U.S. patent application number 16/922858 was filed with the patent office on 2020-10-22 for holographic waveguide eye tracker.
This patent application is currently assigned to DigiLens Inc.. The applicant listed for this patent is DigiLens Inc.. Invention is credited to Alastair John Grant, Milan Momcilo Popovich, Jonathan David Waldern.
Application Number | 20200333606 16/922858 |
Document ID | / |
Family ID | 1000004942756 |
Filed Date | 2020-10-22 |
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United States Patent
Application |
20200333606 |
Kind Code |
A1 |
Popovich; Milan Momcilo ; et
al. |
October 22, 2020 |
Holographic Waveguide Eye Tracker
Abstract
An eye tracker having a first waveguide for propagating
illumination light along a first waveguide path and propagating
image light reflected from at least one surface of an eye along a
second waveguide path. At least one grating lamina for deflecting
the illumination light out of the first waveguide path towards the
eye and deflecting the image light into the second waveguide path
towards a detector is disposed adjacent an optical surface of the
waveguide.
Inventors: |
Popovich; Milan Momcilo;
(Leicester, GB) ; Waldern; Jonathan David; (Los
Altos Hills, CA) ; Grant; Alastair John; (San Jose,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DigiLens Inc. |
Sunnyvale |
CA |
US |
|
|
Assignee: |
DigiLens Inc.
Sunnyvale
CA
|
Family ID: |
1000004942756 |
Appl. No.: |
16/922858 |
Filed: |
July 7, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16277390 |
Feb 15, 2019 |
10712571 |
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16922858 |
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14903249 |
Jan 6, 2016 |
10209517 |
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PCT/GB2014/000197 |
May 19, 2014 |
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16277390 |
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61855625 |
May 20, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 27/0093 20130101;
G06K 9/6256 20130101; G02B 6/34 20130101; G02B 2027/014 20130101;
G02F 1/3132 20130101; A61B 3/113 20130101; G02B 27/0172 20130101;
G02F 1/292 20130101; G02B 27/017 20130101; G02F 2001/311 20130101;
G02B 2027/0187 20130101; G02B 6/4206 20130101; G02B 2027/0174
20130101; G06K 9/00604 20130101 |
International
Class: |
G02B 27/01 20060101
G02B027/01; G02B 27/00 20060101 G02B027/00; A61B 3/113 20060101
A61B003/113; G02B 6/34 20060101 G02B006/34; G02B 6/42 20060101
G02B006/42; G02F 1/29 20060101 G02F001/29; G02F 1/313 20060101
G02F001/313; G06K 9/00 20060101 G06K009/00; G06K 9/62 20060101
G06K009/62 |
Claims
1. An apparatus for tracking the movement of an eye, comprising: a
first waveguide; a light source for illuminating said eye; at least
one detector optically coupled to said waveguide; and disposed
within said waveguide, at least one grating layer comprising a
first plurality of grating elements with a first prescription for
diffracting a first characteristic of light reflected from said eye
into a first total internal reflection (TIR) path to said detector
and a second plurality of grating elements with a second
prescription for diffracting a second characteristic of light
reflected from said eye into a second TIR path to said
detector,
2. The apparatus of claim 1, wherein said first characteristic
provides an image of said eye at said detector.
3. The apparatus of claim 1, wherein said second characteristic
provides an optical signature formed by at least one of reflection
or scatter from at least one of an optical surface or ocular medium
of said eye at said detector.
4. The apparatus of claim 3, wherein said signature is one of
speckle or glint.
5. The apparatus of claim 1, wherein said first waveguide contains
a grating or prism for coupling illumination from said source into
an illumination TIR path in said first waveguide and a grating for
coupling light out of said illumination TIR path onto said eye.
6. The apparatus of claim 1, further comprising a second waveguide
supporting a grating or prism for coupling illumination from said
source into an illumination TIR path in said second waveguide and a
grating for coupling light out of said illumination TIR path onto
said eye.
7. The apparatus of claim 1, wherein at least one of said first
plurality of grating elements or said second plurality of grating
elements comprises at least one switchable grating element having a
diffracting state and a non-diffracting state.
8. The apparatus of claim 1, wherein at least one of said first
plurality of grating elements or said second plurality of grating
elements comprises at least one switchable grating element having a
diffracting state and a non-diffracting state, wherein said element
in said diffracting state deflects light reflected from said eye
into at least one of said first or second TIR paths.
9. The apparatus of claim 1, wherein at least one of said first
plurality of grating elements or said second plurality of grating
elements comprises an array of elongate elements.
10. The apparatus of claim 1, wherein at least one of said first
plurality of grating elements or said second plurality of grating
elements is a two-dimensional array.
11. The apparatus of claim 1, wherein said grating layer comprises
at least one of a switchable Bragg grating, a switchable grating
recorded in a reverse mode holographic polymer dispersed liquid
crystal, a switchable grating recorded in a reverse mode
holographic polymer dispersed liquid crystal, a surface relief
grating or a non-switching Bragg grating.
12. The apparatus of claim 1, wherein said eye surface is at least
one of a cornea, front or rear lens surface, iris, sclera or
retina.
13. The apparatus of claim 1, wherein said detector is a
two-dimensional array.
14. The apparatus of claim 1, further comprising at least one
grating with at least one of optical power or diffusing
properties.
15. The apparatus of claim 1, wherein said detector is connected to
at least one of an image processing apparatus for determining at
least one spatio-temporal characteristic of an eye movement or an
image processing system including a neural network.
16. The apparatus of claim 1, wherein said detector is coupled to
said waveguide by a grating or a prism.
17. The apparatus of claim 1, integrated within a HMD or HUD.
18. The apparatus of claim 1, further comprising a turning
grating.
19. The apparatus of claim 1, wherein said source is a laser or a
light emitting diode.
20. The apparatus of claim 1, wherein said at least one detector
comprises a first detector for recording said first characteristic
of light and a second detector for recording said second
characteristic of light.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This present application is a continuation of U.S. patent
application Ser. No. 16/277,390, entitled "Holographic Waveguide
Eye Tracker" to Popovich et al., filed Feb. 15, 2019 and issued on
Jul. 14, 2020 as U.S. Pat. No. 10,712,571, which is a continuation
of U.S. patent application Ser. No. 14/903,249, entitled
"Holographic Waveguide Eye Tracker" to Popovich et al., filed Jan.
6, 2016 and issued on Feb. 19, 2019 as U.S. Pat. No. 10,209,517,
which is a U.S. National Phase of PCT Application No.
PCT/GB2014/000197, entitled "Holographic Waveguide Eye Tracker" to
Popovich et al, filed May 19, 2014, which claims the benefit of
U.S. Provisional Patent Application No. 61/855,625, entitled
"Apparatus for Eye Tracking" to Popovich et al., filed May 20,
2013, the disclosures of which are incorporated herein by reference
in their entireties.
BACKGROUND
[0002] This invention relates to an eye tracking sensor, and more
particularly to an eye tracker using electrically switchable
gratings.
[0003] Eye tracking is important in Head Mounted Displays (HMDs)
because it can extend the ability of the user to designate targets
well beyond the head mobility limits. Eye tracking technology based
on projecting IR light into the users eye and utilizing the primary
Purkinje reflections and the pupil-masked retina reflection have
been around since the 1980's. The method tracks the relative motion
of these images in order to establish a vector characterizing the
point of regard. Most eye trackers have employed flat beam
splitters in front of the users' eyes and relatively large optics
to image the reflections onto a sensor (generally a CCD or CMOS
camera).
[0004] There is much prior art in the patent and scientific
literature including the following United States filings:
[0005] 1. United Stated Patent Application Publication No.
US2011019874(A1) by Levola et al entitled DEVICE AND METHOD FOR
DETERMINING GAZE DIRECTION;
[0006] 2. U.S. Pat. No. 5,410,376 by Cornsweet entitled Eye
tracking method and apparatus;
[0007] 3. U.S. Pat. No. 3,804,496 by Crane et al entitled TWO
DIMENSIONAL EYE TRACKER AND METHOD FOR TRACKING AN EYE TWO
DIMENSIONAL EYE TRACKER AND METHOD FOR TRACKING AN EYE;
[0008] 4. U.S. Pat. No. 4,852,988 by Velez et al entitled Visor and
camera providing a parallax-free field-of-view image for a
head-mounted eye movement measurement system;
[0009] 5. U.S. Pat. No. 7,542,210 by Chirieleison entitled EYE
TRACKING HEAD MOUNTED DISPLAY;
[0010] 6. United Stated Patent Application Publication No. US
2002/0167462 A1 by Lewis entitled PERSONAL DISPLAY WITH VISION
TRACKING; and
[0011] 7. U.S. Pat. No. 4,028,725 by Lewis entitled HIGH RESOLUTION
VISION SYSTEM.
[0012] The exit pupil of these trackers is generally limited by
either the size of the beamsplitter or the first lens of the
imaging optics. In order to maximize the exit pupil, the imaging
optics are positioned close to the beamsplitter, and represent a
vision obscuration and a safety hazard. Another known limitation
with eye trackers is the field of view, which is generally limited
by the illumination scheme in combination with the geometry of the
reflected images off the cornea. The cornea is an aspheric shape of
smaller radius that the eye-ball. The cornea reflection tracks
fairly well with angular motion until the reflected image falls off
the edge of the cornea and onto the sclera. The need for beam
splitters and refractive lenses in conventional eye trackers
results in a bulky component that is difficult to integrate into a
(HMD). The present invention addresses the need for a slim, wide
field of view, large exit pupil, high-transparency eye tracker for
HMDs.
