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

8 2. Components of Terrestrial Laser Scannerinterferometry flme-öf-flight triangulationindirectßme-of-flightdirecttime-of-flightFMCW(beat frequency)AMCW(phase difference)polarization modulationFigure 2.1: Opticaldistance measurement methods in terrestrial laser scanners.continuous wave, polarisation modulation. The direct time-of-flight method and the amplitude-modulatedcontinuous wave method are widely-used in terrestrial laser scanners, whereas the frequency-modulatedcontinuous wave method and the polarisation modulation method are rarelyused in terrestrial laser scan¬ners.The intensity of the reflected laser beam defines a fourth dimension, in addition to the 3D coordinates.The reflected laser beam is detected by a photodiode, i.e.PIN diode or avalanche photo diode (APD), andthe amplitude of the detected signal corresponds directly to the intensity of the received laser beam. Theintensity of the laser beam is dependent on several parameters, such as the range, the angleof incidence andthe surface properties, e.g. colour, roughness. Unfortunately, the physical significance of this informationis often not exactly defined by the manufacturers [Lichti et al., 2002a]. Therefore, the intensitycan relateto either the amplitude or the energy of the received signal.For avoiding noise caused by ambient light,additional filters are used, e.g.IR Filter.Since laser scanning is an active and prismless method for surveying the environment, it is necessary tocomment on someaspects of the reflection principles. Often, the results of laser scanning are 3D-models,which have to be illuminated within the software packages. Thus, the section is completed by utilizingsome methods for modeling the reflectance. This is especially the case in computer graphics,models are used for shading the digital models to achieve a realistic appearance with respectwhere severalto illumina¬tion.The term reflectance is defined as the ratio of the amount of electromagnetic radiation, usually light,reflected from a surface to the amount originally strikingthe surface.2.1.1 Electromagnetic WavesElectromagnetic radiation contains a broad spectrum from cosmic radiation with very short wavelengths(A < 10~14m) up to radio waves (A > 106m). Electromagneticwaves are used as carrier waves for trans¬mitting signals, which show varying forms, e.g. impulses, patterns. The specific wavelengththat is useddepends on the layer of the atmosphere, e.g. troposphere, ionosphere, and on the range to be measured. De¬lays during the transmission of the electromagnetic wave between emitter and receiver caused by differentconditions of the atmospherehave to be taken into account. Variationsin temperature, air pressure, hu¬midity, and other parameters influence the speed of light and the propagation of the electromagnetic wave.Thus, the speed of light is not a constant and has to be corrected by applying atmospheric conditions.