U.S. patent application number 16/743905 was filed with the patent office on 2020-05-14 for method and apparatus for contact image sensing.
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 | 20200150469 16/743905 |
Document ID | / |
Family ID | 55967320 |
Filed Date | 2020-05-14 |
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United States Patent
Application |
20200150469 |
Kind Code |
A1 |
Popovich; Milan Momcilo ; et
al. |
May 14, 2020 |
Method and Apparatus for Contact Image Sensing
Abstract
A contact image sensor comprises: a waveguiding structure for
propagating light in a first direction comprising, in series, a
first clad medium, a first core, a switchable grating clad, a
second core, and a second clad medium sandwiched by transparent
substrates, patterned parallel electrode elements orthogonally
traversing the waveguides, a light source, a platen and a detector.
Switchable grating regions overlapped by a first voltage-addressed
electrode element diffract TIR light from the first core towards
the platen. Switchable grating region overlapped by a second
voltage-addressed electrode element diffract TIR light reflected
from the platen into a TIR path within the second core.
Inventors: |
Popovich; Milan Momcilo;
(Leicester, GB) ; Waldern; Jonathan David; (Los
Altos Hills, CA) ; Grant; Alastair John; (San Jose,
CA) |
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Applicant: |
Name |
City |
State |
Country |
Type |
DigiLens Inc. |
Sunnyvale |
CA |
US |
|
|
Assignee: |
DigiLens Inc.
Sunnyvale
CA
|
Family ID: |
55967320 |
Appl. No.: |
16/743905 |
Filed: |
January 15, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15562755 |
Sep 28, 2017 |
10591756 |
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PCT/GB2016/000065 |
Mar 30, 2016 |
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16743905 |
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62178041 |
Mar 31, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 5/1828 20130101;
G02F 1/1326 20130101; G02B 26/0808 20130101; G02B 27/4272 20130101;
G02F 1/29 20130101 |
International
Class: |
G02F 1/13 20060101
G02F001/13; G02B 27/42 20060101 G02B027/42; G02B 26/08 20060101
G02B026/08; G02B 5/18 20060101 G02B005/18; G02F 1/29 20060101
G02F001/29 |
Claims
1. A contact image sensor comprising: a waveguiding structure for
propagating light in a first direction comprising, in series
disposed in a layer sandwiched by transparent substrates, a first
clad medium, a first core, a switchable grating clad, a second
core, and a second clad medium; electrodes applied to opposing
surfaces of said substrates at least one patterned into a set of
parallel elements orthogonally traversing said cores; a light
source optically coupled to said first and second cores; a platen
in optical contact with said waveguiding structure; a detector
optically coupled to said first and second core regions; wherein
switchable grating regions overlapped by a first voltage-addressed
electrode element are operative, in their diffracting state, to
diffract TIR light from first core into a path to outer surface of
said platen, wherein switchable gratings region overlapped by a
second voltage-addressed electrode element are operative, in their
diffracting state, diffract TIR light reflected from said platen
into a TIR path to said detector along said second core.
2. The apparatus of claim 1 wherein said waveguiding structure
comprises a multiplicity of said cores and said clads cyclically
arranged.
3. The apparatus of claim 1 wherein said voltages are applied
sequentially, two electrodes at a time, to all electrodes in the
array.
4. The apparatus of claim 1 wherein said diffracting state exists
when no electric field is applied across said SBG element and said
non-diffracting state exists when an electric field is applied or
said diffracting state exists when an electric field is applied
across said SBG element and said non-diffracting state exists when
no electric field is applied.
5. The apparatus of claim 1 wherein when contact is made with an
external material at a region on said platen a portion of the light
incident at the region on said platen contacted by said external
material is transmitted out of said platen, wherein light incident
on the outer surface of said platen in the absence of said contact
with an external material is reflected downwards.
6. The apparatus of claim 1 wherein when contact is made with an
external material at a region on said platen a portion of the light
incident at the region on said platen contacted by said external
material is reflected downwards, wherein light incident on the
outer surface of said platen in the absence of said contact with an
external material. is transmitted out of said platen.
7. The apparatus of claim 1 wherein the output from said detector
is read out in synchronism with the switching of said electrode
elements.
8. The apparatus of claim 1 wherein said light source is one of a
laser or LED and said light is coupled into said waveguiding
structure by one of a grating or a prism.
9. The apparatus of claim 1 wherein said grating or prism are
clocked.
10. The apparatus of claim 1 wherein said switchable grating clad
is a switchable Bragg grating recorded in one of a HPDLC grating,
uniform modulation grating or reverse mode HPDLC grating.
11. The apparatus of claim 1 wherein said switchable grating clad
includes at least one of a fold grating or a multiplexed grating or
a rolled k-vector grating.
12. A method of making a contact image measurement comprising the
steps of: a) providing a waveguiding structure for propagating
light in a first direction comprising, in series disposed in a
layer sandwiched by transparent substrates, a first clad medium, a
first core, a switchable grating clad, a second core, and a second
clad medium; electrodes applied to opposing surfaces of said
substrates at least one patterned into a set of parallel elements
orthogonally traversing said cores; a light source optically
coupled to said first and second cores; a platen in optical contact
with said waveguiding structure; a detector optically coupled to
said first and second core regions; b) coupling said light into a
TIR path in said waveguiding structure; c) an external material
contacting a region on the external surface of said platen; d)
setting first and second electrode elements to a first voltage
state and all other voltage-addressed electrodes set to a second
voltage state; e) switchable grating regions overlapped by a first
electrode element diffracting TIR light from first core into a path
to the platen outer surface; f) switchable grating regions
overlapped by a second electrode element diffracting light
reflected from one of said region or said platen external surface
into a TIR path in said second core; g) transmitting said reflected
light to said detector.
13. The method of claim 12 wherein said first voltage state
corresponds to a voltage being applied and said second voltage
state corresponds to no voltage being applied.
14. The method of claim 12 wherein said first voltage state
corresponds to no voltage being applied and said second voltage
state corresponds to a voltage being applied.
15. The method of claim 12 wherein at least a portion of said light
incident at said region on said platen is transmitted out of said
platen, wherein at least a portion of said second optical path
light not incident at said region is reflected.
16. The method of claim 12 wherein at least a portion of said light
incident at said region on said platen is reflected, wherein at
least a portion of said second optical path light not incident at
said region being transmitted out of said platen.
17. The apparatus of claim 12 wherein said waveguiding structure
comprises a multiplicity of said cores and said clads cyclically
arranged.
18. The apparatus of claim 12 wherein said voltages are applied
sequentially, two electrodes at a time, to all electrodes in the
array.
19. The apparatus of claim 1 wherein the output from detector is
read out in synchronism with the switching of the electrode
elements.
20. The apparatus of claim 1 configured as a finger print scanner
or a touch sensor.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 15/562,755 filed on Sep. 28, 2017, which is a
U.S. National Phase of PCT Application No. PCT/GB2016/000065 filed
on Mar. 30, 2016, which claims the benefit of U.S. Provisional
Patent Application No. 62/178,041 filed Mar. 31, 2015, the
disclosures of which are incorporated herein by reference in their
entireties.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to an imaging sensor, and more
particularly to a contact image sensor using electrically
switchable Bragg gratings.