[0013] The inventors have found that diffractive optical elements
offer a route to providing compact, transparent, wide field of view
eye trackers. One important class of diffractive optical elements
is based on Switchable Bragg Gratings (SBGs). SBGs are fabricated
by first placing a thin film of a mixture of photopolymerizable
monomers and liquid crystal material between parallel glass plates.
One or both glass plates support electrodes, typically transparent
indium tin oxide films, for applying an electric field across the
film. A volume phase grating is then recorded by illuminating the
liquid material (often referred to as the syrup) with two mutually
coherent laser beams, which interfere to form a slanted fringe
grating structure. During the recording process, the monomers
polymerize and the mixture undergoes a phase separation, creating
regions densely populated by liquid crystal micro-droplets,
interspersed with regions of clear polymer. The alternating liquid
crystal-rich and liquid crystal-depleted regions form the fringe
planes of the grating. The resulting volume phase grating can
exhibit very high diffraction efficiency, which may be controlled
by the magnitude of the electric field applied across the film.
When an electric field is applied to the grating via transparent
electrodes, the natural orientation of the LC droplets is changed
causing the refractive index modulation of the fringes to reduce
and the hologram diffraction efficiency to drop to very low levels.
Note that the diffraction efficiency of the device can be adjusted,
by means of the applied voltage, over a continuous range. The
device exhibits near 100% efficiency with no voltage applied and
essentially zero efficiency with a sufficiently high voltage
applied. In certain types of HPDLC devices magnetic fields may be
used to control the LC orientation. In certain types of HPDLC phase
separation of the LC material from the polymer may be accomplished
to such a degree that no discernible droplet structure results.
[0014] SBGs may be used to provide transmission or reflection
gratings for free space applications. SBGs may be implemented as
waveguide devices in which the HPDLC forms either the waveguide
core or an evanescently coupled layer in proximity to the
waveguide. In one particular configuration to be referred to here
as Substrate Guided Optics (SGO) the parallel glass plates used to
form the HPDLC cell provide a total internal reflection (TIR) light
guiding structure. Light is "coupled" out of the SBG when the
switchable grating diffracts the light at an angle beyond the TIR
condition. SGOs are currently of interest in a range of display and
sensor applications. Although much of the earlier work on HPDLC has
been directed at reflection holograms transmission devices are
proving to be much more versatile as optical system building
blocks. Typically, the HPDLC used in SBGs comprise liquid crystal
(LC), monomers, photoinitiator dyes, and coinitiators. The mixture
frequently includes a surfactant. The patent and scientific
literature contains many examples of material systems and processes
that may be used to fabricate SBGs. Two fundamental patents are:
U.S. Pat. No. 5,942,157 by Sutherland, and U.S. Pat. No. 5,751,452
by Tanaka et al. Both filings describe monomer and liquid crystal
material combinations suitable for fabricating SBG devices.
[0015] One of the known attributes of transmission SBGs is that the
LC molecules tend to align normal to the grating fringe planes. The
effect of the LC molecule alignment is that transmission SBGs
efficiently diffract P polarized light (ie light with the
polarization vector in the plane of incidence) but have nearly zero
diffraction efficiency for S polarized light (ie light with the
polarization vector normal to the plane of incidence. Transmission
SBGs may not be used at near-grazing incidence as the diffraction
efficiency of any grating for P polarization falls to zero when the
included angle between the incident and reflected light is
small.
[0016] There is a requirement for a compact, lightweight eye
tracker with a large field of view, and a high degree of
transparency to external light.
SUMMARY
[0017] It is a first object of the invention to provide a compact,
lightweight eye tracker with a large field of view, and a high
degree of transparency to external light.
[0018] It is a second object of the invention to provide a compact,
lightweight eye tracker with a large field of view, and a high
degree of transparency to external light implemented in a thin
optical waveguide.
[0019] The objects of the invention are achieved in one embodiment
of the invention in which there is provided an eye tracker
comprising: a first waveguide for propagating illumination light
along a first waveguide path and propagating image light reflected
from at least one surface of an eye along a second waveguide path;
a source of the illumination light optically coupled to the
waveguide and a detector optically coupled to the waveguide. At
least one grating lamina for deflecting the illumination light out
of the first waveguide path towards the eye and deflecting the
image light into the second waveguide path towards the detector is
disposed adjacent an optical surface of the waveguide. The optical
surface of the waveguide is at least one of an internal surface or
an external surface of the waveguide.
[0020] In one embodiment the grating lamina comprises an output
grating for deflecting the illumination light out of the first
waveguide path towards eye and an input grating for deflecting the
image light into the second waveguide path towards the
detector.
[0021] In one embodiment at least one of the input and output
gratings comprises at least one switchable grating element having a
diffracting state and a non diffracting state.
[0022] In one embodiment the grating lamina comprises at least one
switchable grating element having a diffracting state and a non
diffracting state. An element in the diffracting state deflects
illumination light out of the first waveguide path towards the eye
and deflects the image light into the second waveguide path towards
the detector.
[0023] In one embodiment the switchable grating elements are
elongate with longer dimension aligned perpendicular to at least
one of the first and second waveguide paths.
[0024] In one embodiment the first and second waveguide paths are
parallel.
[0025] In one embodiment the grating lamina further comprises at
least one of: an input grating for deflecting illumination light
from the source into the first waveguide path; and an output
grating for deflecting image light out of the second waveguide path
towards the detector.
[0026] In one embodiment the grating lamina further comprises a
second waveguide containing a linear array of switchable grating
elements optically coupled to the detector and overlaying the
output grating. Each element when in its diffracting state samples
a portion of the light in the first waveguide and deflects it along
the second waveguide towards the detector.
[0027] In one embodiment the grating lamina further comprises a
third waveguide containing a linear array of switchable grating
elements optically coupled to the light source and overlaying the
input grating. Each element when in its diffracting state deflects
light from the third waveguide into the first waveguide.
[0028] In one embodiment the output grating abuts an upper or lower
edge of the output grating along the first waveguide path.
[0029] In one embodiment the output grating comprises upper and
lower gratings disposed adjacent upper and lower edges of the
output grating along the first waveguide path.
[0030] In one embodiment the input grating comprises a first array
of switchable elongate beam deflection grating elements and an
overlapping second array of switchable elongate beam deflection
grating elements. The elements of the first and second arrays are
disposed with their longer dimensions orthogonal.
[0031] In one embodiment at least one of the input and output
gratings is a linear array of elongate switchable beam deflection
elements with longer dimension aligned perpendicular to the first
and second waveguide paths.
[0032] In one embodiment the grating lamina is one of a switchable
Bragg grating, a switchable grating recorded in a reverse mode
holographic polymer dispersed liquid crystal, a surface relief
grating, and a non switching Bragg grating.
[0033] In one embodiment the image light has the characteristics of
a speckle pattern
[0034] In one embodiment the eye surface being tracked is at least
one of the cornea, lens, iris, sclera and retina.
[0035] In one embodiment the detector is a two dimensional
array.
[0036] In one embodiment the at least one grating lamina encodes at
least one of optical power and diffusing properties.
[0037] In one embodiment the detector is connected to an image
processing apparatus for determining at least one spatio-temporal
characteristic of an eye movement.
[0038] In one embodiment an eye tracker comprises: a waveguide for
propagating illumination light reflected from at least one surface
of an eye along a waveguide path; a source of the illumination
light; a detector optically coupled to the waveguide. The waveguide
contains at least one grating lamina for deflecting illumination
light reflected of an eye surface into the second waveguide path
towards the detector.
[0039] In one embodiment the detector is connected to an image
processing apparatus for determining at least one spatio-temporal
characteristic of an eye movement.
[0040] In one embodiment the image light is a Purkinje
reflection.
[0041] In one embodiment the source is a laser.
[0042] In one embodiment the source is a light emitting diode.
[0043] In one embodiment the illumination grating provides
collimated light.
[0044] In one embodiment the illumination grating provides
divergent light.
[0045] In one embodiment the input grating encodes optical
power.
[0046] In one embodiment the output grating encodes optical
power.
[0047] In one embodiment at least one of the grating lamina
includes at least one turning grating.
[0048] In one embodiment the eye tracker image processing system
includes a neural network.
[0049] A more complete understanding of the invention can be
obtained by considering the following detailed description in
conjunction with the accompanying drawings, wherein like index
numerals indicate like parts. For purposes of clarity, details
relating to technical material that is known in the technical
fields related to the invention have not been described in
detail
BRIEF DESCRIPTION OF THE DRAWINGS
[0050] FIG. 1A is a schematic plan view of an eye tracker shown in
relation to a human eye in one embodiment of the invention
[0051] FIG. 1B is a schematic front elevation view showing elongate
grating elements used in the imaging grating in one embodiment of
the invention.
[0052] FIG. 1C is a schematic front elevation view showing a two
dimensional array of grating elements used in the imaging grating
in one embodiment of the invention.
[0053] FIG. 2 is a schematic plan view of the eye tracker shown the
imaging and illumination gratings and input and output gratings in
one embodiment of the invention.
[0054] FIG. 3 is a schematic plan view of the eye tracker shown the
imaging and illumination gratings and input and output gratings in
one embodiment of the invention.
[0055] FIG. 4 is a plan view of the eye tracker shown the imaging
and illumination gratings and input and output gratings in one
embodiment of the invention.
[0056] FIG. 5 is a schematic cross section view of an illumination
grating used in one embodiment of the invention.
[0057] FIG. 6 is a schematic cross section view of an illumination
grating used in one embodiment of the invention.
[0058] FIG. 7 is a schematic cross section view of a first aspect
of an imaging grating used in one embodiment of the invention.
[0059] FIG. 8 is a schematic cross section view of a second aspect
of an imaging grating used in one embodiment of the invention.
[0060] FIG. 9A is a schematic top elevation view of a first layer
of a two layer imaging grating in one embodiment of the
invention.
