Download - Laser Scanner
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Warsaw University of Technology
Institute of Aeronautics and Applied Mechanics
Department of Automation and Aeronautical Systems
2014
Presented By : Sumit Singh
M.Sc Aerospace Engineering
Supervised By : Professor
Janusz Narkiewicz D.Sc, Ph.D
1/13/2014
Laser Scanner - Principle of Operation, Application in Navigation
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List of Symbols = Area, illuminated by the laser beam
c = Speed of light
D = Beam diameter at its source
Dr = Aperture diameter of the receiver
LASER = Light Amplification by the Stimulated Emission of Radiation
n = Index of refraction
nsys = system transmission factor
natm = Atmospheric transmission factor
Pr = Power of radar
Pt = transmitted power
R = Total time in second case of calculation
S = Distance
T = Total time in first case of calculation
t = Time
v = Frequency of light
= Angle of incidence
t = Laser width
= wavelength of light
= angle of light emitted by a laser medium with a single mirror
= Cross-section of the target
= Scattering angle of the target
= Intensity distribution of a beam
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Table of Contents List of Symbols ........................................................................................................................................ 2
1. Introduction ........................................................................................................................................ 4
2. Basic Theory of Laser Scanner............................................................................................................. 5
2.1 Light ............................................................................................................................................... 5
2.2 Light Propagation .......................................................................................................................... 6
2.3 Building a LASER ............................................................................................................................ 7
2.3.1 Population Inversion .............................................................................................................. 7
2.3.2 Light Amplification ................................................................................................................. 7
2.3.3 Beam Characteristics .............................................................................................................. 8
2.3.4 Wavelength ............................................................................................................................ 8
2.3.5 Laser Types ............................................................................................................................. 9
3. Principle of Operation ......................................................................................................................... 9
3.1 Time of Flight ................................................................................................................................ 9
3.2 Triangulation ............................................................................................................................... 10
3.3 Principle of operation of an Airborne Laser Scanner .................................................................. 11
3.4 Block Diagram of an Airborne Laser Scan System ....................................................................... 12
4. Application ........................................................................................................................................ 13
4.1 General Application .................................................................................................................... 13
4.2 Airborne Laser Scanning intensity data correction ..................................................................... 13
5. Problem and Solution ....................................................................................................................... 15
5.1 Assumptions ................................................................................................................................ 15
5.2 Diagram of the problem .............................................................................................................. 15
6. Algorithms ......................................................................................................................................... 16
6.1 Flow Chart ................................................................................................................................... 16
7. MATLAB Script ................................................................................................................................... 17
7.1 Problem Solutions ....................................................................................................................... 17
8. Conclusion ......................................................................................................................................... 18
9. List of References .............................................................................................................................. 19
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1. Introduction
Laser scanning is basically a method of high accuracy mapping or reality
capture. Laser scanner is the device which precisely records three dimensional
information of a real object or environment. Laser scanner has two related but
separate meaning.
i. Controlled deflection of laser beams.
ii. controlled steering of laser beams followed by a distance measurement.
There are different types of scanner in the market but they work on the same basic
principle. A laser scanner emits rapidly pulsing or continuous laser beams. As it
emits the beam the scanner automatically rotates around its vertical axis, and a
rapidly spinning or oscillating mirror also moves the beam up and down, resulting in
a systematic sweeping of the beam over an area. When the beams hit an object
some of its energy bounces back to its scanner, where if the returned energy signal
is strong enough the sensor detects it. The inbuilt timer calculates the distance from
the scanner to the object.
Image-1 (Pulsing laser beams) Image-2 (Oscillating laser beams)
There are more to 3-D scanning than measuring distance. For each distance
measurement additional critical data is recorded including the corresponding
horizontal angle of the rotating laser and the corresponding vertical angle of the
moving mirror. The scanner automatically combines these to calculate a 3-D (X,Y,Z)
co-ordinates position for each point. The resulting scan is a set of 3-D co-ordinate
measurements. It's a detailed 3-D representation of the scene often called 'Point
Cloud'.
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2. Basic Theory of Laser Scanner
2.1 Light
In an atom the nucleus based electrons occupy certain orbits revolving around
the proton. When some energy is added to the atom electron jumps to a new orbit
away from the nucleus. Alternatively when an electron jumps closer to a nucleus the
atom releases energy. One of the most convenient way to release energy is as a
quantum of electromagnetic energy, a Photon. The energy carried by the photon is
equal to the transition energy. Photons of different wavelengths carries different
energy. The control of photon emission is critical for laser. There are two types of
photon emission.
i. Spontaneous
ii. Stimulated
Spontaneous Emission - This type of emission occurs all by itself and is the source
of virtually all light we see in nature, such as: the sun, the stars, the television
monitor, the fluorescent bulb. In the spontaneous emission if the electrons of an
atom are in an energy level above the lowest possible energy level then they can
drop down to a lower energy level, releasing energy often in form of light without
outside intervention.
