U.S. patent application number 14/032415 was filed with the patent office on 2014-03-27 for structured light systems with static spatial light modulators.
This patent application is currently assigned to ALCES TECHNOLOGY, INC.. The applicant listed for this patent is David M Bloom, Matthew A Leone. Invention is credited to David M Bloom, Matthew A Leone.
Application Number | 20140085426 14/032415 |
Document ID | / |
Family ID | 50338453 |
Filed Date | 2014-03-27 |
United States Patent
Application |
20140085426 |
Kind Code |
A1 |
Leone; Matthew A ; et
al. |
March 27, 2014 |
Structured light systems with static spatial light modulators
Abstract
Structured light systems are based on temporally modulated light
sources and static spatial light modulators.
Inventors: |
Leone; Matthew A; (Jackson,
WY) ; Bloom; David M; (Jackson, WY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Leone; Matthew A
Bloom; David M |
Jackson
Jackson |
WY
WY |
US
US |
|
|
Assignee: |
ALCES TECHNOLOGY, INC.
Jackson
WY
|
Family ID: |
50338453 |
Appl. No.: |
14/032415 |
Filed: |
September 20, 2013 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61705000 |
Sep 24, 2012 |
|
|
|
Current U.S.
Class: |
348/46 ; 362/227;
362/259; 362/277 |
Current CPC
Class: |
G02B 26/0816 20130101;
G02B 26/085 20130101; H04N 13/204 20180501; G02B 26/101 20130101;
G01C 3/08 20130101; G02B 26/0841 20130101; G06K 9/2036 20130101;
G02F 1/0128 20130101; G02B 26/0833 20130101; G02F 1/13306 20130101;
F21K 9/60 20160801; G02F 1/155 20130101; G02B 26/0858 20130101;
G09G 3/3433 20130101; H04N 13/254 20180501; G02F 1/29 20130101;
G01B 11/2527 20130101; G02B 26/02 20130101; F21K 9/65 20160801;
G09G 2300/0473 20130101; G02B 26/06 20130101; G09G 2300/0426
20130101; G02F 1/0123 20130101; G02B 26/0808 20130101; G02B 27/0068
20130101 |
Class at
Publication: |
348/46 ; 362/227;
362/277; 362/259 |
International
Class: |
F21K 99/00 20060101
F21K099/00; H04N 13/02 20060101 H04N013/02 |
Claims
1. A structured light system comprising: a first light source for
illuminating a first static spatial light modulator; a second light
source for illuminating a second static spatial light modulator;
and, a third light source for illuminating a third static spatial
light modulator, such that when the first, second and third light
sources are turned on and off in succession, three spatial phases
of one base spatial pattern are projected in succession.
2. The system of claim 1, the first, second and third light sources
being diode lasers.
3. The system of claim 1, the first, second and third light sources
being light emitting diodes.
4. The system of claim 1, the first, second and third static
spatial light modulators being made from glass plates with metal
coatings adhered to them.
5. The system of claim 1, the three spatial phases being 0, 120 and
240 degrees.
6. The system of claim 1 further comprising: a fourth light source
for illuminating a fourth static spatial light modulator, such that
when the first, second, third and fourth light sources are turned
on and off in succession, four spatial phases of one base spatial
pattern are projected in succession.
7. The system of claim 6, the four spatial phases being 0, 90, 180
and 270 degrees.
8. The system of claim 1 further comprising: a camera separated
from the first, second and third static spatial light modulators by
a baseline distance, the camera receiving information from the
first, second and third light sources pertaining to temporal
modulation characteristics of projected patterns.
9. The system of claim 1 further comprising: first, second and
third homogenizers to make illumination of the respective spatial
light modulators by the light sources uniform.
10. A structured light system comprising: a light source; and, a
mechanical carrier that moves four different static spatial light
modulator masks sequentially through an optical system in sync with
temporal modulation of the light source, such that four spatial
phases of one base spatial pattern are projected in succession.
11. The system of claim 10, the light source being a diode
laser.
12. The system of claim 10, the light source being a light emitting
diode.
13. The system of claim 10, the masks being made from glass plates
with metal coatings adhered to them.
14. The system of claim 10, the four spatial phases being 0, 90,
180 and 270 degrees.
