Calibration method for telecentric imaging 3D shape measurement system

• US 10,110,879 B2
• Filed: 02/24/2016
• Issued: 10/23/2018
• Est. Priority Date: 03/05/2015
• Status: Active Grant

• Current Assignee: Shenzhen University (Guangdong Education Department)
• Original Assignee: Shenzhen University (Guangdong Education Department)
• Inventors: Tian, Jindong, Li, Dong, Yan, Sichen, Tian, Yong
• Primary Examiner(s): Le, Peter D
• Application Number:
  US15/052,489
• Publication Number:
  US 20160261851A1
• Time in Patent Office: 972 Days
• CPC Class Codes: G01B 11/2504 Calibration devices
• CPC Class Codes: G01B 11/254 Projection of a pattern, viewing through a pattern, e.g. moiré
• CPC Class Codes: G06T 7/521 from laser ranging, e.g. using interferometry; from the projection of structured light
• CPC Class Codes: G06T 7/80 Analysis of captured images to determine intrinsic or extrinsic camera parameters, i.e. camera calibration
• CPC Class Codes: H04N 13/236 using varifocal lenses or mirrors
• CPC Class Codes: H04N 13/246 Calibration of cameras
• CPC Class Codes: H04N 13/254 in combination with electromagnetic radiation sources for illuminating objects
• CPC Class Codes: H04N 13/296 Synchronisation thereof; Control thereof

Abstract

A calibration method is described for a telecentric imaging 3D shape measurement system, including step S1: establishing a telecentric 3D shape measurement system; S2: 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 S3: 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.

Claims

1. A calibration method for a telecentric imaging 3D shape measurement system, wherein the calibration method comprises the following steps of:

step S1;

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 S2;

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 S3;

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 S2 includes;

step S21;

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 z₀ of the translation stage;

step S22;

using a computer to program a sinusoidal fringe pattern with a period of T₀, 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 2π

L/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;

2. A calibration method for a telecentric imaging 3D shape measurement system, wherein the calibration method comprises the following steps of:

step S1;

establishing a telecentric 3D shape measurement system, and the measurement system including;

a projector with bilateral telecentric lens, a camera with 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 S2;

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 S3;

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 S3 includes;

step S31;

adjusting the translation stage to be at a depth in the common depth of field of the telecentric projection and camera equipment, placing a calibration target on the translation stage, letting the telecentric projector equipment project white light to the calibration target, and collecting the calibration target image by the telecentric camera equipment extracting feature points of the calibration target to obtain series of world coordinates (X_{107 i}, Y<sub>ω

i</sub>, Z<sub>ω

i</sub>) (i=1, 2, . . . , N), N is the total number of feature points) in the world coordinates system and their corresponding series of pixel coordinates (μ

_i, ν

_i) (i=1, 2, . . . , N) in the imaging plane of the telecentric camera equipment; and

step S32;

calibrating parameters of the telecentric camera equipment by using the world coordinates (X_{107 i}, Y<sub>ω

i</sub>, Z<sub>ω

i</sub>) (i=1, 2, . . . , N) and the pixel coordinates (μ

_i, ν

_i) (i=1, 2, . . . , N) of the feature points of the calibration target transform the pixel coordinates (μ

, ν

) into the world coordinates (X₁₀₇ , Y₁₀₇ ), wherein

the step S32 includes;

step S321;

building an orthographic transformation model during the bilateral telecentric lens imaging without consideration of lens distortions;

the orthographic transformation model being;