Stimulated Emission - In such type of emission generally electrons are in higher
energy level. The excited atoms are forced to release their energy in form of light.
The provoked emission of light has the same wavelength and is precisely in phase
with the photon that generated it. In simple word the photon that hits the atom
generates another identical photon, which in turn stimulate another photon emission.
This is how the starter photon creates a cascade of stimulated emission which is
called LASER (Light Amplification by the Stimulated Emission of Radiation).
Image-3 (Cascade of stimulated emission)
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2.2 Light Propagation
The application of laser is closely related to the behaviour of light as it travels
through various material. When light is transmitted through solids, liquids, or gasses
the following propagation of light takes place.
i. Refraction - The part of the light projecting on the surface propagates
through the material. the beam of light changes direction at the point of
entrance to the new medium. Such phenomena is known as refraction.
ii. Reflectance - Part of the light falling on the surface is reflected back from the
surface.
iii. Absorption - Part of the light projecting on the surface is absorbed as it
propagates through material. The electromagnetic beam usually is converted
into heat.
iv. Scattering - In such case light gets diffused in many directions as light
projects on random distribution surfaces (like smoke or fog particles).
v. Polarization - The plane of oscillation of the light beam may change when
light projects on the surface.
Different materials creates different kinds of propagation. Reflectance and Refraction
are the two important light propagation. Refraction is directly related to the speed of
light as it travels through various material. In vacuum light travels faster 300,000
km/s. But when it travels through any material the velocity comes down to 299,920
km/s. Index of refraction (n) is an important measure to be taken into account which
is :
=
The index of refraction is always higher than 1. The wavelength of light is , the
frequency of light wave is v, and the velocity by which the light travels is c. And the
relation between these parameter can be described as =
. The frequency (v) of
light wave as it travels through different media as the velocity and wavelength
changes : =
. As the light passes through one transparent
medium to another transparent medium (air to water) the ray of light bends. The
angle of this light wave bending is called refraction, which is directly related to the
index of refraction. When the light wave passes through and gets back from the first
material the reverse refraction takes place. Reflectance also changes the travelling
direction of light. There are two types of refraction a) Specular b) Diffuse.
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2.3 Building a LASER
2.3.1 Population Inversion
The very basic of stimulated light emission is that the majority of light has to
be in high energy state, unfortunately this high energy state is not the default state
under thermodynamic equilibrium. That's why a prerequisite for Light Amplification by
the Stimulated Emission of Radiation (LASER) is that the distribution of light atoms
be altered so that more atoms become excited to emit energy and where the number
of excited atoms are more than low energy atoms which are ready to absorb energy.
This process of altering the energy distribution within atoms, making sure that most
of the atoms is in high energy state, is known as Population Inversion.
The basic step of creating popular inversion is by selecting the atoms and excite
them to a particular energy level. Light and electron techniques are the most
common technique for laser excitation. Population inversion often creates sublevels
of high and low energy level, which helps to form a multiline laser (multiple
simultaneous wavelengths). By the population inversion the laser medium gets
excited when it is hit by a photon and produces stimulated light.
2.3.2 Light Amplification
A starter photon triggers a cascade stimulated emission when a population of
laser material is inverted. The magnitude of cascading effect is proportional to the
distance light travels through the laser material. In order to build a bright laser light
the light has to travel long distance that is why an elongated rod of a laser material
can be used for bright light, to intensify the beam mirrors are placed in the path of
the laser light before it exits for the laser medium. The figure below shows the basic
principle of the laser rod operation.
Image-4 (Stimulated emission in a Laser rod)
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The formation of narrow laser beam is caused by the oscillation of light in a laser
cavity. The radius of the beam depends on the length, the light has travelled with in
the laser medium. Light emitted by a laser medium with no mirror can be defined by
an angle .
= Sin1( dn
2l ) (1)
Where, d = rod diameter, n = index of refraction of the laser material, l = length or
cavity of the rod.
Light emitted by a laser medium with a single mirror can be defined by an angle .
= Sin1( dn
4l ) (2)
2.3.3 Beam Characteristics
Across the laser beam the distribution of light is not uniform. Normally the
intensity of the laser beam is less in the sides and it is more bright in the middle.