[0061] FIG. 9B is a schematic plan view of a first layer of a two
layer imaging grating in one embodiment of the invention.
[0062] FIG. 9C is a schematic side elevation view of a first layer
of a two layer imaging grating in one embodiment of the
invention.
[0063] FIG. 10A is a schematic top elevation view of a second layer
of a two layer imaging grating in one embodiment of the
invention.
[0064] FIG. 10B is a schematic plan view of a second layer of a two
layer imaging grating in one embodiment of the invention.
[0065] FIG. 10C is a schematic side elevation view of a second
layer of a two layer imaging grating in one embodiment of the
invention.
[0066] FIG. 11A is a schematic cross view of the human eye
illustrating the formation of the Purkinje images.
[0067] FIG. 11B is a schematic cross view of the human eye
illustrating reflections from the retina and iris.
[0068] FIG. 12A is a schematic plan view illustrating a first
aspect of the localization of an eye feature by a two layer imaging
grating each layer comprising elongate elements with the elements
of the two gratings at right angle.
[0069] FIG. 12B is a schematic plan view illustrating a second
aspect of the localization of an eye feature by a two layer imaging
grating each layer comprising elongate elements with the elements
of the two gratings at right angle.
[0070] FIG. 13A is a schematic cross of the human eye in a first
rotational state showing a typical speckle pattern formed by the
cornea and retina.
[0071] FIG. 13B is a schematic cross of the human eye in a first
rotational state showing a typical speckle pattern formed by the
cornea and retina.
[0072] FIG. 14 is a schematic front elevation view of a human eye
show showing the directions of motions of speckle patterns produced
by the retina and cornea.
[0073] FIG. 15A is a schematic cross section view of a human eye in
a first rotational state showing reflection from the retina and
cornea.
[0074] FIG. 15B is a schematic cross section view of a human eye in
a second rotational state showing reflection from the retina and
cornea.
[0075] FIG. 16 is a schematic cross section view of an imaging
grating comprising an array of SBG lens elements with focal length
varying across the exit pupil in one embodiment of the
invention.
[0076] FIG. 17A is a schematic cross section view of an imaging
grating comprising an array of variable power lenses in one
embodiment of the invention.
[0077] FIG. 17B is a detail of FIG. 17A showing a variable power
lens comprising a variable index layer and a diffractive element of
fixed focal length.
[0078] FIG. 18 is a schematic illustrate of an imaging grating in
one embodiment of the invention in which the imaging grating
comprises an array of interspersed grating elements having at least
two different prescriptions.
[0079] FIG. 19 is a schematic cross section view of the
illumination grating of an eye tracker using separate illumination
and imaging gratings in one embodiment of the invention.
[0080] FIG. 20 is a schematic plan view the illumination grating of
an eye tracker using separate illumination and imaging gratings in
one embodiment of the invention.
[0081] FIG. 21 is a schematic cross section view of an alternative
illumination grating of an eye tracker using separate illumination
and imaging gratings in one embodiment of the invention.
[0082] FIG. 22A is a schematic plan view of the imaging grating,
the image sampling grating and the detector module of an eye
tracker using separate illumination and imaging gratings in one
embodiment of the invention.
[0083] FIG. 22B is a schematic plan view of image sampling grating
and the detector module of an eye tracker using separate
illumination and imaging gratings in one embodiment of the
invention.
[0084] FIG. 22C is a schematic cross section view of the imaging
grating and the image sampling grating of an eye tracker using
separate illumination and imaging gratings in one embodiment of the
invention.
[0085] FIG. 22D is a schematic cross section view of image sampling
grating and the detector module of an eye tracker using separate
illumination and imaging gratings in one embodiment of the
invention.
[0086] FIG. 22E is a schematic cross section view of the imaging
grating, the image sampling grating and the detector module of an
eye tracker using separate illumination and imaging gratings in one
embodiment of the invention.
[0087] FIG. 23 is a block diagram showing the principal modules of
an eye tracker system using separate illumination and imaging
gratings in one embodiment of the invention.
[0088] FIG. 24 is a schematic illustration of a grating element
switching scheme provided by the imaging grating and image sampling
grating in one embodiment of the invention.
[0089] FIG. 25 is a schematic plan view of an eye tracker using
common illumination and imaging gratings in one embodiment of the
invention.
[0090] FIG. 26 is a schematic cross section view showing the
imaging and illumination grating and the input, output, image
sampling and detector sampling gratings of an eye tracker using
common illumination and imaging gratings in one embodiment of the
invention.
[0091] FIG. 27A is a schematic plan view of the image sampling
grating of an eye tracker using common illumination and imaging
gratings in one embodiment of the invention.
[0092] FIG. 27B is a schematic cross section view of the
illumination sampling grating, the input grating and laser of an
eye tracker using common illumination and imaging gratings in one
embodiment of the invention.
[0093] FIG. 27C is a schematic plan view image sampling grating and
the detector module with detector overlaid of an eye tracker using
common illumination and imaging gratings in one embodiment of the
invention.
[0094] FIG. 27D is a schematic plan side elevation view showing the
image sampling grating and detector of an eye tracker using common
illumination and imaging gratings in one embodiment of the
invention.
[0095] FIG. 27E is a schematic cross section view of the output
grating and the image sampling grating an eye tracker using common
illumination and imaging gratings in one embodiment of the
invention.
[0096] FIG. 27F is a schematic cross section view of the input
grating and the illumination sampling of an eye tracker using
common illumination and imaging gratings in one embodiment of the
invention.
[0097] FIG. 28 is a simplified representation of the imaging
process an eye tracker using common illumination and imaging
gratings in one embodiment of the invention.
[0098] FIG. 29 provides a system block diagram showing the key
modules of an eye tracker using common illumination and imaging
gratings in one embodiment of the invention.
[0099] FIG. 30 is a flow chart showing the process for determining
eye displacement vectors from the recorded speckle data.
[0100] FIG. 31A is a flowchart for a calibration process for an eye
tracker using common illumination and imaging gratings in one
embodiment of the invention.
[0101] FIG. 31B is a schematic illustration of the initial
calibration procedure used for an eye tracker in one embodiment of
the invention.
[0102] FIG. 32 is a schematic plan view of an eye tracker including
an illumination sampling grating and an image sampling grating each
based on gratings with grating vectors substantially aligned
parallel to the waveguide plane in one embodiment of the
invention.
[0103] FIG. 33A is a schematic plan view illustrating a first
aspect of an eye tracker including an illumination sampling grating
and an image sampling grating in which the illumination light is
angularly offset from the image light.
[0104] FIG. 33B is a schematic plan view illustrating a second
aspect of an eye tracker including an illumination sampling grating
and an image sampling grating in which the illumination light is
angularly offset from the image light.
[0105] FIG. 34 is a block diagram showing the principal modules of
an eye tracker system including a neural network in one embodiment
of the invention.
[0106] FIG. 35 is a block diagram showing the principal modules of
an eye tracker system based common illumination and imaging grating
in which the processing system includes a neural network in one
embodiment of the invention.
DETAILED DESCRIPTION
[0107] The invention will now be further described by way of
example only with reference to the accompanying drawings. It will
apparent to those skilled in the art that the present invention may
be practiced with some or all of the present invention as disclosed
in the following description. For the purposes of explaining the
invention well-known features of optical technology known to those
skilled in the art of optical design and visual displays have been
omitted or simplified in order not to obscure the basic principles
of the invention. Unless otherwise stated the term "on-axis" in
relation to a ray or a beam direction refers to propagation
parallel to an axis normal to the surfaces of the optical
components described in relation to the invention. In the following
description the terms light, ray, beam and direction may be used
interchangeably and in association with each other to indicate the
direction of propagation of light energy along rectilinear
trajectories. Parts of the following description will be presented
using terminology commonly employed by those skilled in the art of
optical design. It should also be noted that in the following
description of the invention repeated usage of the phrase "in one
embodiment" does not necessarily refer to the same embodiment.
[0108] The proposed eye tracker aims to satisfy a suite of
challenging requirements. Since it will eventually be integrated
into a head-worn display, it should make minimum impact on the
overall optical performance. The inventors' design goals are: a
field of view (FOV) of 60.degree. horizontal.times.48.degree.
vertical; 17 mm eye relief; and eye motion box/exit pupil (20
mm..times.10-15 mm). Moreover, the eye tracker must satisfy eye
safety requirements for near-eye visual displays with regard to
weight (minimal), center of gravity (ergonomic), and profile.
Furthermore it should not compromise: pixel resolution, see-through
(.gtoreq.90%) and power consumption (minimal).
[0109] Eye Trackers based on classical Purkinje imaging methods
suffer from high latency resulting mainly from the large delay
incurred by feature recognition and tracking algorithms. The
inventors are strongly motivated by a desire to develop an eye
tracker that firstly simplifies the image processing problems of
classical eye tracking that often result in unacceptably high
latency and secondly can make use of relatively unsophisticated
detector technology. The proposed eye tracker avoids the cost and
complexity of implementing classical Purkinje imaging methods by
tracking eye signatures using low resolution high speed image
sensors. Ideally the detector technology would be equivalent in
specification to that used in the infrared mouse a device which is
now ubiquitous and, more importantly, capable of being manufactured
using sub dollar components. The signatures do not need to be
images of eye features such as pupil edges but can be random
structures such as speckle patterns (including reflections from
multiple surfaces and scatter from the optical media inside the
eye). However, it is important that whatever signature is tracked
has a strong spatio-temporal variation with gaze direction.
[0110] The inventors believe that this approach offers significant
advantages in terms of detector resolution, processing overhead and
power consumption.
