Calibration method for telecentric imaging 3D shape measurement system

Patent metadata

Field	Value
Publication number	US-10110879-B2
Application number	US-201615052489-A
Country	US
Kind code	B2
Filing date	Feb 24, 2016
Priority date	Mar 5, 2015
Publication date	Oct 23, 2018
Grant date	Oct 23, 2018

Abstract

A calibration method is described for a telecentric imaging 3D shape measurement system, including step S 1: establishing a telecentric 3D shape measurement system; S 2: controlling a telecentric projection equipment to project a sinusoidal fringe pattern to a translation stage, and collecting the sinusoidal fringe pattern by a telecentric camera equipment; moving the translation stage to different depth, then obtaining absolute phase values of a pixel for calibration by a phase-shifting method; and conducting linear fitting on the series of absolute phase values of the pixel and the corresponding depths to obtain a phase-depth conversion of the measurement system; and S 3: transforming pixel coordinates on the image plane of the telecentric camera equipment into world coordinates through calibrating parameters of the telecentric camera equipment. A relationship between phase and depth herein is linear, and only needs to calibrate the linearity of one pixel. Therefore, the phase-depth calibration is of small complexity, high precision, good operability and strong practical applicability. Moreover, an orthographic transformation model of camera with bilateral telecentric lens is provided, which simplifies the calibration process.

First claim

The invention claimed is:

1. A calibration method for a telecentric imaging 3D shape measurement system, wherein the calibration method comprises the following steps of:
step S 1 : establishing a telecentric 3D shape measurement system, and the measurement system including: a projector with a bilateral telecentric lens, a camera with a bilateral telecentric lens and a translation stage; letting the optical axis of the camera be vertical to the translation stage that is positioned horizontally, and an optical axis of the projector form a slant angle with the translation stage; controlling the optical axis of the camera and the optical axis of the projector to be in a same plane;
step S 2 : making the translation stage be in the common depth of field of the telecentric projector equipment and the telecentric camera equipment, collecting fringes by the telecentric camera equipment when the telecentric projector equipment projects sinusoidal fringe pattern to the translation stage, selecting any pixel on an imaging plane of the telecentric camera equipment as a pixel for calibration, solving an absolute phase value of the pixel by using a phase-shifting method, and recording a depth value of the translation stage at the moment;
controlling the translation stage to be different depths, which are on a direction along the optical axis of the telecentric camera equipment, and in the common depth of field of the telecentric projection equipment and the telecentric camera equipment, obtaining every absolute phase value of the pixel for calibration respectively when the translation stage is at different depths, and recording each corresponding depth value of the translation stage; and
conducting linear fitting on the different depth values of the translation stage and the corresponding absolute phase values of the pixel for calibration, establishing a conversion between absolute phase values and depth values in the telecentric 3D shape measurement system; and
step S 3 : transforming pixel coordinates on the image plane of the telecentric camera equipment into a world coordinate through calibrating parameters of the telecentric camera equipment,
wherein the step S 2 includes:
step S 21 : controlling the translation stage in the common depth of field of the telecentric projection equipment and the telecentric camera equipment, adjusting the translation stage to be at a lowest position of the system's depth measurement range, and recording a lowest depth z 0 of the translation stage;
step S 22 : using a computer to program a sinusoidal fringe pattern with a period of T 0 , and implanting it into the telecentric projection equipment, so that the telecentric projection equipment can project four sinusoidal fringe patterns with fixed period to the translation stage; conducting phase-shifting operation on the four sinusoidal fringe patterns, and the shifting phase values based on the first pattern are 2πL/4 (L=0, 1, 2, 3); sequentially collecting the four phase-shifting fringe patterns by the telecentric camera equipment; selecting any pixel on the imaging plane of the telecentric camera equipment as a pixel for calibration, and setting the pixel coordinates in the image coordinates system of the telecentric camera equipment as (μ, ν); an expression of light intensity of the pixel for calibration I L (μ, ν) is:

I
L

⁡

(

μ
,
v

)

=

a
⁡

(

μ
,
v

)

+

b
⁡

(

μ
,
v

)

⁢

cos
⁡

[

φ
⁡

(

μ
,
v

)

+

2
⁢
π
⁢

⁢
L

4

]

(
1
)

wherein L=0, 1, 2, 3, α (μ, ν) is background intensity, b (μ, ν) is fringe contrast, and φ (μ, ν) is the Lth absolute phase value of the pixel (μ, ν) for calibration;
according to a four-step phase-shifting method, a wrapped phase ϕ (μ, ν) of the pixel for calibration (μ, ν) can be calculated as :

ϕ
⁡

(

μ
,
v

)

=

arctan
[

∑

L
=
0

3

⁢

⁢

I
L

⁡

Assignees

• Univ Shenzhen

Inventors

• Tian Jindong
• Li Dong
• Yan Sichen
• Tian Yong

Classifications

• G01B11/254

  Projection of a pattern, viewing through a pattern, e.g. moiré · CPC title

• G06T7/521

  from laser ranging, e.g. using interferometry; from the projection of structured light · CPC title

• G01B11/2504 Primary

  Calibration devices · CPC title

• H04N13/296

  Synchronisation thereof; Control thereof · CPC title

• H04N13/236

  using varifocal lenses or mirrors · CPC title