U.S. patent application number 12/372573 was filed with the patent office on 2009-08-20 for system, ophthalmic lens, and method for extending depth of focus.
This patent application is currently assigned to AMO Regional Holdings. Invention is credited to Pablo Artal, Silverstre Manzanera, Patricia Ann Piers, Hendrik Albert Weeber.
Application Number | 20090210054 12/372573 |
Document ID | / |
Family ID | 40535591 |
Filed Date | 2009-08-20 |
United States Patent
Application |
20090210054 |
Kind Code |
A1 |
Weeber; Hendrik Albert ; et
al. |
August 20, 2009 |
SYSTEM, OPHTHALMIC LENS, AND METHOD FOR EXTENDING DEPTH OF
FOCUS
Abstract
System, ophthalmic lens, and method for extending depth of focus
includes an optic having a clear aperture disposed about a central
axis. The optic includes a first surface and an opposing second
surface. The first and second surfaces are configured to introduce
an asymmetric aberration to the eye while maintaining the in-focus
visual acuity.
Inventors: |
Weeber; Hendrik Albert;
(Groningen, NL) ; Piers; Patricia Ann; (Groningen,
NL) ; Artal; Pablo; (Murcia, ES) ; Manzanera;
Silverstre; (Murcia, ES) |
Correspondence
Address: |
ABBOTT MEDICAL OPTICS, INC.
1700 E. ST. ANDREW PLACE
SANTA ANA
CA
92705
US
|
Assignee: |
AMO Regional Holdings
Quarryvale
IE
|
Family ID: |
40535591 |
Appl. No.: |
12/372573 |
Filed: |
February 17, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61029284 |
Feb 15, 2008 |
|
|
|
Current U.S.
Class: |
623/6.11 ;
351/159.52 |
Current CPC
Class: |
G02C 7/02 20130101; A61F
2/1618 20130101; A61F 2/1659 20130101; A61F 2240/002 20130101; A61F
2/1637 20130101; G02C 2202/20 20130101; G02C 2202/22 20130101; A61F
2/14 20130101; A61F 2/1654 20130101; A61F 2/1613 20130101 |
Class at
Publication: |
623/6.11 ;
351/176; 351/177 |
International
Class: |
A61F 2/16 20060101
A61F002/16; G02C 7/02 20060101 G02C007/02 |
Claims
1. An ophthalmic lens for an eye, the eye having an in-focus visual
acuity and a depth of focus, the ophthalmic lens comprising: an
optic having a central axis and a clear aperture disposed about the
central axis, the optic comprising a first surface and an opposing
second surface, the first and second surfaces together configured
to introduce at least some asymmetric aberration to the eye to
extend the depth of focus while maintaining the in-focus visual
acuity.
2. The ophthalmic lens of claim 1, wherein the asymmetric
aberration is a higher order asymmetric aberration.
3. The ophthalmic lens of claim 1, wherein the asymmetric
aberration is selected from a group consisting of: an asymmetric
astigmatism, a higher order astigmatism, a vertical coma, a lateral
coma, and a trefoil.
4. The ophthalmic lens of claim 1, wherein the first and second
surfaces are together configured to introduce a predetermined
degree of coma to the eye while maintaining the in-focus visual
acuity.
5. The ophthalmic lens of claim 1, wherein the optic is selected
from the group consisting of: a monofocal intraocular lens, a
multifocal intraocular lens, an accommodating intraocular lens, and
a toric intraocular lens.
6. The ophthalmic lens of claim 1, wherein one or more of the first
and second surfaces comprises an asymmetric curvature.
7. The ophthalmic lens of claim 1, wherein the eye has an
intraocular lens implanted therein, the intraocular lens having a
predetermined optical characteristic, wherein the in-focus visual
acuity is based at least in part on the optical characteristic, and
wherein the optic and the intraocular lens are together configured
to introduce the asymmetric aberration to the eye while maintaining
the in-focus visual acuity.
8. The ophthalmic lens of claim 1, wherein the optic is a
multifocal intraocular lens comprising a plurality of focal points,
each of the plurality of focal points having a depth of focus, and
wherein the asymmetric aberration extends the depth of focus of
each of the plurality of focal points.
9. The ophthalmic lens of claim 1, wherein the optic is an
accommodating intraocular lens configured to provide the eye with a
functional range of vision, and wherein the asymmetric aberration
extends the functional range of vision.
10. The ophthalmic lens of claim 1, wherein the optic is further
configured to correct for one or more non-asymmetric higher order
aberrations.
11. The ophthalmic lens of claim 10, wherein the one or more
non-asymmetric higher order aberrations is selected from a group
consisting of a spherical aberration and a non-asymmetric
astigmatism.
12. The ophthalmic lens of claim 1, further comprising a binary
phase mask optically coupled with the optic, the binary phase mask
configured to extend the depth of focus by a predetermined amount,
and wherein the asymmetric aberration further extends the depth of
focus beyond the predetermined amount.
13. The ophthalmic lens of claim 1, wherein the optic is an
intraocular lens having a hyperfocal configuration for extending
the depth of focus by a predetermined amount, and wherein the
asymmetric aberration further extends the depth of focus beyond the
predetermined amount.
14. The ophthalmic lens of claim 1, wherein the optic is a zonal
aspheric intraocular lens configured to extend the depth of focus
by a predetermined amount, and wherein the asymmetric aberration
further extends the depth of focus beyond the predetermined
amount.
15. The ophthalmic lens of claim 1, wherein the optic is a
diffractive monofocal intraocular lens, and wherein the asymmetric
aberration is further configured to correct for a chromatic
aberration associated with the diffractive monofocal intraocular
lens.
16. A lens system for an eye, the eye having an in-focus visual
acuity and a depth of focus, the lens system comprising: a first
lens having a first optical axis; and a second lens adjacent the
first lens, the second lens having a second optical axis being
non-aligned with the first optical axis, the first lens and second
lens together configured to introduce at least some asymmetric
aberration to the eye to extend the depth of focus while
maintaining the in-focus visual acuity.
17. A method for modifying a depth of focus of an eye, the method
comprising the steps of: measuring a wavefront aberration of the
eye; determining an in-focus visual acuity of the eye; and
determining an asymmetric aberration to be induced in the wavefront
aberration of the eye, the depth of focus being extended by the
asymmetric aberration when induced in the wavefront aberration and
while maintaining the in-focus visual acuity.