[0003] A contact image sensor is an integrated module that
comprises an illumination system, an optical imaging system and a
light-sensing system--all within a single compact component. The
object to be imaged is place in contact with a transparent outer
surface (or platen) of the sensor. Well known applications of
contact image sensors include document scanners, touch sensors for
computer interfaces, bar code readers and optical identification
technology. Another field of application is in biometric sensors,
where there is growing interest in automatic finger print
detection. Fingerprints are a unique marker for a person, even an
identical twin, allowing trained personel or software to detect
differences bewtween individuals. Fingerprinting using the
traditional method of inking a finger and applying the inked finger
to paper can be extremely time-consuming. Digital technology has
advanced the art of fingerprinting by allowing images to be scanned
and the image digitized and recorded in a manner that can be
searched by computer. Problems can arise due to the quality of
inked images. For example, applying too much or too little ink may
result in blurred or vague images. Further, the process of scanning
an inked image can be time-consuming. A better approach is to use
"live scanning" in which the fingerprint is scanned directly from
the subject's finger. More specifically, live scans are those
procedures which capture fingerprint ridge detail in a manner which
allows for the immediate processing of the fingerprint image with a
computer. Examples of such fingerprinting systems are disclosed in
Fishbine et al. (U.S. Pat. Nos. 4,811,414 and 4,933,976); Becker
(U.S. Pat. No. 3,482,498); McMahon (U.S. Pat. No. 3,975,711); and
Schiller (U.S. Pat. Nos. 4,544,267 and 4,322,163). A live scanner
must be able to capture an image at a resolution of 500 dots per
inch (dpi) or greater and have generally uniform gray shading
across a platen scanning area. There is relevant prior art in the
field of optical data processing in which optical waveguides and
electro-optical switches are used to provide scanned illumination
One prior art waveguide illuminator is disclosed in U.S. Pat. No.
4,765,703. This device is an electro-optic beam deflector for
deflecting a light beam within a predetermined range of angle. It
includes an array of channel waveguides and plural pairs of surface
electrodes formed on the surface of a planar substrate of an
electro-optic material such as single crystal LiNbO.sub.3.
[0004] While the fingerprinting systems disclosed in the foregoing
patents are capable of providing optical or optical and mechanical
fingerprint images, such systems are only suitable for use at a
central location such as a police station. Such a system is clearly
not ideal for law enforcement and security applications where there
is the need to perform an immediate identity and background check
on an individual while in the field. In general current contact
image sensor technilogy tend to be bulky, low in resolution and
unsuitable for operation in the field.
[0005] Thus there exists a need for a portable, high resolution,
lightweight biometeric contact image scanner.
SUMMARY OF THE INVENTION
[0006] It is an object of the present invention to provide a
portable, high resolution, lightweight biometeric contact image
scanner.
[0007] A contact image sensor according to the principles of the
invention comprises: a waveguiding structure for propagating light
in a first direction comprising, in series disposed in a layer
sandwiched by transparent substrates, a first clad medium, a first
core, a switchable grating clad, a second core, and a second clad
medium; electrodes applied to opposing surfaces of the substrates
at least one patterned into a set of parallel elements orthogonally
traversing the cores; a light source optically coupled to the first
and second cores; a platen in optical contact with the waveguiding
structure; and a detector optically coupled to the first and second
core regions. Switchable grating regions overlapped by a first
voltage-addressed electrode element are operative, in their
diffracting state, to diffract TIR light from first core into a
path leading to the outer surface of the platen. Switchable
gratings region overlapped by a second voltage-addressed electrode
element are operative, in their diffracting state, diffract TIR
light reflected from the platen into a TIR path to the detector
along the second core.
[0008] In one embodiment the waveguiding structure comprises a
multiplicity of the cores and the dads cyclically arranged.
[0009] In one embodiment the voltages are applied sequentially, two
electrodes at a time, to all electrodes in the array.
[0010] In one embodiment the diffracting state exists when no
electric field is applied across the SBG element and the
non-diffracting state exists when an electric field is applied.
[0011] In one embodiment the diffracting state exists when an
electric field is applied across the SBG element and the
non-diffracting state exists when no electric field is applied.
[0012] In one embodiment when contact is made with an external
material at a region on the platen a portion of the light incident
at the region on the platen contacted by the external material is
transmitted out of the platen, wherein light incident on the outer
surface of the platen in the absence of the contact with an
external material is reflected downwards.
[0013] In one embodiment when contact is made with an external
material at a region on the platen a portion of the light incident
at the region on the platen contacted by the external material is
reflected downwards. Light incident on the outer surface of the
platen in the absence of the contact with an external material is
transmitted out of the platen.
[0014] In one embodiment the output from the detector is read out
in synchronism with the switching of the electrode elements.
[0015] In one embodiment the light source is one of a laser or LED
and the light is coupled into the waveguiding structure by one of a
grating or a prism.
[0016] In one embodiment the switchable grating clad is a
switchable Bragg grating recorded in one of a HPDLC grating,
uniform modulation grating or reverse mode HPDLC grating.
[0017] In one embodiment the switchable grating clad includes at
least one of a fold grating or a multiplexed grating or a rolled
k-vector grating.
[0018] In one embodiment a method of making a contact image
measurement comprising the steps of: [0019] a) Providing a
waveguiding structure for propagating light in a first direction
comprising, in series disposed in a layer sandwiched by transparent
substrates, a first clad medium, a first core, a switchable grating
clad, a second core, and a second clad medium; electrodes applied
to opposing surfaces of the substrates at least one patterned into
a set of parallel elements orthogonally traversing the cores; a
light source optically coupled to the first and second cores; a
platen in optical contact with the waveguiding structure; a
detector optically coupled to the first and second core regions;
[0020] b) coupling the light into a TIR path in the waveguiding
structure; [0021] c) an external material contacting a region on
the external surface of the platen; [0022] d) setting first and
second electrode elements to a first voltage state and all other
voltage-addressed electrodes set to a second voltage state; [0023]
e) switchable grating regions overlapped by a first electrode
element diffracting TIR light from first core into a path to the
platen outer surface; [0024] f) switchable grating regions
overlapped by a second electrode element diffracting light
reflected from one of the region or the platen external surface
into a TIR path in the second core; and [0025] g) transmitting the
reflected light to the detector.
[0026] In one embodiment the first voltage state corresponds to a
voltage being applied and the second voltage state corresponds to
no voltage being applied.
[0027] In one embodiment the first voltage state corresponds to no
voltage being applied and the second voltage state corresponds to a
voltage being applied.
[0028] In one embodiment at least a portion of the light incident
at the region on the platen is transmitted out of the platen,
wherein at least a portion of the second optical path light not
incident at the region is reflected.
[0029] In one embodiment at least a portion of the light incident
at the region on the platen is reflected, wherein at least a
portion of the second optical path light not incident at the region
being transmitted out of the platen.
[0030] In one embodiment the waveguiding structure comprises a
multiplicity of the cores and the dads cyclically arranged.
[0031] In one embodiment the voltages are applied sequentially, two
electrodes at a time, to all electrodes in the array.
[0032] In one embodiment the output from detector is read out in
synchronism with the switching of the electrode elements.
[0033] In one embodiment the contact image sensor is configured as
a finger print scanner or a touch sensor.
[0034] 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
[0035] FIG. 1A is a schematic side elevation view of a contact
image sensor using a tablet computer as a light source in one
embodiment.
[0036] FIG. 1B is a detail of schematic side elevation view of a
contact image sensor using a tablet computer as a light source in
one embodiment.
[0037] FIG. 1C is schematic plan view of a contact image sensor
using a tablet computer as a light source in one embodiment.
[0038] FIG. 2A is a plan view of a transparent electrode array in
one embodiment.
[0039] FIG. 2B is a scan line displayed on a computer tablet used
as an illumination source in one embodiment.
