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

'94 5. Kinematic Laser Scanningbeam is not punctiform, but has a more complex structure during propagation in the air, cf. Section 2.1.2.If the laser beam hits an object, a footprint of the laser beam is generated.Therefore, the 'real' footprintis much larger than the 'visible' footprint. Furthermore, the sensitivity of the diodes is important. Only asmall part of the laser beam needs to hit the photo diode to start the process.If another part of the laserbeam hits the photo diode directly a second time, the stopping process is performed.Neither does the laserbeam rotate 2tt nor can a reproducible and reasonable rotation time T be derived. To avoid these problems,either a more complex electronic circuit has to be developed or two photothem have to be implemented. The latter was used in this study.diodes with a distance betweenThe use of two photo diodes implicates some specialties with respect to the experimental setup. Addition¬ally, the center angle a has to be determined. Referring to the postulation regarding the precisionof therotation time T, the required precision for the center angle a can be estimated. To achieve the best possibleprecision for the center angle a, the principleof autocollimation was used. InFigure 5.4, the two photodiodes, as well as the autocollimation mirror mounted on a plate, can be seen. This plate can be aligned byan adjusting device.autocollimationmirrordiode(stop)centre laser beam(rotating mirror]diode(start)Figure 5.5: Mathematical relations for deriving the center angle a: the radius r,be known.the base b and the offset Ar has toIn Figure 5.5, the mathematical relation between the desired center angle a, the plate including the photodiodes, and the autocollimation mirror can be seen. The center angle a can be caluclated bya = 2 •arctan-2(r Ar)(5.5)where r is the radius, i.e.distance between center of rotating mirror and autocollimation mirror, Ar is theoffset between the photodiodes and the autocollimation mirror and b is the base i.e.the distance betweenthe two diodes. The offset Ar was measured with a slide gauge several times. As result, the offset Ar is lessthan 0.5 mm with a precision of about 0.1 mm. Considering Equation (5.5), the influence of Ar is negligible,if the radius r is larger than 0.5 m. The reason is that, assuming the slowest angular freqeuncy of 12.5 rps2,the influence on the rotation time T is less than the time precision of 1/xs claimed. Thus, Equation (5.5)simplifies to a = 2 •arctan2r(5.6)2Rps: rotations per second. According to the specifications of the manufacturer, there are three different rotating velocities: ojns12.5rps(Tfa0.08s),w25 fa 25rps (T fa 0.04 s) and w33 fa 33 rps (T fa 0.03 s)