[0111] An eye tracker according to the principles of the invention
provides an infrared illumination channel for delivering infrared
illumination to the eye and an imaging channel for forming an image
of the eye at a sensor. In one embodiment of the invention
illustrated in FIGS. 1-2, the eye tracker comprises a waveguide 100
for propagating illumination light towards an eye 116 and
propagating image light reflected from at least one surface of an
eye; a light source 112 optically coupled to the waveguide; and a
detector 113 optically coupled to the waveguide. Disposed in the
waveguide are: at least one input grating 114 for deflecting
illumination light from the source into a first waveguide path; at
least one illumination grating 102 for deflecting the illumination
light towards the eye; at least one imaging grating 101 for
deflecting the image light into a second waveguide path; and at
least one output grating 115 for deflecting the image light towards
the detector. The inventors also refer to the waveguide 100 as the
DigiLens. The illumination and imaging gratings are arrays of
switchable beam deflection grating elements with the preferred
grating technology being a SBG as described above. In one
embodiment of the invention shown in FIG. 1B the grating elements
in the imaging grating 120 are elongate as indicated by 121 with
longer dimension orthogonal to the beam propagation direction. In
one embodiment of the invention shown in FIG. 1C the imaging
grating may comprise a two dimensional array 122 of elements 123
each having optical power in two orthogonal planes. Typically the
first and second waveguide paths, that is, the imaging and
illumination paths in the waveguide are in opposing directions as
illustrated in FIG. 1A. The illumination light will typically be
fully collimated while the image light will have some divergence of
angle determined by the scattering angle from eye services, the
angular bandwidth of the gratings and the numerical aperture of the
grating elements. As will be discussed later, in one embodiment the
imaging and illumination gratings are provided by a single grating
with the illumination and imaging ray paths. Where separate imaging
and illumination gratings are used the two gratings may employ
different TIR angles within the waveguide. This is advantageous in
terms of avoiding the risk of cross coupling of illumination light
into the detector and image light into the light source.
[0112] In FIG. 1A the illumination light path is illustrated by the
light 1010 from the source which is directed into a TIR path 1011
by the input grating and diffracted out of the waveguide as the
light generally indicated by 1012. Typically the eye tracker will
have a pupil of size 20-30 mm. to allow capture of light reflected
from the eye to continue should the waveguide change position
relative to the eye. Since the eye tracker will usually be
implemented as part of a HMD its pupil should desirably match that
of the HMD. FIG. 1A shows return light 1013 reflected from the
front surface of the cornea 117 and light 1014 reflected from the
retina 118. The corneal and retinal image light enters the
waveguide along tray paths such 1015, 1116 and is deflected into a
TIR path such as 1017 by an active element of the imaging grating
which is switched one element at a time. The light 1017 is
deflected into a ray path 1018 toward the detector by the output
grating. Advantageously, the detector reads out the image signal in
synchronism with the switching of the SBG lens array elements. The
detector is connected to an image processing apparatus for
determining at least one spatio-temporal characteristic of an eye
movement. The image processor, which is not illustrated, detects
pre-defined features of the backscattered signals from the cornea
and retina. For example, the image processor may be used to
determine the centroid of an eye feature such as the pupil. Other
trackable features of the eye will be well known to those skilled
in arts of eye tracker design and visual optics.
[0113] The eye surfaces used for tracking are not necessarily
limited to the front surface of the cornea and the retina. The
invention can be applied using reflections from any of the surfaces
of the lens, iris and sclera including any of the reflections
normally referred to as Purkinje reflections. In one particularly
important embodiment of the invention to be discussed later the
light reflected from the eye is speckle. The speckle may arise from
reflections at any of the above surfaces or from the bulk medium of
the cornea lens and other parts of the eye.
[0114] Advantageously, the light source is a laser emitting in the
infrared band. Typically, the laser emits at a wavelength in the
range 785-950 nm. The choice of wavelength will depend on laser
efficiency, signal to noise and eye safety considerations. Light
Emitting Diodes (LEDs) may also be used. In one embodiment of the
invention the detector is a two dimensional array. However other
types of detector may be used including linear arrays and analogue
devices such as position sensing detectors. In the embodiment shown
in FIG. 1 the illumination grating provides divergent light. In
alternative embodiments of the invention the illumination grating
provides collimated light.
[0115] The gratings may be implemented as lamina within or adjacent
an external surface of the waveguide. In other words the grating
may be disposed adjacent an optical surface of the waveguide.
comprising at least one of an internal surface or an external
surface of the waveguide. For the purposes of discussing the
invention we will consider Bragg gratings disposed within the
waveguide. Advantageously the gratings are switchable Bragg
gratings (SBGs). In certain embodiments of the invention passive
gratings may be used. However, passive gratings lack the advantage
of being able to direct illumination and collect image light from
precisely defined areas of the pupil. In one embodiment the
gratings are reverse mode SBGs. Although the invention is discussed
in relation to transmission gratings it should be apparent to those
skilled in the art that equivalent embodiments using reflection
gratings should be feasible in most cases. The gratings may be
surface relief gratings. However, such gratings will be inferior to
Bragg gratings in terms of their optical efficiency and
angular/wavelength selectivity.
[0116] The input and illumination gratings may be configured in
many different ways. FIG. 2 is a schematic plan view showing one
possible implementation for use with the embodiment of FIG. 1. Here
the input grating comprises two grating elements 114A,114B and the
illumination grating is also divided into the upper and lower
gratings 120A,120B, each providing narrow beam deflecting grating
strips above and below the imaging grating 102. The detector
grating 115 is also indicated. Since the guided beams in the input
and illumination grating are collimated, and likewise the guided
beams in the imaging and detector gratings, there is no cross talk
between the two regions of the waveguide.
[0117] In the embodiment of the invention shown in FIGS. 3-4, which
is similar to the one of FIG. 2, the upper and lower illumination
grating may be arrays of switchable grating elements 121A,121B
comprising switchable grating elements such as 122A,122B. The SBG
deflector arrays scroll illumination across the exit pupil in step
with the activation of the imaging grating elements. Finally, in
the embodiment of FIG. 4 the illumination grating comprises just
one strip 123 containing elements 124 disposed along the top edge
of the imaging grating.
[0118] The invention does not assume any particular configuration
of the grating elements. It is important to note that the SBGs are
formed as continuous lamina. Hence the illumination gratings
elements may be considered to be part of the imaging grating. This
is a significant advantage in terms of fabrication and overall form
factor. In embodiments where the illumination grating is split into
two elements as discussed above the input laser light may be
provided by one laser with the upper and lower beam being provided
by a beam splitting means. Alternatively, two separate laser
modules may be used to provide light that is coupled into the
waveguide via the input gratings 114A,114B are illustrated in FIGS.
3-4. The invention does not assume any particular method for
providing the laser input illumination or coupling the laser light
into the waveguide. Many alternative schemes should be apparent to
those skilled in the art of optical design.
[0119] The illumination grating may provide illumination light of
any beam geometry. For example, the light may be a parallel beam
emitted normally to the surface of the eye tracker waveguide. The
illuminator grating is illustrated in more detail in the schematic
side elevation view of FIG. 5 in which the SBG linear array 130 is
sandwiched between transparent substrates 130A,130B. Note that the
substrate layers extended to cover the entire waveguide and
therefore also act as the substrates for the imaging grating.
Advantageously, the ITO layers are applied to the opposing surfaces
of the substrates with at least one ITO layer being patterned such
that SBG elements may be switched selectively. The substrates and
SBG array together form a light guide. Each SBG array element has a
unique optical prescription designed such that input light incident
in a first direction is diffracted into output light propagating in
a second direction. FIG. 5 shows TIR illumination beam 1020 being
deflected by the active element 131 to provide divergent
illumination light 1021.
[0120] An alternative embodiment of the linear deflector array is
shown in the schematic side elevation view of FIG. 6. In this cases
the array 132 sandwiched by substrates 132A,132B is based on a
lossy grating that diffracts incrementally increasing fractions of
the guided beam out of the waveguide towards the eye. Beam portions
1023A-1023C diffracted by the grating elements 133A-133C are
illustrated. Typically, the index modulation of the grating
elements will be designed to provide uniform extraction along the
array and hence uniform output illumination. Note that the
geometrical optics of FIGS. 5-6 has been simplified for the sake of
simplifying the description.
[0121] Advantageously, the illumination grating elements may encode
optical power to provide sufficient beam spread to fill the exit
pupil with light. A similar effect may be produce by encoding
diffusion characteristics into the gratings. The apparatus may
further comprise an array of passive holographic beam-shaping
diffusers applied to the substrate, overlapping the linear SBG
array, to enhance the diffusion. Methods for encoding beam
deflection and diffusion into diffractive devices are well known to
those skilled in the art of diffractive optics. Cross talk between
the imaging and illumination channels is overcome by configuring
the SBGs such that the illumination TIR path within the eye tracker
lies outside the imaging TIR path.
[0122] In one embodiment of the invention the imaging grating may
also encode optical power. A two layer SBG imaging grating that
encodes optical power is illustrated in FIGS. 7-10. The arrays are
shown in their stacked configuration in FIG. 7. The substrates
136A,136B and 139A,139B together provide the imaging waveguide as
illustrated in FIG. 8 where the ray path from the eye into the
waveguide via an activated SBG element 42 is represented by rays
1025-1028. The arrays are shown in front, plan and side elevation
views in FIGS. 9-10. The arrays comprise linear arrays of column
elements each having the optical characteristics of a cylindrical
lens. The column vectors in the two arrays are orthogonal. The
first array comprises the SBG array 135 sandwiched by the
substrates 136A,136B with one particular element 137 being
indicated. The second array comprises the SBG array 40 sandwiched
by the substrates 139A,139B with one particular element 141 being
indicated.
