U.S. patent application number 12/424562 was filed with the patent office on 2010-10-21 for dental surface imaging using polarized fringe projection.
Invention is credited to Rongguang Liang.
Application Number | 20100268069 12/424562 |
Document ID | / |
Family ID | 42270206 |
Filed Date | 2010-10-21 |
United States Patent
Application |
20100268069 |
Kind Code |
A1 |
Liang; Rongguang |
October 21, 2010 |
DENTAL SURFACE IMAGING USING POLARIZED FRINGE PROJECTION
Abstract
An intra-oral imaging apparatus having a fringe pattern
generator energizable to emit a fringe pattern illumination having
a predetermined spatial frequency, with light in the approximate
350-500 nm range. A polarizer in the path of the fringe pattern
illumination has a first polarization transmission axis. A
projection lens is disposed to direct the polarized fringe pattern
illumination as incident illumination toward a tooth surface. An
imaging lens is disposed to direct light reflected and scattered at
the tooth surface along a detection path. An analyzer is disposed
along the detection path, having a second polarization transmission
axis. A detector disposed along the detection path obtains image
data from the light provided through the analyzer. A control logic
processor is responsive to programmed instructions and actuable to
adjust the intensity over one or more portions of the fringe
pattern illumination according to the image data obtained from the
detector.
Inventors: |
Liang; Rongguang; (Penfield,
NY) |
Correspondence
Address: |
Carestream Health, Inc.;ATTN: Patent Legal Staff
150 Verona Street
Rochester
NY
14608
US
|
Family ID: |
42270206 |
Appl. No.: |
12/424562 |
Filed: |
April 16, 2009 |
Current U.S.
Class: |
600/425 |
Current CPC
Class: |
G06T 2207/10016
20130101; A61B 5/0088 20130101; A61B 5/1079 20130101; G06T 7/0014
20130101; G06T 2207/20212 20130101; G03B 15/14 20130101; A61B
5/4547 20130101; G06T 7/521 20170101; A61B 1/06 20130101; G01B
11/24 20130101; A61B 1/247 20130101; A61C 9/006 20130101; G01B
11/2527 20130101; G06T 2207/30036 20130101; A61B 5/1077 20130101;
G01B 11/25 20130101; G03B 35/08 20130101; A61B 1/00009
20130101 |
Class at
Publication: |
600/425 |
International
Class: |
A61B 5/05 20060101
A61B005/05 |
Claims
1. An intra-oral imaging apparatus comprising: a fringe pattern
generator energizable to emit a fringe pattern illumination having
a predetermined spatial frequency, with light in the approximate
350-500 nm range; a polarizer disposed in the path of the fringe
pattern illumination emitted from the fringe pattern generator and
having a first polarization transmission axis; a projection lens
disposed to direct the polarized fringe pattern illumination as
incident illumination toward a tooth surface; an imaging lens
disposed to direct at least a portion of the light reflected and
scattered from the incident illumination at the tooth surface along
a detection path; an analyzer disposed along the detection path and
having a second polarization transmission axis; a detector disposed
along the detection path for obtaining image data from the light
provided through the analyzer; and a control logic processor
responsive to programmed instructions and actuable to obtain image
data from the detector and to adjust the intensity over one or more
portions of the fringe pattern illumination that is emitted from
the fringe pattern generator according to the obtained image
data.
2. The imaging apparatus of claim 1 wherein the second polarization
transmission axis is parallel to the first polarization
transmission axis.
3. The imaging apparatus of claim 1 wherein the second polarization
transmission axis is orthogonal to the first polarization
transmission axis.
4. The imaging apparatus of claim 1 further comprising an actuator
coupled to either the polarizer or the analyzer and energizable to
rotate the coupled polarizer or analyzer to one of two positions,
substantially 90 degrees apart.
5. The imaging apparatus of claim 1 wherein the analyzer is a
polarization beam splitter and wherein the detector is a first
detector disposed to receive light transmitted through the
polarization beam splitter and further comprising a second detector
disposed to receive light reflected from the polarization beam
splitter.
6. The imaging apparatus of claim 1 wherein the fringe pattern
generator comprises a grating.
7. The imaging apparatus of claim 1 wherein the fringe pattern
generator comprises a spatial light modulator.
8. The imaging apparatus of claim 7 wherein the spatial light
modulator is taken from the group consisting of a digital
micromirror device and a liquid crystal device.
9. The imaging apparatus of claim 1 wherein the fringe pattern
generator comprises an emissive device that forms and emits the
fringe pattern.
10. The imaging apparatus of claim 1 further comprising a filter
disposed along the detection path for transmitting light in the
350-500 nm range.