18. The method of claim 17, further comprising inducing the
asymmetric aberration in the wavefront aberration.
19. The method of claim 18, wherein the step of inducing is
selected from a group consisting of: implanting an intraocular lens
configured to introduce the asymmetric aberration to the eye while
maintaining the in-focus visual acuity; positioning an ophthalmic
lens adjacent the eye, the ophthalmic lens configured to introduce
the asymmetric aberration to the eye while maintaining the in-focus
visual acuity; photoaltering a cornea of the eye to introduce the
asymmetric aberration to the eye while maintaining the in-focus
visual acuity; implanting a corneal tissue portion in the cornea of
the eye, the corneal tissue configured to introduce the asymmetric
aberration to the eye while maintaining the in-focus visual acuity;
inserting an intracorneal implant in the cornea of the eye, the
intracorneal implant configured to introduce the asymmetric
aberration to the eye while maintaining the in-focus visual
acuity.
20. The method of claim 17, wherein the step of determining an
asymmetric aberration comprises: selecting an asymmetric aberration
type from a group consisting of an asymmetric astigmatism, a higher
order astigmatism, a vertical coma, a lateral coma, and a trefoil;
and determining an amount of the asymmetric aberration type to
extend the depth of focus while maintaining the in-focus visual
acuity.
21. The method of claim 17, further comprising forming an
ophthalmic lens configured to introduce the asymmetric
aberration.
22. The method of claim 21, wherein the forming step comprises one
of a group consisting of: lathe-cutting an ophthalmic lens having
first and second opposing surfaces, the first and second opposing
surfaces together configured to introduce the asymmetric aberration
to the eye; and molding an ophthalmic lens having first and second
opposing surfaces, the first and second opposing surfaces together
configured to introduce the asymmetric aberration to the eye.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/029,284, filed Feb. 15, 2008.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to ophthalmic lenses
and more specifically to intraocular lenses having an extended
depth of focus.
[0004] 2. Background
[0005] Intraocular lenses (IOLs) are commonly used to replace the
natural lens of the eye under cataract conditions. Alternatively,
the natural lens may be replaced to correct other visual
conditions, for example, to provide accommodation or
pseudo-accommodation in the event a subject develops presbyopia and
has diminished focusing capability on both distant objects and near
objects. "Accommodation" is the ability of the eye to change focus
from near to far, far to near, and all distances in between. As
presbyopia progresses, accommodation ability generally decreases.
For example, with presbyopia, which usually begins at around age
40, the lens becomes less flexible. As the ciliary muscle contracts
to move the lens forward, the lens typically resists due to
presbyopia. Accommodating and/or multifocal intraocular lenses may
be used to restore at least some degree of accommodative or
pseudo-accommodative ability.
[0006] Accommodating intraocular lenses (AIOLs) are generally
configured to focus on objects over a range of distances typically
by moving axially and/or by changing shape in response to an ocular
force produced by the ciliary muscle, zonules, and/or capsular bag
of the eye. Current accommodating intraocular lenses are capable of
providing about 0.5 diopter of objective accommodation. Multifocal
intraocular lenses (MFIOLs) provide a pseudo-accommodation by
simultaneously providing two or more foci, for example, one to
provide distant vision and the other to provide near vision. This
pseudo-accommodation may have some trade-off, such as dysphotopsia
(e.g., halos or glare), reduced contrast sensitivity due to the
continual presence of defocused light, reduced intermediate vision,
pupil dependent performance, or the like. Over time, patients with
multifocal intraocular lenses generally select the focus that
provides the sharper image and ignore other blurred images.
[0007] Another approach to providing some degree of simulated
accommodation is by extending the depth of focus of a traditional
monofocal lens so that objects over a broader range of distances
are simultaneously resolved. This approach also has some trade-off
with reduced contrast sensitivity. Examples of this approach are
discussed in U.S. Pat. Nos. 6,126,286, 6,923,539, and
7,061,692.
[0008] An intraocular lens is needed that extends the depth of
focus of an eye while minimizing the occurrence of one or more
factors reducing the optical performance of the eye, such as
dysphotopsia, reduced contrast sensitivity, reduced intermediate
vision, pupil dependent performance, or the like. More
particularly, an intraocular lens is needed that extends the depth
of focus of an eye without significantly reducing the in-focus
visual acuity of the eye and while minimizing the occurrence of one
or more factors reducing the optical performance of the eye, such
as dysphotopsia, reduced contrast sensitivity, reduced intermediate
vision, pupil dependent performance, or the like. Further, systems
and methods for extending the depth of focus of the eye while
minimizing the occurrence of one or more factors reducing the
optical performance of the eye are needed.
SUMMARY OF THE INVENTION
[0009] The present invention is generally directed to ophthalmic
devices, systems, and methods for extending the depth of focus of a
subject's vision by introducing at least some higher order
asymmetric aberration in the eye. The ophthalmic device may be an
intraocular lens, a contact lens, a corneal inlay or onlay, a pair
of spectacles, or the like. In some embodiments, the ophthalmic
device may be a part of the structure of the natural eye, for
example, the resulting corneal surface following a refractive
procedure, such as a LASIK or PRK procedure. Embodiments of the
present invention may find particular use in ophthalmic devices
having a multifocal element (e.g., a diffractive or refractive lens
producing two or more foci or images) or having accommodative
capabilities.
[0010] In one aspect of the present invention, a lens for
ophthalmic use, such as an intraocular lens, is provided. The lens
includes an optic having a clear aperture disposed about a central
axis. The optic includes a first surface and an opposing second
surface. The first and second surfaces are together configured to
introduce at least some asymmetric aberration in the eye to
increase the depth of focus while maintaining the in-focus visual
acuity of the eye. Maintaining in-focus visual acuity is referred
to herein as having essentially the same letter acuity or reading
acuity and/or having an identical functional acuity, which is
regarded as normal for a particular age group, and which does not
limit the functional vision. Maintaining in-focus visual acuity
specifically excludes super-acuity, that is, acuity that
significantly exceeds the acuity associated with normal 20/20
vision. In one embodiment, the ophthalmic lens introduces some
degree of coma, or other higher order asymmetric aberration, in the
eye while maintaining in-focus visual acuity of the eye
[0011] In another embodiment, a lens system for an eye is provided,
and the lens system includes a first lens having a first optical
axis and a second lens adjacent the first lens. The second lens has
a second optical axis being non-aligned with the first optical
axis. The first lens and second lens are together configured to
introduce at least some asymmetric aberration to the eye to extend
the depth of focus while maintaining the in-focus visual acuity of
the eye.
