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

3.5 Non-Instrumental Errors 651 o08&06vic0)I 0402001 008v>c0)I 0402°°0 5 10 15 20 25 30 35 40 45 50range [m]Figure 3.37: Intensity of reflected laser beam. Influence of the reflectivity (top) and influence of the angle of incidence(bottom).The relative intensity values in Figure 3.37 do not match the given reflectivity values of the targets. Upto 5 m, the white target shows a relative intensity of « 93 %, the grey target shows a relative intensity of« 52 % and the dark grey target shows a relative intensity of « 38 %. Thus, the given reflectivity valuesindicate system-related reflectivity values more than absolute reflectivity values.Concerning the intensity of laser light, the speckle effect also has to be considered. Specklesare based onvariations of the intensity of coherent laser light produced by the interference of the diffuse scattered lighton rough surfaces with the incident light.This means the intensity of the reflected laser light can varydepending on the intensity of rough surfaces and can be affected by speckle effects.3.5.2 Angle of IncidenceThe angle of incidence is defined as the angle between the incident laser beam and the tangenton the surfacefrom which the laser beam is reflected. Generally, the angle of incidence can be measured in two directions,horizontally and vertically. An angle of incidence of 90 ° describes a laser beam hittingobjectat a normal.the surface of theThe angle of incidence influences the distances to be measured in two different ways.On one hand, thecenter of reference for the distance is shifted because the footprintof the laser beam varies from a circularform to an elliptical form. On the other hand, the footprint of the laser spot increases and covers a largerarea. The reference point for measuringthe distance is shifted and not well-defined because of the distortionof the footprint. Thus, the distance has a systematic offset leading to a false range between objectand laserscanner. Furthermore, the noise of the distances increases with a decreasing angle of incidence because theintensity of the reflected laser beam decreases and the footprint of the laser beam enlarges.