Calibration of a Terrestrial Laser Scanner - Institute of Geodesy and ...
Calibration of a Terrestrial Laser Scanner - Institute of Geodesy and ... Calibration of a Terrestrial Laser Scanner - Institute of Geodesy and ...
12 2. Components of Terrestrial Laser Scanner400 600 800 1000 1200 1400WAVELENGTH, nmFigure 2.5: Eye transmission for electromagnetic waves.aperture and records the highest output power level of the laser beam. Due to the detected power level,lasers are classified. Table 2.1 shows the different laser classes and their characteristics. More informationconcerning laser safety can be found in [Henderson, 1997] and [Young, 1984].Table 2.1: Overview of Laser Safety Classes accordingto IEC 60825-1 and EN 60825-1.ClassDescription1 Eye-safe under all operating conditionsIMSafe for viewing directly with the naked eye, but maybe hazardous to view with the aid ofoptical instruments. In general, the use of magnifying glasses increases the hazard from a widelydivergingbeam, e.g. LEDs and bare laser diodes, and binoculars or telescopes increase the hazardfrom a wide, collimated beam; such as those used in open-beam telecommunications systems.2 These are visible lasers. This class is safe for accidental viewing under all operating conditions.However, it may not be safe for aperson who deliberately stares into the laser beam for longerthan 0.25 s, by overcoming their natural aversion response to the very bright light.2M These are visible lasers. This class is safe for accidental viewing with the naked eye, as long asthe natural aversion response is not overcome as with Class 2, but may be hazardous, even foraccidental viewing, when viewed with the aid of optical instruments, as with class IM.3RRadiation in this class is considered low risk but potentiallyhazardous. The class limit for 3R is5 times the applicable class limit for Class 1 (for invisible radiation) or class 2 (for visible radia¬tion). Hence, continuous wave visible lasers emitting between 1 and 5 mW are normallyVisible class 3R is similar to class IIIA in US regulations.Class 3R.3BRadiation in this class is very likely to be dangerous.For a continuous wave laser the maximumoutput into the eye must not exceed 500 mW. The radiation can be a hazard to the eyeor skin.However, viewing of the diffuse reflection is safe.4 This is the highest class of laser radiation. Radiation in this class is very dangerous, and viewingof the diffuse reflection may be dangerous.materials onto which they are projected.Class 4 laser beams are capable of settingfire toMost of the terrestrial laser scanners fit in class 3, e.g. HDS Leica Geosystems (Switzerland), Trimble (USA),Zoller+Fröhlich GmbH (Germany), FARO Technologies Inc. (USA), I-Site Pty Ltd. (Australia), 3rd Tech Inc.(USA). The laser scanners of Callidus Precision Systems GmbH (Germany), Optech Inc. (Canada) and RieglLaser Measurement Systems GmbH (Austria) are categorized as class 1.eye safety is frequently guaranteed since the operation of laser scanners in the scanningHowever, for all laser scanners,laser beam at a high speed. The laser beam does not hit the eyes long enough to cause damagemode deflects thedue to the
2.1 Distance and Reflectance Measurement System 13rotationAdditionally, if the laser beam describes a constant line without rotation, eyeavoided It is then not only recommended but also prescribed to wear safety gogglescontact has to be2.1.3 Direct Time-of-FlightThe direct time-of-flight method determines the travel time At of an impulse to a surface and back by= - At(2 3)where s is the distance to the surface at which the impulse was reflected and c is the speed of lightThetemporal accuracy has to be high due to fast speed of light (c « 31 mm, a time resolution is required of108 m/s) If the range has to be solved toAt = 2 As 2 0 001m3 108mA12,= 6 7 10 12s= 6 7ps (2 4)Due to the time resolution required for distance measurements withm the millimeter scale, the electronicunits have to produce short pulses or patterns and have to operate very quickly Since a single timed pulseis not very accurate, a large number of pulses is used and averaged to givea more accurate distance Oftenthe direct time-of-flight technique is able to distinguish between a first pulse and a last pulse,detected and belong to the same emitted laser pulsepulse is of interest Furthermore,problems caused at edges and by multipaththat can be avoided m the direct time-of-flight techniquewhich areIt can be decided whether the first pulse or the lasteffects result m biased dataThe advantage of the direct time-of-flight technique m terrestrial laser scanners is the ability to cover longranges up to several hundreds of meters or even up to a kilometer The emitted laser may have high energyfor traveling a long distance because the energy of the received signal for measuring the travel time has to besufficiently highSince the emitted pulses cannot be generated m a short time, the minimum time intervalbetween two laser pulses for determining distances is limited Thus,the frequency of measuring distances isdefined and characterizes the sampling frequency of a laser scanner Typical sampling frequenciesfor laserscanners are between 1 kHz and 30 kHz As discussed before, the range resolution is limited according tothe time resolutionachievableGenerally, a distance accuracy for a single distance measurement below 1 cm is rarelyFigure 2 6 shows schematically the direct time-of-flight principledistance «emitterpulsereceiver 4-start sto 0clockobject /reflectorFigure 2.6: Direct time-of-flight principle schematically according to [Zetsche, 1979]2.1.4 Amplitude-Modulated Continuous Wave (AMCW)The amplitude-modulated continuous wave method (AMCW) modulates the carrier smusoidally or rec¬tangularly The returned energy waveform is delayed by the travel time At and appears proportionatelyphase-shifted m comparison to the emitted energy The range s is proportional to the phase shift A