[0012] In another embodiment, a method is provided for modifying a
depth of focus of an eye. The method includes measuring a wavefront
aberration of the eye, determining an in-focus visual acuity of the
eye, and determining an asymmetric aberration to be induced in the
wavefront aberration of the eye. The depth of focus is extended by
the asymmetric aberration when induced in the wavefront aberration
and while maintaining the in-focus visual acuity.
[0013] In other embodiments, the present invention may be used in
concert with a multifocal intraocular lens to extend all of the
focal points thereof, with an accommodating intraocular lens to
extend the functional range of vision available to the patient,
with other extended depth of focus techniques, with targeted
correction of other higher-order aberrations, with chromatic
aberration correction, and the like.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Embodiments of the present invention may be better
understood from the following detailed description when read in
conjunction with the accompanying drawings. Such embodiments, which
are for illustrative purposes only, depict the novel and
non-obvious aspects of the invention. The drawings include the
following figures, with like numerals indicating like parts:
[0015] FIG. 1 is a schematic drawing of a human eye after
implantation with an intraocular lens;
[0016] FIG. 2 is a schematic drawing of a thin lens model that
approximates the human eye of FIG. 1;
[0017] FIG. 3 is a plot of defocus versus minimum readable letter
size, for a variety of aberration corrections, for a first
subject;
[0018] FIG. 4 is a plot of defocus versus minimum readable letter
size, for a variety of aberration corrections, for a second
subject;
[0019] FIG. 5 is a plot of defocus versus minimum readable letter
size, for a variety of aberration corrections, for a third
subject;
[0020] FIG. 6 is a plot of depth of focus versus the variety of
aberration corrections shown in FIGS. 3-5, for each of the
subjects;
[0021] FIG. 7 is a graph of the depth of focus versus the variety
of aberration corrections shown in FIGS. 3-5 illustrating the
average focus depth for each of the variety of aberration
corrections;
[0022] FIG. 8 is a graph of the minimum readable letter size versus
the variety of aberration corrections shown in FIGS. 3-5
illustrating the minimum readable letter size for each of the
variety of aberration corrections;
[0023] FIG. 9 is a Modulation Transfer Function (MTF) illustrating
an MTF volume in one embodiment;
[0024] FIG. 10 illustrates comparisons of inverse letter size
versus defocus for various aberration correction types of the first
subject to Modulation Transfer Function volume versus defocus for
the respective aberration correction types of the first
subject;
[0025] FIG. 11 illustrates comparisons of inverse letter size
versus defocus for various aberration correction types of the
second subject to Modulation Transfer Function volume versus
defocus for the respective aberration correction types of the
second subject;
[0026] FIG. 12 illustrates comparisons is a plot of inverse letter
size versus defocus illustrating depth of focus determination at a
threshold in one example;
[0027] FIG. 13 is a plot of depth of focus versus the aberration
correction types determined from the pschophysical measurement and
determined from theoretical calculation of MTF volume shown in FIG.
10 of the first subject;
[0028] FIG. 14 is a plot of depth of focus versus the aberration
correction types determined from the pschophysical measurement and
determined from theoretical calculation of MTF volume shown in FIG.
11 of the second subject;
[0029] FIG. 15 is a Modulation Transfer Function illustrating an
MTF area in one embodiment;
[0030] FIG. 16 illustrates comparisons of inverse letter size
versus defocus for various aberration correction types of the first
subject to Modulation Transfer Function area versus defocus for the
respective aberration correction types of the first subject;
[0031] FIG. 17 illustrates comparisons of inverse letter size
versus defocus for various aberration correction types of the
second subject to Modulation Transfer Function area versus defocus
for the respective aberration correction types of the second
subject;
[0032] FIG. 18 is a plot of depth of focus versus the aberration
correction types shown in FIG. 16 of the first subject and a MTF
area threshold of 0.1;
[0033] FIG. 19 is a plot of depth of focus versus the aberration
correction types shown in FIG. 17 of the second subject and a MTF
area threshold of 0.2;
[0034] FIG. 20 is a Modulation Transfer Function illustrating a
threshold frequency in one embodiment;
[0035] FIG. 21 illustrates comparisons of inverse letter size
versus defocus for various aberration correction types of the first
subject to threshold frequency versus defocus for the respective
aberration correction types of the first subject;
[0036] FIG. 22 illustrates comparisons of inverse letter size
versus defocus for various aberration correction types of the
second subject to threshold frequency versus defocus for the
respective aberration correction types of the second subject;
[0037] FIG. 23 is a plot of depth of focus versus the aberration
correction types determined from the pschophysical measurement and
determined from theoretical calculation of the threshold frequency
as shown in FIG. 21 of the first subject;
[0038] FIG. 24 is a plot of depth of focus versus the aberration
correction types determined from the pschophysical measurement and
determined from theoretical calculation of the threshold frequency
as shown in FIG. 22 of the second subject;
[0039] FIG. 25 is a Modulation Transfer Function illustrating a
method for determining a Modulation Transfer threshold that is
determined from MT values calculated for the 10' letter size
(termed .+-.x) in one embodiment.
[0040] FIG. 26 is a plot of depth of focus versus the aberration
correction types determined from the pschophysical measurement and
determined from theoretical calculation of .+-.x of the first
subject;
[0041] FIG. 27 is a plot of depth of focus versus the aberration
correction types shown determined from the pschophysical
measurement and determined from theoretical calculation of .+-.x of
the second subject;
[0042] FIG. 28 is a Modulation Transfer Function illustrating an
MTF volume within a frequency range in one embodiment;
[0043] FIG. 29 illustrates comparisons of inverse letter size
versus defocus for various aberration correction types of the first
subject to MTF volume within a frequency range versus defocus for
the respective aberration correction types of the first
subject;
[0044] FIG. 30 illustrates comparisons of inverse letter size
versus defocus for various aberration correction types of the
second subject to MTF volume within a frequency range versus
defocus for the respective aberration correction types of the
second subject;
[0045] FIG. 31 is a plot of depth of focus versus the aberration
correction types determined from the pschophysical measurement and
determined from theoretical calculation of MTF volume within a
frequency range shown in FIG. 29 of the first subject;
[0046] FIG. 32 is a plot of depth of focus versus the aberration
correction types determined from the pschophysical measurement and
determined from theoretical calculation of MTF volume within a
frequency range shown in FIG. 30 of the second subject;
[0047] FIG. 33 is a plot of measured depth of focus versus the
aberration correction types for the first and second subjects;
[0048] FIG. 34 is a plot comparing depth of focus measured
pschophysically to depth of focus calculated with all theoretical
methods versus the aberration correction types for the first
subject; and
[0049] FIG. 35 is a plot comparing depth of focus measured
pschophysically to depth of focus calculated with all theoretical
methods versus the aberration correction types for the second
subject.