Another characteristic of the beam is it's divergence which means the light spreads
out from the laser cavity. The angle of divergence , depends on the wavelength of
the laser , the beam diameter at its source is D.
Intensity distribution of a beam can be described by .
= Sin1(K
D ) (3)
K is a constant which depends on the distribution of light intensity in a cross section
of a laser beam.
2.3.4 Wavelength
Several factors are there which determine the wavelength of a laser emits.
The most important of them is the emitted laser wave length depends on the laser
material and its quantum properties. The wavelength of stimulated emission is
directly affected by the process of achieving popular inversion.
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2.3.5 Laser Types
According to the type of material used as a laser medium, a laser can be
divided into three different categories - 1) Gas Laser, 2) Solid - state Laser, 3)
Semiconductor Laser.
Laser Medium Main Wavelength
Other Wavelengths
Gas Helium-Cadmium vapour 441.6 nm 325 nm (UV)
Copper vapour 511 nm 578 nm
Solid-state Laser
Alexandraite 700-830 nm (IR) 325 nm (UV)
Erbium 1540 nm (IR) 850 nm (IR)
Semiconductor Laser
GaN/SiC 423 nm 405-425 nm
GaInPAs/GaAs 670-680 nm
3. Principle of Operation All of the measurement of distance of objects using the laser beam utilizes the
three basic attributes of laser light :
i. Clear light travels through straight lines.
ii. The velocity of light when travelling through space is known.
iii. The light produced by laser generally is a single wavelength light and that's
why it's easy to detect.
There are two traditional ways of measuring distance with laser :
i. Time-of-Flight.
ii. Triangulation.
3.1 Time of Flight
The principle behind this technique is to measure the total time (t) a laser
pulse takes to travel to an object and return to the receiver. Since the speed of light
(c) is known that's why it is possible to determine the distance travelled by the laser
pulse. Assuming the distance travelled is S, then the relation between distance
travelled, speed of light and time taken could be shown as following :
= (
2) (4)
In a Time -of-Flight (TOF) measurement system a scanner typically emits pulses of
laser electromagnetic radiation. The emitted pulse is focused, narrowed and pointed
at a targeted object. The object reflects part of the laser pulse electromagnetic
radiation back to the photosensitive sensor fixed on the scanner. An internal clock
measures the time take during the whole operation i.e. laser electromagnetic
radiation starting from the scanner and return back to the scanner after hitting by the
targeted object. A built-in microprocessor performs the aforementioned time-of-flight
measurements and reports the distance of the target object relative to the laser
scanner.
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Image-4 (Time-of-Flight Illustration)
In the Image-4 it can be seen that there is a little angle between laser emission and
reception but in reality it has negligible effect on the time-of-flight measurement. For
the high velocity of light it is possible to take thousands of measurements per
second. Multiple pulses are often used in a least square fitting to improve reliability.
Distance is then computed by comparing the phase shift between the emitted
wavelength and received laser pulse.
3.2 Triangulation
In such method two laser beams are used instead of one laser beam. There
are options to produce two laser beams 1) Using two laser scanner 2) splitting one
beam in two. The two beam intersect at the targeted object, the position of two laser
beam source is known. The angle formed at the intersection can be measured and
from this value it is possible to calculate the distance to the target object.
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3.3 Principle of operation of an Airborne Laser Scanner
Airborne laser scanner creates disk images of acquired 3-D data with the help
of many laser scanner combined with an attitude and position measurement system
on a platform mounted on an aircraft. The main goal of airborne laser scanner is to
take three dimensional data from earth's surface by taking the following
considerations into account -
1) Over large areas it is very time efficient for data acquisition.
2) In a common co-ordinate system the 3-D data registers automatically.
3) High accuracy and resolution of the registered data.
Normally an airborne laser scanning system comprises the following parts -
i. At least one laser scanner which offers full waveform echo digitalization and
provides a two dimensional line scan mode.
ii. A Global Navigation Satellite System (GNSS) which measures the attitude
and position of an aircraft with in the World Geodic System'84 (WGS84). At
least one terrestrial Global Positioning System (GPS) is mandatory in order to
acquire high quality GPS data.
iii. A software is normally used for monitoring of coverage and real time system
control.
iv. A software is also used to merge the scanned data with the position and the
orientation data of the scanning platform.
v. For the sensors a shock absorbing and rigid platform is necessary.