[0123] FIG. 11A illustrates the principles of the formation of the
first four Purkinje images corresponding to reflections off the
front of the cornea1033,1043; the back of the cornea 1032, 1042;
the front of the eye lens1031,1041; and the back of the eye lens
1030,1040. FIG. 11B illustrates the formation of images of the
retina by rays 1034,1044 and the iris by rays 1035,1045.
[0124] FIG. 12 shows how the first and second SBG lens arrays of
FIGS. 7-10 may be used to localize an eye feature such as by
scanning row and column SBG elements such as 142 and 143.
[0125] With regard to the use of speckle as an eye signature FIG.
13 illustrates how the size of speckle feature as recorded in two
captured speckle images may vary with the eye orientation and
displacement with respect to the eye optical axis 1050. FIG. 13A
illustrates speckle formed by illuminating the eye along the
direction 1050A which is initially parallel to the eye optical
axis. The components of the corneal and retinal speckle light
parallel to the eye optical axis are indicated by 1050B,1050C. FIG.
14A shows the formation of speckle with the eye rotated in the
plane of the drawing. The detected corneal and retinal speckle
light 1050D,1050E parallel to the direction 1050 which is now no
longer parallel to the eye optical axis is shown. As shown by the
insets 1051,1053 the size and spatial distribution of the speckles
changes as the eye rotates. Correlation of the two speckle patterns
will provide a measure of the eye rotation. Note that, typically,
the speckle patterns recorded at the detector will combine separate
speckle patterns from the cornea and retina as well as other
surfaces and biological media interacting with the illumination
beam. In one embodiment of the invention the eye tracker processor
compares the speckle images due to light being scattered from the
retina and cornea. When the eye is panned horizontally or
vertically the relative position of the speckle pattern from the
cornea and retina change accordingly allowing the direction of gaze
to be determined from the relative trajectories of the reflected
light beams.
[0126] FIG. 14 represents the front of the eye 146 cornea 147 and
illuminated region 148 of the retina illustrates the direction of
movement of corneal and retinal speckle features as indicated by
the vectors 149,150 correspond to the ocular displaces illustrated
in FIG. 15. In general the ray reflection vectors directions will
be closely linked to eye rotation. FIG. 15A represents the
reflection of rays from the cornea 1056,1057 and retina 1054,1055
for one eye position. FIG. 15B shows the reflection paths from the
cornea 1058,1059 and the retina 1060,1061 after a horizontal (or
vertical) eye rotation. Reflection from the cornea has a strong
secular component. Retinal reflection is more diffuse. The size of
the corneal reflected angles would ordinarily require a large
angular separation between the illumination and detection optical
axes. This would make eye tracking using corneal reflections over
large FOVs very difficult. The invention avoids the problem of
imaging large reflection angles (and dealing with are lateral and
vertical eye movements which can arise from slippage) by using
matched scrolling illumination and detection. Hence the reflection
angle becomes relatively small and can be approximated to:
.PSI..about.2[ (D/r-1) .PHI.+d/r ] where r is the cornea radius
.PHI. is the eye rotation and D is the distance of the eye centre
from the displaced centre of curvature of the cornea and d is the
lateral displacement of the eye centre.
[0127] In one embodiment of the invention based on the one of FIGS.
7-10 the imaging grating comprises an SBG array 143 in which the
lens elements 144 have varying focal length across the exit pupil.
In the embodiment of FIG. 16 grating elements of first and second
focal length indicated by the divergent beams 1062,1064 and
1063,1065 are uniformly interspersed . In one embodiment
illustrated in FIG. 17A the imaging waveguide comprises arrays 145
of variable power lens elements 146. As shown in the detail of FIG.
17B a variable power lens would be provided by combining a
diffractive element 147 of fixed focal length with a variable index
layer 148.
[0128] In one embodiment of the invention shown in the schematic
view of FIG. 18 the imaging grating comprises a single layer two
dimensional SBG array. A group of elements labelled 152 which
comprises interspersed elements such as 153,154. The group forms
the image region 151 at the detector 110. Each SBG element is
characterized by one from a set of at least two different
prescriptions. FIG. 18 does not show the details of the waveguide
and the illumination and input/output gratings. At least one of the
SBG prescriptions corresponds to a lens for forming an image of the
eye on the detector. At least one prescription is optimized for
imaging a signature formed by a surface of the eye. Hence the
embodiment of FIG. 18 allows eye tracking to be performed using
speckle patterns and conventional features such as Purkinje
reflections.
[0129] FIGS. 19-24 provide schematic illustrations of aspects of an
eye tracker based on the principles of FIGS. 1-6. In this
embodiment of the invention the earlier described imaging,
illumination, input and output gratings are augmented by an
additional grating to be referred to as an image sampling grating
which overlays the output grating. FIG. 19 shows a side elevation
view of the illumination grating 163. FIG. 20 is a plan view
showing the imaging grating 165, the illumination grating 163 and
the image sampling grating 170 overlaid on the output grating 164.
FIG. 21 is a side elevation view of an alternative embodiment of
the illumination grating163. FIG. 22A is a plan view of the imaging
grating, the image sampling grating 14 and the detector module180.
FIG. 22B is a plan view of the image sampling grating and the
detector module. FIG. 22C is a cross sectional view showing the
imaging grating and the image sampling grating. FIG. 22D is a cross
sectional view of the image sampling grating and the detector
module. Finally, FIG. 22E is a cross sectional view of the imaging
grating, the image sampling grating and the detector module. To
assist the reader the projection plane of each illustration is
referred to a Cartesian XYZ reference frame.
[0130] The imaging grating 165 comprises an array of column-shaped
SBG elements, such as the one labelled 167, sandwiched by
substrates 168,169. Column elements of the imaging grating 165 are
switched on and off in scrolling fashion backwards and forward
along the direction indicated by the block arrow 1320 in FIG. 20
such that only one SBG column is in its diffractive state at any
time .
[0131] The illuminator array 163 is shown in detail in FIG. 19
comprises substrates 161A,161B sandwiching an array of SBG
rectangular elements such as 163A,163B. The SBG elements may have
identical diffracting characteristics or, as shown in FIG. 19, may
have characteristics that vary with position along the array. For
example, the element 163A provides a diffusion distribution 1310
centered on a vector at ninety degrees to the array containing rays
such as 1311. However, the element 63B provides an angled
distribution 1312 containing rays such as 1313. In an alternative
embodiment shown in FIG. 21 the diffusion polar distributions may
have central ray directions that varying in a cyclic fashion across
the array as indicated by the rays 1313-1318.
[0132] The image sampling grating 170, comprising an array of
rectangular SBG beam deflecting elements 173 such as 176 (shown in
its diffracting state in FIG. 22C) sandwiched by substrates
174,175. The waveguide containing the imaging grating 165,
illumination grating 163 and the output grating 164 is separated
from the image sampling grating 170 by a medium (not illustrated)
which may be air or a low refractive index transparent material
such as a nanoporous material.
[0133] Infrared light from a surface of the eye is coupled into the
waveguide by an active imaging grating element, that is, by a
diffracting SBG column. The guided beam undergoes TIR in the
waveguide up to the output grating. As shown in FIG. 22C the output
grating 164 deflects the beam through ninety degrees into the
direction 1322 towards the image sampling grating 170. As shown in
FIG. 22C a portion of the beam 1322 is deflected into the image
sampling grating by an active SBG element 176 where it undergoes
TIR in the direction indicated by the ray 1323 (and also by block
arrow 1321 in FIG. 20). The light that is not sampled by the image
sampling grating indicated by 1320 1321 is trapped by a suitable
absorbing material, which is not illustrated. The TIR beam is
deflected in the detector module 180 by a first holographic lens
172 to provide out image light 1325. Turning now to FIG. 22D we see
that the detector module contains mirror surfaces 177A,177B and a
further holographic lens 178 which forms an image of the eye
features or speckle pattern that is being tracked on the detector
array 166. Note the holographic lens 172,178 may be replaced by
equivalent diffractive elements based on Bragg or surfaces relief
gratings. Conventional refractive lens elements may also be used
where size constraints permit.
[0134] FIG. 23 is a system block diagram of the eye tracker of
FIGS. 19-22. The system modules comprise the imaging grating 300,
illumination grating 301, illumination grating driver 302,
illumination sampling grating 303, imaging grating driver 304,
detector driver 30, image-sampling array driver 306, detector 166
and processor 307. The apparatus also comprises a laser driver
which is not illustrated. The optical links from the image grating
to the image sampling array and the image sampling array to the
detector are indicated by the block arrows 329,330. The processor
307 comprises a frame store 308 or other image memory device for
the storage of captured eye image or speckle pattern frames and an
image processor 309 further comprising hardware or software modules
for noise subtraction 310 and image analysis 311. The processor
further comprises hardware control module 312 for controlling the
illumination, imaging and image sampling grating drivers, all said
modules operating under the control of a main processor 313. Data
and control links between components of the system are indicated by
319-325. In particular, each driver module contains switching
circuitry schematically indicated by 326-328 for switching the SBG
elements in the imaging grating, illumination grating array, and
image sampling grating.
[0135] FIG. 24 illustrates the switching scheme used in the imaging
grating and image sampling grating. The illumination grating
elements are switched in phase with the imaging grating columns.
Column element 165A of the imaging grating array 165 and element
170A of the readout array 170 are in their diffracting states. The
projection (indicated by 170B) of element 170A on the column 65A
defines an active detection aperture. Using such as scheme it is
possible to track features of the eye using a X,Y localization
algorithm aided by predictions obtained from analysis of
displacement vectors determined from successive frames. Methods for
implementing such search schemes will be known to those skilled in
the art. The invention does not rely on any particular algorithm or
processing platform.
