Laser Beam Analysis
Laser Line Generators for Light­- sectioning in Rail Inspection
3D-Measurement and process control for research
and industrial environments
Modern measuring techniques often involve using lasers specifically designed for the demanding requirements
of industrial tasks. An increasing number
of laser diodes have become available in
the green and blue region of the visible
spectrum so that laser-diode based modules now cover the full range from UV to
IR. A variety of beam-shaping techniques
can be used to produce the particular
beam profile necessary for a specific measuring task. One application is the continuous monitoring of rail profiles during
rail production or of their deterioration in
use by mounting the test system on the
train itself. The geometry of the rail can
be determined by using laser lines and the
laser light-sectioning method. The in situ
measurement of wear ensures that the
repair of the affected rail sections is performed in time.
Laser light-sectioning is one of the main inspection techniques used for example during the rail production and when the tracks
are in use. A laser line is directed at the rail
at a particular angle and a camera records
an image that visualizes the upper rail head
geometry (Fig. 1). Plastic deformations, for
example, of the rail head (i.a. squats) that
arise from stress during operation (e.g. from
high axle load of freight trains or in curves)
are detected and provide continuous feedback about rail wear.
A signal with high signal-to-noise-ratio
and high precision is i.a. achieved by using
high power laser lines with well-defined line
geometry, providing spatial resolutions in the
micrometer range.
In general, not only the rail head but the
complete rail profile is examined and this information is then stored with the exact position information, so that rail maintenance can
be performed systematically.
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
the Authors
Anja Krischke
Anja Krischke studied
Physics at the University
of Würzburg with a
focus on the description
of ultrashort laser pulses and quantum
control. She joined Schäfter+Kirchhoff in
2011 and now works in optics development.
Peter Gips
Peter Gips studied
Information Engineering
at the Technical
University Dresden. He
joined Schäfter+Kirchhoff in 1991 after
research in the field of digital signal processing and is now responsible for image
processing and software development.


Anja Krischke
Schäfter+Kirchhoff GmbH
22525 Hamburg, Germany
Tel: +49 40 853 997 - 0
E-mail: A.Krischke@SuKHamburg.de
Peter Gips
Schäfter+Kirchhoff GmbH
22525 Hamburg
Tel: +49 40 853 997 - 0
E-mail: P.Gips@SuKHamburg.de
Christian Knothe
Christian Knothe first
studied Physics at the
University of Freiburg
i. Br. with a focus on laser spectroscopy before
completing his doctoral
thesis in fiber optics at
the Technical University of Hamburg-Harburg. Since he joined Schäfter+Kirchhoff in
2005, he has been responsible for the advanced fiber optic applications.
Ulrich Oechsner
Dr. Ulrich Oechsner
studied Physics before
completing his doctoral
thesis at the University of
Hamburg. After research
in the fields of electrophysiology and physiological optics, he joined Schäfter+Kirchhoff
in 2000 where he is responsible for optical
design and system development.


Dr. Christian Knothe
Schäfter+Kirchhoff GmbH
22525 Hamburg, Germany
Tel: +49 40 853 997 - 0
E-mail: Ch.Knothe@SuKHamburg.de
Laser modules for industry
Industrial applications, such as the inspection of rails during production and operation
place specific demands on the laser source.
The laser module 40TE-PO (Fig. 1) is encased
in a rugged full-metal, potential free ESDprotected housing and has an integrated
microcontroller and serial RS232 interface.
The controller regulates i.a. laser diode cur-
Dr. Ulrich Oechsner
Schäfter+Kirchhoff GmbH
22525 Hamburg, Germany
Tel: +49 40 853 997 - 0
E-mail: U.Oechsner@SuKHamburg.de
rent and temperature. If the inserted laser diode is without its own TE-cooler, the casing
is additionally equipped with a temperature
sensor and Peltier element to adjust the diode temperature.