11. A method for obtaining a surface image of a tooth comprising:
obtaining a reference image of a portion of the tooth surface
having at least a lighter area and a darker area; generating and
projecting one or more fringe patterns onto the portion of the
tooth surface, wherein the intensity of the fringe pattern
illumination that is directed toward the darker area exceeds the
intensity of the fringe pattern illumination that is directed
toward the lighter area and wherein the fringe pattern has the same
spatial frequency for pattern features over the lighter and darker
areas; and obtaining one or more images from the one or more
projected fringe patterns.
12. The method of claim 11 wherein obtaining the reference image
further comprises directing a structured illumination onto the
tooth surface.
13. The method of claim 11 wherein projecting the one or more
fringe patterns comprises modulating light of a wavelength in the
range from 350-500 nm.
14. The method of claim 11 wherein projecting the one or more
fringe patterns comprises modulating polarized light.
15. The method of claim 11 wherein obtaining the reference image
further comprises changing the transmission axis of an analyzer or
a polarizer.
16. The method of claim 11 wherein generating the one or more
fringe patterns comprises actuating a spatial light modulator.
17. The method of claim 11 wherein obtaining the reference image
comprises obtaining light directed from a polarization
beamsplitter.
18. The method of claim 11 wherein generating the one or more
fringe patterns further comprises shifting the relative spatial
position of two or more of the generated fringe patterns.
19. The method of claim 18 wherein shifting the relative spatial
position of two or more of the generated fringe patterns comprises
shifting by a fraction of the period of the fringe pattern.
20. An intra-oral imaging apparatus comprising: a fringe pattern
generator comprising a spatial light modulator and energizable to
emit a fringe pattern illumination having a plurality of pattern
features with a predetermined spatial frequency; a polarizer
disposed in the path of the fringe pattern illumination emitted
from the fringe pattern generator and having a first polarization
transmission axis; a projection lens disposed to direct the
polarized fringe pattern illumination as incident illumination
toward a tooth surface; an imaging lens disposed to direct at least
a portion of the light reflected and scattered from the incident
illumination at the tooth surface along a detection path; an
analyzer disposed along the detection path and having a second
polarization transmission axis; a detector disposed to form image
data from light that is obtained from the tooth along the detection
path; a filter disposed along the detection path for transmitting
light in a range from approximately 350-500 nm and attenuating
light outside the range; and a control logic processor responsive
to programmed instructions and actuable to obtain image data from
the detector and to adjust the intensity of light over one or more
portions of the pattern features of the fringe pattern illumination
that is emitted from the fringe pattern generator according to the
obtained image data.
Description
FIELD OF THE INVENTION
[0001] The invention relates generally to the field of diagnostic
imaging using structured light and more particularly relates to a
method for three-dimensional imaging of the surface of teeth and
other structures using fringe projection.
BACKGROUND OF THE INVENTION
[0002] Fringe projection imaging uses patterned or structured light
to obtain surface contour information for structures of various
types. In fringe projection imaging, a pattern of lines of an
interference fringe or grating is projected toward the surface of
an object from a given direction. The projected pattern from the
surface is then viewed from another direction 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 spatially
shifted for obtaining additional measurements at the new locations,
is typically applied as part of fringe projection imaging, used in
order to complete the contour mapping of the surface and to
increase overall resolution in the contour image.
[0003] Fringe projection imaging has been used effectively for
surface contour imaging of solid, highly opaque objects and has
been used for imaging the surface contours for some portions of the
human body and for obtaining detailed data about skin structure.
However, a number of technical obstacles have prevented effective
use of fringe projection imaging of the tooth. One particular
challenge with dental surface imaging relates to tooth
translucency. Translucent or semi-translucent materials in general
are known to be particularly troublesome for fringe projection
imaging. Subsurface scattering in translucent structures can reduce
the overall signal-to-noise (S/N) ratio and shift the light
intensity, causing inaccurate height data. Another problem relates
to high levels of reflection for various tooth surfaces. Highly
reflective materials, particularly hollowed reflective structures,
can effectively reduce the dynamic range of this type of
imaging.
[0004] In fringe projection imaging overall, contrast is typically
poor, with noise as a significant factor. To improve contrast, many
fringe projection imaging systems take measures to reduce the
amount of noise in the contour image. In general, for accurate
surface geometry measurement using fringe imaging techniques, it is
important to obtain the light that is directly reflected from the
surface of a structure under test and to reject light that is
reflected from material or structures that lie beneath the surface.
This is the approach that is generally recommended for 3D surface
scanning of translucent objects. A similar approach must be used
for intra-oral imaging.
[0005] From an optics perspective, the structure of the tooth
itself presents a number of additional challenges for fringe
projection imaging. As noted earlier, light penetrating beneath the
surface of the tooth tends to undergo significant scattering within
the translucent tooth material. Moreover, reflection from opaque
features beneath the tooth surface can also occur, adding noise
that degrades the sensed signal and thus further complicating the
task of tooth surface analysis.