[0040] FIG. 3 is a plan view of a fold grating in one
embodiment
[0041] FIG. 4 is schematic side elevation view of a contact image
sensor using a bidirectional waveguide in one embodiment.
[0042] FIG. 5 is schematic side elevation view of a contact image
sensor using a bidirectional waveguide in one embodiment.
[0043] FIG. 6 is schematic plan view of a waveguide containing a
fold grating used in a contact image sensor in one embodiment.
[0044] FIG. 7A is a schematic drawing of a waveguide structure used
in contact image sensor in one embodiment.
[0045] FIG. 7B is a detail of a waveguide structure used in contact
image sensor using a unidirectional illumination and detection
waveguide in one embodiment.
[0046] FIG. 8A is a schematic side elevation view of a contact
image sensor using a unidirectional illumination and detection
waveguide in one embodiment.
[0047] FIG. 8B is a schematic plan view of a contact image sensor
using a unidirectional illumination and detection waveguide in one
embodiment.
[0048] FIG. 9 is a schematic plan view of a detail of contact image
sensor using a unidirectional illumination and detection waveguide
in one embodiment.
[0049] FIG. 10A is a cross section view showing ray propagation in
a waveguide core in one embodiment
[0050] FIG. 10B is three dimensional view showing ray propagation
in a waveguide core in one embodiment
[0051] FIG. 11A is a schematic plan view of a contact image sensor
using a unidirectional illumination and detection waveguide in one
embodiment.
[0052] FIG. 11B is a schematic cross section view of a contact
image sensor using a unidirectional illumination and detection
waveguide in one embodiment.
[0053] FIG. 12A is a schematic cross section view showing a stage
in the process of fabricating a unidirectional illumination and
detection waveguide in one embodiment.
[0054] FIG. 12B is a schematic cross section view showing a stage
in the process of fabricating a unidirectional illumination and
detection waveguide in one embodiment.
[0055] FIG. 12C is a schematic cross section view showing a stage
in the process of fabricating a unidirectional illumination and
detection waveguide in one embodiment.
[0056] FIG. 12D is a schematic plan view of an electrode structure
in one embodiment
[0057] FIG. 13 is a schematic plan view of a waveguide structure
for a unidirectional illumination and detection waveguide in one
embodiment.
[0058] FIG. 14 is a schematic plan view of an electrode array
coated substrate for a unidirectional illumination and detection
waveguide in one embodiment.
[0059] FIG. 15 is a schematic plan view of a waveguide structure
for a unidirectional illumination and detection waveguide in one
embodiment.
[0060] FIG. 16 is a flow chart for making a contact image
measurement using a unidirectional illumination and detection
waveguide in one embodiment.
[0061] FIG. 17 is a schematic cross section view of a contact image
sensor using a bidirectional waveguide and a multiplexed grating
beam control layer in one embodiment.
[0062] FIG. 18 is a schematic plan view of a detail of the
embodiment of FIG. 17.
[0063] FIG. 19 is a detail of a multiplexed grating beam control
layer used in the embodiment of FIG. 17.
[0064] FIG. 20 is a grating characteristic of the embodiment of
FIG. 0.17.
DETAILED DESCRIPTION OF THE INVENTION
[0065] It will be 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.
[0066] In the following description SBG (Switchable Bragg Grating)
will refer to a Bragg grating that can be electrically switched
between an active or diffracting state and an inactive or
non-diffractive state. In the embodiments to be described below the
preferred switchable grating will be a Switchable Bragg Grating
(SBG) recording in a Holographic Polymer Dispersed Liquid Crystal
(HPDLC) material. The principles of SBGs will be described in more
detail below. For the purposes of the invention a non switchable
grating may be one based on any material or process currently used
for fabricating Bragg gratings. For example the grating may be
recorded in a holographic photopolymer material. In some
embodiments a non switchable grating may be provided by a surface
relief grating.
[0067] An (SBG) is formed by recording a volume phase grating, or
hologram, in a polymer dispersed liquid crystal (PDLC) mixture.
Typically, SBG devices are fabricated by first placing a thin film
of a mixture of photopolymerizable monomers and liquid crystal
material between parallel glass plates. Techniques for making and
filling glass cells are well known in the liquid crystal display
industry. One or both glass plates support electrodes, typically
transparent indium tin oxide films, for applying an electric field
across the PDLC layer. A volume phase grating is then recorded by
illuminating the liquid material with two mutually coherent laser
beams, which interfere to form the desired grating structure.
During the recording process, the monomers polymerize and the HPDLC
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 PDLC layer. When an electric
field is applied to the hologram 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
resulting in for a "non diffracting" state. Note that the
diffraction efficiency of the device can be adjusted, by means of
the applied voltage, over a continuous range from near 100%
efficiency with no voltage applied to essentially zero efficiency
with a sufficiently high voltage applied. U.S. Pat. Nos. 5,942,157
and 5,751,452 describe monomer and liquid crystal material
combinations suitable for fabricating SBG devices.
[0068] To simplify the description of the invention the electrodes
and the circuits and drive electronics required to perform
switching of the SBG elements are not illustrated in the Figures.
Methods for fabricated patterned electrodes suitable for use in the
present invention are disclosed in PCT US2006/043938. Other methods
for fabricating electrodes and schemes for switching SBG devices
are to be found in the literature. The present invention does not
rely on any particular method for fabricating transparent switching
electrodes or any particular scheme for switching arrays of SBGs.
Although the description makes reference to SBG arrays the
invention may be applied using any type of switchable grating. To
clarify certain geometrical of aspects of the invention reference
will be made to A Cartesian (XYZ)coordinate system where
appropriate.
[0069] In one embodiment illustrated in FIGS. 1-3 there is provided
a contact image sensor in which line scanned illumination is
provided by a computer tablet screen. The apparatus further
comprises a platen 102, a waveguide layer 101 and a lower substrate
103 in contact with the screen of the table 104. The platen and
lower substrate together provide a waveguide cell. Note that in
practice it will be advantageous for the waveguide to be fabricated
in a separate cell which is then laminated to the platen. As shown
in the cross section detail of FIG. 1B and the plan view of FIG.
1C. The waveguide layer comprises alternating of strips of
switchable SBG cladding 105 and polymer cores 106. Electrodes are
applied to opposing faces of the platen and lower substrate. As
shown in FIG. 2A at least one electrode 109 is patterned into
column-shaped elements 109 disposed in an orthogonal direction to
the waveguide cores. The electrodes are used to switch portions of
the clad between non-diffracting states. A clad region in its
diffracting states couples light reflected from the platen surface
into an adjacent core. All core regions under an addressed
electrode are switched simultaneously. FIG. 1C shows one such
grating region indicated by 114 lying under the voltage addressed
electrode 109. This grating region diffracts the reflected light
1107 into the TIR light path 1108. As will be discussed later the
diffracting state may occur with or without an applied voltage
across the grating according to the type of holographic material
system used. Note that the light has been illustrated as undergoing
TIR in the plane of the drawing. However, in practice, the grating
orientation (as defined by the grating k-vector) and reflected beam
vector will result in a more complicated TIR path which will
typically result in rays undergoing a spiral TIR path down each
core. The signals from the waveguide cores are collected by a
linear detector at the end of the waveguide. In the simplest
embodiment each waveguide core abuts a pixel of the detector.
