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

2.1 Distance and Reflectance Measurement System 11Concerning terrestrial laser scanners, the spotbeam waist and the range.size of the laser beam is afunction of the wavelength, theThe minimum spot size is not at the emitting laser diode but also somewherealong the path of the laser beam. But, the beam waist can be shifted along the path by means of focussingin order to minimize the spot size according to the object range.Beam Waist7Figure 2.4: Beam waist and divergence angle of a laser beam according to [Andrews and Phillips, 1998].Refraction, Scintillation and DispersionAn electromagnetic wave, which propagates through a random medium, e.g. the atmosphere,is influencedby varying conditions, e.g. temperature, pressure, humidity. Thus, propagation characteristics change withrespect to the geometry and the intensity. The varying geometry is caused by refraction, which makesthe beam move or quiver. The beam propagation is no longer a straight line but a curve. In addition,the variation in the intensity of the laser beam caused by the varying atmosphere isFurthermore, these intensity fluctuations are affected by optical turbulence.Opticalcalled scintillation.turbulence can bedefined as the fluctuations in the refractive index resulting from atmospheric turbulence [Beland, 1993].Another effect is known as dispersion. Dispersion here is defined as the dependence of the propagationvelocity on the wavelength.4Since each laser light consists of a narrow spectrum, which is so close thatthe laser light can be considered monochromatic, the propagation velocity is defined by the group velocityof differing frequencies or wavelengths. Consequently, a grouprefractive coefficient must be considered.Depending on the group refractive coefficient, the distance to be measured has to be corrected and this isknown as the first velocity correction.Further details concerning atmospheric influences on the propaga¬tion of electromagnetic waves can be found in [Joeckel and Stober, 1991] and [Rüeger, 1996].The described influences are relatively small for short ranges and can be neglected. However, for long rangelaser scanners covering distances of up to several hundreds of meters, the influences due to the atmosphereand the refractive index have to be taken into account. They affect the distances in a systematic way andcause methodological errors.Laser SafetyDue to power density, most lasers are dangerous and can cause damage to the skin or the eyes. Dependingon the wavelength of the laser, different elements of the eyesare affected. Lasers are generally catego¬rized into four classes according to the laser's ability to cause damage to the eyes.The eye transmissioncan be seen in Figure 2.5. For the visible area and some infrared frequencies, the eyecharacteristic of nearly100 %.has a transmissionThe classification procedure of lasers is carried out in a specific setup as defined in [IEC, 2001]. The laserbeam is focused on anaperture located in a predefined distance. A detector is placed justbehind the4Dispersion is used in different ways: For example, in optics, dispersion is the separation of a wave into its spectral componentsdepending on the wavelength