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

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16 2. Components of Terrestrial Laser Scanner2.1.6 Overview of Distance Measurement Techniques in Terrestrial Laser ScannersTable 2.2 gives an overview of the distance techniquesthat are used in terrestrial laser scanners. The corre¬lation between the distance measurement technique on one hand and the range and data acquisition rateon the other can be seen.Table 2.2:Overview of distance measurement techniques in terrestrial laser scanners. The data acquisition rate, i.e.frequency, and the range depend on the distance measurement technique.Laser Scanner Manufacturer Distance Technique Range[m] Frequency [kHz]CPW 8000 Callidus direct TOF 80 28LS 880 FARO AMCW 76 120HDS 3000 Leica Geosystems direct TOF 300 1.8LMS-Z420i Riegl direct TOF 800 12GX Trimble direct TOF 350 5Imager 5003 Zoller+Frohlich AMCW 53 6252.1.7 Avalanche Photo Diode (APD)An avalanche photodiode (APD) is a photodiode that internally amplifies the photocurrent by an avalancheprocess. A large reverse-bias voltage, typically over 100 volts, is applied across the active region.This volt¬age causes the electrons initially generated by the incident photons to accelerate as they move through theAPD active region. As these electrons collide with other electrons in the semiconductor material, they causea fraction of them to become part of the photocurrent. This process is known as avalanche multiplication.Avalanche multiplicationcontinues to occur until the electrons move out of the active area of the APD.Figure 2.10: Layout of silicon photodiodes: conventional design (left) and pin layout (right) according to [Meschede,2004].The gain of the APD can be changed by changing the reverse-bias voltage.A larger reverse-bias voltageresults in a larger gain. However, a larger reverse-bias voltagealso results in increased noise levels. Excessnoise resulting from the avalanche multiplication process places a limit on the useful gainof the APD.The avalanche process introduces excess noise because every photo-generated carrier does not undergo thesame multiplication.An APD is compact and immune to magnetic fields, requires low currents, is difficult to overload and hashigh quantum efficiency that can reach 90 %. The noise properties of an APD are affected bythat the APD is made of. Typicalthe materialssemiconductor materials used in the construction of low-noise APDsinclude silicon (Si), indium gallium arsenide (InGaAs) and germanium (Ge). More information concerningphotons and APD can be found in [Saleh and Teich, 1991] and [Meschede, 2004].
2.1 Distance and Reflectance Measurement System 172.1.8 Reflection PrinciplesThe reflection of electro-optical waves is based on different types of reflection. It is distinguished between:•specular, i.e. mirror-like, reflection• diffuse, i.e. Lambertian, reflectionThe specular or mirror-like reflection is characterized so that the angle of reflection 6r is identical to theangle of incidence 9t with respect to the surface normal0r = 0t. (2.9)The incoming ray is reflected in only one single direction. The specular reflection appears if the roughnessof the surface is relatively small in comparison to the wavelength of the incoming ray [Gerthsen and Vogel,1993]. The specular reflection is illustrated in Figure 2.11 (left). The complement to specular reflectionis the diffuse reflection, as illustrated in Figure 2.11 (middle). The incoming ray fans out as a bundle ofreflecting rays, covering the hemisphere surrounding the surface of the object. This reflection requires thatthe roughness of the surface be uneven or granular with respect to the wavelength of the incoming ray. Ifa surface is completely non-specular, the reflected light will spread over the hemisphere surrounding thesurface.The combination of specularray isreflection and diffuse reflection leads to a mixed reflection. Theincomingdivided into several reflecting rays covering the hemisphere surrounding the surface of the object.However, the intensity of the ray, with its angle of reflection equalling the angle of incidence, mirrored atthe surface normal, is the largest. One approximation of a mixed reflection is shown in Figure 2.11 (right).Some surfaces exhibit retroreflection. The structure of these surfaces is such that lightis returned in thedirection from which it came reagardless of the angle of incidence. Such reflection is caused by prisms,which are used for precisedistance measurements of total stations.specular reflection diffuse reflection "mixed" reflectionFigure 2.11: Types of reflection: specular reflection (left), diffuse reflection (middle) and mixed reflection, consistingof combined diffuse-specular reflection (right).2.1.9 Reflectance ModelsSpecular reflection and diffuse reflection are simple approximations. In reality, surfaces exhibit varioustypes of reflection. Other properties of electromagnetic waves, e.g. polarization, influence the way ofreflection and complicate a mathematical description. In computer graphics several models were developedfor describingthe reflectance. Inthe following,three models are introduced: the Lambertian reflectance
Page 1: DISS. ETH NO. 17036Calibration of a
Page 5 and 6: ZusammenfassungSeit einigen Jahren
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136 A. Imaging System of Imager 500
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138 B. Technical Data of Imager 500
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-GenauigkeitsbetrachtungenInvestiga
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152 BibliographyWunderlich, T. A. [
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