[0006] One corrective measure that has been attempted to make
fringe projection workable for contour imaging of the tooth is
application of a coating that changes the reflective
characteristics of the tooth surface itself. Here, to compensate
for problems caused by the relative translucence of the tooth, a
number of conventional tooth contour imaging systems apply a paint
or reflective powder to the tooth surface prior to surface contour
imaging. For the purposes of fringe projection imaging, this added
step enhances the opacity of the tooth and eliminates or reduces
the scattered light effects noted earlier. However, there are
drawbacks to this type of approach. The step of applying a coating
powder or liquid adds cost and time to the tooth contour imaging
process. Because the thickness of the coating layer is often
non-uniform over the entire tooth surface, measurement errors
readily result. More importantly, the applied coating, while it
facilitates contour imaging, can tend to mask other problems with
the tooth and can thus reduce the overall amount of information
that can be obtained.
[0007] Even where a coating or other type of surface conditioning
of the tooth is used, however, results can be disappointing due to
the pronounced contours of the tooth surface. It can be difficult
to provide sufficient amounts of light onto, and sense light
reflected back from, all of the tooth surfaces. The different
surfaces of the tooth can be oriented at 90 degrees relative to
each other, making it difficult to direct enough light for
accurately imaging all parts of the tooth.
[0008] There have been a number of attempts to adapt structured
light surface-profiling techniques to the problems of tooth
structure imaging. For example, U.S. Pat. No. 5,372,502 entitled
"Optical Probe and Method for the Three-Dimensional Surveying of
Teeth" to Massen et al. describes the use of an LCD matrix to form
patterns of stripes for projection onto the tooth surface. A
similar approach is described in U.S. Patent Application
Publication 2007/0086762 entitled "Front End for 3-D Imaging
Camera" by O'Keefe et al. U.S. Pat. No. 7,312,924 entitled
"Polarizing Multiplexer and Methods for Intra-Oral Scanning" to
Trissel describes a method for profiling the tooth surface using
triangularization and polarized light, but needing application of a
fluorescent coating for operation. Similarly, U.S. Pat. No.
6,885,464 entitled "3-D Camera for Recording Surface Structures, In
Particular for Dental Purposes" to Pfeiffer et al. discloses a
dental imaging apparatus using triangularization but also requiring
the application of an opaque powder to the tooth surface for
imaging.
[0009] It can be appreciated that an apparatus and method that
provides accurate surface contour imaging of the tooth, without the
need for applying an added coating or other conditioning of the
tooth surface for this purpose, would help to speed reconstructive
dentistry and could help to lower the inherent costs and
inconvenience of conventional methods, such as those for obtaining
a cast or other surface profile for a crown, implant, or other
restorative structure.
SUMMARY OF THE INVENTION
[0010] It is an object of the present invention to advance the art
of diagnostic imaging, particularly for intra-oral imaging
applications. With this object in mind, the present invention
provides an intra-oral imaging apparatus comprising: a fringe
pattern generator energizable to emit a fringe pattern illumination
having a predetermined spatial frequency, with light in the 350-500
nm range; a polarizer in the path of the fringe pattern
illumination emitted from the fringe pattern generator and having a
first polarization transmission axis; a projection lens disposed to
direct the polarized fringe pattern illumination as incident
illumination toward a tooth surface; an imaging lens disposed to
direct at least a portion of the light reflected and scattered from
the incident illumination at the tooth surface along a detection
path; an analyzer disposed along the detection path and having a
second polarization transmission axis; a detector disposed along
the detection path for obtaining image data from the light provided
through the analyzer; and a control logic processor responsive to
programmed instructions and actuable to obtain image data from the
detector and to adjust the intensity over one or more portions of
the fringe pattern illumination that is emitted from the fringe
pattern generator according to the obtained image data.
[0011] It is a feature of the present invention that it applies
light of suitable polarization and wavelength along with fringe
projection patterning of varying brightness to the task of tooth
contour imaging.
[0012] An advantage offered by the apparatus and method of the
present invention relates to improved imaging of tooth surfaces and
at lower cost over conventional contour imaging methods. Unlike
conventional methods, no powder or other opaque substance must be
applied to the tooth as a preparatory step for contour imaging.
[0013] 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 disclosed invention may occur or become apparent to
those skilled in the art. The invention is defined by the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] 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. The elements of the drawings are not
necessarily to scale relative to each other.
[0015] FIG. 1 is a schematic diagram of an imaging apparatus using
polarized fringe projection imaging in one embodiment.
[0016] FIG. 2A is a block diagram showing the use of an analyzer
with its polarization axis in parallel to the polarizer of a
polarized fringe projection imaging apparatus.
[0017] FIG. 2B is a block diagram showing the use of an analyzer
with its polarization axis orthogonal to the polarizer of a
polarized fringe projection imaging apparatus.
[0018] FIG. 3A shows the polarization-dependent reflection and
scattering of illumination incident on the tooth.
