Optics: measuring and testing – By light interference – Having light beams of different frequencies
Reexamination Certificate
2002-02-20
2003-08-05
Tarcza, Thomas H. (Department: 3662)
Optics: measuring and testing
By light interference
Having light beams of different frequencies
C356S498000, C356S601000
Reexamination Certificate
active
06603561
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a method of making measurements in three-dimensions using the chromatic dispersion of a diffraction grating.
2. Related Art
Diffraction range finders are devices which determine distance by correlating the relationship between the distances of a diffraction grating from an illuminated target surface with the respective relative displacements of high-order diffraction images from the position of the respective zero-order image as observed through the diffraction grating. The target must be self-illuminated or illuminated by a secondary source of energy propagated by periodic waves. Higher-order diffraction images of a target are reconstructed at a receiver which has a means to focus the radiation onto a transducer that can sense the position of the higher-order diffraction images. As a target is moved toward or away from a grating surface, the relative displacement of a higher-order image from both the zero-order image and other higher-orders images can be measured to take target range.
Chromatic dispersion has previously been used within structured illumination projectors to light a surface being ranged through a triangulation or parallax method.
The “Rainbow Range Finder” and its principles of operation are discussed U.S. Pat. Nos. 4,864,395; 5,200,792; and 5,157,487. Zheng Jason Geng holds U.S. Pat. Nos. 5,675,407; 6,028,672; and 6,147,760 for related inventions.
Rainbow range finders take range readings by projection of a pattern of colors onto a target and then taking the further step of correlating the colors on the target with the distances to a receiver that can discriminate the colors. All published embodiments of rainbow range finder presume a structured illumination source that projects a pattern of unique color hues onto a target surface. Typically, a rainbow projector will have a diffraction grating inside the projector that coverts the radiation from an incandescent light bulb into a broad spectrum. Said spectrum is then focused onto a target surface. The receiver can be an ordinary color video camera that has separate sensors for red, green and blue, as is typical of most television cameras. As asserted in these patents, there are well understood techniques of colorimetry for making determinations of a unique color at each pixel site in the camera by measuring the relative intensity of the primary colors. The present inventor has demonstrated such a method for such color discrimination using television cameras with red, green and blue channels (“Pantomation—A System for Position Tracking,” Tom DeWitt and Phil Edelstein,
Proceedings of the Second Symposium on Small Computers in the Arts,
1982, IEEE Computer Society, No. 455, pp. 61-70).
The Rainbow Range Finder relies on triangulation to make range measurements and therefore suffers from the intrinsic limitations of a parallax-based range finder. Among these limitations are perspective foreshortening which results in an inverse square relationship of accuracy to distance. Triangulation also suffers from the liability that occluded regions can occur between the projector and receiver causing obscured regions devoid of readings. Furthermore, as applied to profilometry, all triangulation devices make a trade-off between target height and depth sensitivity.
The limitations endemic to triangulation ranging methods as found, for example, in the Rainbow Range Finder led to the development of an improved method of range finding that uses a diffraction grating in the receiver.
Patents that teach how a range finder can be made with diffraction gratings are:
U.S. Pat. No. 4,678,324 awarded to Tom DeWitt (now known as Tom Ditto, the inventor of the present invention) on Jul. 7, 1987 for “Range Finding by Diffraction.”
U.S. Pat. No. 5,076,698 granted to Smith et al. on Dec. 31, 1991 for “Sensing the Shape of an Object.”
PCT/US1997/02384, priority date Dec. 30, 1996, laid open as WIPO WO1999/044013 and published as Canadian Patent Application CA2277211, “VARIABLE PITCH GRATING FOR DIFFRACTION RANGE FINDING SYSTEM,” inventors Ditto and Lyon.
The '324 patent supra teaches “It has been found that the objects of the present invention may be realized by projecting a monochromatic pencil beam of light at a target, viewing the illuminated target through a diffraction grating, and measuring the displacement of the higher order diffraction images from the position of the zero order image lines,” [column 4, lines 56-61].
In
FIG. 1
, adapted from Thomas D. DeWitt and Douglas A. Lyon, “A Range Finding Method Using Diffraction Gratings,”
Applied Optics,
May 10, 1995, Vol. 34 No. 14, pp. 2510-2521, the authors describe a mathematical relationship in the diffraction range finder whereby range can be determined by measuring the displacement x
104
of a higher-order diffraction image formed at the focal plane of a camera
130
. The displacement x
104
is measured with respect to point
107
located at the center of the focal plane of the camera
130
. The distance D
100
from the target
150
to grating
120
can be measured along a line of light from a laser
110
. The relationships of a diffraction range finder be described geometrically as:
D
=
(
1
-
(
n
λ
p
-
sin
(
ρ
+
arctan
(
x
F
)
)
)
2
n
λ
p
-
sin
(
ρ
+
arctan
(
x
F
)
)
)
(
d
tan
(
ρ
+
arctan
(
x
F
)
)
-
s
)
cos
(
α
)
-
(
1
-
(
n
λ
p
-
sin
(
ρ
+
arctan
(
x
F
)
)
)
2
n
λ
p
-
sin
(
ρ
+
arctan
(
x
F
)
)
)
-
sin
(
α
)
(
1
)
In relation to FIG.
1
and Equation (1), a laser
110
transmits monochromatic light to a target
150
along a line of illumination
115
. The target
150
redirects said light to a diffraction grating
120
, and the diffraction grating
120
diffracts said light into a diffraction pattern. The diffraction pattern is passed through a lens
140
of a camera
130
and is recorded on a focal plane of the camera
130
. Other parameters appearing in FIG.
1
and Equation (1) are as follows:
D
100
is the range along the line of illumination
115
from the target
150
to the diffraction grating
120
.
d
101
is the distance from the lens
140
to the diffraction grating
120
.
s
102
is the distance from the lens
140
to the line
117
, wherein the line
117
is normal to grating plane of the grating
120
and passes through the intersection
118
of the illumination ray
115
with the grating plane.
n is an integer denoting the diffraction order (n=0 denotes zero-order diffraction, while n>0 and n<0 denotes high order diffraction)
&lgr; is the wavelength of the light transmitted by the laser
110
.
p is the pitch of the grating
120
.
F
103
is the focal length of the lens
140
.
x
104
is the position on the focal plane where the diffraction image forms.
&agr;
105
is the angle of a laser relative the line
117
.
&rgr;
106
is the angle of the baseline of the camera
130
relative to the line
117
.
An example of the related art is shown in
FIG. 2. A
step block
230
is a target that is illuminated by a laser
210
. The laser
210
produces a sheet of monochromatic light
220
. On the target
230
surface, the sheet of light
220
is diffused as wave fronts
222
back toward a diffraction grating
240
. Examples of diffused light rays are shown as
224
and
225
. The light diffused from the target
230
strikes the grating
240
which is in the field-of-view of a monochrome camera
250
with array sensor
255
. Examples of diffracted rays are shown as extensions of rays
224
and
225
. If the camera signal is viewed on a television monitor
255
, it will show points
257
of horizontal displacement across the screen proportional to target range. The correlated positions on the monitor of example rays
224
and
225
are indicated.
In
Andrea Brian
Schmeiser Olsen & Watts
Tarcza Thomas H.
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