IMPROVING THE 3D SCAN PRECISION OF LASER

TRIANGULATION

Dr. Athinodoros Klipfel AT – Automation Technology GmbH

THE PRINCIPLE OF LASER TRIANGULATION

X

Y Z

Image of Target Object

Triangulation Geometry Example

Sensor Image of Laser Line

3D LASER TRIANGULATION EXAMPLE OF AN AT 3D CAMERA

Image Acquisition by Means of

High Speed CMOS Sensor Integrated Image Processing

using FPGA technology

Output of Profile Data Over GigE-Vision

ADVANTAGE Profile Frequency up to 72000 Hz!

3D SCAN ACQUISITION

3D Scan

Conveyance of Target Object

- Linear Stage

- Conveyor Belt

- Turntable

Scan Direction

COMMON LINE DETECTION ALGORITHMS

MAX TRSH COG

Reflection of laser light on the object surface causes a Gauss shaped intensity distribution on

sensor image

Finds Position of Maximum Intensity

Finds Threshold Points Finds Center of Gravity

Intensity distribution along a column of the sensor image

PARAMETERS AFFECTING 3D SCAN PRECISION: IMAGE RESOLUTION

Example with AT Camera C5-2040-GigE and α=30°

FoV = 50 mm

dx = 24 µm

dz = 48 µm

dz_COG = 0.75 µm (with Output of 6 Subpixels)

Resolution depends on imaging scale and triangulation angle

Height Resolution: dz=dx/sin(α)

Lateral Resolution: dx=FOV / Pixels per row

COG Height Resolution with 6 Subpixels (factor 26=64): dz_cog=dz/64

PARAMETERS AFFECTING 3D SCAN PRECISION: NOISE DUE TO LASER SPECKLES PHENOMENON

Laser Speckles

The waves of laser light reflected from different parts of the target surface impinge on the sensor

with different phase, which is caused by the different optical path.

The interference of these waves on the sensor causes intensity variations of the laser line image

Laser light reflection impinging on the sensor with different phase

PARAMETERS AFFECTING 3D SCAN PRECISION: NOISE DUE TO LASER SPECKLES PHENOMENON

Typical Sensor Image of 660nm Laser Line. Intensity variations caused by Speckles

Maximum Intensity per Sensor Image Column

Distorted Intensity Distribution along a column due to Speckles

PARAMETERS AFFECTING 3D SCAN PRECISION: NOISE DUE TO LASER SPECKLES PHENOMENON

Example of height profile detected by COG algorithm (laser 660nm)

Standard deviation of noise: ca. 14 µm

3D Setup: AT C5-2040-GigE 2048 Pixels/Profile, FoV=50mm, α=30°, dx=24µm, dz_COG=0.75µm

IMPROVING THE 3D SCAN PRECISION WITH THE FINITE IMPULSE RESPONSE FILTER (FIR)

Smoothing Filter • Numerical Average

• Gauss Average

FIR-Filter (Savitzky-Golay)

Raw intensity data

Smoothed intensity data Selection 5, 7 or 9 Coefficients

COG

Improving the 3D Scan Precision with the Finite Impulse Response Filter (FIR)

Height profile detected by COG algorithm with FIR smoothing filter (Laser 660nm)

Standard deviation of noise: ca. 11 µm

20% improvement

Intensity distribution along a column of sensor image after application of FIR filter in smoothing mode

3D Setup: AT C5-2040-GigE 2048 Pixels/Profile, FoV=50mm, α=30°, dx=24µm, dz_COG=0.75µm

IMPROVING THE 3D SCAN PRECISION WITH THE FIR-PEAK ALGORITHM

Detection of “Zero Crossing” outputs the position of Gauss

curve at a resolution of 6 subpixels (1/64 pixel)

FIR-Filter (Savitzky-Golay)

Raw intensity data

Differential Filter

FIR-PEAK

Selection 5, 7 or 9 Coefficients

IMPROVING THE 3D SCAN PRECISION WITH THE FINITE IMPULSE RESPONSE FILTER (FIR)

Standard deviation of height values in

the profile (µm) Surface Material COG with FIR FIR-PEAK Anodized Aluminum 13 13

Machined Aluminum 10 9 Galvanized Steel 9 9

White Plastic 8 10 Black Plastic 12 12

Comparison between COG with FIR smoothing filter and FIR-PEAK

3D Setup: AT C5-2040-GigE 2048 Pixels/Profile, FoV=50mm, α=30°, dx=24µm, dz_COG=0.75µm

Both algorithms result to almost the same precision.

The COG with FIR filter works better with “thick” lines.

The FIR-PEAK is more precise in applications with “thin” lines.

IMPROVING THE 3D SCAN PRECISION BY MEANS OF LASER WITH SHORTER WAVELENGTH

The Speckle effect depends on the laser wavelength.

Shorter laser wavelength causes less speckles and reduces the noise.

Image of 660nm Laser Line

Image of 405nm Laser Line

IMPROVING THE 3D SCAN PRECISION BY MEANS OF LASER WITH SHORTER WAVELENGTH

3D Setup: AT C5-2040-GigE 2048 Pixels/Profile, FoV=50mm, α=30°, dx=24µm, dz_COG=0.75µm

Height profile of white plastic surface acquired using COG algorithm with FIR smoothing filter

Standard deviation of noise: ca. 6 µm

Laser 405nm

Laser 660nm

Standard deviation of noise: ca.4 µm

30% improvement

IMPROVING THE 3D SCAN PRECISION OF LASER TRIANGULATION

Summary

The 3D scan precision of Laser triangulation depends on:

•Image scaling / resolution

•Laser Speckle effect

The precision can be improved by means of:

• sophisticated algorithms such as the Finite Impulse Response smoothing Filter (FIR) and the FIR-PEAK detection method

•Lasers with shorter wavelength such as 405nm

IMPROVING THE 3D SCAN PRECISION OF LASER TRIANGULATION

Thank you for your attention!

CONTACT INFORMATION

Dr. Athinodoros Klipfel Sales Manager 3D Sensors

AT – Automation Technology GmbH Bad Oldesloe, Germany Tel. +49-4531-88011-0

E-mail: klipfel@automationtechnology.de Web: http://www.automationtechnology.de/