[0019] FIG. 3B is a diagram showing the relative intensities of
reflected light and the scattered light from incident
illumination.
[0020] FIGS. 4A, 4B, and 4C are perspective views of a tooth imaged
with fringe projection imaging, using non-polarized light,
cross-polarized light, and co-polarized light, respectively.
[0021] FIG. 5A is a diagram showing wavelength-dependent
penetration of illumination incident on the tooth.
[0022] FIG. 5B is a schematic diagram showing relative intensities
of reflected and scattered light with different wavelengths.
[0023] FIG. 6 is a schematic diagram showing an imaging apparatus
for obtaining both co-polarized and cross-polarized light in fringe
projection imaging.
[0024] FIG. 7 is a block diagram showing components of an
intra-oral imaging system according to one embodiment.
[0025] FIG. 8 is a schematic diagram showing how increased
brightness can be applied for improved imaging over a portion of
the imaging field with contoured surfaces.
[0026] FIGS. 9A and 9B show exemplary projected light patterns
generated for contour imaging in one embodiment.
[0027] FIG. 10 is a logic flow diagram that shows the sequence for
obtaining a contour-compensated image.
[0028] FIG. 11 is a schematic block diagram showing components of a
pattern generator in one embodiment.
[0029] FIG. 12 is a schematic diagram of an imaging apparatus using
polarized fringe projection imaging in one embodiment.
DETAILED DESCRIPTION OF THE INVENTION
[0030] The figures provided herein are given in order to illustrate
key principles of operation and component relationships along their
respective optical paths according to the present invention and are
not drawn with intent to show actual size or scale. 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 of the invention itself. In the drawings
and text that follow, like components are designated with like
reference numerals, and similar descriptions concerning components
and arrangement or interaction of components already described are
omitted.
[0031] In the context of the present disclosure, the term "fringe
pattern illumination" is used to describe the type of structured
illumination that is used for fringe projection imaging or
"contour" imaging. The fringe pattern itself can include, as
pattern features, multiple lines, circles, curves, or other
geometric shapes that are distributed over the area that is
illuminated and that have a predetermined spatial frequency,
recurring at a given period.
[0032] Two portions of a line of light or other feature in a
pattern of structured illumination can be considered to be
substantially "dimensionally uniform" when their line width is the
same over the length of the line to within no more than +/-15
percent. As is described in more detail subsequently, dimensional
uniformity of the pattern of structured illumination is needed to
maintain a uniform spatial frequency.
[0033] As noted above in the background section, conventional
approaches for fringe projection imaging fall short of providing
good results for tooth tissue for a number of reasons. Apparatus
and methods of the present invention address the problems of
obtaining images of the tooth when using fringe projection imaging
with fringe pattern illumination by selection of favorable light
properties and by techniques that improve light delivery to the
highly contoured tooth surface.
[0034] Referring to the schematic block diagram of FIG. 1, there is
shown an embodiment of an intra-oral imaging apparatus 10 for
obtaining surface contour information from a tooth 20 using
structured light. A fringe pattern generator 12 is energizable to
form the structured light as a fringe pattern illumination and
project the structured light thus formed as incident light toward
tooth 20 through a polarizer 14 and projection lens 16. Light
reflected and scattered from tooth 20 is provided to a detector 30,
through an imaging lens 22 and an analyzer 28. Detector 30 is
disposed along a detection path 88, at the image plane of imaging
lens 22. A control logic processor 34 accepts feedback information
from detector 30 and, in response to this and other data, is
actuable to effect the operation of pattern generator 12, as
described in more detail subsequently.
[0035] One function of control logic processor 34 for fringe
projection imaging is to incrementally shift the position of the
fringe and trigger the detector to take images that are then used
to calculate three-dimensional information of tooth surface. For
the phase shifting fringe projection method, at least three images
are typically needed in order to provide enough information for
calculating the three-dimensional information of the object. The
relative positions of the fringes for these three projected images
are typically shifted by one-third of the fringe period. Control
logic processor 34 can be a computer, microprocessor, or other
dedicated logic processing apparatus that executes programmed
instructions.
[0036] Intra-oral imaging apparatus 10 of FIG. 1 uses polarized
light for surface imaging of tooth 20. Polarizer 14 provides the
fringe pattern illumination from fringe pattern generator 12 as
linearly polarized light. In one embodiment, the transmission axis
of analyzer 28 is parallel to the transmission axis of polarizer
14. With this arrangement, only light with the same polarization as
the fringe pattern is provided to the detector 30. In another
embodiment, analyzer 28, in the path of reflected light to detector
30, is rotated by an actuator 18 into either of two orientations as
needed: [0037] (a) Same polarization transmission axis as polarizer
14. In this "co-polarization" position, detector 30 obtains the
specular light reflected from the surface of tooth 20, and most of
the light scattered and reflected from the superficial layer of
enamel surface of tooth 20, as well as some of the light scattered
back from sub-surface portions of the tooth. The co-polarization
orientation of the analyzer 28 axis is shown in FIG. 2A. Parallel
or co-polarization provides improved contrast over other
configurations. [0038] (b) Orthogonal polarization transmission
axis relative to polarizer 14. Using the orthogonal polarization,
or cross-polarization, helps to reduce the specular component from
the tooth surface and obtain more of the scattered light from inner
portions of the tooth. The cross-polarization orientation of the
analyzer 28 axis is shown in FIG. 2B.
