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

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18 2. Components of Terrestrial Laser Scannermodel, the Phong model and the Torrance-Sparrow model. More information concerningcan be found in [Luhmann et al., 2006], [Janser et al., 1996] and [Tonnies and Lemke, 1994].reflection modelsLambertian Reflectance ModelOne common model for diffuse reflection is Lambertian reflectance, where the light is reflected with equalluminance in all directions according to Lambert's cosine law [Gerthsen and Vogel, 1993]. Lambert's cosinelaw states that the reflected or transmitted luminous intensity in anydirection J from an element of aperfectly diffusing surface varies as the cosine of the angleof the surface:a between that direction and the normal vectorJ = Jocos(a),(2.10)where Jo is the intensity radiation in normal direction. As a consequence, the luminance of the surface isthe same regardless of the viewing angle and the angleof incidence.Phong ModelPhong's model is one of the most common lighting models in computer graphics.The intention of thePhong reflection model is to produce a 3D model with a certain degree of realism. Therefore, it combinesthe common reflection principles, specular and diffuse, with ambient lighting. In Figure 2.12, the imageand the Phong model of a sphere can be seen. The modeled spheresphere. More information can be found in [Phong, 1975].becomes a realistic illustration of a realFigure 2.12: Phong model: Picture of a sphere (left) and sphere displayed by Phong model (right), adapted from[Phong, 1975].Torrance-Sparrow Reflectance ModelThe Torrance and Sparrow reflectance model is close to the Phong illumination model with some differ¬ences. These differences are that the intensity of the specular reflection varies with the direction of the lightsource and the angle of peak specular reflection is not always exactly at the angle of incidence [Torranceand Sparrow, 1967].2.2 Angle Measurement SystemThe orientation of the laser beam in terrestrial laser scanners is measured electro-optically by encoders withrespect to a horizontal and a vertical direction. The electro-optical method is based on transmitted light oron reflected light. A glass arc with diameters of 5 cm to 7 cm is coded by a pattern of alternate opaqueand transparent areas. Incident light from an internal source falls on the glass arc and photodiodes convert
2.2 Angle Measurement System 19Figure 2.13: Comparison between Phong model (left) and Torrance and Sparrow model (right), adaptedfrom [Bhnn,19771.the light energy transmitted by or reflected from the pattern into electrical energy, e.g. pulses or pixelinformation. The digital signal then has to be converted from an analog signal to a digital output readableas a decimal number, cf. Figure 2.14.In most terrestrial laser scanners, binary encoding is implemented since an initialization is not required andthe angular resolution is sufficiently high for laser scanning. However, for higher resolution, within somemilligons, incremental encodinghas to be used.^^**emmm»^^glass arcelectro-optical barrieroutput signalFigure 2.14: Glass arc, electro-optical barrier and output signal schematically (left) and angle transmitter ofHeidenhainGmbH, Germany (right).2.2.1 Incremental EncodingIncremental encoding is also called the relative method since the reference base is not marked on the circle.Using this method, only a relative rotation of the glass circle is measurable. Light from a source is passedthrough the graduated glass circle prepared by transparent and opaque strips of equal width. The smallestgraduation unit is defined by the width of one strip. The quantity of light received by the photodiodedepends on the rotation and changes are nearly sinusoidal. The periodsbe counted and the amount of rotation can be determined. Incremental encoding requiresof the sinusoidal oscillation canan initialisation.Therefore, the glass arc is marked with an additional code that differs from the pattern containing the angleinformation. The additional code has to pass the electro-opticalThe determination of the angle is carried out bybarrier and the counter is then initialized.a coarse measurement and a fine measurement. The coarsemeasurement counts the number of passing transparent and opaque strips. Therefore, the output signal ofthe photo diode is triggered and the number of pulses, i.e. transitions between positive and negative states,are counted. Due to the pattern consisting of up to 20,000 elements, the rotation angle has a resolution upto 0 02 gon. The fine measurement uses the Moiré effect. The pattern of several opaque and transparent
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
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68 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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''iI98 5. Kinematic Laser ScanningA
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102 5. Kinematic Laser Scanning5.3
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—1.104 5. Kinematic Laser Scannin
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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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