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3.4 Block Diagram of an Airborne Laser Scan System
System
Control
Airborne
Scanner
Shock absorbing rigid platform
IMU GPS
Antenna
Raw scan data Position and
attitude IMU/GPS
raw data
Terrestrial GPS-reference
network
GPS-reference network
raw data
Third party
IMU/GPS data
processing
software
Scan data
processing and
adjustment
Position and Attitude
Flight data in WGS84
Laser scan data in
WGS84
Airb
orne
Dat
a A
cqui
sitio
n
GP
S r
efe
ren
ce D
ata
Acq
uisi
tion
Dat
a P
roce
ssin
g
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4. Application
4.1 General Application
Nowadays laser technology is used in many areas starting from items we use
on daily basis. Instead of a needle to read data from a disk now days laser
technology is used to read data from a compact disk, such as music CDs. Bar code
scanner uses laser to illuminate barcodes to read special strips. Holograms are
produced by laser. Telecommunication data is transferred as a form of a pulsed laser
beams through fibre optic cables.
Laser is also used in corrective eye surgery which is achieved by quickly and
preciously burning the tissue of lens of the eye, correcting deformities that cause
myopia. By penetrating a laser deep into a tissue and coagulating a tissue area,
bleeding can be stopped. Harmful skin blemish are also removed by using laser
treatment. Laser is also used in spectrograph with the help of its controlled
wavelength and high resolution.
In military laser is also used to destroy short and medium range tactical missiles
with in the air before the missiles hit the ground. Military prefers laser as a defence
weapon for the capabilities of their fast retargeting an object by a speed of mach
velocities.
4.2 Airborne Laser Scanning intensity data correction
Airborne Laser Scanner (ALS) produces point cloud where each point cloud
has three coordinates (X,Y,Z) and which is determined using the following systems -
Global Navigation Satellite System (GNSS), Inertial measurement Unit (IMU) and
laser range finder. Laser scanning intensity values comprises number of photons
which are striking the detector, and they are recorded digitally. The intensity values
are also related to which comes back to the receiver. It is possible to use power
equation for radar as the basic physical operation is same for ALS and Radar.
Pr =PtDr
2
4R4t2 nsysnatm (5)
Where, Pr is the intensity or received power, Pt is the transmitted power, Dr is the
aperture diameter of the receiver, t is the laser width, nsys is system transmission
factor, natm is the atmospheric transmission factor, is the cross-section of the target. From the equation (5) it can easily be seen that the intensity or received
power is directly related to the power which has been transmitted, the atmospheric
and system transmission and the footprint of the laser beam.
The cross section of a laser beam can be explained with the equation below -
=4
(6)
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In the previous equation (6) is the scattering angle of the target, is reflectance of
the surface, is the area which is illuminated by the laser beam. can also be
written as a function of beam width and distance R, which is shown below -
=2
2
4 (7)
Substituting the value of from equation (7) to equation (6) one can obtain the following equation -
= 22cos () (8)
Substituting the value of into equation (5) one can obtain the following laser scanning intensity equation -
Pr =PtDr
2
4R2 nsysnatmcos() (9)
nsys is the optical transmission efficiency of any type of optical components in an
Airborne Laser Scanning (ALS) system. It is a constant value for certain ALS system
but may vary with different type of sensors. The Dr component also varies with
different type of sensors. The is an angle of incidence which is described as an angle between the surface normal and the incoming laser beam. The figure below
represents the Lambertian scattering of surface, topographic effect on angle of
incidence.
Image-5 (Topographic Effect)
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5. Problem and Solution A laser beam has to travel from point P1 to point P2. At the middle of both
points there is an obstacle circle. So the laser has to avoid the circular obstacle and
reach at point P2 from P1 and the total time elapsed during the whole process has to
be calculated.
5.1 Assumptions
Where distance between P1 and P2 is 100000 m, Diameter of the circle is
20000 m, Distance at first stopping point from P1 to S1 is (d1) = 30000 m. Distance
at second stopping point from S1 to S2 is (d2) = 22360.68 m (2 = 2 + 2),
Distance at third stopping point from S2 to S3 is (d3) = 22360.68 m. Distance at last
stopping point from S3 to P2 is (d4) = 30000 m. a = 20000 m, b = 10000 m.
Using the formula of Time of Flight =
2 the total time (T) taken by the laser to
travel from P1 to P2 has been calculated in Matlab. The Matlab script is given below.
Where S = distance, t = time, c = speed of light. Circular obstacle is an assumption.
5.2 Diagram of the problem
P1
P2
S1
S2
S3
a
b
F1
F2
k
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In the above diagram the time measurement has been calculated with the change in
distance at second time the first stopping point is F1 from P1 and the distance is
(m1) = (m4) = 20000 m, and distance from F1 to S2 is (m2) = 31622.7766 m = (m3).
