar) and are
tilted relative to the corresponding prisms of FIG. 1 so that entry
and exit angles of the light beams are equal. Such an arrangement
can offer improved performance depending upon the lateral
dimensions of the array and the number of deflector elements
therein.
[0043] FIG. 3 is a more detailed side elevation of instrument 10 of
FIG. 1 or the variant 10a of FIG. 2 in which the principal
components of unit 20 are shown separately. A light source 30
directs a collimated source beam 32 via a beam-splitter 34 to an
oscillating mirror scanner 36 that is moved by actuator 37 to
generate illuminating beams 22 that are scanned from deflector to
deflector in array 12 to generate the sequence of interrogating
beams 24 that are directed into eye-system 18 and onto the retina
38 over the desired range of incident angles. Scanning mirror 36
thus forms a point source or common point for beams 24 and a common
point (indicated at X) for all return beams. Thus, each return beam
23 returned from retina 38 passes back via deflector array 12 and
scanner mirror 36 to beam-splitter 34 by which it is diverted via a
focusing system 42 to a photo detector 44. System 42 includes a
moveable lens assembly 43 that can be moved axially back and forth
through a focus range, as indicated by arrows 46. While the source
beam 32 (and, thus, the illuminating, interrogating and return
beams 22, 24 and 23) can have any desired spot, disc or annular
cross-section desired, an annular cross-section like that commonly
used in known autorefractors (such the Shin-Nippon SRW-5000
mentioned above) is preferred as it can be analysed and processed
in a substantially standard manner.
[0044] Each return beam 23--or more correctly its image 48 at
detector 44--thus contains information of the refractive status of
the eye-system that is captured or quantified by detector 44, which
is preferably a two-dimensional array of photo sensors.
[0045] Finally, unit 20 includes a central processor and controller
49 that may conveniently comprise a dedicated PC and is connected
to accept and analyse the output of detector 44 and to drive lens
assembly 43 under servo-control. Processor 49 is also connected to
control scanner driver 37 and to ensure correct timing of
illumination and return signal detection. A connection between
light source 30 and processor 49 is also shown as it will be
convenient to ensure that source beam 32 is correctly configured
and that a representation of the current source beam sectional
pattern is stored for comparison with image 48.
[0046] While each return beam 23 is being received, focusing lens
assembly 42 is moved along the direction of the optical axis to
vary the focus size and shape of the image 48. Commonly, three
positions of the focussing assembly 42 are recorded for each of
three return beam image shapes: one position where the image (spot
or ring) appears smallest and in sharpest focus, a second position
where the image appears maximally elongated in one meridian and a
third position where the image is maximally elongated in a
different meridian, usually one that is orthogonal to the first
meridian. The three positions of lens assembly 42 respectively
indicate the spherical equivalent power of the eye, the sagital
astigmatic component and the tangential astigmatic component of the
refraction. The significance of spot/image size in relation to
spherical equivalent power of eye 18 can be understood in the
following elementary way. Since the interrogating beam 24 that
enters eye 18 is collimated, a normal or emmetropic eye will return
a parallel collimated beam, a myopic eye will return a convergent
beam and a hyperopic eye will return a divergent beam, both of
which will result in larger images sizes.
[0047] FIG. 4 shows some features that may be added to enhance the
performance of the instrument 10 or variant 10a of FIGS. 1 and 2. A
movable fixation target 50 is located on a gaze beam path 52 that
is optically coupled by a first additional beam-splitter 54 into
return beam path 23 and on optical axis 16. Fixation target 50
aligns the gaze or axis of the eye. with optical axis 16 of the
system and controls accommodation. A second additional
beam-splitter 56 in gaze path 52 directs an image of eye 18 onto a
CCD detector 58, allowing gaze direction and eye-alignment to be
monitored since CCD detector 58 receives the ocular image via
beam-splitters 54 and 56. Optical or acoustical distance sensors 60
can be used to (alternately or additionally) indicate when eye 18
appears to be axially aligned. Sensors 60, along with detector 58
if desired, can be connected to processor 49 (FIG. 3)--as indicted
by arrows marked P--so that initiation of a measurement cycle can
be automatic.
[0048] FIG. 5 shows the optical layout of an instrument 60 that
comprises the second example of the invention, which has a
different deflector array 62 than that of the first example but,
otherwise, may be substantially the same. The same reference
numerals will therefore be used for elements of instrument 60 that
are essentially the same as those of FIGS. 1 or 2 of the first
example. FIG. 5 is a plan view like that of FIGS. 3 and 4.
[0049] As in the first example, array 62 of instrument 60 comprises
a row of beam deflector elements 63 but, in this case, each
deflector element 63 comprises a mirror (or optionally a prism) 64
that can be tilted by an actuator 66. [It will be appreciated that,
if mirrors 64 are fixedly located, instrument 60 will be
substantially identical to instrument 10 of the first example in
which the scanning function is performed by scanner means in source
and detector unit 20, and/or by an LCD shutter 26 or LCD shutter 28
(see FIG. 1). ] Actuator 66 can be a known solid-state device such
as a barium titanate piezoelectric actuator. This allows instrument
60 of the second example to function quite differently from that of
the first example, because each element 63 can be operated as a
shutter or scanner. A number of different operational modes are
envisaged.