DETAILED DESCRIPTION
[0050] An ophthalmic lens, an ophthalmic system, and a method of
modifying optical characteristics of an eye are provided in
accordance with the present invention. In general, at least some
asymmetric aberration is introduced in the eye to increase the
depth of focus of the eye while maintaining in-focus visual acuity.
Maintaining in-focus visual acuity is referred to herein as having
essentially the same letter acuity or reading acuity or having an
identical functional acuity, which is regarded as normal for a
particular age group, and which does not limit the functional
vision. Maintaining in-focus visual acuity specifically excludes
super-acuity, that is, acuity that significantly exceeds the acuity
associated with normal 20/20 vision.
[0051] In one embodiment, the ophthalmic lens introduces a higher
order asymmetric aberration (e.g., some degree of coma or the like)
in the eye while maintaining the in-focus visual acuity of the eye.
Due to the near-spherical or substantially spherical geometry of
the anterior surface of the cornea, two types of aberrations,
spherical aberration and coma, may occur. The term "coma" is
referred to herein as an optical aberration in which the image of a
point source is generally a comet-shaped figure. Spherical
aberration and coma are similar to one another by inadequately
imaging or focusing rays at the same point. Coma differs from
spherical aberration, however, in that a point object is imaged not
as a circle but as a comet-shaped figure. Nevertheless, both cases
are characterized by a loss of definition at the focal spot. By
inducing an asymmetric aberration (e.g., coma or other higher order
asymmetric aberration) to the wavefront aberration of a
pseudophakic eye, the depth of focus may be increased.
[0052] In one embodiment, an ophthalmic lens with pre-determined
bending factors (e.g., to produce asymmetric aberrations)
introduces asymmetric aberration in the eye while maintaining
in-focus visual acuity. For example, the ophthalmic lens may be
formed with an asymmetric curvature on the anterior side of the
corresponding optic, the posterior side of the corresponding optic,
or a combination of the anterior and posterior side of the optic.
In another embodiment, an ophthalmic lens may be lathe-cut (e.g.,
the surface of the lens may be lathed) to be rotationally
asymmetric. In another embodiment, an ophthalmic lens may be molded
to be rotationally asymmetric. For example, U.S. Pat. No.
5,620,720, the entire disclosure of which is incorporated herein,
discloses a cast molding technique for forming intraocular
lenses.
[0053] In another embodiment, the lens may be mechanically
configured to be tilted or de-centered in the eye (e.g., by a
controlled and pre-determined degree). For example, U.S. Pat. Nos.
5,567,365 and 5,571,177 and U.S. patent application Ser. No.
12/239,462 filed Sep. 26, 2008, to Deacon et al, the entire
disclosures of which are incorporated herein, disclose various
methods for modifying the orientation of an implanted intraocular
lens.
[0054] In another embodiment, an Alvarez lens can be used and
positioned to introduce a pre-determined degree of asymmetric
aberration. For example, U.S. Pat. No. 3,305,294 discloses an
Alvarez lens with lens elements that are movable relative to each
other transversely to the optical axis of the lens and PCT Pub. No.
WO/2006/025726 discloses an Alvarez-type intraocular lens, both of
which are incorporated in entirety herein. In another embodiment, a
dual lens system (e.g., axially positioned with respect to one
another) that is de-centered with respect to one another may be
used.
[0055] Other higher order asymmetrical aberrations may be used to
extend or increase the depth of focus including, but not
necessarily limited to, astigmatism, high-order astigmatism,
vertical coma, lateral coma, trefoil, and the like, and
combinations thereof may also be used. Examples of ophthalmic
lenses include, but are not necessarily limited to, intraocular
lenses, external lenses, contact lenses, intrastromal lens
implants, implantable shaped corneal tissue, and the like.
[0056] Because each individual vision typically has a unique
wavefront characteristic, the ophthalmic lens may similarly have a
variety of configurations to introduce the asymmetric aberration
while maintaining in-focus visual acuity. Detailed information
about the wavefront characteristics associated with the eye (e.g.,
optical aberrations) may be acquired. Examples of such detailed
information include, but are not necessarily limited to, the extent
of a desired refractive correction, the location in the eye
associated with the correction (e.g., where the correction can be
made most effectively), and the like. Wavefront analysis
techniques, made possible by devices such as a Hartmann-Shack type
sensor, can be used to generate maps of refractive power. Other
wavefront analysis techniques and sensors may also be used. The
maps of refractive power, or similar refractive power information
provided by other means, such as corneal topographs or the like,
can then be used to identify and locate the optical aberrations
that require correction.
[0057] The ophthalmic lens may also have multifocal
characteristics. With a multifocal lens embodiment, the introduced
asymmetric aberration preferably extends the depth of focus
associated with all of the focal points of the multifocal lens. In
other embodiments, the introduced asymmetric aberration can extend
the depth of focus in either the near or the far focus position. In
an accommodating lens embodiment, the lens with asymmetric
aberration extends the functional range of vision available to the
patient. Furthermore, the introduction of a pre-determined degree
of asymmetric aberration (e.g., while maintaining in-focus visual
acuity) can be combined with other extended depth of focus
concepts, such as binary phase masks, lenses that utilize
hyperfocality, zonal aspheric lenses, low-add multifocal lenses,
and the like, with targeted correction of other higher-order
aberrations, such as spherical aberration and/or astigmatism (e.g.,
using a toric lens), and/or with chromatic aberration correction
(e.g., using a diffractive monofocal lens).
[0058] Referring to the drawings, a human eye 10 is shown in FIG. 1
after an intraocular lens 1 has been inserted. Light enters (e.g.,
from the left of FIG. 1) and passes through a cornea 14, an
anterior chamber 15, an iris 16, and enters a capsular bag 17.