[0136] FIGS. 25-27 provide schematic illustrations of aspects of an
eye tracker that extends the embodiment of FIGS. 19-24 by
introducing a further grating component to be referred to as an
illumination sampling grating which overlays the input grating. The
other feature of this embodiment is that the illumination grating
is no longer separate from the imaging gratings. Instead the two
are combined in a bi-directional waveguide in which a common
switchable column grating is used to illuminate and image the eye
with the illumination and image wave-guided light propagating in
opposing directions. The combined gratings will be referred to as
the illumination and imaging grating. As will be explained below
the function of the illumination sampling grating, which is similar
in structure to the image sampling grating, is to concentrate the
available illumination into region of the eye selected by the image
sampling grating. This confers the dual benefits of light
efficiency and avoidance of stray light from regions of the eye
that are not being tracked.
[0137] Turning now to the drawings, FIG. 25 is a plan view showing
the imaging and illumination grating 190, the image sampling
grating 194, illumination sampling grating 195 the input grating
193 and output grating 192 and the detector module 200 . Column
elements of the illumination and imaging grating are switched on
and off in scrolling fashion backwards and forward such that only
one SBG column is in its diffractive state at any time. The counter
propagating beam paths are indicated by 1341,1342. FIG. 26 shows
the components of FIG. 25 in a side elevation view. FIG. 27A is a
plan view of the illumination sampling grating. FIG. 27B is a cross
sectional view of the illumination sampling grating 195 including
the input grating 193 and the laser 205. FIG. 27C is a plan view of
the image sampling grating 194 showing the detector module 200 and
detector 205 overlaid.
[0138] FIG. 27D is a side elevation view showing detector module
200 in more detail. The detector 205 and a cross section of the
image sampling grating 194 are included. FIG. 27E is a cross
sectional view of the output grating 192 and the image sampling
grating 194. FIG. 27F is a cross section view of the input grating
193 and the illumination sampling grating 194. To assist the reader
the projection plane of each illustration is referred to a
Cartesian XYZ reference frame.
[0139] The illumination and imaging grating comprises the array 190
of column-shaped SBG elements, such as the one labelled 191
sandwiched by the transparent substrates 190A,190B. The input and
output grating which are disposed in the same layer are labelled by
193,192 respectively. The detector module 200 is delineated by a
dotted line in FIGS. 25-26 and in more detail in FIG. 27D. The
image sampling grating 194, comprises an array of rectangular SBG
beam deflecting elements (such as 197) sandwiched by substrates
194A,194B. Typically the imaging grating and image sampling grating
are separated by a medium 198 which may be air or a low refractive
index transparent material such as a nanoporous material. The
illumination sampling grating 195 which is has a very similar
architecture to the image sampling grating comprises an array of
rectangular SBG beam deflecting elements (such as 196) sandwiched
by substrates 195A,195B. Typically the imaging grating and image
sampling grating are separated by a medium 199 which may be air or
a low refractive index transparent material such as a nanoporous
material.
[0140] Referring to FIG26 and FIG. 27F illumination light 1350 from
the laser is directed into the illumination sampling grating by a
coupling grating 207. The light then proceeds along a TIR path as
indicated by 1350A, 1350B up to an active element 208 where it is
diffracted into the direction 1351 towards the input grating. Not
that the image sampling grating directs all of the illumination
light through the active element of the illumination sampling
grating the elements of which are switched in synchronism with the
elements of the image sampling grating to ensure that at any time
the only the region of the that is being imaged receives
illumination. The illumination path in the waveguide is indicated
by 1352-1354.
[0141] Infrared light 1356 (also illustrated as the signature 1355)
from one or more surfaces of the eye is coupled into the waveguide
by a diffracting SBG column such as 191. The guided beam indicated
by 1357,1358 undergoes TIR in the waveguide up to the output
grating 192. The output grating deflects the beam through ninety
degree into the direction 1359 towards the image sampling grating.
As shown in FIG. 27E the beam in direction 1359 is deflected into
the image sampling grating by an active SBG element 197 where it
undergoes TIR along the ray path indicated by 1360, 1361. The TIR
beam is deflected into the detector module 200 as light 1363 by a
first holographic lens 203. Any light that is not sampled by the
image sampling grating is trapped by a suitable absorbing material,
which is not illustrated. The absorbing material may be a prism,
prism array, an infrared absorbing coating or some other means
known to those skilled in the art.
[0142] The detector module contains mirror surfaces 201,202 and a
further holographic lens 204 which forms an image of the eye
signature that is being tracked on the detector array 205. The ray
path from the image sampling grating to the detector is indicated
by the rays 1363-1365. Advantageously, the mirror surfaces are
coatings applied to opposing faces of a prismatic element. However,
the invention does not rely on any particular scheme for steering
the image light towards the detector array. Note that the
holographic lens 203,204 may be replaced by equivalent diffractive
elements based on Bragg or surfaces relief gratings. Conventional
refractive lens elements may also be used where size constraints
permit.
[0143] In one embodiment of the invention illumination light from
laser module is converted into S-polarized light which is coupled
into the eye tracker waveguide by the input grating. This light is
then converted into circularly polarized light using a quarter wave
plate. An active SBG column will then diffract the P-component of
the circularly polarized wave guided light towards the eye, the
remaining P-polarized light being collected in a light trap. The
P-polarized light reflected back from the eye (which will be
substantially P-polarized) is then diffracted into a return TIR
path by the active SBG column and proceeds to the detector module
as described above. This scheme ensures that image and illumination
light is not inadvertently coupled into the input and output
gratings respectively. In other embodiments of the invention the
unwanted coupling of the image and illumination light may be
overcome by optimizing the TIR angles, the angular bandwidths of
the imaging and illumination gratings, the spacings along the
waveguide of the input and output gratings, and the illumination
and imaging beam cross sections. In one embodiment the illumination
light which will typically in most embodiments of the invention be
collimated may be angled such that the waveguide propagation angle
of the illumination beam differs from the waveguide angles of the
image light.
[0144] An important feature of the invention is that elements of
the illumination sampling grating are switched to allow
illumination to be localized to a small region within the active
column of the DigiLens ensuring that the illumination is
concentrated exactly where it is needed. This also avoids stray
light reflections a problem which can consume significant image
processing resources in conventional eye tracker designs. Since the
illumination is scrolled the cornea and retina are not exposed to
continuous IR exposure allowing higher exposures levels to be used
leading to higher SNR. A safety interlock which is not illustrated
may be included to switch off the laser when no tracking activity
has been detected for a predefined time.
[0145] The proposed scheme for switching the columns and readout
elements in the embodiments of FIGS. 25-27 is based on tracking the
movement of the pupil using a X,Y localization algorithm similar to
the one illustrated in FIG. 24 which shows how the ith activated
column of DigiLens and jth activated element of the readout array
are used to select the speckle pattern region (X,Y).
[0146] FIG. 28 is a simplified representation of the detection path
starting with the collimated rays 1400 from an active column
element 370 of the imaging array. The rays 1400 are sampled by an
element 371 of the detector grating to provide the rays 1402 which
are imaged by the holographic lens 372 to provide the rays 1403
incident on the detector 205.
[0147] FIG. 29 provides a system block diagram of the eye tracker
of FIGS. 26-27. The system modules comprise the illumination and
imaging grating 190, image sampling grating 194, illumination
sampling grating 195, detector 205, laser 206, illumination
sampling array driver 340, image sampling array driver 341,
detector driver 342, laser driver 343, illumination and imaging
grating driver 344 and processor 345. The processor 345 comprises a
frame store or other image storage media 346 for the storage of
captured eye image or speckle pattern frames and an image processor
347 further comprising hardware or software modules for noise
subtraction 348 and image analysis 349. The processor further
comprises hardware control module 350 for controlling the
illumination, imaging and image sampling grating drivers, all said
modules operating under the control of a main processor 351. The
above described modules are connected by communication and control
links schematically indicated by 360-369 include control lines for
switching the SBG elements in the imaging grating, illumination
sampling grating array, and image sampling grating 367-369.
[0148] In one embodiment of the invention the detector array is a
detector array of resolution 16.times.16 with a framing rate of
2300 fps of the type commonly used in infrared mouse equipment. In
alternative embodies similar sensor technology of resolution
64.times.64 operating at 670 fps may be used. The selection of a
particular sensor will depend on factors such as the required
tracking resolution and accuracy and the update rate of the eye
tracker. Exemplary sensors are manufactured by Pixart Inc. The
detector optical prescription will be determined by a detailed
ray-tracing analysis and will require trade-offs of speckle size,
F-number and DigiLens column width. In the case of speckle tracking
the detector lens aperture defines the limiting speckle size. The
detector field of view is determined by the detector size and the
detector lens focal length. However, the invention could be applied
with any currently available imaging sensor technology. In one
embodiment the DigiLens provides 25 SBG scrolling columns.times.17
SBG readout elements. The Agilent device can be programmed to
switch 2300 fps So a complete scan of the FOV will take
(25.times.17)/2300 s.=185 ms. However, in practice the eye tracker
will use a more sophisticated X-Y search process that localizes the
pupil using column and readout element coordinates. It is
anticipated that on average around 10 search steps may be needed to
converge on the pupil position resulting in a latency of 4.3 ms. On
this basis the latency of the tracker is potentially .times.100
lower than that of comparable image processing-based Purkinje-type
eye trackers. It is also anticipated that the correlation process
will be implemented in hardware resulting in a relatively modest
data processing latency. The detected eye signature is stored and
compared with other saved patterns to determine the eye gaze
trajectory and to make absolute determinations of the gaze
direction (bore sighting). Initial calibration (that is, building
up the database of saved patterns) is carried out by directing the
user to look at test targets at predefined points in the field of
view (FOV) over which the eye gaze is to be tracked. Since the
frames are of low resolution large numbers of samples may be
collected without significant computational overhead.
