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

68 3. Calibration of Terrestrial Laser Scannerangle of incidence [°]30 30angle of incidence [°]Figure 3.41: Influence of the angle of incidence on the precision of the distance measurement to a planar object.particle. This particle causes a reflection of the ray inside the material, cf. Figuredistance is always longer than the desired distance to the surface of the object.3.42. Thus, the measuredincident rayreflected rays2ntl reflectionparticlesFigure 3.42: Surface backscattering and refractional effects in inhomogeneous semitransparent materials, e.g.foam, wood, marble [Ingensand, 2006].styro¬The effect of refraction and intrusion of the laser beam in materials is verified by comparingdetected by the laser scanner and the nominal distances provided bythe distancesthe interferometer on the calibrationtrack line. The experimental setup is analogous to the investigationof the 'static mode' of the distancemeasurement system, cf. Section 3.2.1. Theresultingresiduals between nominal and measured distancescan be interpreted regarding systematic effects caused by the intrusion of the laser beam. The followingmaterials were investigated: styrofoam (thickness of 2 cm and 5 cm), wood and metal.The results are given in Figure 3.43 and Figure 3.44. Figure 3.43 shows the accuracy for styrofoamand woodin comparison to a white paper used as a reference target.The findingsare clear: the laser beam intrudesthe styrofoam and wood. The depth of the intrusion depends on the granulation and the roughness. Forthe materials used, the intrusion depth shows a value of nearly 15 mm for styrofoam and nearlywood.5 mm for