- Page 1: DISS. ETH NO. 17036Calibration of a
- Page 5 and 6: ZusammenfassungSeit einigen Jahren
- Page 7 and 8: ContentsAbstractiiiZusammenfassungv
- Page 9 and 10: Contentsix6.1.3 Results Ill6.2 Stat
- Page 11 and 12: 1Introduction1.1 Terrestrial Laser
- Page 13: 1.2 Motivation 3Precision [m10~1 "1
- Page 16 and 17: 6 1. Introduction
- Page 18 and 19: 8 2. Components of Terrestrial Lase
- Page 20 and 21: 10 2. Components of Terrestrial Las
- Page 24 and 25: 14 2. Components of Terrestrial Las
- Page 26 and 27: 16 2. Components of Terrestrial Las
- Page 28 and 29: 18 2. Components of Terrestrial Las
- Page 30 and 31: 20 2. Components of Terrestrial Las
- Page 32 and 33: 22 2. Components of Terrestrial Las
- Page 34 and 35: 24 3. Calibration of Terrestrial La
- Page 36 and 37: 26 3. Calibration of Terrestrial La
- Page 38 and 39: '28 3. Calibration of Terrestrial L
- Page 40 and 41: 30 3. Calibration of Terrestrial La
- Page 42 and 43: 32 3. Calibration of Terrestrial La
- Page 44 and 45: "[m] of system 90%) 3540 distance b
- Page 46 and 47: !36 3. Calibration of Terrestrial L
- Page 48 and 49: 138 3. Calibration of Terrestrial L
- Page 50 and 51: 40 3. Calibration of Terrestrial La
- Page 52 and 53: 42 3. Calibration of Terrestrial La
- Page 54 and 55: 44 3. Calibration of Terrestrial La
- Page 56 and 57: 46 3. Calibration of Terrestrial La
- Page 58 and 59: 48 3. Calibration of Terrestrial La
- Page 60 and 61: 50 3. Calibration of Terrestrial La
- Page 62 and 63: 52 3. Calibration of Terrestrial La
- Page 64 and 65: 54 3. Calibration of Terrestrial La
- Page 66 and 67: 56 3. Calibration of Terrestrial La
- Page 68 and 69: Inormal axis,intersectionpoint axis
- Page 70 and 71: 60 3. Calibration of Terrestrial La
2.1 Distance <strong>and</strong> Reflectance Measurement System 13rotationAdditionally, if the laser beam describes a constant line without rotation, eyeavoided It is then not only recommended but also prescribed to wear safety gogglescontact has to be2.1.3 Direct Time-<strong>of</strong>-FlightThe direct time-<strong>of</strong>-flight method determines the travel time At <strong>of</strong> an impulse to a surface <strong>and</strong> back by= - At(2 3)where s is the distance to the surface at which the impulse was reflected <strong>and</strong> c is the speed <strong>of</strong> lightThetemporal accuracy has to be high due to fast speed <strong>of</strong> light (c « 31 mm, a time resolution is required <strong>of</strong>108 m/s) If the range has to be solved toAt = 2 As 2 0 001m3 108mA12,= 6 7 10 12s= 6 7ps (2 4)Due to the time resolution required for distance measurements withm the millimeter scale, the electronicunits have to produce short pulses or patterns <strong>and</strong> have to operate very quickly Since a single timed pulseis not very accurate, a large number <strong>of</strong> pulses is used <strong>and</strong> averaged to givea more accurate distance Oftenthe direct time-<strong>of</strong>-flight technique is able to distinguish between a first pulse <strong>and</strong> a last pulse,detected <strong>and</strong> belong to the same emitted laser pulsepulse is <strong>of</strong> interest Furthermore,problems caused at edges <strong>and</strong> by multipaththat can be avoided m the direct time-<strong>of</strong>-flight techniquewhich areIt can be decided whether the first pulse or the lasteffects result m biased dataThe advantage <strong>of</strong> the direct time-<strong>of</strong>-flight technique m terrestrial laser scanners is the ability to cover longranges up to several hundreds <strong>of</strong> meters or even up to a kilometer The emitted laser may have high energyfor traveling a long distance because the energy <strong>of</strong> the received signal for measuring the travel time has to besufficiently highSince the emitted pulses cannot be generated m a short time, the minimum time intervalbetween two laser pulses for determining distances is limited Thus,the frequency <strong>of</strong> measuring distances isdefined <strong>and</strong> characterizes the sampling frequency <strong>of</strong> a laser scanner Typical sampling frequenciesfor laserscanners are between 1 kHz <strong>and</strong> 30 kHz As discussed before, the range resolution is limited according tothe time resolutionachievableGenerally, a distance accuracy for a single distance measurement below 1 cm is rarelyFigure 2 6 shows schematically the direct time-<strong>of</strong>-flight principledistance «emitterpulsereceiver 4-start sto 0clockobject /reflectorFigure 2.6: Direct time-<strong>of</strong>-flight principle schematically according to [Zetsche, 1979]2.1.4 Amplitude-Modulated Continuous Wave (AMCW)The amplitude-modulated continuous wave method (AMCW) modulates the carrier smusoidally or rec¬tangularly The returned energy waveform is delayed by the travel time At <strong>and</strong> appears proportionatelyphase-shifted m comparison to the emitted energy The range s is proportional to the phase shift A
Change language