[0039] When the tooth is imaged with an imaging system and sensor,
the light that is available to the sensor can be (i) light
reflected from the tooth top surface; (ii) light scattered or
reflected from the near surface volume or portion of the tooth; and
(iii) light scattered inside the tooth. In the context of the
present disclosure, the "near-surface volume" of the tooth is that
portion of the tooth structure that lies within no more than a few
hundred .mu.m of the surface.
[0040] It is known that the light reflected from the tooth surface
(i), the specular light, maintains the polarization state of the
incident light. As the incident light propagates further into the
tooth, the light is increasingly depolarized.
[0041] Disadvantageously, some portion of the specular light (i)
for a contour pattern may be incident on more highly reflective
portions of the tooth surface, even causing some amount of
saturation that degrades light detection. In contrast to
conventional approaches that use all the light from the tooth,
methods of the invention use at least portions of both the specular
light (i) and the near-surface reflected light (ii), and avoid the
light scattered deep inside the tooth (iii). Applicants have found
that the near-surface light (ii), particularly for blue light and
shorter wavelengths, is still substantially polarized. Thus, for
example, a large portion of the light scattered and reflected from
the superficial layer of the tooth enamel also has the same
polarization state as the incident light and as the specular light
(i).
[0042] FIG. 3A shows why the apparatus and method of the present
invention use scattered near-surface light from just beneath the
surface of the tooth. When a polarized light P0 with small
dimension illuminates the tooth, some of the light P1 is reflected
from the surface of the tooth in specular fashion and has the same
polarization state as the illumination light P0. The other portion
of the illumination light P0 goes into the tooth, is subject to
scattering and depolarizes. Some of the scattered light P2 escapes
the tooth surface near the illumination region and can reach
detector 30 (FIG. 1).
[0043] Of particular interest, the spatial "footprint" of the
scattered light P2, which relates to the dimensions of pattern
features of the structured light, such as line thicknesses, shows
an increase over the corresponding spatial footprint of reflected
light P1. For example, where the structured light pattern consists
of parallel lines of light of a given thickness, the reflected
light P1 from these pattern features has lines of substantially the
same thickness as the projected pattern. However, the scattered
light P2 is detected as lines of slightly increased thickness. That
is, since light P2 has been scattered inside the tooth, the
projected footprint on the tooth surface is broader than that of
the specular reflected light, which is the same size as the
illumination beam. The graph of FIG. 3B shows the difference
between the footprint of the light from the tooth surface (P1) and
the light from inside the tooth (P2). To reduce the measurement
error that can result, the light detected from inside the tooth
should be minimized. Applicants have found that polarization
provides an effective discriminator for separating the specular
light (P1) from the tooth surface from the scattered light from
inside the tooth, while still taking advantage of a portion of the
scattered light (P2).
[0044] The group of contour images shown in FIGS. 4A-4C gives a
comparison of approaches for obtaining and using light returned
from the tooth using fringe projection. FIG. 4A shows a contour
image of tooth 20 obtained using unpolarized light. FIG. 4B shows a
somewhat poorer image using cross-polarized light, but not
exhibiting specular reflection. FIG. 4C shows the improvement in
the image contrast when using co-polarized light. Areas of high
brightness in this image are due to specular reflection. As these
images show, fringe contrast improves when the cross-polarization
light is blocked from the image detector.
[0045] In addition to taking advantage of favorable properties of
polarized light, embodiments of the present invention also take
advantage of different amounts of reflection that correspond to the
wavelength of light directed toward the tooth. FIG. 5A shows three
different wavelengths .lamda.1, .lamda.2, and .lamda.3 as directed
toward tooth 20. The shortest wavelength at .lamda.1 penetrates the
tooth the shortest distance. The next longest wavelength at
.lamda.2 penetrates the tooth an additional distance. Finally, the
longest wavelength at .lamda.3 penetrates the tooth the farthest
distance. The graph of FIG. 5B shows how scattering affects the
footprint of the light on the tooth surface from each wavelength.