[0149] Although the invention may be used to detect any type of eye
signature, speckle is attractive because it avoids the image
analysis problems of identifying and tracking recognizable features
of the eye that are encountered in Purkinje imaging schemes.
Prerequisites for measuring eye displacement vectors (rotational
and/or translational) include achieving an adequate level of
speckle contrast (after detector noise and ambient light have been
subtracted from the detected signal) and being able to resolve
individual speckle grains. A high signal to noise ratio (SNR) is
essential for detecting variations in speckle properties at
required angular resolution. The SNR depends on the speckle
contrast, which is defined as the ratio of the root means square
(rms) variation of the speckle intensity to the mean intensity. The
speckle contrast lies between 0-1 assuming Gaussian statistics. The
detector should have low noise and a short integration time. If the
motion of the eye is appreciably faster than the exposure time of
the CCD camera rapid intensity fluctuations of the speckle pattern
will occur, the average of the detected patterns resulting in a
blurred image with reduced speckle contrast. The smallest speckle
size is set by the diffraction limit. Applying the well-known
formula from diffraction theory: w=.about.2.44 D/a (assuming: a
detector lens to detector distance D.about.70 mm.; IR wavelength
1=785 nm.; and detector lens aperture a .about.3 mm.) we obtain a
diffraction limited speckle diameter w at the detector of .about.64
microns. The resolution of a typical mouse sensor is around 400-800
counts per inch (cpi), with rates of motion up to 14 inches per
second (fps). Hence the limiting speckle size is equivalent to one
count per 64 micron at 400 cpi which is roughly compatible with the
expected speckle size.
[0150] The proposed strategy for processing speckle data captured
by the eye tracker is based on the following assumptions.
[0151] Speckle patterns provide unique "fingerprints" of regions of
the cornea and retina.
[0152] Unlike speckle interferometry which requires that the
speckle motion is less than speckle size, speckle imaging using a
detector array requires that the speckle displacement from frame to
frame is greater than the speckle size
[0153] A displacement of the cornea and retina relative to the
detector will result in a shift of the speckle pattern by the same
amount
[0154] The shifts of the corneal and retinal speckle patterns will
be in opposite directions.
[0155] The motion of the speckles can be determined from the
correlation of two consecutive frame speckle patterns. This
information together with the relative motion of the corneal and
retinal speckle patterns can be used to determine eye displacement
vectors.
[0156] The correlation and image analysis processes may take
advantage standard techniques already developed in applications
such as radar, biological imaging etc.
[0157] The speckle contrast and speckle size at the detector are
compatible with the detector resolution and SNR.
[0158] The following characteristics of the speckle image may also
be used to assist the tracking of the eye use speckle: speckle
grain size; speckle brightness (either individual or collective
brightness); speckle shape; rate of change of any of the preceding
characteristics with ocular movement; and relative directions of
corneal and retinal bema displacements. It is further recognized
that each of these aspects of the speckle image will be dependent
on the illumination beam direction (scanning or static); the
detection optics and the focal length of the imaging optics. The
rate of change of the corneal versus retinal speckles will depend
on the focal length.
[0159] The flow chart in FIG. 30 summarizes the process for
determining eye displacement vectors from the recorded speckle
data. The process relies on a database of frame data collected
during initial calibration and noise characteristics. The
calculation of the displacement vectors uses inputs from a suite of
mathematical models that simulate the first order eye optics, the
eye tracker optics and the eye dynamics. The process may be
interrupted by the user or automatically when a DigiLens failure
occurs. The process also includes DigiLens hardware control to
enable X,Y addressing of DigiLens columns and readout elements. The
correlation process for obtaining the eye displacement vector from
two detected frames in one embodiment may be summarized as follows.
Each frame is subdivided into small sub frames. The sub-frame
coordinates may be predefined or alternatively may be determined by
an interactive scheme using the output from an Eye Dynamics Model.
A 2D correlation map between the sub images from the two frames is
calculated starting with a one pixel step in the x and y directions
and repeat the calculation increasing the step size by one pixel at
a time. Other statistical metrics may also be computed at this
stage to assist in refining the calculation. We then repeat the
correlation process for another selected frame region. A
displacement vector is then computed using (for the time period
between the two analyzed frames) using the peaks of the correlation
maps. Ideally the sub frames should be entirely within the corneal
or retinal fields, the two being distinguished by their opposing
directions. Data which does not yield clear separation of the two
will be rejected) at this stage. The calculation is refined using
data from an Eye Optical Model which models of the eye dynamics and
an Eye Tracker Model which models the optical system. The verified
displacement vector is used to determine the next search X,Y
coordinates (ie SBG column, row) for the Eye Tracker using
predicted gaze trajectory calculated using an Eye Dynamics Model.
The basic ray optics used in the Eye Model in particular the
relationship of the first order corneal and retinal reflection
paths of the eye may be modelled using ray-tracing programs such as
ZEMAX. Standard eye models well known to those skilled in the art
will be adequate for this purpose. Further models may be used to
simulate speckle from the retina and the cornea. The Eye Dynamics
Model carries out a statistical analysis of the displacement
vectors from previous frames to determine the most optical next X,Y
search location (ie the columns and readout elements to be
activated in the DigiLens.
[0160] Initial calibration is carried out by directing the user to
look at test targets at predefined points in the FOV. The
bore-sighting process is illustrated in FIG. 32 which shows a
flowchart (FIG. 32A) and a schematic illustrates of the initial
calibration procedure (FIG. 32B). According to FIG. 31A the bore
sighting procedure 400 comprises the following steps:
[0161] At step 401 present targets to the eye at location j;
[0162] At step 402 capture a series of frames at location j;
[0163] At step 403 store the capture frames;
[0164] At step 404 move to the next target position in the field of
view (FOV);
[0165] At step 405 repeat the process while j is less than a
predefined integer N; otherwise end the process (at step 406).
[0166] Referring to FIG. 31B we see that initial calibration will
be carried by presenting targets (typically lights sources,
resolution targets etc.) to the viewer at different points
1.ltoreq.j.ltoreq.N in the field of view 410 (the point also being
labelled as 411-413) and capturing and storing frames of signature
images at each location. The targets may be presented sequentially
along the sweep path labelled by 414. However, other presentation
schemes may be used. The stored frames will be processed to enhance
SNR and extract statistical metrics (such as histograms,
probability density functions for speckle size etc.) for subsequent
"on-the-fly" frame comparison. Each frame provides a "fingerprint"
for the region of the FOV concerned. The signatures will vary in:
relative positions of the corneal and retinal reflections, or where
speckle patterns are used: speckle contrast; and speckle size
distribution (which is linked to optical magnification).
[0167] In relation to the embodiment of FIG. 25 we have described
the use of an image sampling grating overlaying the output grating.
The image sampling grating comprises a linear array of switchable
grating elements, each element when in its diffracting state
sampling a portion of the light in the waveguide and deflecting it
along the image sampling grating towards said detector. In a
similar fashion an illumination sampling grating overlays the input
grating. The illumination sampling grating is optically coupled to
the light source and comprises a linear array of switchable grating
elements. Each element when in its diffracting state deflects light
from the illumination sampling grating into the waveguide. Turning
to FIG. 32 we next consider an embodiment that implements image and
illuminations sampling grating using a single grating layer. The
eye tracker 420 comprises a DigiLens 420, image sampling gating 422
illumination sampling grating 423 containing elements such as 424
and 425 respectively. Output and input gratings 426,427 link the
sampling gratings to the detector and light sources respectively.
As indicated by the shading pattern of the grating elements each
element comprising a switchable grating with Bragg fringes slanted
at 45 degrees with grating vectors in the plane of the drawing;
that is, in a plane parallel to the waveguiding surfaces. The
inventors refer to these gratings as turning gratings. Hence
illumination ray 1422 undergoing TIR in the DigiLens is deflected
through an angle of ninety degrees by the active element 425 into
the ray direction 1423. Similarly the image ray 1420 is deflected
through an angle of ninety degrees in the direction 1421 by the
active element 424. It should also be apparent from consideration
of the drawing that all of the gratings may be formed in a single
layer in a single waveguide (with the appropriate electrode
patterning of the sandwiching substrates. It should also be
apparent that the turning grating principle may be applied in any
of the above described embodiments including those in which the
DigiLens comprises separated overlapping illumination and imaging
gratings. The sampling gratings may overlap. The design of the
turning gratings may be based on the teachings of U.S. Pat. No.
8,233,204 entitled OPTICAL DISPLAYS which is incorporated herein by
reference in its entirety.
[0168] A challenge in a single layer eye tracker design of the type
described above is to provide adequate eye illumination without
compromising the ability of the DigiLens to collected scattered
light from the eye. Most attempts to use gratings for light
management in bi-directional waveguides fail because of the
fundamental principle of grating reciprocity. In practical terms
this means that some of the image light almost always ends up
getting coupled into the illumination path to the source by the
input grating. In the reciprocal process some of the illumination
light is diffracted into the imaging path to the detector by the
output grating. The amount of this cross coupling will depend on
the beam divergence and waveguide dimensions. The proposed solution
which is illustrated in FIG. 33 assumes the common illumination and
imaging waveguide architecture discussed above and, in particular,
the one illustrated in FIG. 25. The apparatus comprises the
DigiLens 450 which comprises an array of SBG columns such as 451
and a waveguide component 451 comprising the illumination sampling
and imaging sampling gratings 452,453 and containing grating
elements (which we may refer to as pixels) such as 454,455. A cross
section of the illumination sampling grating is provided by 456.
