U.S. patent application number 16/310674 was filed with the patent office on 2019-08-29 for extended depth of field intraoral imaging apparatus.
The applicant listed for this patent is Carestream Dental Technology Topco Limited. Invention is credited to Yiyi Guan, James R. Milch, Victor C. Wong.
Application Number | 20190261843 16/310674 |
Document ID | / |
Family ID | 56363922 |
Filed Date | 2019-08-29 |
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United States Patent
Application |
20190261843 |
Kind Code |
A1 |
Guan; Yiyi ; et al. |
August 29, 2019 |
EXTENDED DEPTH OF FIELD INTRAORAL IMAGING APPARATUS
Abstract
An apparatus for intraoral imaging has an illumination source
that directs light to an object. An imaging apparatus forms an
image at an image sensor array from reflected light from the
object, the imaging apparatus having an optical stop along an
optical axis. A phase modulator is disposed at or near the optical
stop. An image processor conditions data from the image sensor
array and provides processed image data of the object.
Inventors: |
Guan; Yiyi; (Pittsford,
NY) ; Wong; Victor C.; (Pittsford, NY) ;
Milch; James R.; (Penfield, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Carestream Dental Technology Topco Limited |
London |
|
GB |
|
|
Family ID: |
56363922 |
Appl. No.: |
16/310674 |
Filed: |
June 15, 2016 |
PCT Filed: |
June 15, 2016 |
PCT NO: |
PCT/US2016/037483 |
371 Date: |
December 17, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61B 1/24 20130101; G06T
5/003 20130101; G02F 1/0102 20130101; A61B 1/00009 20130101; G01B
11/2513 20130101; G02F 2203/50 20130101; A61B 1/00197 20130101;
H04N 5/2256 20130101; G06T 5/20 20130101; H04N 2005/2255 20130101;
A61B 1/00172 20130101 |
International
Class: |
A61B 1/24 20060101
A61B001/24; A61B 1/00 20060101 A61B001/00; H04N 5/225 20060101
H04N005/225; G06T 5/00 20060101 G06T005/00; G02F 1/01 20060101
G02F001/01; G01B 11/25 20060101 G01B011/25 |
Claims
1. An apparatus for intraoral imaging comprising: a) an
illumination source that directs light to an object; b) an imaging
apparatus that forms an image at an image sensor array from
reflected light from the object, the imaging apparatus having an
optical stop along an optical axis; c) a phase modulator disposed
at or near the optical stop; and d) an image processor that
conditions data from the image sensor array and provides processed
image data of the object.
2. The apparatus of claim 1 wherein the image processor provides
deblurring of the image data.
3. The apparatus of claim 1 wherein the optical stop is between an
objective lens that directs the reflected light through the optical
stop and a second lens that directs light from the optical stop to
the image sensor array.
4. The apparatus of claim 1 further comprising a display for
rendering the processed image data.
5. The apparatus of claim 1 wherein the phase modulator is a cubic
phase modulator.
6. The apparatus of claim 1 wherein the phase modulator, from a
view along the optical axis, is rectangular or circular in
shape.
7. The apparatus of claim 1 wherein the phase modulator has
different phase functions in orthogonal x- and y-directions.
8. The apparatus of claim 1 wherein the illumination source
includes a solid-state light emitter.
9. The apparatus of claim 1 wherein the image sensor array is a
color or monochromatic device.
10. The apparatus of claim 1 wherein the illumination source
generates patterned illumination for surface contour imaging of the
object.
11. An apparatus for intraoral imaging comprising: a) an
illumination source that directs light to an object; b) an imaging
apparatus that forms an image from reflected light from the object
at an image sensor array, the imaging apparatus having an optical
stop along an optical axis between an objective lens that directs
the reflected light through the optical stop and a second lens that
directs light from the optical stop to the image sensor array. c) a
phase modulator disposed at or near the optical stop; d) an image
processor that is programmed with instructions to condition data
from the image sensor array for deblurring according to phase
function characteristics of the phase modulator; and provides
processed image data of the object; and e) a display in signal
communication with the image processor for rendering the processed
image data.
12. The apparatus of claim 11 wherein the phase modulator is a
cubic phase plate to increase a depth of focus for the intraoral
imaging apparatus.