The longer the wavelength, the larger the footprint, resulting in
larger measurement error. Wavelength .lamda.1 could be near-UV or
blue light in the range of 350 to 500 nm, for example. Wavelength
.lamda.2 could be green light in the range of 500 to 700 nm, for
example. Wavelength .lamda.3 could be red or IR light in the range
of 700 nm or higher, for example. Thus, blue or near UV light in
the approximate 350-500 nm range, because it provides the least
penetration into the tooth structure, proves to be a suitable light
source for fringe projection imaging in one embodiment.
[0046] For the embodiment of FIG. 1, spatial light modulators can
be used as part of fringe pattern generator 12 to provide the
needed shifting motion for polarized fringe projection imaging, as
described in more detail subsequently. The fringe pattern itself is
shifted to at least one alternate position during imaging, more
preferably to two or more alternate positions. This shifting of the
light pattern can be caused by a separate actuator (not shown in
FIG. 1), such as a piezoelectric or other type of actuator that is
part of fringe pattern generator 12 for achieving precision
incremental movement. Alternately, where fringe pattern generator
12 uses a spatial light modulator, this shifting can be performed
electronically, without mechanical movement of parts within fringe
pattern generator 12. In addition, another actuator 18 can be
positioned for providing 90 degree rotation to either polarizer 14
or analyzer 28 (such as is shown in FIG. 1) in order to obtain both
co-polarization and cross-polarization images. Polarization can
also be rotated when using an LCD spatial light modulator.
[0047] FIG. 6 shows an embodiment of an intra-oral imaging
apparatus 40 that obtains images using both parallel and
cross-polarization without requiring rotation of either polarizer
14 or analyzer 28 between image captures. A polarization beam
splitter 36 separates the reflected and scattered light, reflecting
the cross-polarized light to a detector 30b and transmitting the
co-polarized light to a detector 30a.
[0048] Because the co-polarized and cross-polarized light provide
different types of information about the surface and near-surface
of the tooth, imaging apparatus 40 of FIG. 6 offers the advantage
of using both polarizations without the need for mechanical
movement of analyzer 28 or polarizer 14, combining the results from
orthogonal polarizations in order to obtain improved surface
contour data.
[0049] Detectors 30, 30a, or 30b in the embodiments described
herein can be any of a number of types of image sensing array, such
as a CCD device, for example. Polarizers and analyzers can be
wire-grid or other polarizer types.
[0050] In one embodiment of the present invention, the imaging
apparatus is packaged in the form of a hand-held probe that can be
easily positioned within the patient's mouth with little or no
discomfort. Referring to FIG. 7, there is shown an intra-oral
imaging system 42 that includes imaging apparatus 10 in the form of
a probe. The probe communicates, over a wired or wireless data
communication channel, with control logic processor 34 that obtains
the images from either or both co-polarized and cross-polarized
projection fringes. Control logic processor 34 provides output
image data that can be stored as a data file and displayed on a
display 38.
[0051] As noted in the background section, the pronounced contours
of the tooth include surfaces that are steeply sloped with respect
to each other, complicating the task of directing enough light onto
each surface. As a result, some surfaces of the tooth may not
provide 3-D information that is sufficient. Referring to FIG. 8,
this problem is represented relative to a rear surface 26 of tooth
20. Patterned light from imaging apparatus 10 generates a
contour-detecting fringe pattern 44 onto tooth 20, as shown in box
B. Fringe pattern 44 is sufficiently bright for obtaining 3-D image
content over a top surface area, as outlined over an area 52;
however, the back surface area corresponding to rear surface 26 of
tooth 20 and outlined as a darker area 54 is very dimly lit. This
allows only a coarse estimation, at best, of the contour of rear
surface 26.
[0052] In order to compensate for this lack of brightness using
conventional fringe projection patterning techniques, an embodiment
of the present invention selectively increases the light intensity
of the fringe pattern illumination over a given area. In FIG. 8, a
fringe pattern 50 is shown with two different areas, differentiated
by their relative light intensities. In fringe pattern 50, a first
intensity 56 is provided for fringe projection imaging of surfaces
such as top surface area that are more readily accessible for
contour imaging. A second intensity 58, higher than first intensity
56 for the example shown and as indicated by darker lines in FIG.
8, is provided for the back surface area of the tooth. It should be
observed that the actual pattern feature spacing and thickness of
the projected contour lines that are the pattern features in this
example is not changed in this embodiment. The same spatial
frequency of fringe pattern 50 is preserved. This means that the
contour pattern, fringe pattern 50, remains dimensionally uniform,
with individual lines or other pattern features changed only in
intensity, rather than in dimension or spacing (period). Only the
relative intensity of the fringe pattern illumination over one or
more areas is increased where needed. For example, along any one
line within structured light fringe pattern 50, there can be any
number of intensities, such as the two shown as first and second
intensities 56 and 58 in FIG. 8. The line thickness within the
fringe pattern does not change; the spatial frequency of the fringe
pattern is preserved.