15. A structured light system comprising: a light source; a static
spatial light modulator illuminated by the light source; a frame;
and, a projection lens, the lens projecting an image of a part of
the static spatial light modulator that appears within the frame,
wherein the static spatial light modulator moves with respect to
the frame such that different spatial phases of a one-dimensional
periodic pattern are projected in sync with modulation of the light
source.
16. The system of claim 15, the light source being a diode
laser.
17. The system of claim 15, the light source being a light emitting
diode.
18. The system of claim 15, the static spatial light modulator
having successive phases of the one-dimensional periodic spatial
pattern carried on a wheel and the wheel rotating in sync with
modulation of the light source.
19. The system of claim 15 further comprising: a camera separated
from the static spatial light modulator by a baseline distance, the
camera receiving information from the light source pertaining to
temporal modulation characteristics of projected patterns.
Description
RELATED APPLICATIONS
[0001] This application claims priority benefit from U.S.
61/705,000, "Structured light systems", filed on Sep. 24, 2012 and
incorporated herein by reference.
TECHNICAL FIELD
[0002] The disclosure is related to structured light systems and
static spatial light modulators.
BACKGROUND
[0003] Structured light systems project known light patterns onto
an object. Surface contours of the object make the patterns appear
distorted when viewed with a camera at a vantage point separated
from the pattern projector by a baseline distance. Geometrical
relationships are used to interpret the distortions to determine
the distance from the projector to points on the object. In this
way, three dimensional spatial coordinates of the surface of the
object may be obtained.
[0004] Many conventional structured light systems are based on
projecting patterns that are periodic in one dimension, such as
stripe patterns. Successive spatially phase-shifted replicas of a
pattern are projected. Conventional projectors, such as those based
on digital-micromirror-array spatial light modulators, are able to
produce grayscale patterns at approximately 200 Hz. The update rate
is limited by the use of pulse width modulation to produce
grayscale (analog pixel brightness) from a digital light modulator
(binary pixel brightness).
[0005] Depth resolution of a structured light system depends on how
well the spatial phase of a periodic pattern can be resolved, and
that in turn depends on accurate measurements of pixel brightness
across an image. When the update rate of a pattern projector is
limited to a few hundred Hertz, noise from low-frequency sources
such as 60 Hz lighting may degrade structured light system
performance.
[0006] What are needed are structured light systems with faster
pattern projection rates that enable synchronous detection of the
phase of spatial patterns.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a conceptual diagram of a structured light
projector with multiple light sources and static spatial light
modulators.
[0008] FIG. 2 is a conceptual diagram of a structured light
projector with one light source and multiple static spatial light
modulators.
[0009] FIG. 3 is a conceptual diagram of a structured light
projector with one light source and a spatial light modulator based
on a moving mask.
[0010] FIG. 4 illustrates different positions of the moving mask of
FIG. 3.
[0011] FIG. 5 illustrates an alternative mask scheme for a
projector such as that of FIG. 3.
[0012] FIG. 6 illustrates a structured light system using any of
the projectors of FIGS. 1-3.
DETAILED DESCRIPTION
[0013] Structured light systems with static spatial light
modulators are capable of projecting light patterns much faster
than is possible with conventional, reconfigurable spatial light
modulators. A static spatial light modulator is one that has a
fixed spatial pattern that it imparts to light passing through it.
An example of a static spatial light modulator is a photomask: a
glass plate with a metal coating adhered to it that varies from
opaque to partially transmitting to clear in different regions of
the plate. A static spatial light modulator may be moveable, but
its spatial pattern cannot be reconfigured.
[0014] Three examples of structured light systems based on static
spatial light modulators are discussed below. In one example,
multiple light sources each illuminate different static spatial
light modulators. This system permits pattern rates as fast as the
light sources can be temporally modulated, well into the megahertz
range. In two other examples, one temporally modulated light source
is combined with a movable, static spatial light modulator.
Different parts of the modulator are illuminated at different
times. The speed of these systems depends on how fast different
parts of a static modulator can be moved in front of a light
source. Such systems may permit pattern rates in the kilohertz
range.
