Calibration of a Terrestrial Laser Scanner - Institute of Geodesy and ...

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10 2. Components of Terrestrial Laser ScannerFigure 2.3: Casing (left) and structure (right) of a laser diode, adaptedfrom [Hinderung, 2004]Laser Beam PropagationThe propagation of optical and infrared waves can be described by various models These models use basicgeometries, such as [Andrews and Phillips, 1998] and [Meschede, 2004]• Plane wave An unbounded wave with constant amplitude and constant phase A planewave modelis defined as a model in which the phase fronts form parallel planesThis model is used to describethe properties of starlight and other exo-atmospheric sources at a ground-based receiver• Spherical wave An unbounded wave associated with apoint source The sphericalwave model ischaracterized by concentric spheres forming the phasefronts This model is sometimes used for asmall-aperturesource withm or near turbulent mediums• Gaussian-beam wave A wave of finite extent with focusing capabilitiesThis model is used in mostbeam wave analysisThe medium of propagation is in most cases the turbulent atmosphere, which changes the conditions per¬manently These fluctuations of the atmosphere, which have an influence on the refraction index along thepropagation path of the laser beam, cause a variety of deleterious effects in the wave, eg disruptioncoherence, broadening of the beam, redistribution of the beam energy [Andrews and Phillips, 1998]Considering diffraction, a natural wave phenomenon of all light waves, the propagationof theof the laser beamis disturbed inmany ways The beam diameter is changed and thus, the amount of energy within anyspotsize inside the beam diameter deteriorates Theamount of beam spreading varies dependingon thewavelength, the shape of the phase front, and the size of the emitting aperture [Andrews and Phillips,1998] An estimation on the amount of beam spreading at large distances from the beam waist is given bythe beam divergence angle ©_b, cf Figure 2 4 Therefore, the minimum value for the spotsize is assumedto be at the beam waist Beyond the beam waist, the beam diverges according to [Weichel, 1990]WZ=W0\ 1+ ( 2(22)where wz is the beam radius at range z, w0 is the beam waist and A is the wavelength The geometrical formof the laser footprint can have various forms The most common forms are cyclical and ellipticalWithin thefootprints, the irradiance, l e the power per unit, is not uniform but Gaussian, accordingto the Gaussianbeamwave [Andrews and Phillips, 1998] The beam divergence is assumed to be linear as defined by thebeam divergence angle Ob Furthermore, the beam diameter also depends on the wavelength
2.1 Distance and Reflectance Measurement System 11Concerning terrestrial laser scanners, the spotbeam waist and the range.size of the laser beam is afunction of the wavelength, theThe minimum spot size is not at the emitting laser diode but also somewherealong the path of the laser beam. But, the beam waist can be shifted along the path by means of focussingin order to minimize the spot size according to the object range.Beam Waist7Figure 2.4: Beam waist and divergence angle of a laser beam according to [Andrews and Phillips, 1998].Refraction, Scintillation and DispersionAn electromagnetic wave, which propagates through a random medium, e.g. the atmosphere,is influencedby varying conditions, e.g. temperature, pressure, humidity. Thus, propagation characteristics change withrespect to the geometry and the intensity. The varying geometry is caused by refraction, which makesthe beam move or quiver. The beam propagation is no longer a straight line but a curve. In addition,the variation in the intensity of the laser beam caused by the varying atmosphere isFurthermore, these intensity fluctuations are affected by optical turbulence.Opticalcalled scintillation.turbulence can bedefined as the fluctuations in the refractive index resulting from atmospheric turbulence [Beland, 1993].Another effect is known as dispersion. Dispersion here is defined as the dependence of the propagationvelocity on the wavelength.4Since each laser light consists of a narrow spectrum, which is so close thatthe laser light can be considered monochromatic, the propagation velocity is defined by the group velocityof differing frequencies or wavelengths. Consequently, a grouprefractive coefficient must be considered.Depending on the group refractive coefficient, the distance to be measured has to be corrected and this isknown as the first velocity correction.Further details concerning atmospheric influences on the propaga¬tion of electromagnetic waves can be found in [Joeckel and Stober, 1991] and [Rüeger, 1996].The described influences are relatively small for short ranges and can be neglected. However, for long rangelaser scanners covering distances of up to several hundreds of meters, the influences due to the atmosphereand the refractive index have to be taken into account. They affect the distances in a systematic way andcause methodological errors.Laser SafetyDue to power density, most lasers are dangerous and can cause damage to the skin or the eyes. Dependingon the wavelength of the laser, different elements of the eyesare affected. Lasers are generally catego¬rized into four classes according to the laser's ability to cause damage to the eyes.The eye transmissioncan be seen in Figure 2.5. For the visible area and some infrared frequencies, the eyecharacteristic of nearly100 %.has a transmissionThe classification procedure of lasers is carried out in a specific setup as defined in [IEC, 2001]. The laserbeam is focused on anaperture located in a predefined distance. A detector is placed justbehind the4Dispersion is used in different ways: For example, in optics, dispersion is the separation of a wave into its spectral componentsdepending on the wavelength
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
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Page 18 and 19: 8 2. Components of Terrestrial Lase
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60 3. Calibration of Terrestrial La
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74 4. Static Laser Scanning4.1.2 Mi
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76 4. Static Laser ScanningOptical
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78 4. Static Laser Scanningmaximum
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80 4. Static Laser Scanningand gene
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82 4. Static Laser Scanning• Ther
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84 4. Static Laser Scanning4.3.3 NU
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86 4. Static Laser Scanning
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88 5. Kinematic Laser Scanningmirro
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90 5. Kinematic Laser ScanningThe a
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92 5. Kinematic Laser Scanninghas a
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'94 5. Kinematic Laser Scanningbeam
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propagationresults series Taylorter
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108 5. Kinematic Laser Scanningonly
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cf.aroundframe regardingsetupusinga
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112 6. Applications of Terrestrial
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acquisitiondescribingFinally, thatM
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132 7. Summaryrange, achievable by
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134 7. Summary
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136 A. Imaging System of Imager 500
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138 B. Technical Data of Imager 500
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u140 C. Adjustment of SphereA =dfi,
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142 C. Adjustment of Sphere
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144 D. Electronic Circuit for Deter
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aMulti-SensorPlatform,PhDGeodesy Sw
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Accuracy-AeinTPS1200 AG, -Version 1
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-GenauigkeitsbetrachtungenInvestiga
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152 BibliographyWunderlich, T. A. [
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List of Figures3.22 Residuals of th
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156 List of Figures
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158 List of Tables
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200220072002Curriculum VitaePersona
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