As the
6. Algorithms
First of all using the Pythagorean formula 2 = 2 + 2), distance between
S1 and S2 then S2 and S3 has been calculated. Later using Time of Flight formula
=
2 time has been calculate. By adding each time the total time has been
calculated.
So, 1 = 2
=
2310^4
310^4= 2 10^(4) , in the same way all the other t2, t3, and
t4 has been calculated and added together to obtain the total elapsed time T, and r1,
r2, r3, r4 has been calculated and added together to obtain the total elapsed time R
for the time calculation with different distance.
6.1 Flow Chart
Input values of distances
d1, d2, d3, d4, m1, m2,
m3, m4
Application of formulas
2 = 2 + 2),
=
2 ,
Output values of times
t1, t2, t3, t4, T, r1, r2,
r3, r4, R
END
START
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7. MATLAB Script clc clear all %Distance between point P1 and P2 [meter] da = 10^5; %Distance between point P1 and first stopping point S1 and m1 [meter] d1 = 3*10^4; m1 = 2*10^4; %Distance between first stopping point S1/F1 and second stopping point S2
[meter] a = 2*10^4; k = 3*10^4; b = 10^4; d2 = sqrt(a^2+b^2); m2 = sqrt(k^2+b^2); %Distance between second stopping point S2 and third stopping point S3/F2
[meter] d3 = d2; m3 = m2; %Distance between third stopping point to point P2 [meter] d4 = d1; m4 = m1; %% Using Time of Flight formulation, calculating total time elapsed %Speed of light [meter/sec] c = 3*10^8; %Time taken to reach from point P1 TO S1/F1 [sec] t1 = (2*d1)/c r1 = (2*m1)/c %Time taken to reach from point S1/F1 to S2 [sec] t2 = (2*d2)/c r2 = (2*m2)/c %Time taken to reach from point S2 to S3/F2 [sec] t3 = (2*d3)/c r3 = (2*m3)/c %Time taken to reach from point S3/F2 to P2 [sec] t4 = (2*d4)/c r4 = (2*m4)/c %Total time elapsed to reach from point P1 to point P2 [sec] T = (t1+t2+t3+t4) R = (r1+r2+r3+r4)
7.1 Problem Solutions Case 1 Case 2
t1 [sec] t2 [sec] t3 [sec] t4 [sec] T [sec] r1 [sec] r2 [sec] r3 [sec] r4 [sec] R [sec] 2.0000e-004
1.4907e-004
1.4907e-004
2.0000e-004
6.9814e-004
1.3333e-004
2.1082e-004
2.1082e-004
1.3333e-004
6.8830e-004
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8. Conclusion After reviewing the above information on laser scanner it has been
understood that the total operation of laser scanner is dependent on laser and which
is dependent on the energy level of an atom. Propagation of light is very important is
laser scanning, different materials creates different kinds of propagation. In order to
build a bright laser light the light has to travel long distance that is why an elongated
rod of a laser material can be used for bright light, to intensify the beam mirrors are
placed in the path of the laser light before it exits for the laser medium. Time of flight
is the most common way to calculate distance between two objects, and where the
distance is known the time can be measured through this method, like the problem
which has been solved in the report. A starter photon triggers a cascade stimulated
emission when a population of laser material is inverted. By the oscillation of light in
a laser cavity the narrow laser in generally created. Across the laser beam the
distribution of light is not uniform. The most important of them is the emitted laser
wave length depends on the laser material and its quantum properties. The
wavelength of stimulated emission is directly affected by the process of achieving
popular inversion.
In the problem as the obstacle is a proper circle and it is situated at right middle of
both points P1 and P2 that is why it is easier to calculate the distance of the
divergence from point S1 (in first case) and from pint F1 (in second case). From the
calculated total time of the both cases it can be easily understood that if the laser
light divert from point F1 (according to the second case), then the laser light is taking
less time (R = 6.8830e-004 sec)to reach point P2 from Point P1 compared to the first
case of laser light divergence (T = 6.9814e-004 sec). This same method can be to
determine the distance between objects such as buildings, monuments, landscapes
etc, where time and speed of light shall be a known factor.
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9. List of References
1. http://repository.upenn.edu/cgi/viewcontent.cgi?article=1083&context=ci
s_reports
2. http://www.riegl.com/nc/products/principle-of-operation-
detail/poo/airborne-laser-scan-systems/
3. http://www.fig.net/pub/athens/papers/ts26/TS26_1_Schulz_Ingensand.p
df
4. http://www.fig.net/pub/athens/papers/ts26/TS26_1_Schulz_Ingensand.p
df