However, other coupling schemes should be apparent to those skilled
in ther art. At any time the tablet 112 displays a column 115 of
width a few pixels against a black background 116 as illustrated in
FIG. 2B. The bright column is scrolled continuously across the
tablet screen. The width of the column may be just one pixel. In
practice a width of several pixels may be required to achieve an
adequate signal level. The tablet based on LCoS or LED technology
will normally emit light over a large cone angle as indicated by
ther rays 1100,1101. A small portion of this light lying within a
small solid angle will be totally internally reflected at the outer
surface of the platen as indicated by the rays 1103-1104. The
reflected light 1104 is then coupled into a waveguide by an active
region of the waveguide as discussed above. In one embodiment
wherever an external body such as a finger touches the platen, it
"frustrates" the reflection process, causing light to leak out of
the platen. Thus, the parts of the skin that touch the platen
surface reflect very little light, forming dark pixels in the
image. The image is built up line by line into a finger print
image. A key advantage of this embodiment is that the tablet
eliminates the need for a separate scanner layer allow making the
sensor thinner, cheaper and lower power consumption. However, the
use of visible light may preclude its application in many security
applications.
[0070] The complex beam steering required to couple the light
reflected from the platen in the waveguide cores requires a grating
structure referred to by the inventors as a fold grating. This type
of grating is normally used for changing beam direction and
providing beam expansion within a waveguide. Gratings designed for
coupling light into or out of a waveguides are tilted around an
axis lying in the waveguide plane. Fold gratings have a more
generalized tilt. In their simplest implementation, as used in the
present invention, they are tilted around an axis perpendicular to
the waveguide plane such they deflect beams in the waveguide plane.
More generally, they may have tilts defined by two rotation angles
so that, for example, light can be coupled into the waveguide and
deflected into an orthogonal direction inside the waveguide, all in
one step. FIG. 3 is a plan view of a basic fold grating 118. When
the set of rays 1110 encounter the grating, they diffract in a
manner that changes the direction of propagation by 90.degree..
Note that when a ray encounters the grating, regardless of whether
it intersects the grating from above or below, a fraction of it
changes direction and the remainder continues unimpeded. A typical
ray will interact many times with vertically (in the Y direction)
while some light will be moving laterally (in the X direction).
From a design perspective, it is desirable to engineer the amount
of light 1111 emerging from the output edge of the grating to be
uniformly distributed laterally and the amount of light 1112
emerging from the side edge of the grating to be as small as
possible.
[0071] In one embodiment illustrated in FIG. 4 an illumination
layer sensor for providing line scanned illumination and a detector
layer from receiving reflected light from a platen and transmitting
it to an infrared detector are combined in a single SBG array
waveguide layer. The contact image sensor comprises a waveguide
grating layer 161 a transmission grating 162 a platen 163, a light
source 165 and a detector 166. In one embodiment the transmission
grating and waveguide are air separated. In one embedment the
transmission grating and waveguide are separated by a think layer
of low index nanoporous material. Light from the source is coupled
into a TIR path 1140 in the waveguide by a coupling grating 167.
The waveguide contains an array of switchable grating elements such
as 164 which is shown in its diffracting state. The grating
elements switch in scrolling fashion, each element in its
diffracting state diffracting light output out of the waveguide
toward the transmission grating. For example the active element 164
diffracts the TIR light into the direction 1141, typically normal
to the waveguide. The transmission grating deflects the light into
the direction 1142 inside the platen meeting the platen surface TIR
conditions. Light reflected from the platen 1143 passes through the
transmissive grating without deviation since it is now off-Bragg
(that is, it lies outside the diffraction efficiency angular
bandwidth of the grating as predicted by Kogelnik theory). The
optical surfaces of the waveguide are roughened to ensure that at
least a small portion of the reflected light from the platen enters
the TIR path 1144 to the detector. Advantageously, the source and
detector operate in the infrared. The detector is typically a
linear detector.
[0072] In one embodiment illustrated in the cross sectional view of
FIG. 5 a contact image sensor similar in concept to the one of FIG.
4 comprises a waveguide 170, a first passive transmission Bragg
grating 171 a second passive transmission Bragg grating 174, and a
platen 175. The waveguide contains a SBG grating array 176
comprising column shaped elements, an input coupling grating 177
for coupling light from an infrared source 178 into the waveguide.
The apparatus further comprises a light output coupling grating 180
for directing light onto a linear infrared detector array 181. The
transmission Bragg gratings may be recorded in a holographic
photopolymer or in a HPDLC material, the latter providing an
attractive passive medium owing to its high index modulation
capability. At any time two elements of the grating array, such as
the ones indicated by 176A,176B are in a diffracting state. We next
consider the light path through the contact image sensor. Light
1150 from the source is coupled into a TIR path 1151 in the
waveguide by the input grating. The active grating element 176B
deflects the light out of the waveguide into the direction 1152
which is typically normal to the waveguide. The second passive
grating 173 diffracts the light into the platen in the direction
1153. After reflection from the platen outer surface the light 1154
is diffracted toward the waveguide by the first passive grating 171
in the direction 1155 which is substantially normal to the
waveguide. The light is coupled into the TIR path 1156 by the
active grating element 176A and is finally deflected out of the
waveguide by the output coupling grating onto the infrared
detector. In one embodiment the apparatus of FIG. 5 further
comprises polarization components such as half wave plates and
quarter wave plates for controlling beam polarization to achieve
more efficient bidirectional transmission of light within the
waveguide as disclosed in PCT Application No.: PCT/GB2014/000295
entitled METHOD AND APPARATUS FOR CONTACT IMAGE SENSING
[0073] In one embodiment illustrated in FIG. 6 a contact image
sensor waveguide comprises an array of switching SBG columns 182
and fold grating 183. Signal light 1160, 1161 from the SBG array is
deflected into an orthogonal direction by the fold grating and
transmits light towards the linear detector array 184. The
advantage of this scheme is the output beam can be tailored to
match the length of standard linear arrays. This allows more
flexibility in the specification of the platen area and aspect
ratio.
[0074] In one embodiment illustrated in FIGS. 7-15 there is
provided a contact image sensor in which the receiver and transmit
functions are combined in a single unidirectional waveguide.
Unidirectional here means that the illumination and the reflected
light from the platen propagated in the same direction (but not in
the same waveguide core). Referring to the FIG. 7A and FIG. 7B, the
waveguide 190 comprises a layer 191 containing a cyclically
repeated waveguiding structure 192 for propagating light in a first
direction comprising, a first clad medium 193, a first core 194, a
switchable grating clad 195, a second core 196, and a second clad
medium 197. The light propagation direction z is indicated. The
first clad medium 193 and second clad medium 197 have a low
refractive index to ensure that there is no cross talk between
adjacent waveguide cores. Ideally, the first clad and second clad
media are air or a low index nanoporous material (ideally with a
refractive index well below 1.3). The switchable grating clad
contains a fold grating with a k-vector orientated in 3D space. The
waveguide layer is sandwiched by transparent substrates 1991A,
199B. In one embodiment of the substrates provides a platen.
However, advantageously, the waveguide would be fabricated in a
separate cell which is then laminated to the platen. Transparent
electrodes are applied to opposing surfaces of the substrates at
least one being patterned into a set of parallel elements
orthogonally traversing the cores. An infrared source is optically
coupled to the first and second cores. Finally a detector is
optically coupled to the first and second core regions. The dashed
lines 1190, 1191 in FIG. 7A indicate the continuation of the
waveguide core and clad elements. For the sake of simplicity we
shall consider the minimum configuration ie first and seconds cores
a switchable grating clad and first and second clad media as
discussed above. The switching electrodes have very small gaps. In
one embodiment, the basic core clad group 192 has a total width of
50.8 um to provide a 500 dpi repeat pattern. In one embodiment the
voltages are applied sequentially, two electrodes at a time, to all
electrodes in the array. The output from the detector is read out
in synchronism with the switching of the electrode elements. The
first core provides an illumination (or transmitter) waveguide
while the second core provides a signal (or receiver) waveguide.