Vital data about the laser module and diode are stored and can be read out, such as
the laser diode‘s hours of operation or the
diode current consumption, which provides
information about diode degradation. Re-
www.laser-journal.de LTJ 41 Laser Beam Analysis
Figure 1: High power laser line generator 13LRM+40TE-PO. The module is specially
designed to meet the requirement for industrial measurement tasks, e.g. the measurement of rail deterioration using laser light-sectioning.
ducing the diode temperature slows diode
degradation and increases its life time. Furthermore, the electronics include reverse polarity and overvoltage protection, a power
and current limitation, as well as digital (up
to 250 kHz) and simultaneous analog modulation (up to 10 Hz).
Both singlemode and multimode diodes
in a variety of different diode casings are used
as light sources in the 40TE -PO, with the singlemode diodes producing thinner lines. The
complete system is built in a modular fashion
with casing, diode, appropriate collimation
lens and carefully selected beam-shaping optics. The optics can be optionally protected
by a window made from scratch-resistant
sapphire glass. Focus settings are adjusted
easily, with both line length and line width
increasing in direct proportion with greater
working distance.
Laser light-sectioning with laser
lines
Laser light-sectioning is a 3D-measurement
technique that uses laser triangulation to determine the height profile of an incident section (Fig. 2). A camera measures the lateral
displacement and distortion of the incident
laser line. The image contains the complete
height information of the section defined
by the incident laser beam. The 3D profile
is acquired by continuous scanning, as the
object passes through the detection system.
A complete rail profile is, for example, obtained by positioning several laser line generators and cameras radially around the rail (Fig.
3). By evaluating the height variations detected by the cameras the complete geometry
of the profile is extracted. If different laser lines
42 LTJ January 2013 No. 1
overlap, e.g. laser lines with different wavelengths and adequate filters for the detecting
cameras are used to separate the signals.
The reflective characteristics of the object
are critical for the success of the measurement. If reflection from the object is specular
(mirror-like), no signal reaches the camera.
At least partly diffusive reflection is needed.
Most technical surfaces exhibit a reflective behavior consisting of both diffuse and specular
reflection, with the fraction of diffuse reflection (itself isotropic) decreasing for higher
incidence angles α.
However, the homogeneity of the laser
line directed on a diffusively reflecting object
is disrupted by laser speckle, a multi-interference effect on optically rough surfaces. The
intensity centers of the laser line are displaced
laterally from the beam axis in a stochastic
manner, disturbing the measurement. Laser speckle can be prevented by using a low
coherence light source, e.g. a superluminescent diode. Choosing the right camera lens
aperture setting contributes to minimizing
FIGURE 3: Light-Sectioning setup using
two laser lines. Cameras and lasers are
in Scheimpflug configuration. An region
of interest of 500 × 100 pixels is set for
the Photon Focus cameras that allows
1447 fps. The f-stop does not need to be
increased, which means that an inspection train can travel at least twice as fast
without losing light intensity.
laser speckle as well. The aperture acts as a
spatial frequency filter, with a larger aperture
producing a high frequency speckle pattern
(Fig. 4A) that is less disturbing when compared with the low frequency pattern with
larger gaps caused by a smaller aperture setting (Fig. 4B).
The height-measuring range and the resolution are determined by the triangulation
angle α between the planes of the dissecting
laser line and the optical axis of the camera
lens (Fig. 2). Larger angles α cause larger line
displacements and, thus, a higher resolution
for the same height change in comparison
with smaller incident angles. The height-measuring range decreases with α.
In order to ensure constant signal amplitude at the sensor, the depths of focus of both
the dissecting laser line and the detecting
camera system must be large enough to cover
the required height-measurement range.
However, since the signal to be acquired
is non-parallel with the camera sensor the
depth of focus of the camera is often not
large enough to produce a sharp image of the
whole dissecting plane. This problem can be
overcome by using the Scheimpflug configuration for the camera/laser setup (see Figure
3 and Box 1, Fig. 6).
Choosing the right laser line:
micro and macro laser lines
FIGURE 2: Laser triangulation principle.
The angle α between the laser beam and
the camera determines the resolution and
the height measurement range.
Every laser line is focussed at a predefined
working distance from the beam source.
At distances outside of this point, the laser
line becomes wider and its power density
decreases.