Prior to insertion, the natural lens (not shown) occupies
essentially the entire interior of the capsular bag 17. After
insertion, the capsular bag 17 may house the intraocular lens 1, in
addition to a fluid that occupies the remaining volume and
equalizes the pressure in the eye 10. The intraocular lens 1 is
preferably constructed to introduce an asymmetric aberration in the
eye 10 without significantly reducing the in-focus visual acuity
thereof. After passing through the intraocular lens 1, light exits
a posterior wall 18 of the capsular bag 17, passes through a
posterior chamber 11, and strikes the retina 12, which detects the
light and converts it to a signal transmitted through the optic
nerve to the brain.
[0059] The intraocular lens 1 has an optic 1a with a refractive
index greater than the surrounding fluid. The optic 1a has an
anterior surface 2 facing away from the retina 12 and a posterior
surface 3 facing toward the retina 12. In this embodiment, the
anterior surface 2 and posterior surface 3 are shaped to induce a
predetermined amount of coma in the eye 10. In one embodiment, the
anterior surface 2 is rotationally asymmetric with respect to the
posterior surface 3. The optic 1a is held in place by a haptic 19,
which couples the optic 1a to the capsular bag 17 after insertion.
In the illustrated embodiment, the optic 1a is suspended within the
capsular bag 17, for example, to allow accommodative movement of
the optic 1a of the intraocular lens 1 along the optical axis, such
as may be found with accommodative intraocular lenses.
Alternatively, the intraocular lens 1 may be disposed adjacent to,
and even biased against, the posterior wall 18, for example, to
reduce cellular growth on the optic 1a. The optic 1a may be either
a monofocal intraocular lens or a multifocal intraocular lens.
[0060] A well-corrected eye typically forms an image at the retina
12. If the lens 1 has too much or too little power, the image
shifts axially along the optical axis away from the retina 12,
toward or away from the lens. The power required to focus on a
close or near object is generally greater than the power required
to focus on a distant or far object. The difference in optical
power between the farthest and nearest object that may be brought
into focus by a particular lens or lens system is typically
referred to as an "add power" (e.g., in the case of a multifocal
intraocular lens) or a "range of accommodation" or "accommodative
range" (e.g., in the case of an accommodating intraocular lens that
responds to ciliary muscle contraction to move axially and/or
deform so as to change the optical power of the optic). A normal
range of add power or accommodation is generally about 4 Diopters
at the plane of the optic 1a, although this number may be as low as
3 or fewer Diopters or as high as 6 or more Diopters based on the
geometry of the eye.
[0061] In many cases, the optical system of the eye may be well
approximated by a thin lens model, shown schematically in FIG. 2.
Such a thin lens system 20 may be used to predict the location of
an image for a given object distance, Z. In addition, the thin lens
system 20 may also be used to predict the power required of a lens
to bring objects at the object distance, Z, into focus on the
retina. This may be used to predict or determine in-focus visual
acuity for a particular optical system or eye.
[0062] A marginal light ray 29 originates at the base of an object
21, where the ray 29 crosses an optical axis 28. The ray 29 passes
through an optional spectacle 22 having a power, .PHI.spectacle,
and enters the eye. The eye itself is represented by a cornea 23
with a power, .PHI.cornea, an aperture stop (or pupil) 24, an
intraocular lens 25 with a power, .PHI.lens, and a retina 26. An
image 27 is formed of the object 21 at the location where the
marginal ray 29 intersects the optical axis 28. If the object 21 is
"in focus," then the image 27 is formed at the retina 26. If the
object is "out of focus," then the image is translated axially away
from the retina 26, being either too close to the lens or too far
from the lens. The space between the object 21 and the cornea 23 is
assumed to be filled with air, having a refractive index of
n.sub.air (e.g., typically 1). The space between the cornea 23 and
the retina 26 is assumed to be filled with a fluid having a
refractive index of n.sub.eye.
[0063] One exemplary figure of merit for tracking the performance
of visual systems is known as a Modulation Transfer Function (MTF).
The MTF generally indicates the ability of an optical system to
reproduce (e.g., transfer) various levels of detail (e.g., spatial
frequencies) from the object to the image. MTF is particularly
desirable as a figure of merit because it may be both predicted by
simulation and approximately measured through the visual response
of real patients.
[0064] The MTF is related to the apparent contrast of alternating
bright and dark bars of an image. If the MTF is 1, then the bright
areas generally appear completely bright, and the dark areas
generally appear completely dark. If the MTF is 0, both areas
appear as gray, with generally little to no distinction between
bright and dark areas. Typical MTF values lie between 0 and 1 with
some light bleeding into the dark areas and some darkness bleeding
into the light areas.
[0065] The MTF has a dependence on spatial frequency, which is
inversely related to the width of the alternating bright and dark
bars in the image. Note that MTF is particularly suited for human
vision testing, in that the spatial frequency may be controlled
during a test by controlling the size of a letter "E," where the
widths of the prongs in the "E" have a prescribed size. MTF is
measured along two orthogonal axes (e.g., an x-axis and a y-axis or
a horizontal axis and a vertical axis).
[0066] Spatial frequency is typically reported in units of line
pairs per mm at the retina. At low spatial frequencies (e.g.,
represented with wider bars), the MTF is generally higher than at
high spatial frequencies (e.g., represented with narrower bars).
For frequencies greater than a predetermined cutoff spatial
frequency, the MTF is 0. This is a property governed by the physics
of diffraction. MTF may be calculated in a straightforward
numerical manner, either by a ray-tracing program such as Oslo or
Zemax, by another existing simulation tool, or by self-written
code, all of which provide generally equivalent results with
varying degrees of sophistication.
[0067] FIG. 3 is a plot of minimum readable letter size versus
defocus, for a variety of aberration corrections, for a first
subject (SM). FIG. 4 is a plot of defocus versus minimum readable
letter size, for a variety of aberration corrections, for a second
subject (EV). FIG. 5 is a plot of defocus versus minimum readable
letter size, for a variety of aberration corrections, for a third
subject (HW). Six cases were used for comparison: case 1 is based
on the naturally occurring higher-order aberrations of the subject
with only lower-order astigmatism and defocus corrected; case 2 is
based on a correction of all aberrations (e.g., no wavefront
aberrations); case 3 is based on a correction of all aberrations
except for a positive spherical aberration (e.g., 0.22 .mu.m); case
4 is based on a correction of all aberrations except for a negative
spherical aberration (e.g., -0.22 .mu.m)); case 5 is based on a
correction of all aberrations except for a coma aberration ((e.g.,
0.22 .mu.m)); and, case 6 is based on a correction of all
aberrations except for an astigmatism aberration (e.g., 0.22
.mu.m). As best shown in FIGS. 3-5, the introduction of coma (e.g.,
case 5) provided the greatest depth of focus for all three
subjects.