[0015] FIG. 1 is a conceptual diagram of a structured light
projector with multiple light sources and static spatial light
modulators. In FIG. 1, four light sources 105, 106, 107 and 108
illuminate four static spatial light modulators 115, 116, 117 and
118, respectively. Homogenizers 110, 111, 112 and 113 serve to make
the illumination of the modulators uniform. The homogenizers may be
realized as light tunnels and spatial filters, for example.
Finally, four lenses 120, 121, 122 and 123 project images of the
spatially modulated light onto an object 125.
[0016] Light sources 105-108 may be diode lasers or light emitting
diodes. These light sources are modulated temporally with standard
diode driver electronics. The modulation frequency may be as high
as 1 MHz or more, although modulation that fast may not be
necessary. Static spatial light modulators 115-118 may be
photomasks with variable thickness metal coatings. Areas on the
mask where the coating is thin or nonexistent pass the most light
while areas where the coating is thicker pass less light. Lenses
120-123 may be individual lenses as illustrated, lenses in a lens
array, or part of a more complex optical system.
[0017] In the figure, the masks are shown with stripe patterns
having sinusoidal optical density variations in one direction.
Although the masks are shown as if they were in the plane of the
page, in fact they are perpendicular to the page.
[0018] Light sources 105-108 are modulated such that they are
turned on and off in succession. Thus the projected pattern that is
incident upon object 125 changes from an image of static modulator
115 to one of static modulator 116 to one of static modulator 117,
etc. The system of FIG. 1 uses four light sources and static
modulators to project four successive patterns. The patterns are
four spatial phases, e.g. 0, 90, 180 and 270 degrees, of one base
pattern. Structured light depth capture may also be performed with
three phases, e.g. 0, 120 and 240 degrees. In that case only three
light sources, masks and associated optics are needed.
[0019] FIG. 2 is a conceptual diagram of a structured light
projector with one light source and multiple static spatial light
modulators. The system of FIG. 2 is similar to that of FIG. 1
except that in FIG. 2 a mechanical carrier holds the four different
static light modulator masks. The carrier moves the masks
sequentially through an optical system. The masks are otherwise the
same as 115-118 of FIG. 1.
[0020] In FIG. 2, light source 205 illuminates one of masks 216,
217, 218 or 219 that are fixed in carrier 215. Homogenizer 210
ensures that the light source illuminates a mask uniformly. As in
the system of FIG. 1, the homogenizer may be realized as a light
tunnel and spatial filter. Lens 220 projects an image of a mask
onto object 225.
[0021] Masks or static spatial light modulators 216-219 move back
and forth in the apparatus of FIG. 2 as indicated by the arrows and
in sync with modulation of light source 205. Thus the light source
is turned on for a brief time while mask 216 is in the light path
from homogenizer 210 to lens 220. Next the light source is turned
on for a second brief time when mask 217 is in the light path,
etc.
[0022] Light source 205, which may be a diode laser or light
emitting diode, can be modulated at high speed. However, the system
can only produce new images as fast as carrier 215 can position
masks in the light path. This may limit pattern projection to
kilohertz rates. Comparing the systems of FIGS. 1 and 2, the former
permits much higher pattern rates, but the latter uses fewer
optical components. As mentioned in connection with FIG. 1, in FIG.
2 carrier 215 and its masks are shown as if they lie in the plane
of the page when in fact they are perpendicular to the plane of the
page.
[0023] A third example of a structured light system based on a
static spatial light modulator is shown in FIG. 3 which is a
conceptual diagram of a structured light projector with one light
source and a spatial light modulator based on a moving mask. The
system of FIG. 3 is similar to that of FIG. 2 except that one mask
with extra periods of a one-dimensional periodic pattern is moved
in front of an aperture or frame to produce different spatial
phases of the pattern.
[0024] In FIG. 3, light source 305 illuminates mask 312.
Homogenizer 310 ensures uniform illumination of the mask. Lens 320
projects an image of the mask onto object 325. However, the image
only includes the part of mask 312 that lies within frame 315. As
in the systems of FIGS. 1 and 2, light source 305 may be a diode
laser or light emitting diode, homogenizer 310 may include a light
tunnel and spatial filter, and mask 312 and frame 315 are shown as
if they lie in the plane of the page, when in fact they are
oriented perpendicular to the plane of the page and perpendicular
to the direction of propagation of light from light source 305 to
lens 320.