13. An intraoral imaging method comprising: a) directing light to
an object from an illumination source; b) directing reflected light
from the object along an optical axis through an objective lens
that directs the light through an optical stop; c) disposing a
phase modulator along the optical axis, at or near the optical
stop; d) forming image data at an image sensor array from the
imaged reflected light from the object; e) conditioning the image
data for deblurring; and f) rendering the conditioned image data on
a display.
14. The method of claim 13 wherein the optical stop is positioned
between the objective lens and a second lens, and where the phase
modulator causes the imaging system to have similar point spread
functions over a range of object distances.
15. The method of claim 13 wherein conditioning the data for
deblurring comprises digitally filtering the data based on a point
spread function.
Description
FIELD OF THE INVENTION
[0001] The disclosure relates generally to the field of dental
diagnostic imaging and more particularly to an apparatus and method
for intraoral scanning.
[0002] The disclosure relates generally to methods and apparatus
for intraoral imaging and more particularly to an intraoral imaging
apparatus that has defocus correction to provide an extended depth
of field.
BACKGROUND OF THE INVENTION
[0003] The intraoral camera is increasingly used as a diagnostic
tool to support a range of applications for accurate
characterization of shape and condition of teeth and supporting
structures and tissues. In order to provide image content of
sufficient resolution and accuracy for diagnostic use, the
intraoral camera must meet demanding requirements for image
quality.
[0004] The design of the portable, handheld intraoral camera must
address a number of inherent challenges related to overall
usability as well as to the constraints of the intraoral
environment. The camera must be sized and shaped for ease of use
and configured to allow access to different regions of the mouth.
Sufficient illumination must be provided, within tight spacing and
size constraints, for any type of reflectance imaging. This
includes illumination for a camera that performs either contour
imaging with patterned illumination or 2-D image acquisition with
full-field imaging, or both.
[0005] One image aberration that can compromise image quality for
the intraoral camera is defocus. Space and packaging constraints
force camera optics to be of reduced size, even with the
above-noted illumination constraints. Conventional methods for
increasing the depth of field (DOF) and thereby reducing focus
aberration cannot be applied without reducing illumination in a
characteristically light-starved system.
[0006] Thus, it can be seen that there is need for improved
solutions that increase the DOF of the intraoral camera without
compromising the illumination levels available for imaging.
SUMMARY OF THE INVENTION
[0007] It is an object of the present disclosure to advance the art
of diagnostic imaging and to address the need for improved imaging
performance. An object of the present disclosure is to address the
need for accurate characterization of intraoral surfaces. Exemplary
method and/or apparatus embodiments in this application can relax
requirements for positioning an intraoral imaging apparatus at the
focal distance of an imaged tooth or other object.
[0008] These objects are given only by way of illustrative example,
and such objects may be exemplary of one or more embodiments of the
invention. Other desirable objectives and advantages inherently
achieved by the may occur or become apparent to those skilled in
the art. The invention is defined by the appended claims.
[0009] According to an aspect of the present disclosure, there is
provided an apparatus for intraoral imaging that can include a) an
illumination source that directs light to an object; b) an imaging
apparatus that forms an image at an image sensor array from
reflected light from the object, the imaging apparatus having an
optical stop along an optical axis; c) a phase modulator disposed
at or near the optical stop; and d) an image processor that
conditions data from the image sensor array and provides processed
image data of the object.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The foregoing and other objects, features, and advantages of
the invention will be apparent from the following more particular
description of the embodiments of the invention, as illustrated in
the accompanying drawings.
[0011] The elements of the drawings are not necessarily to scale
relative to each other. Some exaggeration may be necessary in order
to emphasize basic structural relationships or principles of
operation. Some conventional components that would be needed for
implementation of the described embodiments, such as support
components used for providing power, for packaging, and for
mounting and protecting system optics, for example, are not shown
in the drawings in order to simplify description.
[0012] FIG. 1 is a schematic diagram that shows an intraoral
camera.
[0013] FIG. 2 is a schematic diagram that shows the use of
patterned light for characterizing surface contour.
[0014] FIG. 3 shows surface imaging using a pattern with multiple
lines of light.
[0015] FIG. 4 shows a transparent phase plate that serves as a
phase modulator.
[0016] FIG. 5 shows schematic diagrams for an imaging lens system
with a stop.
[0017] FIGS. 6A, 6B, and 6C show point spread functions (PSFs) for
the imaging system of FIG. 5, but without phase modulation.
[0018] FIGS. 7A, 7B, and 7C show PSFs for the imaging system of
FIG. 5 using a phase mask modulator according to an embodiment of
the present disclosure.