The SBG clad essentially `opens` a side of the waveguide wall,
allowing light to bounce up onto the platen, onto the finger, and
back down onto the SBG fold, but significantly further up the
waveguide (by an amount determined by the platen thickness). In the
embodiment illustrated the electrodes are orthogonal to the cores
and clad. In one embodiment the electrodes are tilted at a small
angle to the normal to the waveguide direction (in the plane of the
waveguide) to overcome the risk of cross coupling between
neighboring cores. In one embodiment the SBG clad is in direct
contact with the platen layer. In other embodiments the SBG clad
may be separated from the platen layer by intermediate transparent
layers. In some embodiments it may be advantageous to have LC
alignment layers in proximity to the SBG clad to promote more
optimal alignment of the LC directors in the SBG clad. In some
embodiments polarization control layers may be provided in
proximity to the waveguide layer.
[0075] The operation of the contact image sensor will now be
discussed in more detail. FIG. 8A shows a schematic cross section
of the platen 200 and waveguide 190. FIG. 8B is a plan view showing
the electrode array 201 and finger 202 in contact with the platen.
First and second simultaneously voltage-addressed electrodes are
indicated by 203,204. The light propagation direction z is
indicated by an arrow. Considering the cross section view of FIG.
8A, light incident in one waveguide cores is indicated by 1190.
This light is diffracted upwards towards the platen by a SBG clad
region overlaid by the first voltage addressed electrode. The
reflected light 1192 is diffracted into a second core by the SBG
clad region overlaid by the second voltage-address electrode. FIG.
8B shows the sideways displacement of the guided light. The input
path in the first waveguide is deflected sideways and up to the
platen by the SBG clad region overlaid by the first
voltage-addressed electrtode 203 as indicated by the rays
1195,1196. On reflection the downward light 1197 undergoes a
further sideways shift when it interacts with the SBG clad region
overlaid by the second voltage-addressed electrode 204 as indicated
by the rays 1197,1198. Finally the light proceeds along a TIR path
along the second waveguide core up to the detector.
[0076] The coupling of light between two cores via the SBG clad
region is illustrated in plan view in FIG. 9 which shows first and
second cores 210,211 separated by the SBG clad 212. Two regions of
the SBG clad simultaneously addressed by first and second
electrodes (not shown) are indicated by shaded areas 213,214. The
wave guided beam 1201 (from the infrared source) in the first core
is coupled by the SBG grating region overlapped by the first
voltage-addressed electrode into a path up to the platen. At the
platen the beam undergoes total internal reflection and propagates
into the SBG region overlapped by the second voltage-addressed
electrode. Here it is diffracted into the TIR path 1202 in the
second core (leading to the detector)
[0077] Examples of the TIR paths in the cores are illustrated in
the core cross section view of FIG. 10A and the three dimensional
view of a portion of a core in FIG. 10B. The grating orientation
(as defined by the grating k-vector) and the reflected beam vector
results in the spiral TIR path indicated by the ray paths 1203,1204
in FIG. 10A and the ray paths 1207,1208 shown interacting with the
reflected faces of the core at points 1209-1212 in FIG. 10B.
[0078] FIG. 11A is a plan view of the waveguide showing the
propagation of illumination and signal light between neighboring
waveguide cores. The section of the waveguide array illustrated
comprises a first clad region 222, a first core 223, a SBG clad
224, a second core 225, a second clad region 226. Transparent
electrodes 227-230 traverse the cores and dads. The illumination
light in first core follows TIR path 1220-1222. Electrodes 227,230
are simultaneously voltage addressed such that SBG regions 231,232
are switched into their diffracting states. The TIR ray 1120 is
coupled into the first core is diffracted upwards towards the
platen as indicated by the rays 1222, 1223. After reflection at the
outer surface of the platen the reflected ray 1224 is diffracted by
the SBG region 231 as indicated by the rays 1225 and coupled into a
TIR path 1226 within the second core 225 which transmits the signal
light to an element of the linear infrared detector (not shown).
FIG. 11B is a cross section of the waveguide in the YZ plane
showing a projection of the ray paths in the first core, SBG clad
and second core. FIG. 11B uses the labelling of FIG. 11A. The TIR
path in the first core is indicated by the rays 1230, the coupling
of the TIR light and diffraction up to the platen is represented by
the ray 1234; the reflection of the light from the platen and
coupling into the second core is represented by 1235; and the TIR
path in the second core is represented by 1236.
[0079] FIG. 12 illustrates three stages in the fabrication of the
waveguiding structures. In a first stage illustrated in FIG. 12A
patterned electrodes are applied to a transparent substrate. In a
second stage shown in FIG. 12B a lower substrate is etched to form
cavities for the clad material and electrodes coatings are applied
to the bases of the cavities to be filled with SBG recording
material. In a next key stage of the process the SBG clad cavities
are filled with holographic material and exposed to form fold
gratings. After curing of the hologram the upper and lower
substrates are laminated as shown in FIG. 12C. The electrode
pattern used for at least one of the coatings applied to the
substrates in FIG. 12A or FIG. 12B is shown in FIG. 12D. Either the
upper or lower substrate electrodes may be patterned in this way
with the opposing substrate surface having a continuous electrode
coating. In one embodiment both electrodes are patterned according
to FIG. 12D. The processes for etching substrates and applying
patterned electrodes are well known to those skilled in the art.
Other intervening steps will be required for coating etching
holographic recording and gluing and illuminination, as should be
apparent to those skilled in the art. Examples of a waveguide plate
and a patterned electrode plate based on the principles of FIG. 12
are shown in FIGS. 13-14.
[0080] FIG. 15 shows elements of the waveguide structure 250 in one
embodiment comprising a waveguide array layer 251, an infrared
source module, beam expansion and waveguide coupling optics 252, an
infrared detector module 253, comprising a linear infrared detector
array 254. The apparatus further comprises a first substrate with
patterned electrode coating 256, flexible electrical connectors
257,258 linking the electrodes to a drive module and power supply
(not shown), and a second substrate 260 with patterned electrode
coating 261, with flexible electrical connectors 262,263 linking
the electrodes to the drive module and power supply. Note that one
of the substrates may comprise the waveguide substrate described in
FIG. 12.
[0081] In one embodiment the diffracting state of the SBG clad
exists when no electric field is applied across the SBG element and
the non-diffracting state exists when an electric field is applied.
In one embodiment the diffracting state exists when an electric
field is applied across the SBG element and the non-diffracting
state exists when no electric field is applied. Materials having
this property are referred to as reverse mode.
[0082] In one embodiment the SBG clad is recorded in a reverse mode
HPDLC material. Reverse mode HPDLC differs from conventional HPDLC
in that the grating is passive when no electric field is applied
and becomes diffractive in the presence of an electric field. The
reverse mode HPDLC may be based on any of the recipes and processes
disclosed in PCT Application No.: PCT/GB2012/000680, entitled
IMPROVEMENTS TO HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL
MATERIALS AND DEVICES.