[0050] First, many deflector elements 63 may be used in array 62
since their component mirrors 64 and actuators 66 can be made very
small and mounted very precisely. They can be arranged much closer
together than prisms 14 of array 12 in the first example, at least
for sectors of the eye that are of particular interest. While light
source, detector and processor unit 20 can be operated to scan the
illuminating beam 22 along array 63 in a similar manner as
described with reference to FIG. 3, it may be difficult to ensure
that scanned beam 22 does not illuminate more than one of the
closely spaced deflector mirrors at one time. Accordingly,
actuators 66 can be operated to (i) `correctly` angle only the
intended mirror 64 to direct its interrogating beam 24 into eye 18
and (ii) `incorrectly` angle nearby mirrors 64 to ensure that any
beams that they generate are not directed into eye 18. This allows
scanner 36/37 of FIG. 3 to effect coarse scanning leaving fine
scanning to be undertaken by mirrors 64. [As will be understood
from the description of the first example, a mirror that is
correctly angled to direct an interrogating beam 22 in to eye 18
will also be correctly angled to direct return beam 23 from the
retina 38 to the detector of unit 20.] Thus, in the operational
mode just described, beam deflector elements 63 (comprising mirrors
64 and their actuators 66) can be operated like shutter 26 (or 28)
of FIG. 1.
[0051] Second, the beam deflector elements 63 of a multiple element
array 62 can be operated to perform some or all of the scanning
functions of scanner mirror 36 of the first example. For example,
instead of mounting small mirrors close together to finely
interrogate eye areas of particular interest, a series of larger
mirrors 64 can be successively illuminated by coarse scanning of
illuminating beam 22 employing a coarse scanner in unit 20 (similar
to mirror 36 described in the first example) and each illuminated
mirror 64 can be moved by its actuator 66 to effect fine-scanning
of interrogation beam 24 over a small range of angles.
[0052] FIG. 6 illustrates an instrument 80 and a method that form
the third exemplary embodiment of the invention and enable the
measurement of eye length as well as wavefront aberrations and
peripheral refraction. Instrument 80 incorporates the instrument of
FIGS. 3 and 4 for measurement of wavefront aberration and
peripheral refraction and adds thereto an interferrometer beam path
82 for the measurement of eye length. The same reference numerals
will be used for those parts of instrument 80 that have
substantially the same function as instrument 10 and will not be
separately described here.
[0053] The interferrometer beam path 82 is arranged substantially
at right angles to the source beam 32a that is emitted by light
source 30. It comprises (i) an additional beam-splitter 84 arranged
in beam 32 before scanning mirror 36, (ii) a dispersion
compensation element 86, (iii) an additional moveable mirror 88 and
(iv) an additional photo detector 90. As indicated by arrows 92,
mirror 88 is moveable along beam path 82 toward and away from photo
detector 90, by an actuator 93 under the control of processor 49
(FIG. 3). Preferably, actuator 93 is operated to reciprocate mirror
88 back and forth.
[0054] It will be assumed in what follows that the axial length of
eye-system 18 is of interest, so axial illuminating, interrogating
and return beams 25, 27 and 29 are those under consideration. In
use, source beam 32a travels through additional beam-splitter 84
and is split at point A into two emerging beam portions, beam 32
which continues (as before described) to scanning mirror 36 and a
reference beam 94 that is reflected by splitter 84 into beam path
82 on to reciprocating mirror 88 from which it is reflected back
via point A to detector 90. Since the portion of interferometer
path 82 between point A and detector 90 is also travelled by return
beam portions 29a and 29b, which are reflected to detector 90 by
splitter 84, reference beam 94 can interfere or beat with return
beam portions 29a and 29b. It is of course necessary that the
travel of mirror 88 during reciprocation is sufficient to cause
interference between both return beam portions 29a and 29b. These
interferences are detected by detector 90 and transmitted to
processor 49 along with the precise position of mirror 88 (as
indicated by arrows with the letter P. For convenience, it is
assumed that interference with return beam portion 29a occurs when
mirror 88 is at point D1 and that interference with return beam
portion 29b occurs when mirror 88 is at point D2.
[0055] More specifically, the interference will appear if the
optical distances [A, B, C1]+[A, D2] and [A, B, C2]+[A, D2] are
equal. Since the relative distance between D1 and D2 is accurately
known from the mirror positions, the optical distances between
points C1 and C2 are also known. The physical distance between
cornea and retina surfaces can then be computed by using well known
group refractive index values of ocular media to convert the
optical distances into physical distances. Measurement accuracy can
be improved by the use of the dispersion compensating element 86
into beam path 82, such devices being known in the art. It will be
appreciated that, while the general optical techniques employed in
additional beam path 82 to indicate eye length are not new (see,
for example, the Schmid references identified above), the
particular combination with instrument 10 is most useful, elegant
and novel. However it will also be appreciated that other known
techniques of optically indicating eye length may also be used
separately or in combination with the optical characterisation
systems disclosed herein.
[0056] While a number of examples of the invention, along with a
number of variants, have been described, it will be appreciated
that many others are possible without departing from the scope of
the invention as defined by the following claims. The specific
terms and arrangements used in the examples have been for
illustration rather than limitation.
* * * * *