[0053] Maintaining dimensional uniformity and spatial frequency of
the fringe pattern is advantageous for contour imaging because it
provides a uniform resolution over the full image field. Other
techniques have been proposed for changing the pattern dimensions
itself, such as thickening the pattern lines over specific areas;
however, because the spatial frequency of the fringe pattern
changes when using such a technique, the resulting resolution of
the contour image that is obtained is non-uniform. With respect to
the example fringe pattern 50 given in FIG. 8, it is instructive to
observe that if the area indicated as second intensity 58 actually
used thicker lines, the resulting contour image would suffer
reduced resolution over this area. By maintaining the lines of
fringe pattern 50 as dimensionally uniform and only increasing the
intensity of light to provide second intensity 58 in this example,
embodiments of the present invention provide an increased
illumination without loss of resolution over the darker region.
[0054] The schematic diagram of FIG. 8 showed a simple case in
which fringe pattern 50 compensates for surface steepness by using
two different intensities 56 and 58. FIGS. 9A and 9B show examples
of other possible arrangements that use more than two light
intensities. In FIG. 9A, for example, light for the fringe pattern
illumination can be of first intensity 56, second intensity 58, or
a third intensity 66, represented as the highest intensity in this
example. In FIG. 9B, light can be of first, second, or third
intensities 56, 58, or 66 respectively, or of an even higher fourth
intensity 68 as shown. The light intensity can vary along any
individual pattern feature, such as along a single line in the
projected fringe pattern 50.
[0055] In addition to increasing the light intensity over darker
areas of the tooth surface relative to the position of imaging
apparatus 10, it is also possible to reduce the light intensity
over areas where there may be highly specular reflection that
otherwise causes saturation of the detector. Again, it must be
emphasized that what changes is the light intensity over one or
more portions of the projected light pattern; line thickness and
spacing, both related to the spatial frequency, remain the same for
different intensities.
[0056] Referring again to FIGS. 1 and/or 6, the light intensity
over the projected pattern can be changed by controlling fringe
pattern generator 12 by means of commands from control logic
processor 34, in response to programmed instructions, and by means
of signals provided from control logic processor 34 to related
control components. In one embodiment, fringe pattern generator 12
is a digital micromirror device (DMD). Intensity can then be
increased over any portion of projected fringe pattern 50 by
increasing the effective duty cycle of the rotatable mirrors of the
DMD using Pulse-Width Modulation (PWM), so that the source
illumination is provided for a suitable amount of time over a
particular portion of the fringe pattern. Other methods of
illumination intensity adjustment would apply for LCD and for other
transmissive and emissive spatial light modulators, using light
modulation techniques familiar to those skilled in the imaging
arts.
[0057] Referring again to FIG. 7, control logic processor 34 is
programmed with instructions that automatically adapt the local
intensities of lines or other features in fringe pattern 50
according to imaging conditions.
[0058] The logic flow diagram of FIG. 10 shows a sequence of steps
that are used for adaptive fringe projection imaging in one
embodiment. In an initial step 60 a first reference image is
obtained. The reference image can be a contour image, formed by
projecting structured light onto the tooth surface. Alternately,
the reference image can be a conventional two-dimensional image
obtained from projection of a uniform field of light onto the tooth
surface. The reference image that is obtained can be at full
resolution; alternately, since the reference image is not used
directly for imaging but instead to determine the overall amount of
light that is returned over each surface area, the reference image
can be at lower resolution.
[0059] Still referring to FIG. 10, an analysis step 64 follows, in
which areas from the sensed reference image that are not
sufficiently bright are identified. For dental imaging
applications, analysis step 64 can take advantage of known data
about tooth structure. The operator, for example, may identify the
tooth by number or provide other information that is used in
analysis step 64. A map generation step 70 is then executed, in
which areas of greater or lesser intensity are defined according to
the first reference image. With respect to FIGS. 9A and 9B, step 70
then sets up variable intensity fringe pattern 50. An image
acquisition step 74 then uses the generated fringe pattern 50 for
obtaining a contour image with added brightness as described with
respect to FIG. 8. Image acquisition step 74 may be followed by an
optional looping step 76 that repeats the analysis of map
generation step 70 in order to generate a second or other
additional mappings so that the projected structured illumination
pattern can be shifted, with appropriate changes in intensity, one
or more times. This shifting is done in order to obtain a more
accurate evaluation of tooth contour using fringe projection
techniques. The individually obtained contour images are combined
to obtain surface structure information, using techniques well
known in the imaging arts. In one embodiment, image acquisition
step 74 also includes energizing actuator 18 (FIG. 1) in order to
obtain images using both co-polarization (as in FIG. 2A) and
cross-polarization (FIG. 2B).