[0083] In one embodiment when contact is made with an external
material at a region on the platen a portion of the light incident
at the region on the platen contacted by the external material is
transmitted out of the platen. Light incident on the outer surface
of the platen in the absence of the contact with an external
material is reflected downwards. In one embodiment when contact is
made with an external material at a region on the platen a portion
of the light incident at the region on the platen contacted by the
external material is reflected downwards. Light incident on the
outer surface of the platen in the absence of the contact with an
external material. is transmitted out of the platen.
[0084] The light source in any of the embodiments may be a laser or
LED desirably operating in the infrared. Light is coupled into the
waveguiding structure by one of a grating or a prism. In one
embodiment the coupling grating or prism is clocked. In the case of
a grating this means that the projection of the k-vector in the
plane of the waveguide layer is an angle to the waveguide long
axis. Clocking the input coupler will normally result in a
spiralling of the TIR path within the waveguide. In combination
with the fold grating in the SBG layer this property can be used to
control the grating coupling efficiency and angular response.
[0085] In one embodiment the SBG clad is recorded in a uniform
modulation holographic material system. Exemplary uniform
modulation liquid crystal-polymer material systems are disclosed in
United State Patent Application Publication No.: US2007/0019152 by
Caputo et al and PCT Application No.: PCT/EP2005/006950 by Stumpe
et al. both of which are incorporated herein by reference in their
entireties. Uniform modulation gratings are characterized by high
refractive index modulation (and hence high diffraction efficiency)
and low scatter.
[0086] In one embodiment the SBG clad includes at least one of a
fold grating or a multiplexed grating or a rolled k-vector grating.
Multiplexed gratings and rolled k-vectors may be used to improve
the angular response of the SBG clad. The application of
multiplexing, and spatial varying thickness, k-vector directions
and diffraction efficiency in the present invention may be based on
the embodiments, drawings and teachings provided in U.S. patent
application Ser. No. 13/506,389 entitled COMPACT EDGE ILLUMINATED
DIFFRACTIVE DISPLAY, U.S. Pat. No. 8,233,204 entitled OPTICAL
DISPLAYS, PCT Application No.: US2006/043938, entitled METHOD AND
APPARATUS FOR PROVIDING A TRANSPARENT DISPLAY, PCT Application No.:
GB2012/000677 entitled WEARABLE DATA DISPLAY, U.S. patent
application Ser. No. 13/317,468 entitled COMPACT EDGE ILLUMINATED
EYEGLASS DISPLAY, U.S. patent application Ser. No. 13/869,866
entitled HOLOGRAPHIC WIDE ANGLE DISPLAY, and U.S. patent
application Ser. No. 13/844,456 entitled TRANSPARENT WAVEGUIDE
DISPLAY.
[0087] In one embodiment there is provided a method of making a
contact image measurement comprising the steps of: [0088] a)
providing a waveguiding structure for propagating light in a first
direction comprising, in series disposed in a layer sandwiched by
transparent substrates, a first clad medium, a first core, a
switchable grating clad, a second core, and a second clad medium;
electrodes applied to opposing surfaces of the substrates at least
one patterned into a set of parallel elements orthogonally
traversing the cores; a light source optically coupled to the first
and second cores; a platen in optical contact with the waveguiding
structure; a detector optically coupled to the first and second
core regions; [0089] b) coupling the light into a TIR path in the
waveguiding structure; [0090] c) an external material contacting a
region on the external surface of the platen; [0091] d) setting
first and second electrode elements to a first voltage state and
all other voltage-addressed electrodes set to a second voltage
state; [0092] e) switchable grating regions overlapped by a first
electrode element diffracting TIR light from first core into a path
to the platen outer surface; [0093] f) switchable grating regions
overlapped by a second electrode element diffracting light
reflected from one of the region or the platen external surface
into a TIR path in the second core; and [0094] g) transmitting the
reflected light to the detector.
[0095] In one embodiment the first voltage state corresponds to a
voltage being applied and the second voltage state corresponds to
no voltage being applied. In one embodiment the first voltage state
corresponds to no voltage being applied and the second voltage
state corresponds to a voltage being applied. This applies to
reverse mode materials.
[0096] In one embodiment at least a portion of the light incident
at the region on the platen is transmitted out of the platen,
wherein at least a portion of the second optical path light not
incident at the region is reflected. In one embodiment at least a
portion of the light incident at the region on the platen is
reflected, wherein at least a portion of the second optical path
light not incident at the region being transmitted out of the
platen.
[0097] In one embodiment the waveguiding structure comprises a
multiplicity of the cores and the dads cyclically arranged.
[0098] In one embodiment the voltages are applied sequentially, two
electrodes at a time, to all electrodes in the array.
[0099] In one embodiment the output from detector is read out in
synchronism with the switching of the electrode elements.
[0100] A method of a method of making a contact image measurement
in one embodiment of the invention in accordance with the basic
principles of the invention is shown in the flow diagram in FIG.
14. Referring to the flow diagram 500, we see that the said method
comprises the following steps. [0101] At step 501 providing a
waveguiding structure for propagating light in a first direction
comprising, in series disposed in a layer sandwiched by transparent
substrates, a first clad medium, a first core, a switchable grating
clad, a second core, and a second clad medium; electrodes applied
to opposing surfaces of the substrates at least one patterned into
a set of parallel elements orthogonally traversing the cores; a
light source optically coupled to the first and second cores; a
platen in optical contact with the waveguiding structure; a
detector optically coupled to the first and second core regions;
[0102] At step 502 coupling the light into a TIR path in the
waveguiding structure; [0103] At step 503 an external material
contacting a region on the external surface of the platen; at step
504 setting first and second electrode elements to a first voltage
state and all other voltage-addressed electrodes set to a second
voltage state; [0104] At step 505 switchable grating regions
overlapped by a first electrode element diffracting TIR light from
first core into a path to the platen outer surface; [0105] At step
506 switchable grating regions overlapped by a second electrode
element diffracting light reflected from one of the region or the
platen external surface into a TIR path in the second core; and
[0106] At step 507 transmitting the reflected light to the
detector.
[0107] In one embodiment shown in FIG. 17 a contact image sensor
300 comprises a platen 301 overlaying multiplexed Bragg grating
layer 302 containing the multiplexed gratings 303,303 which have
opposing slant angles and differing grating pitches, a
bidirectional waveguide 305 contain switching bidirectional grating
306, input grating 307 an output put grating 309 grating elements
309 and 310 are shown in their diffracting states. The input and
output gratings are passive gratings. The bidirectional waveguide
is sandwich by the layers 311, 312 which are shown in more detail
in FIG. 17. The input and output gratings are coupled to a laser
source 313 and detector 314. Then input grating couples the light
1240 from the laser into the TIR path 1241. The active grating
element 309 couples this light toward the multiplexed grating layer
in the beam direction 1242. The multiplexed grating 303 layer
diffracts a portion of the light into the direction 1243 and the
grating 304 diffracts a portion of the light into the direction
1244. The first beam is directed to a light trap, which is not
illustrated. The second beam is reflected at the outer platen
surface a beam 1255 which interacts with the multiplexed grating a
second time and is diffracted by the grating 304. This light is
coupled into a TIR path in the bidirectional waveguide by the
active element 310. Note that the separation of the addressed
grating elements is determined by the reflection path. The
in-coupled light follows the TIR path 1248 up the output grating
and is diffracted in to the output beam 1249 toward the detector.
The platen and passive transmission multiplexed grating are
separated from the bidirectional waveguide by either an air gap or,
ideally, a low index layer. In one embodiment the SBG array is
comprised of 1600 column SBG elements each overlapped by 48.8
micron wide etched ITO electrodes on a glass substrate (the element
pitch is 50.8 micron pitch equivalent to 500 elements/inch). In the
waveguide, light from the backlight is trapped by total internal
reflection, unless one of the column-shaped grating segments
happens to be active.