[0019] FIG. 8 is a schematic diagram that shows illumination and
imaging optical paths for an imaging apparatus configured to
provide an extended depth of field (DOF).
[0020] FIG. 9A shows an example of an image acquired at some
distance from the focal plane of the imaging apparatus, using phase
modulation in the imaging path, as described herein.
[0021] FIG. 9B shows the same image of FIG. 9A using phase
modulation in the imaging path, after applying digital restoration
as described herein.
[0022] FIG. 10 shows a sequence for forming a digital filter.
DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0023] The following is a description of exemplary method and/or
apparatus embodiments, reference being made to the drawings in
which the same reference numerals identify the same elements of
structure in each of the several figures.
[0024] Where they are used in the context of the present
disclosure, the terms "first", "second", and so on, do not
necessarily denote any ordinal, sequential, or priority relation,
but are simply used to more clearly distinguish one step, element,
or set of elements from another, unless specified otherwise.
[0025] As used herein, the term "energizable" relates to a device
or set of components that perform an indicated function upon
receiving power and, optionally, upon receiving an enabling
signal.
[0026] In the context of the present disclosure, the term "optics"
is used generally to refer to lenses and other refractive,
diffractive, and reflective components or apertures used for
shaping and orienting a light beam. An individual component of this
type is termed an optic.
[0027] In the context of the present disclosure, the terms
"viewer", "operator", and "user" are considered to be equivalent
and refer to the viewing practitioner, technician, or other person
who may operate a camera or scanner and may also view and
manipulate an image, such as a dental image, on a display monitor.
An "operator instruction" or "viewer instruction" is obtained from
explicit commands entered by the viewer, such as by clicking a
button on the camera or scanner or by using a computer mouse or by
touch screen or keyboard entry.
[0028] In the context of the present disclosure, the phrase "in
signal communication" indicates that two or more devices and/or
components are capable of communicating with each other via signals
that travel over some type of signal path. Signal communication may
be wired or wireless. The signals may be communication, power,
data, or energy signals. The signal paths may include physical,
electrical, magnetic, electromagnetic, optical, wired, and/or
wireless connections between the first device and/or component and
second device and/or component. The signal paths may also include
additional devices and/or components between the first device
and/or component and second device and/or component.
[0029] In the context of the present disclosure, the term "camera"
relates to a device that is enabled to acquire a reflectance, 2-D
digital image from reflected visible or NIR (near-infrared) light,
such as structured light that is reflected from the surface of
teeth and supporting structures; in addition, the camera can
operate in single-image mode or a continuous acquisition or video
mode. In the context of the present disclosure, the terms "camera"
and "scanner" can be used interchangeably to describe the same
device, since the device can obtain different image types.
[0030] The term "subject" refers to the tooth or other portion of a
patient that is being imaged and, in optical terms, can be
considered equivalent to the "object" of the corresponding imaging
system. The term "lens" can be used to identify a single-element
lens or a lens group, such as a doublet or other arrangement in
which lenses are positioned adjacently, for example.
[0031] The phrase "at or near" for placement of an optical
component describes component placement at a position along an
optical path where it performs its intended function, within
tolerances acceptable in optical fabrication practice. Placement of
a phase modulator near the optical stop means positioning the phase
modulator at a position close enough to the stop to provide
suitable phase modulation to yield an extended depth of field for
imaging, as described in more detail subsequently. For a hand-held
optical imaging apparatus, a position that is at least within a few
mm of the stop can be sufficient for positioning an optical phase
modulator.
[0032] In the context of the present disclosure, a reflectance
image is a 2D image of a subject obtained by illuminating the
subject with a field of light and obtaining the reflected light
from the subject. A reflectance image can be monochrome or
polychromatic and can use full field illumination or patterned
light, such as for surface contour characterization. A
polychromatic reflectance image can be obtained using a monochrome
sensor with illumination fields of different colors, that is, of
different wavelength bands.
[0033] FIG. 1 is a schematic diagram showing an imaging apparatus
70 that can operate as a still image or video camera 24 for
polychromatic reflectance image data capture as well as a scanner
28 for projecting and imaging functions that characterize surface
contour using structured light patterns 46. A handheld imaging
apparatus 70 can use camera 24 for image acquisition for both
contour scanning and image capture functions according to an
embodiment of the present disclosure.