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Laser Beam Analysis
FIGURE 4: Laser speckle behavior imaged
with a small f-number k=2.8 (A) and with
a large f-number k=22 (B).
Laser micro line generators produce
narrow laser lines with a Gaussian intensity
profile across the laser line. By convention,
the depth of focus is defined as the range
in which the laser line does not increase by
more than a factor of 1.41. For a laser line
with line width B (at the 13.5 % level) and
wavelength λ, the depth of focus is defined
as the Rayleigh range 2zR where:
2 zR =
π B2
.
2λ
In contrast, laser macro line generators produce laser lines with an extended depth of
focus 2zM:
2 zM = 1.75
πB 2
2λ
with the intensity profile across the laser line
approximately Gaussian with lobes on the
side.
For a particular line width B, the depth of
focus of a macro line is almost twice that of a
micro line. At the same working distance A,
macro lines are 2 to 5-times wider and have a
depth of focus 7 to 35-times larger than the
equivalent micro line. Compared with a micro line (Fig. 5A) the power density of a macro
line is reduced (Fig. 5B), but does not change
significantly over a greater range. Micro lines
have a higher power density close to their focus but line width increases and power density falls drastically outside the focus .
Laser line power density is often crucial for
good signal quality. For measurements from
a fast moving train (>100 km/h, low integration times, high test frequency), a good signal
quality can only be achieved with high power
laser lines. A high laser power also enhances
signal contrast in bright environment lighting
and ensures a sufficiently high power density
when using longer laser lines at greater working distances.
Since the height information is obtained
from the displacement of the laser line, the
line width does not generally restrict the resolution of the height measurement (often the
displacement can even be measured more
precisely). However, defects in the scanning
direction will remain undetected if the line
width is larger than the defect itself.
The width of the laser line B (conventionally defined at the 1/e²-level) cannot be infinitesimally small but is limited for ideal optics to
Figure 5: Micro lines (A) exhibit a high
power density at the focus, but line width
increases and power density decreases
considerably outside this point. Macro
lines (B) have a lower power density but
an extended depth of focus (7-35-times
larger).
B=
4λ A
,
ØS π
for a particular wavelength λ, working distance A and incident beam diameter ØS at
the line optics perpendicular to the laser line.
In order to produce very thin lines, the beam
diameter at the line optics needs to be large.
A compromise must be found for each
application between the benefits of a larger
depth of focus accompanied by a compara-
infobox
Scheimpflug configuration for image acquisition
When acquiring an image, commonly the
object, lens and image planes are parallel
with each other. For laser light-sectioning,
the object defined by the angle of incidence is non-parallel with the camera lens
and sensor. Often the depth of focus needs
to be increased to acquire a fully sharp image.
The depth of focus 2zK depends on the fnumber k, the interpixel distance ∆x and
the magnification β (= sensor size/image
area):
2 z K = 2 ∆x k
1+ β
.
β2
The depth of focus can be increased by
simply reducing the aperture size. However, the signal amplitude decreases by a
factor of 2 for each additional f-stop, and
the optical resolution also decreases. The
Scheimpflug configuration allows a fully
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
focussed image of an oblique image if the
object and the image plane both meet the
lens plane along one straight line (Figure 6):
tan γ =
a'
tan α
a
α and γ are the angle between object and
lens plane, and between the lens and image plane, respectively. The parameters a
and a‘ are the distances of the major planes
of the object and lens, and the lens and the
image.
The magnification for the height difference
h is not constant
h=
ax cos γ
a 'sin α − x cos (α − γ )
and the relationship between height difference h and peak position x on the sensor
(relative to the optical axis of the lens) is
non-linear.
FIGURE 6: Scheimpflug configuration: a
sharp image is produced if the object,
lens and image planes all meet in one
straight line.
www.laser-journal.de LTJ 43 Laser Beam Analysis
multimode edge emitter, which produces a
line width of 1.41 mm. The 3D and 2D profile
(Fig. 7D/7E) show a homogeneous intensity
distribution along the laser line, the intensity
distribution across the line is non-Gaussian.
The difference in line width results from the
way the emitter is aligned to the grid lens.
For a singlemode diode, the minor emitter
axis is imaged and the major axis is stretched.