[0108] When an active segment is encountered, a thin lamina of
light is diffracted out of the waveguide. By rapidly switching the
columns ON and OFF in sequence, a thin sheet of light can be
engineered to sweep across the surface of the scanner and into the
tilt grating. During a scan, the user's four fingers are placed
onto the platen surface. Wherever the skin touches the platen, it
"frustrates" the reflection process, causing light to leak out of
the platen. Thus, the parts of the skin that touch the platen
surface reflect very little light, forming dark pixels in the
image. The image is built up line by line into a 500 dpi,
FBI-approved industry standard picture. The second function of the
bidirectional waveguide is to provide a waveguide path to propagate
signal light from the platen to a detector array.
[0109] FIG. 18 shows a detail 320 of the bidirectional waveguide
305 of FIG. 17. The waveguide is sandwiched by the layers 312 and
313. Confinement of the reflected beams from the platen is achieved
by applying thin stripes of infrared (IR) absorbing material 322
separated by clear regions 323 to the top substrate 312 and
roughening or frost-etching the bottom substrate into striped
regions 325 separated by clear regions 326 overlapping the IR
absorbing regions. Note that the IR absorbing and frost-etched
strips are typically much narrower than the adjacent transparent
regions. The IR stripes define parallel propagation channels
terminating at a high resolution infrared linear detector array.
Collimated reflected beams from the platen enter the detector layer
in the gaps between the IR absorbing stripes and undergo TIR within
up to the detector array. Hence the beam propagation is analogous
to that provided by waveguide cavities. Light scattered out of a
give channel is scattered by the frosted layer and absorbed by the
IR coating. Any forward scattered light or multiple scatter between
near neighbouring channels will tend to diminish in intensity with
each ray surface interaction and will form a background noise level
that can be subtracted from the fingerprint signature by the
processing software.
[0110] The key challenge in the embodiment of FIG. 17 is how to
separate the upward-diffracted and platen-reflected beams
(illumination and signal beams) in the bi-directional waveguide. If
the signal beam follows a path parallel to that of the illumination
beam it will end up being diffracted back towards the illuminator.
This is a consequence of the symmetry of diffraction gratings. To
overcome this problem we use two different grating prescriptions in
the waveguide. This can be accomplished in a single grating layer
with all the necessary grating functionality being encoded into the
holographic master during fabrication. A first grating prescription
provides the array used to scan the beam; TIR illumination light
being diffracted vertically upwards. The second grating
prescription, which is used in the portion of the SBG layer
overlapping the illumination source (that is, the grating 307) is
designed to have a narrow angular bandwidth. By narrow angular
bandwidth we mean that the diffraction efficiency versus angle
profile has a narrow FWHM (full width half maximum) width. If the
signal beam is offset from the illumination beam by a small angle
it will not re-interact with the upward-diffracting gratings but
will be diffracted by the detector grating. This narrower angular
bandwidth is achieved by making the grating thicker. The purpose of
the multiplex gratings is to provide the required angular offset.
One of the two multiplexed gratings is designed to tilt the
illumination beam into the correct angle for TIR at the platen
surface the second multiplexed gratings diffracts the light
reflected off the platen, the signal beam, into a direction at a
small angle (around 3-4 degrees) to the vertical. This light is
rediffracted by the scanner grating which has a bandwidth large
enough to diffract the light and has a grating vector biassed to a
give a strong DE for the signal beam. (Typically, the required
angular bandwidth is achieved by limiting the grating thickness to
around 3 microns). The signal light proceeds to TIR along the
waveguide until it is diffracted out of the waveguide towards an
infrared detector array by the detector grating. Owing to the low
angular bandwidth of the illumination input grating very little of
the signal light will be diffracted towards the illumination
backlight. Cross-coupling between the detector and illumination
channels is thus avoided.
[0111] Cross coupling must also be addressed in the multiplex
grating layer in the embodiment of FIG. 17. FIG. 19 shows a detail
of the multiplexed grating layer (labelled 331 which is shown
sandwiched between the platen 301 and a further transparent
substrate 334. Note that for ease of fabrication a further
transparent substrate can be disposed between the multiplexed
grating layer and the platen so that the multiplexed grating
components can be recorded in a separated cell that is laminated
with the paten. FIG. 18 illustrates the basic ray optics showing
the two multiplex gratings 332,333 also labelled by G1,G2 with
grating vectors (normal to the grating fringes) K.sub.1,K.sub.2.
The useful signal corresponds to the ray paths in which
illumination light interacts first with G1 and then with G2 that is
the ray path comprises the rays 1265-1268 emerging from the
multiplexed grating layer at angle .theta. to the surface normal
1269. These reflections satisfy the Bragg condition and are
therefore called on-Bragg (as opposed to ray-grating interactions
such as those at points A, B which do not satisfy the Bragg
condition and are called off-Bragg). The unwanted light (or noise
signal) corresponds to a second set of paths in which the
illumination light interacts first with G2 and then with G1 for
example the rays 1260-1263 emerging from the multiplexed grating
layer at angle .theta. to the surface normal 1264. From grating
symmetry the first and second paths illustrated are reciprocal. The
input beams for the signal and noise paths are each nominally at
0.degree. to the platen. Noise is suppressed by designing G1 and G2
to have slightly different slant angles and pitches (fringe
spacings). The noise and signal beams are the same offset angles
but are engineered to suppress the noise and maximize the signal.
G2 weakly in-couples illumination light at 0.degree. but strongly
out-couples the waveguide angle .theta.. On the other hand, G1
weakly out-couples light at angle .beta.-.delta..theta. to the
platen normal (where .beta. is the total internal reflection angle
at the platen for the ray path 1260-1263 and is a small angular
increment). However the grating G1 strongly in-couples signal at
waveguide angle. In one embodiment the grating G2 has construction
angles 4.degree. and 47.degree. and grating G1 has construction
angles 0.degree. and 47.degree.. In each case the wavelength is 785
nm, the average index of the hologram is 1.53, the hologram
thickness is 15 micron and the holographic refractive index
modulation is 0.002. One issue to be addressed is that the
multiplexed grating causes a split platen interrogation signal
(owing to diffracted and 0-order non-diffracted light being
produced at each beam-grating interaction. This process will result
in some light loss out of the platen. However, this signal loss can
be mitigated by using a powerful laser and by making the
multiplexed grating thicker. A further issue is that stray
(0-order) light reflected vertically downwards from the platen
could contribute to the noise level. This problem can be avoided by
tilting the illumination beam diffracted from the bidirectional
waveguide by up to say 10.degree.. Any directly reflected light
would then be at 10.degree. relative to the SBG detector layer, and
then would fall outside the angular bandwidth of the SBG, pass
straight through and be dumped. Stray light reflected inside the
platen and multiplexed grating layer can be trapped using prismatic
elements (or other equivalent means.
[0112] The inventors are confident that they can fabricate high
quality multiplexed gratings that deliver the above functionality.
SBG Labs has already demonstrated multiplexed gratings in more
challenging applications. In the present case the gratings are at
opposing slant angles which is generally considered to be most
favourable consideration for producing high diffraction efficiency
and uniformity while avoiding the grating formation competition
that can occur which gratings are slanted at similar directions.
Our mastering and replication processes for multiplexed gratings
are at any advance stage of development
[0113] The characteristics of a typical SBG grating for use in the
in the bi-directional waveguide of FIG. 17 are illustrated in FIG.