[0068] FIG. 6 is a plot of depth of focus versus the variety of
aberration corrections shown in FIGS. 3-5, for each of the
subjects. FIG. 7 is a graph of the depth of focus versus the
variety of aberration corrections shown in FIGS. 3-5 illustrating
the average focus depth for each of the variety of aberration
corrections. FIG. 8 is a graph of the minimum readable letter size
(e.g., in the best-focus position) versus the variety of aberration
corrections shown in FIGS. 3-5 illustrating the minimum letter size
for each of the variety of aberration corrections.
[0069] FIGS. 3-5 illustrate examples of induced aberrations that
increase the depth of focus, while maintaining the in-focus acuity.
in-focus acuity is explicitly shown in FIG. 8. For example, the
cases 1 and 5 in FIG. 8 show the same in-focus acuity (letter
size), while the depth of focus of these cases differ (such as
shown in FIG. 7). Similarly, cases 5 and 6 in FIG. 8 show the same
in-focus acuity (letter size), while the depth of focus of these
cases differ (FIG. 7). As a third example, cases 2 and 3 in FIG. 8
show the same in-focus acuity (e.g., based on letter size), while
the depth of focus of these cases differ (as shown in FIG. 7). This
demonstrates that by adding aberrations and/or changing the
aberrations in the eye, the depth of focus of the eye can be
increased, without changing the in-focus acuity.
[0070] FIG. 9 is a Modulation Transfer Function (MTF) illustrating
an MTF volume in one embodiment. FIG. 10 illustrates comparisons of
inverse letter size versus defocus for various aberration
correction types of the first subject to Modulation Transfer
Function volume versus defocus for the respective aberration
correction types of the first subject. For example, inverse letter
size versus defocus for the first aberration correction type of the
first subject is compared to Modulation Transfer Function volume
versus defocus for the first aberration correction type of the
first subject, inverse letter size versus defocus for the second
aberration correction type of the first subject is compared to
Modulation Transfer Function volume versus defocus for the second
aberration correction type of the first subject, inverse letter
size versus defocus for the third aberration correction type of the
first subject is compared to Modulation Transfer Function volume
versus defocus for the third aberration correction type of the
first subject, inverse letter size versus defocus for the fourth
aberration correction type of the first subject is compared to
Modulation Transfer Function volume versus defocus for the fourth
aberration correction type of the first subject, inverse letter
size versus defocus for the fifth aberration correction type of the
first subject is compared to Modulation Transfer Function volume
versus defocus for the fifth aberration correction type of the
first subject, and inverse letter size versus defocus for the sixth
aberration correction type of the first subject is compared to
Modulation Transfer Function volume versus defocus for the sixth
aberration correction type of the first subject. The peak of the
MTF curves is at zero defocus. In each of the comparisons, a
pschophysical measurement (e.g., "psicoph") is compared with a
theoretical calculation (e.g., "optical").
[0071] FIG. 11 illustrates comparisons of inverse letter size
versus defocus for various aberration correction types of the first
subject to Modulation Transfer Function area versus defocus for the
respective aberration correction types of the first subject. For
example, inverse letter size versus defocus for the first
aberration correction type of the second subject is compared to
Modulation Transfer Function volume versus defocus for the first
aberration correction type of the second subject, inverse letter
size versus defocus for the second aberration correction type of
the second subject is compared to Modulation Transfer Function
volume versus defocus for the second aberration correction type of
the second subject, inverse letter size versus defocus for the
third aberration correction type of the second subject is compared
to Modulation Transfer Function volume versus defocus for the third
aberration correction type of the second subject, inverse letter
size versus defocus for the fourth aberration correction type of
the second subject is compared to Modulation Transfer Function
volume versus defocus for the fourth aberration correction type of
the second subject, inverse letter size versus defocus for the
fifth aberration correction type of the second subject is compared
to Modulation Transfer Function volume versus defocus for the fifth
aberration correction type of the second subject, and inverse
letter size versus defocus for the sixth aberration correction type
of the second subject is compared to Modulation Transfer Function
volume versus defocus for the sixth aberration correction type of
the second subject.
[0072] A depth of focus for a lens may be defined based on any
number of criteria, such as a threshold of any of the MTF curves, a
particular increase in spot size or wavefront error, a particular
decrease in Strehl Ratio, or any other suitable criterion. FIG. 12
is a plot of inverse letter size versus defocus illustrating a
depth of focus determination at a threshold, in one example. There
are many possible definitions of depth of focus that many be used,
as well as many other figures of merit that may be used for the
definitions. For instance, any or all of the following optical
metrics may be used: MTF at a particular spatial frequency, MTF
volume (integrated over a particular range of spatial frequencies,
either in one dimension or in two dimensions), Strehl ratio,
encircled energy, RMS spot size, peak-to-valley spot size, RMS
wavefront error, peak-to-valley wavefront error, and edge
transition width. Given the many possible figures of merit, there
are several ways to evaluate them to define a depth of focus.
[0073] One way is to define an absolute threshold, where the
crossings of the figure of merit with the threshold define the
depth of focus. For instance, the depth of focus may be defined as
the region over which the MTF or MTF volume exceeds a threshold of
0.1. Alternatively, any suitable MTF absolute threshold may be
used, such as 0.15, 0.2, 0.25, 0.3 and so forth. Alternatively, the
depth of focus may be defined as the region over which the RMS spot
size is less than a particular threshold value.
[0074] FIG. 13 is a plot of depth of focus versus the aberration
correction types determined from the pschophysical measurement and
determined from theoretical calculation of MTF volume shown in FIG.
10 of the first subject. FIG. 14 is a plot of depth of focus versus
the aberration correction types determined from the pschophysical
measurement and determined from theoretical calculation of MTF
volume shown in FIG. 11 of the second subject.
[0075] FIG. 15 is a Modulation Transfer Function illustrating an
MTF area in one embodiment. The radial average (e.g., the averaged
curvature at the center of the MTF curve) is used to determine the
MTF area.
[0076] FIG. 16 illustrates comparisons of inverse letter size
versus defocus for various aberration correction types of the first
subject to Modulation Transfer Function area versus defocus for the
respective aberration correction types of the first subject. For
example, inverse letter size versus defocus for the first
aberration correction type of the first subject is compared to
Modulation Transfer Function area versus defocus for the first
aberration correction type of the first subject, inverse letter
size versus defocus for the second aberration correction type of
the first subject is compared to Modulation Transfer Function area
versus defocus for the second aberration correction type of the
first subject, inverse letter size versus defocus for the third
aberration correction type of the first subject is compared to
Modulation Transfer Function area versus defocus for the third
aberration correction type of the first subject, inverse letter
size versus defocus for the fourth aberration correction type of
the first subject is compared to Modulation Transfer Function area
versus defocus for the fourth aberration correction type of the
first subject, inverse letter size versus defocus for the fifth
aberration correction type of the first subject is compared to
Modulation Transfer Function area versus defocus for the fifth
aberration correction type of the first subject, and inverse letter
size versus defocus for the sixth aberration correction type of the
first subject is compared to Modulation Transfer Function area
versus defocus for the sixth aberration correction type of the
first subject.
[0077] FIG. 17 illustrates comparisons of inverse letter size
versus defocus for various aberration correction types of the
second subject to Modulation Transfer Function area versus defocus
for the respective aberration correction types of the second
subject. For example, inverse letter size versus defocus for the
first aberration correction type of the second subject is compared
to Modulation Transfer Function area versus defocus for the first
aberration correction type of the second subject, inverse letter
size versus defocus for the second aberration correction type of
the second subject is compared to Modulation Transfer Function area
versus defocus for the second aberration correction type of the
second subject, inverse letter size versus defocus for the third
aberration correction type of the second subject is compared to
Modulation Transfer Function area versus defocus for the third
aberration correction type of the second subject, inverse letter
size versus defocus for the fourth aberration correction type of
the second subject is compared to Modulation Transfer Function area
versus defocus for the fourth aberration correction type of the
second subject, inverse letter size versus defocus for the fifth
aberration correction type of the second subject is compared to
Modulation Transfer Function area versus defocus for the fifth
aberration correction type of the second subject, and inverse
letter size versus defocus for the sixth aberration correction type
of the second subject is compared to Modulation Transfer Function
area versus defocus for the sixth aberration correction type of the
second subject.
[0078] FIG. 18 is a plot of depth of focus versus the aberration
correction types shown in FIG. 16 of the first subject and a MTF
area threshold of 0.1. FIG. 19 is a plot of depth of focus versus
the aberration correction types shown in FIG. 17 of the second
subject and a MTF area threshold of 0.2.
[0079] FIG. 20 is a Modulation Transfer Function illustrating a
threshold frequency in one embodiment. The radial average is used
to determine the threshold frequency from a threshold MT.
[0080] FIG. 21 illustrates comparisons of inverse letter size
versus defocus for various aberration correction types of the first
subject to threshold frequency versus defocus for the respective
aberration correction types of the first subject. For example,
inverse letter size versus defocus for the first aberration
correction type of the first subject is compared to threshold
frequency versus defocus for the first aberration correction type
of the first subject, inverse letter size versus defocus for the
second aberration correction type of the first subject is compared
to threshold frequency versus defocus for the second aberration
correction type of the first subject, inverse letter size versus
defocus for the third aberration correction type of the first
subject is compared to threshold frequency versus defocus for the
third aberration correction type of the first subject, inverse
letter size versus defocus for the fourth aberration correction
type of the first subject is compared to threshold frequency versus
defocus for the fourth aberration correction type of the first
subject, inverse letter size versus defocus for the fifth
aberration correction type of the first subject is compared to
threshold frequency versus defocus for the fifth aberration
correction type of the first subject, and inverse letter size
versus defocus for the sixth aberration correction type of the
first subject is compared to threshold frequency versus defocus for
the sixth aberration correction type of the first subject.
[0081] FIG. 22 illustrates comparisons of inverse letter size
versus defocus for various aberration correction types of the
second subject to threshold frequency versus defocus for the
respective aberration correction types of the second subject. For
example, inverse letter size versus defocus for the first
aberration correction type of the second subject is compared to
threshold frequency versus defocus for the first aberration
correction type of the second subject, inverse letter size versus
defocus for the second aberration correction type of the second
subject is compared to threshold frequency versus defocus for the
second aberration correction type of the second subject, inverse
letter size versus defocus for the third aberration correction type
of the second subject is compared to threshold frequency versus
defocus for the third aberration correction type of the second
subject, inverse letter size versus defocus for the fourth
aberration correction type of the second subject is compared to
threshold frequency versus defocus for the fourth aberration
correction type of the second subject, inverse letter size versus
defocus for the fifth aberration correction type of the second
subject is compared to threshold frequency versus defocus for the
fifth aberration correction type of the second subject, and inverse
letter size versus defocus for the sixth aberration correction type
of the second subject is compared to threshold frequency versus
defocus for the sixth aberration correction type of the second
subject.
[0082] FIG. 23 is a plot of depth of focus versus the aberration
correction types determined from the pschophysical measurement and
determined from theoretical calculation of the threshold frequency
as shown in FIG. 21 of the first subject. FIG. 24 is a plot of
depth of focus versus the aberration correction types determined
from the pschophysical measurement and determined from theoretical
calculation of the threshold frequency as shown in FIG. 22 of the
second subject.
[0083] FIG. 25 is a Modulation Transfer Function illustrating a
method for determining a Modulation Transfer threshold that is
determined from MT values calculated for the 10' letter size
(termed .+-.x) in one embodiment.
[0084] FIG. 26 is a plot of depth of focus versus the aberration
correction types determined from the pschophysical measurement and
determined from theoretical calculation of .+-.x of the first
subject. FIG. 27 is a plot of depth of focus versus the aberration
correction types shown determined from the pschophysical
measurement and determined from theoretical calculation of .+-.x of
the second subject.
[0085] FIG. 28 is a Modulation Transfer Function illustrating an
MTF volume within a frequency range in one embodiment.
[0086] FIG. 29 illustrates comparisons of inverse letter size
versus defocus for various aberration correction types of the first
subject to MTF volume within a frequency range versus defocus for
the respective aberration correction types of the first subject.
For example, inverse letter size versus defocus for the first
aberration correction type of the first subject is compared to MTF
volume within a frequency range versus defocus for the first
aberration correction type of the first subject, inverse letter
size versus defocus for the second aberration correction type of
the first subject is compared to MTF volume within a frequency
range versus defocus for the second aberration correction type of
the first subject, inverse letter size versus defocus for the third
aberration correction type of the first subject is compared to MTF
volume within a frequency range versus defocus for the third
aberration correction type of the first subject, inverse letter
size versus defocus for the fourth aberration correction type of
the first subject is compared to MTF volume within a frequency
range versus defocus for the fourth aberration correction type of
the first subject, inverse letter size versus defocus for the fifth
aberration correction type of the first subject is compared to MTF
volume within a frequency range versus defocus for the fifth
aberration correction type of the first subject, and inverse letter
size versus defocus for the sixth aberration correction type of the
first subject is compared to MTF volume within a frequency range
versus defocus for the sixth aberration correction type of the
first subject.
[0087] FIG. 30 illustrates comparisons of inverse letter size
versus defocus for various aberration correction types of the
second subject to MTF volume within a frequency range versus
defocus for the respective aberration correction types of the
second subject. For example, inverse letter size versus defocus for
the first aberration correction type of the second subject is
compared to MTF volume within a frequency range versus defocus for
the first aberration correction type of the second subject, inverse
letter size versus defocus for the second aberration correction
type of the second subject is compared to MTF volume within a
frequency range versus defocus for the second aberration correction
type of the second subject, inverse letter size versus defocus for
the third aberration correction type of the second subject is
compared to MTF volume within a frequency range versus defocus for
the third aberration correction type of the second subject, inverse
letter size versus defocus for the fourth aberration correction
type of the second subject is compared to MTF volume within a
frequency range versus defocus for the fourth aberration correction
type of the second subject, inverse letter size versus defocus for
the fifth aberration correction type of the second subject is
compared to MTF volume within a frequency range versus defocus for
the fifth aberration correction type of the second subject, and
inverse letter size versus defocus for the sixth aberration
correction type of the second subject is compared to MTF volume
within a frequency range versus defocus for the sixth aberration
correction type of the second subject.
[0088] FIG. 31 is a plot of depth of focus versus the aberration
correction types determined from the pschophysical measurement and
determined from theoretical calculation of MTF volume within a
frequency range shown in FIG. 29 of the first subject. FIG. 32 is a
plot of depth of focus versus the aberration correction types
determined from the pschophysical measurement and determined from
theoretical calculation of MTF volume within a frequency range
shown in FIG. 30 of the second subject.
[0089] FIG. 33 is a plot of measured depth of focus versus the
aberration correction types for the first and second subjects.
[0090] FIG. 34 is a plot comparing depth of focus measured
pschophysically to depth of focus calculated with all theoretical
methods versus the aberration correction types for the first
subject. FIG. 35 is a plot comparing depth of focus measured
pschophysically to depth of focus calculated with all theoretical
methods versus the aberration correction types for the second
subject.
[0091] In some embodiments, other ophthalmic devices and designs
may additionally be incorporated to extend the depth of focus of
monofocal, multifocal, or even accommodating intraocular lenses.
Such ophthalmic devices and designs include, but are not limited
to, those disclosed in U.S. Pat. No. 6,126,286 (Portney) and U.S.
Pat. No. 6,923,539 (Simpson et al.), and U.S. Patent Application
Number 20060116763A1 (Simpson), all of which are herein
incorporated by reference in their entirety. In certain
embodiments, the surface profile may initially have something
similar to those taught in U.S. Pat. No. 6,126,286 or U.S. Pat. No.
6,923,539, or U.S. Pub. No. 20060116763A1. This may be used in
combination with the introduction of asymmetric aberration to
provide both an extended depth of focus and a predetermined visual
acuity performance.
[0092] In some embodiments, an extended or expanded depth of focus
is provided by an ophthalmic lens or optic comprising a
phase-affecting, non-diffractive mask to increase the depth of
focus of an ophthalmic lens. In such embodiments, the ophthalmic
lens may include one or more spatially low frequency phase
transitions, for example, as disclosed in U.S. Pat. No. 7,061,693,
which is herein incorporated by reference in its entirety. Such a
non-diffractive mask may be used in combination with at least one
of the surfaces 2, 3, either on the same or an opposite surface to
provide an optic that provides an extended depth of focus with a
predetermined optical performance or visual acuity
characteristic.
[0093] Analysis and storage of the wavefront characteristics of the
eye as well as the evaluation, determination, and implementation of
asymmetric aberration inducement (i.e., for extending the depth of
focus) may be maintained by a control system including computer
hardware and/or software, often including one or more programmable
processing units operable to execute machine readable program
instructions or code for implementing some or all of one or more of
the methods described herein. The code is often embodied in a
tangible media such as a memory (optionally a read only memory, a
random access memory, a non-volatile memory, or the like) and/or a
recording media (such as a floppy disk, a hard drive, a CD, a DVD,
a memory stick, or the like). The code and/or associated data and
signals may also be transmitted to or from the control system via a
network connection (such as a wireless network, an Ethernet, an
internet, an intranet, or the like) to the system, and some or all
of the code may also be transmitted between components of the
system and/or within the system via one or more bus, and
appropriate standard or proprietary communications cards,
connectors, cables, and the like will often be included in the
system. The system is often configured to perform the calculations
and signal transmission steps described herein at least in part by
programming with the software code, which may be written as a
single program, a series of separate subroutines or related
programs, or the like. Standard or proprietary digital and/or
analog signal processing hardware, software, and/or firmware may be
utilized, and will typically have sufficient processing power to
perform the calculations described herein during treatment of the
patient. The system optionally includes a personal computer, a
notebook computer, a tablet computer, a proprietary processing
unit, or a combination thereof. Standard or proprietary input
devices (such as a mouse, keyboard, touchscreen, joystick, etc.)
and output devices (such as a printer, speakers, display, etc.)
associated with computer systems may also be included, and
processors having a plurality of processing units (or even separate
computers) may be employed in a wide range of centralized or
distributed data processing architectures.
[0094] The description of the invention and its applications as set
forth herein is illustrative and is not intended to limit the scope
of the invention. Variations and modifications of the embodiments
disclosed herein are possible, and practical alternatives to and
equivalents of the various elements of the embodiments would be
understood to those of ordinary skill in the art upon study of this
patent document. These and other variations and modifications of
the embodiments disclosed herein may be made without departing from
the scope and spirit of the invention. The invention, therefore, is
not to be restricted except in the spirit of the following
claims
* * * * *