[0060] FIG. 11 is a schematic block diagram showing components of
fringe pattern generator 12 in one embodiment. A spatial light
modulator 84, such as a digital micromirror device (DMD), liquid
crystal device (LCD), or other type of light modulator array or
grating forms a pattern according to control signals from control
logic processor 34. A light source 80 provides incident light to
spatial light modulator 84, conditioned by one or more optical
elements 82, such as a light uniformizer and lens elements. Spatial
light modulator 84 in this embodiment may be a transmissive device
as shown in FIG. 11 or a reflective device, such as a DMD. Control
logic processor 34 responds to pattern 44 of light brightness that
is returned in the initial reference image as was described earlier
with reference to FIG. 8 to control the intensity of pattern
features in the fringe pattern that it forms on spatial light
modulator 84.
[0061] In the embodiment shown in FIG. 11, light source 80 can be a
solid-state light source, such as a Light-Emitting Diode (LED) or
laser, or can be a lamp or other light source. Blue or near UV
light in the 350-500 nm range is used for providing usable image
content from near-surface portions of the tooth, as described
earlier. In an alternate embodiment, light source 80 is not used
and an emissive array, such as an Organic LED (OLED) is used for
pattern generation from a single component.
[0062] The schematic diagram of FIG. 12 shows another embodiment of
the present invention wherein a filter 90, such as a bandpass
filter that transmits blue or near UV light in the 350-500 nm range
and attenuates other light, is placed in the imaging path. This
embodiment can be less sensitive to factors in the environment,
such as stray light from other equipment in the room. In this
embodiment, light source 80 within fringe pattern generator 12
(FIG. 11) can be either broadband, extending well beyond the
350-500 nm range, or narrow-band, primarily emitting blue and
near-UV light.
[0063] Embodiments of the present invention provide improved
contour imaging for teeth by taking advantage of properties of
light and capabilities of spatial light modulators for forming an
adaptive fringe projection pattern having suitable light intensity
that is responsive to variability in tooth surface characteristics.
The apparatus and methods of the present invention compensate for
problems related to the translucence of the tooth by using
short-wavelength light and by employing principles of polarized
light. When light of suitable wavelength and polarization state is
provided with an adaptable intensity arrangement, a more accurate
indicator of the highly contoured tooth surface can be
achieved.
[0064] The surface contour image that is obtained using the
apparatus and methods of the present invention can be used in a
number of ways. Contour data can be input into a system for
processing and generating a restorative structure or can be used to
verify the work of a lab technician or other fabricator of a dental
appliance. This method can be used as part of a system or procedure
that reduces or eliminates the need for obtaining impressions under
some conditions, reducing the overall expense of dental care. Thus,
the imaging performed using this method and apparatus can help to
achieve superior fitting prosthetic devices that need little or no
adjustment or fitting by the dentist. From another aspect, the
apparatus and method of the present invention can be used for
long-term tracking of tooth, support structure, and bite
conditions, helping to diagnose and prevent more serious health
problems. Overall, the data generated using this system can be used
to help improve communication between patient and dentist and
between the dentist, staff, and lab facilities.
[0065] Advantageously, the apparatus and method of the present
invention provide an intra-oral imaging system for 3-D imaging of
teeth and other dental features without requiring the use of a
special powder or application of some other temporary coating for
the tooth surface. The system offers high resolution, in the 25-50
.mu.m range in one embodiment.
[0066] The invention has been described in detail with particular
reference to a presently preferred embodiment, but it will be
understood that variations and modifications can be effected within
the spirit and scope of the invention. For example, any of a number
of different types of spatial light modulator could be used as part
of the fringe pattern generator. The presently disclosed
embodiments are therefore considered in all respects to be
illustrative and not restrictive. The scope of the invention is
indicated by the appended claims, and all changes that come within
the meaning and range of equivalents thereof are intended to be
embraced therein.
PARTS LIST
[0067] 10. Imaging apparatus [0068] 12. Fringe pattern generator
[0069] 14 Polarizer [0070] 16. Lens [0071] 18. Actuator [0072] 20.
Tooth [0073] 22. Lens [0074] 26. Rear surface [0075] 28. Analyzer
[0076] 30, 30a, 30b. Detector [0077] 34. Control logic processor
[0078] 36. Polarization beam splitter [0079] 38. Display [0080] 40.
Imaging apparatus [0081] 42. Intra-oral imaging system [0082] 44.
Pattern [0083] 50. Fringe pattern [0084] 52, 54. Area [0085] 56.
First intensity [0086] 58. Second intensity [0087] 60. Initial step
[0088] 64. Analysis step [0089] 66. Third intensity [0090] 68.
Fourth intensity [0091] 70. Map generation step [0092] 74. Image
acquisition step [0093] 76. Looping step [0094] 80. Light source
[0095] 82. Optical element [0096] 84. Spatial light modulator
[0097] 88. Detection path [0098] 90. Filter [0099] B. Box [0100]
P0, P1, P2. Polarized light [0101] .lamda.1, .lamda.2,.lamda.3.
Wavelength
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