20 which shows a plot 1280 of the diffraction efficiency versus
angle 1281. The input angle is indicated by 1282 and the output
angle by 1283. Normally, the SBG grating is designed to provide an
input angle of 0.degree. with a FWHM angular bandwidth 1290 greater
than 4.degree. (corresponding to the optimum multiplexed grating
offset angle). This is will within the range of angular bandwidths
that can be achieved with our gratings. The SBG angular bandwidth
is broad enough to both output illumination light from the
bi-directional waveguide, and re-couple signal light from the
platen at the input angle plus the 4.degree. degree offset
introduced by the multiplexed gratings. The SBG chief ray is offset
by half of the multiplexed grating offset angle, that is, by
2.degree.. In one embodiment the grating has construction angles
2.degree. and 47.degree.; an operating wavelength of 785 nm. a
hologram average refractive index of 1.53, a hologram thickness of
4 micron, a grating period of 670 nm. and an index modulation of
0.056. A grating thickness of 4-5 micron is found to switch
effectively.
[0114] In any of the above embodiments, during a scan, the user's
four fingers are placed onto the platen surface. Wherever the skin
touches the platen, it "frustrates" the reflection process, causing
light to leak out of the platen. Thus, the parts of the skin that
touch the platen surface reflect very little light, forming dark
pixels in the image. The image is built up line by line into a 500
dpi, FBI-approved industry standard picture. In the absence of
finger contact the light incident on the platen outer surface is
totally internally reflected downwards towards the wave guiding
structure 50 and then on to the detector. The X coordinate of the
contacting feature is given by the detector array element providing
the dark-level or minimum output signal. The latter will be
determined by the noise level of the detector. The Y coordinate of
the contacting feature is computed from the geometry of the ray
path from the last SBG element in the first SBG array that was in a
diffracting state just prior to TIR occurring in the platen and a
signal from the reflected light being recorded at the detector. In
one embodiment of the invention an alternative detection scheme
based on the principle that in the absence of any external pressure
art the platen/air interface the incident light is transmitted out
of the platen. Now, external pressure from a body of refractive
index lower than the platen (which may a feature such as a finger
print ridge or some other entity) applied on the outer side of the
platen layer causes the light to be totally internally reflected
downwards towards the wave guiding structure. Hence the X
coordinate of the contacting feature is now given by the detector
array element providing the peak output signal. The procedure for
computing the Y coordinate remains unchanged.
[0115] The contact image sensor requires a switchable "cladding"
grating that give the following two states: an "ON" state with high
index .about.ne for waveguide coupling, when switched; and an "OFF"
state with low index .about.no to operated as a cladding layer,
when not switched. In one embodiment this is achieved by grating
optimisation. Ideally, this would work without the need for an
additional alignment layer. The two states would rely purely on the
difference in LC orientation that between the rest state and active
state of the grating. In one embodiment the twos states are
facilitated by controlling the LC director alignment. This will be
based on improvements to chemistry, improvements to current
polyimide alignment layers or a combination of both. Chemistry
improvements using low polymer concentration; low molecular weight
LC, uniform modulation material and reactive monomer-based HPDLC
recipes. In one embodiment nanoparticle-doped polyimide; and
reactive mesogen alignment layers are used to control the alignment
of the SBG clad. Uniform modulation (droplet-free) gratings may
give good alignment when used in conjunction with a suitable 3D
aligning layer. In one embodiment electric fields may be used for
alignment of the SBG clad.
[0116] An exemplary infrared detector for use in the invention is
the M206 contact image sensor available from CMOS Sensor Inc.
(www.csensor.com.). The resolution is selectable between 300 dpi to
600 dpi resolution. The device has a very fast scanning rate,
typically around 0.1 ms/line/color for 300 dpi resolution and 0.2
ms/line/color for 600 dpi resolution. The pixel readout rate is 16
Mega-pixel/sec. It provides 0.2 ms/line/color for 600 dpi
resolution.
[0117] In a typical mobile application of the invention the
preferred software platform would be a ruggedized computer tablet
such as, for example, the Panasonic Android Toughpad. Desirably,
any platform should provide an integrated GPS module. The system
components implemented on a software platform would typically
comprise an executive program, biometric software, hardware
control, finger print server, fingerprint database, graphical user
interface (GUI) and communication interfaces. The biometric
software will typically provide 1:1 and 1;N comparisons; noise
removal, matching algorithms, image enhancement and options for
saving images. The hardware control module includes software for
control the electronics for detector channel switching and readout,
illuminator component switching, laser control and basic functions
such as an on/off switch. Communication interfaces will typically
include LAN, WAN and INTERNET. System Development Kits (SDKs) for
implementing the required functionalities are currently available.
They can be categorized into low and high level tools. While low
level tools can provide rapid integration they still require the
development of a robust fingerprint reader software matching server
and other vital elements for dealing with problems such as
exception handling and system optimization, which makes embedding
them into applications problematic. When modifications or
enhancements are made to either the host application or to the
fingerprint SDK the host software must be recompiled with the
fingerprint SDK, leading to ongoing support and maintenance
problems. High level SDKs free the user from needing to understand
the parameters involved with fingerprint comparison, how they work,
why they are significant, and how data needs to be extracted from
an image as well as data type mapping, database management, data
synchronization, exception handling. The ability to perform 1:N
comparison for large databases is a highly desirable feature
important feature; opening a record set from the database and
matching one-by-one will not produce fast results. In general high
level SDKs will be better at handling poor image quality, bad image
acquisition, and unpredictable user input. Desirably the SDK should
support a variety of development environments including: C++, VB,
NET, Delphi, PowerBuilder, Java, Clarion, and web applications.
High level SDKs avoid the need for development of special DLLs
which can consume 6-12 months in development.
[0118] In applications such as finger print sensing the
illumination light is advantageously in the infrared. In one
embodiment of the invention the laser emits light of wavelength 785
nm. However, the invention is not limited to any particular
illumination wavelength.
[0119] In fingerprint detection applications the invention may be
used to perform any type "live scan" or more precisely any scan of
any print ridge pattern made by a print scanner. A live scan can
include, but is not limited to, a scan of a finger, a finger roll,
a flat finger, a slap print of four fingers, a thumb print, a palm
print, or a combination of fingers, such as, sets of fingers and/or
thumbs from one or more hands or one or more palms disposed on a
platen. In a live scan, for example, one or more fingers or palms
from either a left hand or a right hand or both hands are placed on
a platen of a scanner. Different types of print images are detected
depending upon a particular application. A flat print consists of a
fingerprint image of a digit (finger or thumb) pressed flat against
the platen. A roll print consists of an image of a digit (finger or
thumb) made while the digit (finger or thumb) is rolled from one
side of the digit to another side of the digit over the surface of
the platen. A slap print consists of an image of four flat fingers
pressed flat against the platen. A palm print involves pressing all
or part of a palm upon the platen.
[0120] The present invention essentially provides a solid state
analogue of a mechanical scanner. The invention may be used in a
portable fingerprint system which has the capability for the
wireless transmission of fingerprint images captured in the field
to a central facility for identity verification using an automated
fingerprint identification system.
[0121] Although this application has addressed automatic
fingerprint detection, it is equally relevant to other well-known
applications of contact image sensors including document scanners,
touch sensors for computer interfaces, bar code readers and optical
identification technology.
[0122] It should be emphasized that the drawings are exemplary and
that the dimensions have been exaggerated.
[0123] 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.
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