The cross section of the DigiLens is also shown and is indicated by
458. Gratings for coupling the image and illumination light to the
detector and laser are indicated by 458,459. Finally, an eye is
represented by 460. The input and output gratings, which will
typically overlap the sampling gratings as discussed earlier, are
not illustrated. We next consider the ray paths, first defining a
normal to the illumination waveguide as indicated by 1430. The path
of an incident beam at an angle U1 up the eye is indicated by the
rays 1431-1436 comprising the TIR path 1432, coupling into the
DigiLens via the active element 455 as indicated by the ray 1433,
propagating up to the active column element 451 as indicated by ray
1434, diffraction towards the eye along1435, and light 1436
striking a surface of the eye. The reflection light path from the
eye to the detector is indicated by the rays 1437-1440 with
scattered light from the eye indicated by 1437 entering the
DigiLens as 1438 and propagating along the path 1439 before being
diffracted into the image sampling grating via the element454 and
proceeding along the path 1440 leading the detector. FIG. 33B shows
the corresponding ray paths 1441,1442 for an incident ray 1441
launched at the angle U2 (greater than U1) which terminates at the
detector , the ray paths following the logic of FIG. 33A. In one
embodiment of the invention the method illustrated in FIG. 33
eliminates unwanted light coupling by applying a small tilt to the
input beam angle by an amount equivalent to at least 1 pixel of the
eye tracker imaging matrix, for a specular beam. In other
embodiments larger pixel offsets may be useful for better
discrimination. A similar tilt is required in the case of diffuse
beams. Gratings are currently the preferred option for producing
the tilt. However, alternative methods based on prisms may be used.
In one embodiment the method illustrated in FIG. 33 is used to
provide different grating tilts for the upper and lower halves of
the waveguide, thereby preventing over sizing of the lower portion
of the waveguide.
[0169] In the description of the eye tracker data processing
architecture we have discussed how initial calibration will be
carried by presenting targets (typically lights sources, resolution
targets etc.) to the viewer at different points in the field of
view and capturing and storing frames of speckle pattern images at
each location. These images are used aid the processing of live
data when the eye tracker is normal use. It is proposed that the
process could be aided by incorporating an artificial neural
network within the processor. The bore sighting process would
correspond to training the networks. The network could be used to
compensate at least part of any systematic measurements errors
occurring in the processing. In one embodiment of the invention
shown in the block diagram of FIG. 34 the eye tracker system
comprises the eye tracker waveguide 430, detector 431, processor
comprising: main processor 432, waveguide SBG control module 433,
neural network 434 and image database 435. The system modules are
connected by communication and control links referenced by numerals
436-442. A more detailed architecture incorporating a neural
network is shown in FIG. 35 This is architecture is intended for
use with a common illumination and imaging grating eye tracker
designs such as the one of FIG. 25.
[0170] Although the description of the invention has emphasized the
detection of speckle patterns it should be apparent from
consideration of the description and drawings that the same optical
architecture and indeed many features of the processing
architecture may be used to perform eye tracking using other
optical signatures from the eye. For example features such as
bright or dark pupils and glint may provide suitable signatures.
The blurring of the eye feature being tracked does not present an
impediment providing that the detected image contains enough
content for correlations to be made between captured frames and
stored images capture in the bore sighting (or neural network
training) stage.
[0171] The optical design requires careful balancing of the high
source flux needed to overcome throughput inefficiencies arising
from the small collection angles, low transmission thorough the
DigiLens and the low reflectivity of the eye (.about.2.5% at the
surface of the cornea) with the requirement for eye-safe IR
illumination levels. Typically, for applications in which the eye
tracker is used for hours at a time under continuous IR exposure
the eye irradiance should not exceed around 1 mW/cm2. The
appropriate standards for eye safe infrared irradiance are well
known to those skilled in the art. Since the proposed eye tracker
scrolls the illumination across the eye the cornea and retina are
not exposed to continuous IR exposure allowing higher exposures
levels to be used leading to higher speckle contrast level and
therefore higher SNR at the detector. In a switchable grating based
design there is the risk of a switching malfunction causing the
laser beam scanning to freeze resulting in all of the available
output laser power being concentrated into a small area of the eye.
To overcome this problem a safety interlock may be provided to
switch off the laser when no tracking activity has been detected
for a predefined time of, typically, a few minutes. During this
dead time the IR exposure may be allowed to increase significantly
without exceeding the safety threshold.
[0172] The proposed eye tracker avoids the cost and complexity of
implementing classical Purkinje imaging methods by tracking eye
signatures using low resolution high speed image sensors. The
signatures do not need to be images of eye features such as pupil
edges but can be random structures such as speckle patterns.
However, it is important that whatever signature is tracked has a
strong spatio-temporal variation with gaze direction. Conventional
iris image capture systems are an indicator the level of processing
that will be required in an eye tracker. The iris image is
typically acquired by a camera using infrared light in the 700
nm-900 nm band resolving in the region of 100-200 pixels along the
iris diameter. The first step is usually to detect and remove stray
light before proceeding to determine the boundaries of the iris.
Typically the centers and radii of iris and pupil are approximated
initially by applying a circular edge detector. High accuracy and
rapid response times require high-performance and high-cost
microprocessors that are beyond the scope of consumer products.
Traditional image processing designs based on software are too
slow. It is known that significant improvements may result from an
iris recognition algorithms based on a hardware-software co-design
using low-cost FPGAs. The system architecture consists of a 32-bit
general purpose microprocessor and several dedicated hardware
units. The microprocessor executes in software the less
computationally intensive tasks, whereas the coprocessors speed-up
the functions that have higher computational cost. Typically,
depending on the function implemented, coprocessors speed-up the
processing time by a factor greater than 10 compared to its
software execution. However, the best latency achieved with
hardware-software co-designs, is typically in the range 500-1000
ms. It should be noted that an eye tracker is a much more demanding
proposition for an image processor. Detecting a clean iris image is
only the first step. Applying the edge detection algorithms as the
eye moves around the eye box will require several frames to be
analyzed adding to the overall latency.
[0173] The proposed eye tracker is compatible with many display
applications in consumer products, avionics and other fields such
as Augmented Reality by enabling the features of: wide field of
view; large exit pupil; thin form factor; low inertia; and easy
integration with near-eye display technologies.
[0174] It should be emphasized that the drawings are exemplary and
that the dimensions have been exaggerated. For example thicknesses
of the SBG layers have been greatly exaggerated.
[0175] In any of the above embodiments the waveguides may be curved
or formed from a mosaic of planar or curved facets.
[0176] An eye tracker based on any of the above-described
embodiments may be implemented using plastic substrates using the
materials and processes disclosed in PCT Application No.:
PCT/GB2012/000680, entitled IMPROVEMENTS TO HOLOGRAPHIC POLYMER
DISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES.
[0177] Advantageously, the SBGs are recorded in a reverse mode
HPDLC material in which the diffracting state of SBG occurs when an
electric field is applied across the electrodes. An eye tracker
based on any of the above-described embodiments may be implemented
using reverse mode materials and processes disclosed in PCT
Application No.: PCT/GB2012/000680, entitled IMPROVEMENTS TO
HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND
DEVICES.
[0178] While the invention may be applied with gratings of any type
including switching or non-switching gratings based on Bragg
(volume) holograms, or surface-relief gratings the preferred
grating technology is a SBG, which offers the advantages of fast
switching, high optical efficiency and transparency and high index
modulation.
[0179] With regard to the use of grating arrays it should be
appreciated the number of elements used in an array need not be
very large, depending on the FOV over which gaze is to be
tracked.
[0180] It should also be noted that the gratings used in the eye
tracker are not necessarily all switching gratings. Switching
gratings may be used in combination with passive grating
technologies. As has been indicated by the description and drawings
more than one grating layer (lamina) may be used. The grating
layers discussed above are SBGs disposed between internal waveguide
surfaces (or in other words sandwiched between transparent
substrates that combine to form the waveguide. However in
equivalent embodiments some of the gratings layers could be applied
to external waveguide surfaces. This would apply in the case of
surface relief gratings.
[0181] A glass waveguide in air will propagate light by total
internal reflection if the internal incidence angle is greater than
about 42 degrees. Thus the invention may be implemented using
transmission gratings if the internal incidence angles are in the
range of 42 to about 70 degrees, in which case the light extracted
from the light guide by the gratings will be predominantly
p-polarized.
[0182] Using sufficiently thin substrates the eye tracker could be
implemented as a long clear strip applique running from the nasal
to ear ends of a HMD with a small illumination module continuing
laser dies, light guides and display drive chip tucked into the
sidewall of the eyeglass. A standard index matched glue would be
used to fix the display to the surfaces of the HMD.
[0183] The method of fabricating the SBG pixel elements and the ITO
electrodes used in any of the above-described embodiments of the
invention may be based on the process disclosed in the PCT
Application No. US2006/043938, entitled METHOD AND APPARATUS FOR
PROVIDING A TRANSPARENT DISPLAY.
[0184] The invention does not rely on any particular methods for
introducing light from a laser source into the eye tracker and
directing light scattered from the eye onto a detector. In the
preferred embodiments of the invention gratings are used to perform
the above functions. The gratings may be non switchable gratings.
The gratings may be holographic optical elements. The gratings may
be switchable gratings. Alternatively, prismatic elements may be
used. The invention does not rely on any particular method for
coupling light into the display.
[0185] It should be understood by those skilled in the art that
while the present invention has been described with reference to
exemplary embodiments, it is to be understood that the invention is
not limited to the disclosed exemplary embodiments. Various
modifications, combinations, sub-combinations and alterations may
occur depending on design requirements and other factors insofar as
they are within the scope of the appended claims or the equivalents
thereof.
* * * * *