[0034] As shown in FIG. 1, a control logic processor 80, or other
type of computer may be part of camera 24, controlling the
operation of an illumination array 10 that generates the structured
light and directs the light toward a surface position and
controlling operation of an imaging sensor array 30. Image data
from a surface 20, such as from a tooth 22, is obtained from
imaging sensor array 30 and stored as image data in a memory 72.
Imaging sensor array 30 is part of a sensing apparatus 40 that
includes a lens assembly 34 and associated elements for acquiring
image content. Control logic processor 80, in signal communication
with camera 24 components that acquire the image, processes the
received image data and stores the mapping in memory 72. The
resulting image from memory 72 is then optionally rendered and
displayed on a display 74, which may be part of another computer 75
used for some portion of the processing described herein. Memory 72
may also include a display buffer. One or more sensors 42, such as
a motion sensor, can also be provided as part of scanner 28
circuitry. The image sensor array 30 can be a charge-coupled device
(CCD) or a complementary metal oxide semiconductor (CMOS) array,
for example.
[0035] In structured light imaging, a pattern of lines or other
shapes is projected from illumination array 10 toward the surface
of an object from a given angle. The projected pattern from the
illuminated surface position is then viewed from another angle as a
contour image, taking advantage of triangulation in order to
analyze surface information based on the appearance of contour
lines. Phase shifting, in which the projected pattern is
incrementally shifted spatially for obtaining additional
measurements at the new locations, is typically applied as part of
structured light imaging, used in order to complete the contour
mapping of the surface and to increase overall resolution in the
contour image.
[0036] The schematic diagram of FIG. 2 shows, with the example of a
single line of light L, how patterned light is used for obtaining
surface contour information by a scanner using a handheld camera or
other portable imaging device. A mapping is obtained as an
illumination array 10 directs a pattern of light onto a surface 20
and a corresponding image of a line L' is formed on an imaging
sensor array 30. Each pixel 32 on imaging sensor array 30 maps to a
corresponding pixel 12 on illumination array 10 according to
modulation by surface 20. Shifts in pixel position, as represented
in FIG. 2, yield useful information about the contour of surface
20. It can be appreciated that the basic pattern shown in FIG. 2
can be implemented in a number of ways, using a variety of
illumination sources and sequences for light pattern generation and
using one or more different types of sensor arrays 30. Illumination
array 10 can utilize any of a number of types of arrays used for
light modulation, such as a liquid crystal array or digital
micromirror array, such as that provided using the Digital Light
Processor or DLP device from Texas Instruments, Dallas, Tex. This
type of spatial light modulator is used in the illumination path to
change the light pattern as needed for the mapping sequence.
[0037] By projecting and capturing images that show structured
light patterns that duplicate the arrangement shown in FIG. 2
multiple times, the image of the contour line on the camera
simultaneously locates a number of surface points of the imaged
object. This speeds the process of gathering many sample points,
while the plane of light (and usually also the receiving camera) is
laterally moved in order to "paint" some or all of the exterior
surface of the object with the plane of light.
[0038] A synchronous succession of multiple structured light
patterns can be projected and analyzed together for a number of
reasons, including to increase the density of lines for additional
reconstructed points and to detect and/or correct incompatible line
sequences. Use of multiple structured light patterns is described
in commonly assigned U.S. Patent Application Publications No.
US2013/0120532 and No. US2013/0120533, both entitled "3D INTRAORAL
MEASUREMENTS USING OPTICAL MULTILINE METHOD" and incorporated
herein in their entirety.
[0039] FIG. 3 shows surface imaging using a pattern with multiple
lines of light. Incremental shifting of the line pattern and other
techniques help to compensate for inaccuracies and confusion that
can result from abrupt transitions along the surface, whereby it
can be difficult to positively identify the segments that
correspond to each projected line. In FIG. 3, for example, it can
be difficult over portions of the surface to determine whether line
segment 16 is from the same line of illumination as line segment 18
or adjacent line segment 19. If line features are blurred or
defocused, it can be much more difficult to identify and trace line
segments.
[0040] The thin lens equation provides an idealized approximation
of object distance (O) and image distance (I) for a focal length
f:
1 I = 1 f - 1 O ( 1 ) ##EQU00001##
[0041] Applying the relationship shown in (1), it can be
appreciated that the image distance I shifts when the object
distance O changes. The defocus wave-front aberration that is
caused by such an image shift corresponds to:
W = ( I 1 - I 2 ) 8 ( F / # ) 2 ( 2 ) ##EQU00002##
[0042] Given the relationship shown in (2), it can be appreciated
that one solution for decreasing the defocus aberration W is
increasing the F/# value, such as by using a smaller aperture.