[0025] Frame 315 is stationary with respect to the light source,
homogenizer and lens. Mask 312 moves as indicated by the double
arrow. Mask 312 moves such that successive spatial phases of the
mask pattern are projected. The mask may move in steps and dwell in
positions corresponding to specific spatial phases, or it may move
smoothly and appear frozen at different spatial phases by short
pulses of light emitted by the light source.
[0026] FIG. 4 illustrates different positions of the moving mask
312 of FIG. 3 with respect to frame 315. Inspection of the figure
shows that the pattern of the mask is periodic and that the four
positions shown in the figure correspond to four spatial phases of
the pattern being aligned with the frame.
[0027] In one mechanical arrangement the mask stops at each
position (0, 90, 180, 270) briefly. During the time the mask is
stopped, a light source (e.g. 305) is turned on to illuminate the
mask. The light source is then turned off while the mask is moved
to a new position. Alternatively the light source may be modulated
so that it emits short pulses of light. The mask may move smoothly
in the direction indicated by the double arrow. The light pulses
may be made short enough that movement of the mask is negligible
during each pulse.
[0028] The short pulse approach simplifies the mechanics of moving
the mask since in that case it need only oscillate back and forth.
On the other hand, stopping and starting the mask at each phase may
allow longer duration illumination in each position and thereby
make signal detection easier.
[0029] FIG. 5 illustrates an alternative mask scheme for a
projector such as that of FIG. 3. In FIG. 5, masks 510, 512, 514,
etc are carried on a wheel 515. The wheel may be inserted in an
apparatus such as that shown in FIG. 3 in place of mask 312. The
wheel operates analogously to a color wheel in a color image
projector that uses only one (reconfigurable) spatial light
modulator and projects successive red, green and blue images. When
the diameter of the wheel is small successive phases of a spatial
pattern (e.g. 0, 90, 180 as shown in the figure) may be carried on
it. On the other hand if the diameter is made larger, then a
continuous pattern of stripes radiating out from the center of the
wheel and having, e.g. sinusoidal, variation in the tangential
direction may provide a substitute for discretely framed
patterns.
[0030] Any of the projectors of FIGS. 1-3, including any of the
variations discussed in connection with FIGS. 4 and 5, may be used
in a structured light system such as that illustrated in FIG. 6. In
FIG. 6 a pattern projector located at "P" generates a sinusoidal
stripe pattern 605 that illuminates a three-dimensional surface
located a distance, z, away from the projector. A camera located at
point "C" views the pattern 610 that stripes 605 make when they
illuminate the surface. The camera records the (X, Y) location of
points in pattern 610 as they appear on the camera's image sensor
615. For example point 620 on the surface corresponds to point 621
in the camera. The camera is separated from the projector by
baseline distance, d.
[0031] Pattern projector "P" may be any of the projectors based on
static spatial light modulators as described above. Frequency and
phase information describing the temporal modulation
characteristics of projected patterns is communicated between
projector P and camera C by a SYNC connection 630. Camera C (and
associated processors and memory, not shown) demodulate the spatial
phase of patterns that appear on the camera's image sensor. This
spatial phase demodulation is aided by temporal demodulation of the
same signal. Since projector P is capable of projecting images at
kilohertz or even megahertz frame rates, the temporal demodulation
allows the camera to separate the desired signal (i.e. the image
intensity at each camera pixel) from lower frequency noise.
[0032] Static spatial light modulators may be used in systems that
project grayscale light patterns much faster than is possible with
conventional, reconfigurable spatial light modulators since the
pattern repetition rate depends on the speed of light modulation
(or in some cases mask movement) rather than the time required to
reconfigure light modulator elements. These systems may be
especially useful when the diversity of spatial objects to be
measured is limited. Depth capture in an industrial production line
setting can be a more predictable environment than gesture
recognition for games, as an example.
[0033] The above description of the disclosed embodiments is
provided to enable any person skilled in the art to make or use the
invention. Various modifications to these embodiments will be
readily apparent to those skilled in the art, and the principles
defined herein may be applied to other embodiments without
departing from the scope of the disclosure. Thus, the disclosure is
not intended to be limited to the embodiments shown herein but is
to be accorded the widest scope consistent with the principles and
novel features disclosed herein.
* * * * *