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

3.2 Distance Measurement System 313.2 Distance Measurement SystemThe distance measurement system of the laser scanner Imager 5003 of Zoller+Frohlich is based on the prop¬agation of the modulated laser light. If the laser beam hits an object in the environment, then the laser beamis partially reflected and can be detected by an APD. This photodiode is coupled with a signal amplifierand provides a dynamic range in reflectivity from 5% up to 99% [Fröhlich et al., 2000]. The method usedfor determining distances is the phase difference principle, cf. Section 2.1.4. Because of the ambiguity insignals, e.g. sine signals, a maximum range by means of a modulated wavelength Amax has to be defined.The challenge by defining Amax is to achieve both a long range interval and a high resolution in the distancemeasurement. Therefore, a resolution of up to 1/8000 of Amax is possible, cf. [Rueger, 1996] and [Kahmen,1997]. To guarantee long range as well as high resolution, the emitted laser signal is simultaneouslyintensity-modulated with two different sinusoidal frequencies [Fröhlich et al., 2000]. These two frequenciesdefine a coarse channel component, i.e. low frequency signal Tfs', and a fine channel component, i.e. highfrequency signal 'hfs'. The detected laser light contains the phase shifts of both modulation frequencies:Tfs' limits the maximum range and provides a coarse value for the desired distance and 'hfs' delivers aprecise but ambiguous range. Depending on the desired maximum range for 'hfs', two different modescan be chosen: mode 'close' (Aifs « 54 m), and mode 'far' (Aifs « 108 m). Thus, the distance between thescanner and the object is limited to 27 m and 54 m, respectively, 'hfs' has a wavelength of Ahfs« 6.7m andallows for a resolution of less than 1 mm. The carrier wave of the emitted laser light is in the near infrared(A « 700 nm). Based on constant emitter signals, the intensity of the detected light is equivalentto thereflectivity value of the object. The simultaneous detection of both range and reflectance values guaranteesthat they correspond directly to the same data point in 3D. Furthermore, it has to be mentioned that thisdistance measurement system is (mostly) independent of the lighting condition, e.g. ambient light, becauseof the active illumination of the laser light.The reflectivity of an object is not the only parameter which influences the intensity of the reflected lasersignal. Other parameters also affect the reflectivity value (/), such as [Gerthsen and Vogel, 1993]:• distance d, which is inversely proportional to the square of the distance: / ~ 1/d2•angle of incidence• surface properties, e.g. colour, roughness, reflectanceThe laser scanner canoperate in two different modes: a so called 'scanningmode' and a so called 'staticmode'. The 'scanning mode' means that the deflection unit is rotating. In contrast, the deflectingthe 'static mode' is not rotating and the laser beam can be aligned manually. The usual operationunit inmode forthe laser scanner is the 'scanning mode'. Therefore, the laser beam is deflected by the rotating deflectionunit and is scanning the 3D environment. The 'static mode' should only be used for investigation andcalibration purposes and for some specific applications because the laser beam is permanently aligned inone direction and is reflected by the same object point in the environment. In 'static mode', the laser beamis dangerous for the human eyes since the laser class is 3R and can cause permanent damage to the retina,cf. Section 2.1.2.The operator as well as people involved in the surrounding have to wear safety gogglesaccording to laser safety instructions to avoid irreparable accidents concerning human vision. However,some important aspectscan be gained in 'static mode', for example:• precision and accuracy•long-run behaviour• frequency stability, e.g. caused bythermal drifts