However, the use of a smaller aperture further reduces the
illumination in a light-starved system and may also decrease camera
resolution.
[0043] Certain exemplary method and/or apparatus embodiments of the
application describes a solution for decreasing defocus aberration
in the optical path using phase manipulation or wavefront coding.
Wavefront coding inserts a phase-mask modulator, phase plate
modulator, or, simply, phase modulator, at the aperture or stop of
the imaging system in order to modulate the wavefront form,
effectively correcting misfocus of the image over a range of focal
distances.
[0044] The schematic view of FIG. 4 shows one type of transparent
phase plate that has the following transmittance:
f ( r ) = e j .pi. ( r 2 .lamda. f ) ( 3 a ) ##EQU00003##
[0045] This exemplary phase plate bends the plane wave position at
r by an angle:
.theta..sub.r=sin.sup.-1(-r/f)
[0046] With this behavior, a phase plate 50 can act like a
spherical lens with focal length f. Phase plate 50 can have an
arrangement of areas having different optical indices or thickness,
for example.
[0047] One exemplary straightforward modulation function of the
phase modulator as the phase plate 50 can be expressed as
follows:
P(x,y)=.alpha.(x.sup.3+y.sup.3) (4)
[0048] The coefficient a is determined by optimization, using
techniques familiar to those skilled in the optical design art. The
phase modulator can have various shapes such as being rectangular
or circular in shape. The phase modulator can have different phase
functions in one or multiple dimensions such as orthogonal x- and
y-directions. The phase modulator can have symmetric or
non-symmetric phase functions. Certain exemplary method and/or
apparatus embodiments according to the application can provide a
phase modulator to generate blurred images or a PSF (Point Spread
Function) that is very similar at multiple focus distances and
different field positions along the intraoral imaging apparatus
optical axis. In exemplary embodiments, digital filters can be used
to restore such blurred images at different (e.g., increased) focus
distances.
[0049] Referring to FIG. 5, an imaging lens system 60 with an
aperture stop 62 can include two lenses or lens groups: an
objective lens L1 preceding and a second lens L2 following the stop
along optical axis OA. The imaging lens system of FIG. 5 can have
the overall arrangement of a conventional 4-f (four focal length)
imaging system. However, the geometrical focal distances do not
rigidly constrain image quality when using the approach of the
present disclosure. A phase modulator 50 (here, a transparency such
as phase plate or phase mask) can be added to modulate the
wave-front form at or near stop 62. This effectively enables the
imaging system to obtain improved focus or clear images over a
range (e.g., larger) of focal distances.
[0050] In practice, the transmittance function f(r) for perfect
focus could be highly complex and difficult to manufacture; it is
even possible that no ideal transmittance function solution can be
found. Phase plate or phase modulator 50 (FIG. 4), however, need
not provide perfect focus at any location. Instead, phase modulator
50 can be positioned at or near the stop 62 (FIG. 5) in order to
obtain a well-defined, uniform PSF (Point Spread Function) that is
very similar at multiple distances along the optical axis OA. Then,
since the PSF is well-defined and very similar PSF characteristics
apply over a range of distances, in one exemplary embodiment the
same digital restoration filtering can be used for correcting
misfocus.
[0051] According to some exemplary embodiments of the application,
the phase mask or other phase modulator 50 can be a rectangular
phase plate with non-symmetric free-form, a circular phase plate
with symmetric free-form or the like. The imaging system with a
phase modulator 50 added is designed or optimized to provide PSFs
at different object distances and field positions, wherein the
different PSFs preferably exhibit a high degree of similarity and
are of generally small size. Coefficients of the free-form surface
of the phase mask or other type of phase modulator can be defined
according to this design or optimization. Various aberrations are
considered in the optimization; clear images can be restored using
a digital filter.
[0052] It should be noted that the stop need not be between two
lenses or lens groups, as shown for stop 62 in the exemplary
configuration of FIG. 5. In some imaging optics systems, the stop
can be at some other location along the optical axis OA.
[0053] Reference is hereby made to the following: [0054] Edward R.