This does not work for the multimode diode,
as the multimode intensity distribution produces inhomogeneity in the laser line. As a
consequence the major axis of the emitter
is imaged and the minor axis is stretched,
producing a homogeneous but wider high
power laser line.
Conclusion
FIGURE 7: Beam analysis setup (A) with line scan camera (1) and a high power laser
line generator 13LRM+40TE-PO (2) mounted on a translation stage (3). The 3D beam
profile (B, D) shows an almost homogeneous intensity distribution for a 220 mW singlemode diode (B) and good linearity (C, please note the non-quadratic grid). The line
produced with the 450 mW multimode emitter is very homogeneous (D) but line width
is increased (E).
tively larger line width and a lower power density, and smaller lines with a high power density and a smaller depth of focus. A detailed
laser beam analysis reveals further characteristics of micro and macro laser lines.
Laser Beam Analysis
Most laser diodes have a divergent radiation cone with an elliptical cross-section. A
standard cylinder lens transforms it into a
laser line by stretching the major semi-axis
in relation to the minor semi-axis. The elliptical characteristics as well as the Gaussian intensity distribution along the line remain. A
closer look at the laser line with the help of a
camera reveals the elliptical character, characterized by a greater intensity and width at
the center of the line. The useful range for
light-sectioning is restricted.
Aspherical cylinder lenses and grid lenses
are used to produce laser lines with constant line width and homogeneous intensity
profile along the line, which improves the
performance capabilities of light-sectioning
considerably.
Quality control of laser lines is performed using a setup consisting of a
Schäfter+Kirchhoff line scan camera (SK8160GKO-LB, monochrome, 5 µm pixels
and 60 MHz pixel frequency), software for
image acquisition and evaluation, and the
44 LTJ January 2013 No. 1
laser line generator (40TE -PO) mounted in a
xyz-adjustable console on a translation stage
(Fig. 7A). The line scan camera sensor and
the laser line axis are aligned perpendicular
to one another, so that the camera acquires
the cross-section of the laser line point for
point. By moving the laser line generator on
the translation stage, a scanned beam profile over the whole laser line is acquired. Line
length, line width and the linearity of the laser
line are evaluated. An algorithm determines
the position of the intensity centers of the line
for each measurement point. The maximum
difference between the intensity centers then
serves as a measure for the linearity of the
laser line. Due to inaccuracies the laser line
can be bent („banana-shaped“). Figure 7B-E
shows the beam profile of a 40TE -PO with a
grid lens (13LR M25-M125-1.5, A=110 mm
macro line) as beam-shaping optics and two
different laser diodes. The first laser diode is a
829 nm 220 mW singlemode edge emitter.
After defining the usable laser line range the
software depicts the 3D image (Fig. 7B) that
reveals an almost homogeneous intensity
distribution along the laser line. The intensity
distribution across the laser line is Gaussian,
with a line width of 0.115 mm. Over a line
length of 334 mm, the intensity centers show
a maximum deviation of 0.1 mm (Fig. 7C).
The line width is narrow compared with the
same laser module using a 640 nm 450 mW
High power laser line generators with carefully selected laser line beam-shaping optics
(available for the wavelength range 4051550 nm) are e.g. used for laser light-sectioning of rail profiles. Depending on the
prerequisites for depth of focus of the laser
line, either micro lines with a high power
density and smaller depth of focus or macro
lines with larger line width and increased
depth of focus are appropriate. Integrated
electronics allows the monitoring of safety
and life time data of the laser diode via serial
interface, making maintenance easier.
Schäfter+Kirchhoff also supplies micro
and macro focus as well as laser pattern generators.
the company
Schäfter+Kirchhoff GmbH
Hamburg, Germany
Schäfter+Kirchhoff has accumulated extensive experience in the development
of opto-mechanical and opto-electronic
systems for use in research, aviation and
in space, as well as for demanding medical and industrial applications.
Schäfter+Kirchhoff designs and manufactures their own CCD line scan camera
systems, laser sources, beam-shaping
optics and fiber-optic components for
customers worldwide.
www.SuKHamburg.com
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim