Calibration method for fringe projection systems based on plane mirrors

Patent metadata

Field	Value
Publication number	US-11808564-B2
Application number	US-202017631542-A
Country	US
Kind code	B2
Filing date	Aug 7, 2020
Priority date	Aug 9, 2019
Publication date	Nov 7, 2023
Grant date	Nov 7, 2023

Abstract

A calibration method for fringe projection systems based on plane mirrors. Firstly, two mirrors are placed behind the tested object. Through the reflection of mirrors, the camera can image the measured object from the front and other two perspectives, so as to obtain 360-degree two-dimensional information of the measured object. The projector projects three sets of phase-shifting fringe patterns with frequencies of 1, 8, and 64. The camera captures the fringe image to obtain an absolute phase map with a frequency of 64 by using the phase-shifting method and the temporal phase unwrapping algorithm. By using the calibration parameters between the projector and the camera, the absolute phase map can be converted into three-dimensional information of the measured object. Then, the mirror calibration is realized by capturing a set of 3D feature point pairs, so that the 3D information from different perspectives is transformed into a unified world coordinate system. The calibration method does not need to artificially fix the feature pattern on plane mirrors, only needs to capture a set of 3D feature point pairs by the camera to directly realize the mirror calibration that it avoids the loss of measurement accuracy and realizes high-precision panoramic three-dimensional measurement.

First claim

The invention claimed is:

1. A calibration method for fringe projection systems based on plane mirrors, comprising the steps of:
Step 1: placing two mirrors including a left mirror and a right mirror behind a measured object and using a camera to obtain 360-degree two-dimensional information of the measured object through reflections off the two mirrors;
Step 2: projecting three sets of phase-shifting fringe patterns to the measured object by using a projector, and capturing the projected fringe patterns synchronously with the camera and the two mirrors, wherein an intensity map is obtained for each of the three sets of phase-shifted fringe patterns;
Step 3: calculating wrapped phase maps with different frequencies by using a phase shifting method to obtain three sets of wrapped phase maps, performing phase unwrapping on the obtained three sets of wrapped phase maps, and finally obtaining an absolute phase map by using calibration parameters representing a relationship between each pixel of the projector and each pixel of the camera, and converting the absolute phase map into three-dimensional (3D) information of the measured object;
Step 4: realizing a mirror calibration for each of the two mirrors by capturing a set of 3D feature point pairs so that the 3D information of the measured object is transformed into a unified world coordinate system, so as to achieve high-precision panoramic 3D measurement.

2. The method according to claim 1 wherein the phase-shifting fringe patterns projected by the projector are represented by the following equation:

I
n
p

⁡

(

x
p

,

y
p

)

=

1
⁢
2
⁢
8

+

127
⁢

cos
⁡

[

2
⁢
π
⁢

⁢

fx
p

W

-

2
⁢
π
⁢

⁢
n

N

]

where I n p (x p ,y p ) is the phase-shifting fringe pattern projected by the projector, n represents a phase-shifting index of the phase-shifting fringe patterns, n=0, 1, 2, . . . , N−1, N represents a total number of phase-shifting steps, (x p ,y p ) is pixel coordinates of a projector plane, W is a horizontal resolution of the projector, f is a frequency of the phase-shifting fringe patterns, and
wherein the intensity map is represented by the following equation:
I n ( x,y )= A ( x,y )+ B ( x,y )cos[Φ( x,y )−2π n/N]

where I n (x,y) is the intensity map of the corresponding phase-shifting fringe image, (x,y) is pixel coordinates of a camera plane, A(x,y) is a background intensity, B(x,y) is a modulation of fringes, Φ(x,y) is a phase to be calculated.

3. The method according to claim 1 , wherein the step 3 comprises the sub-steps of:
(1) obtaining the wrapped phase map φ(x,y) by using the following formula:

φ
⁡
(

x
,
y

)

=

arc
⁢
tan
⁢

∑

n
=
0

N
-
1

I
n

(

x
,
y

)

⁢

sin
⁡
(

2
⁢
π
⁢
n

N

)

∑

n
=
0

N
-
1

I
n

(

x
,
y

)

⁢

cos
⁡
(

2
⁢
π
⁢
n

N

)

wherein the obtained phase map φ(x,y) is a wrapped phase with a range of [0,2π], and the absolute phase map Φ(x,y) is represented by the following equation:

Φ
⁡

(

Assignees

• Univ Nanjing Sci & Tech

Inventors

• Zuo Chao
• Yin Wei
• Chen Qian
• Feng Shijie
• Sun Jiasong
• Tao tianyang
• Hu Yan
• Zhang Liang
• Qian Jiaming

Classifications

• G01B11/2504 Primary

  Calibration devices · CPC title

• G01B11/2513

  with several lines being projected in more than one direction, e.g. grids, patterns · CPC title

• G01B11/2527

  with phase change by in-plane movement of the patern · CPC title