Dowski, Gregory E. Johnson, "Wavefront Coding: A modern method of
achieving high performance and/or low cost imaging systems" CDM
Optics, Inc., Boulder, Colo., pp. 1-9. [0055] Manjunath Somayaji,
Vikrant R.Bhakta, and Marc P. Christensen, "Experimental evidence
of the theoretical spatial frequency response of cubic phase mask
wavefront coding imaging systems", Optics Express, vol. 20, no. 2,
16 Jan., 2012. pp. 1878-1895. [0056] Hsin-Yueh Sung, Sidney S.
Yang, Horng Chang, "Design of Mobile Phone Lens with Extended Depth
of Field Based on Point-spread Function Focus Invariance" Proc. of
SPIE Vol. 7061, pp. 706107-1, 706107-11. [0057] Tingyu Zhao, Thomas
Mauger, and Guoqiang Li, "Optimization of wavefront-coded
infinity-corrected microscope systems with extended depth of
field", Biomedical Optics Express, 31 Jul. 2013, pp. 1464-1471.
[0058] Sara Bradburn, Wade Thomas Cathey, and Edward R. Dowski, Jr.
"Realizations of Focus Invariance in Optical/Digital Systems with
Wavefront Coding", Imaging Systems Laboratory, Optoelectronic
Computing Systems Center, Dept. of Electrical and Computer
Engineering, University of Colorado at Boulder. pp. 1-22. [0059]
Edward R. Dowski, Jr. and Wade Thomas Cathey, "Extended Depth of
Field Through Wave-front Coding" Applied Optics, vol. 34, No. 11,
10 Apr. 1995, pp. 1859-1866.
[0060] By way of example, FIGS. 6A, 6B, and 6C show PSFs for the
imaging system of FIG. 5 without phase modulation at stop 62. FIG.
6A shows the PSF for a point source at the focal plane of the
imaging lens system. FIG. 6B shows the PSF for the point source
object advanced +12 mm from the focal point, clearly showing an
out-of-focus condition. FIG. 6C shows the PSF with the point source
object at -12 mm from the focal point, yielding an even more
pronounced mis-focus. From the example PSFs of FIGS. 6A-6C, it is
clear that, in the absence of phase modulator 50, only when an
object is positioned at or near the object focal plane is the
imaging system likely to provide an acceptable image.
[0061] FIGS. 7A, 7B, and 7C show PSFs for the imaging system of
FIG. 5 using phase modulator 50. The focal distances in FIGS. 7A-7C
correspond to distances in FIGS. 6A-6C, that is, with the point
source object at 0, +12 mm, and -12 mm from focus, respectively.
Noticeably, the PSF distribution shown over this range is not the
ideal PSF for focus as in FIG. 6A. However, the PSF energy
distribution, which can be termed the PSF characteristic, is very
nearly the same when the focal distance is varied over an
appreciable distance. This means that the same digital filtering
can be used for image correction; that is, the same digital
filtering can be applied for deblurring the image whether the
object is +12 mm out of focus, in focus, or -12 mm out of focus. In
one exemplary embodiment, different digital filtering can be
applied to different defocusing distances.
[0062] The schematic view of FIG. 8 shows exemplary illumination
and imaging optical paths for imaging apparatus 70 of FIG. 1,
configured to provide an extended depth of field (DOF). An
illumination source 66 directs light to tooth 22 through an
illumination path. Phase modulator 50 is disposed at the stop 62 in
the imaging path, between lenses (or lens groups) L1 and L2. Sensor
array 30 forms image data from the received light. An image
processor 68 is in signal communication with sensor array 30. Image
processor 68 is programmed with instructions to condition data from
the image sensor array to correct for blurring in the received
image and to provide conditioned image data that can be rendered on
a display 52 that is in signal communication with processor 68.
Digital processing by processor 68 can occur in real time, that is,
as soon as the image data is acquired, allowing a practitioner or
other operator to view processed imaging results as they are
obtained, without perceptible delay for image processing, as long
as processor 68 speed is sufficient. Illumination source 66 can
include a solid-state light emitter, such as a light-emitting
diode, for example.
[0063] FIG. 9A shows an example of an image acquired at some
distance from the focal plane of the imaging apparatus, using the
phase modulator 50 as described previously. The slightly blurred
image of FIG. 9A can be acquired at any number of distances on each
side of the ideal object plane; in each case, over the extended
DOF, the acquired image will look substantially the same as shown,
requiring some amount of de-blurring. FIG. 9B shows the same image
generated as described for FIG. 9A, but following application of a
digital filter in accordance with an exemplary embodiment. The
image has noticeably improved contrast, resolution, and/or dynamic
range after digital image processing. Significantly in one
exemplary embodiment, the same digital filtering algorithms can be
applied for images captured over a range of focal distances.
[0064] In the imaging system of the 3D intra-oral camera without
phase modulation, PSFs change at different object distances (FIGS.
6A, 6B, and 6C). In the imaging system with phase modulation added,
PSFs are similar and keep a compact size at different object
distances (FIGS. 7A, 7B, and 7C). The clear image can be restored
by a digital processing filter.
Cubic Phase Plate as Phase Modulator
[0065] One non-symmetry free-form of phase plate is the cubic phase
mask (CPM). The CPM provides straightforward design and inexpensive
fabrication, and is robust. The modulation function of the CPM can
be expressed as given in equation (4) above.
[0066] In the imaging system of a 3D intra-oral camera 24 of FIG.
1, PSFs change at different object distances, as was shown in the
examples of FIGS. 6A-6C. By adding phase modulation, the Applicants
have modified imaging system performance to allow similar PSFs over
a range of object distances, as shown in the examples of FIGS.
7A-7C, with clear imaging capable of being restored using digital
filtering, as described with reference to FIGS. 9A and 9B.
Blurred Image Restoration
[0067] A number of methods can be used to provide blurring
correction, also termed de-blurring, for images obtained using the
extended DOF of the present disclosure. The following gives a
summary of theoretical background and basis for blurred image
restoration.
[0068] For an imaging system with an incoherent source, the
observed image Ui(xi, yi) is given by the standard image formation
equation:
Ui(xi, yi)=ff h(xi, yi; xo, yo)Uo(xo, yo) dxodyo+n(xo, yo) (5)
Where Uo (xo, yo) is the object brightness distribution, h(xi, yi;
xo, yo) is the point spread function (PSF) of the imaging system,
n(xo, yo) is the noise term.
[0069] Fourier transformation to the frequency domain yields the
following:
Ui(u, v)=H(u, v)Uo(u, v)+n(u, v) (6)
[0070] Here, noise term n (u, v) is mainly from high frequencies,
predominantly noise content, with some signal content. For some
applications, clear images are mainly from the low frequency
signals, such as multiline patterns for contour imaging using the
3D intra-oral camera of FIG. 1. One way to remove the noise term
n(u, v) is to apply a high-frequency cut-off filter. Application of
a high-frequency filter yields:
Ui ( u , v ) = { H ( u , v ) U 0 ( u , v ) u , v < ( u , v )
cutoff 0 u , v > ( u , v ) cutoff ( 7 ) ##EQU00004##
[0071] The object field in the spatial domain can be calculated
as:
Uo(xo, yo)=FFT.sup.-1[Ui(u, v)/H(u, v)]; |u, v|<(u, v)cutoff
(8)
[0072] As a result of this digital filter processing, blurring is
corrected and clear images are restored.
[0073] Unsharp masking or other type of deblurring filter can
alternately be used for image restoration.
Implementation Sequence
[0074] An implementation of the intra-oral imaging apparatus can be
formed using the following sequence: [0075] (i) Begin with a
standard imaging lens system having an aperture stop. [0076] (ii)
Add a free-form phase modulator, such as a cubic phase plate to the
aperture stop. The free-form phase plate can be expressed as a
polynomial expression, such as the cubic phase a(x.sup.3+y.sup.3).
[0077] (iii) The correlation operand of PSFs at different object
distances and field positions are defined (with a user-defined
operand) to get a high degree of similarity of PSFs at these
distances and fields through optimization. [0078] (iv) A
minimization operand of PSF size is defined to get a compact PSF
size at different object distances and field positions through
optimization. The reference PSF is PSF at field (0,0), and at the
best focal distance of the standard imaging lens system. [0079] (v)
Design an imaging lens system with a phase plate at its stop after
optimization; determine appropriate polynomial coefficients of the
phase plate. [0080] (vi) According to the resulting PSFs from
optimization and the reference PSF, design a digital filter to
restore blurred images. Steps of FIG. 10 form a suitable digital
filter that can be integrated into the image processing logic.
[0081] Steps of FIG. 10 show a sequence for forming a suitable
digital filter, applying techniques known to those skilled in the
image processing art. A measurement step S100 measures PSFs at the
desired focal distances and fields. The PSFs can then be averaged
or otherwise combined. An inversion step S110 then inverts the
averaged PSF and performs a Fourier transform to the frequency
domain. A multiplication step S120 then multiplies the PSF function
by a high-frequency cutoff filter. A second inversion step S130
performs an inverse Fourier transform back to the spatial domain,
which forms a clear image. A filter forming step S140 then forms a
single digital filter from results of this processing.
[0082] Digital filtering can not only correct image blurring due to
defocus, but can also correct or remedy other aberrations such as
spherical aberration and coma, for example.
[0083] Consistent with an embodiment of the present invention, a
computer program utilizes stored instructions that perform on image
data that is accessed from an electronic memory. As can be
appreciated by those skilled in the image processing arts, a
computer program for operating the imaging system in an embodiment
of the present disclosure can be utilized by a suitable,
general-purpose computer system, such as a personal computer or
workstation. However, many other types of computer systems can be
used to execute the computer program of the present invention,
including an arrangement of networked processors, for example. The
computer program for performing the method of the present invention
may be stored in a computer readable storage medium. This medium
may comprise, for example; magnetic storage media such as a
magnetic disk such as a hard drive or removable device or magnetic
tape; optical storage media such as an optical disc, optical tape,
or machine readable optical encoding; solid state electronic
storage devices such as random access memory (RAM), or read only
memory (ROM); or any other physical device or medium employed to
store a computer program. The computer program for performing the
method of the present disclosure may also be stored on computer
readable storage medium that is connected to the image processor by
way of the internet or other network or communication medium. Those
skilled in the art will further readily recognize that the
equivalent of such a computer program product may also be
constructed in hardware.
[0084] It should be noted that the term "memory", equivalent to
"computer-accessible memory" in the context of the present
disclosure, can refer to any type of temporary or more enduring
data storage workspace used for storing and operating upon image
data and accessible to a computer system, including a database, for
example. The memory could be non-volatile, using, for example, a
long-term storage medium such as magnetic or optical storage.
Alternately, the memory could be of a more volatile nature, using
an electronic circuit, such as random-access memory (RAM) that is
used as a temporary buffer or workspace by a microprocessor or
other control logic processor device. Display data, for example, is
typically stored in a temporary storage buffer that is directly
associated with a display device and is periodically refreshed as
needed in order to provide displayed data. This temporary storage
buffer is also considered to be a type of memory, as the term is
used in the present disclosure. Memory is also used as the data
workspace for executing and storing intermediate and final results
of calculations and other processing. Computer-accessible memory
can be volatile, non-volatile, or a hybrid combination of volatile
and non-volatile types.
[0085] It will be understood that the computer program product of
the present disclosure may make use of various image manipulation
algorithms and processes that are well known. It will be further
understood that the computer program product embodiment of the
present disclosure may embody algorithms and processes not
specifically shown or described herein that are useful for
implementation. Such algorithms and processes may include
conventional utilities that are within the ordinary skill of the
image processing arts. Additional aspects of such algorithms and
systems, and hardware and/or software for producing and otherwise
processing the images or co-operating with the computer program
product of the present disclosure, are not specifically shown or
described herein and may be selected from such algorithms, systems,
hardware, components and elements known in the art.
[0086] Certain exemplary method and/or apparatus embodiments
according to the application can increase a depth of focus for an
intraoral imaging apparatus. Further, fluorescence imaging can be
used with exemplary embodiments of the application. Exemplary
embodiments according to the application can include various
features described herein (individually or in combination).
[0087] While the invention has been illustrated with respect to one
or more implementations, alterations and/or modifications can be
made to the illustrated examples without departing from the spirit
and scope of the appended claims. In addition, while a particular
feature of the invention can have been disclosed with respect to
only one of several implementations/embodiments, such feature can
be combined with one or more other features of the other
implementations/embodiments as can be desired and advantageous for
any given or particular function. The term "at least one of" is
used to mean one or more of the listed items can be selected. The
term "about" indicates that the value listed can be somewhat
altered, as long as the alteration does not result in
nonconformance of the process or structure to the illustrated
embodiment. Finally, "exemplary" indicates the description is used
as an example, rather than implying that it is an ideal. Other
embodiments of the invention will be apparent to those skilled in
the art from consideration of the specification and practice of the
invention disclosed herein. It is intended that the specification
and examples be considered as exemplary only, with a true scope and
spirit of the invention being indicated by at least the following
claims.
* * * * *