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

20 2. Components of Terrestrial Laser Scannerstrips of up to 200 is overlapped by the corresponding diametrically opposed pattern.Based on the Moirépattern, the resolution can be increased by measuring the phase. Therefore, the reading index has to bepositioned within the last period of the Moiré pattern. Further details can be found in [Ingensand, 1998]and [Schlemmer, 1996].2.2.2 Binary EncodingThe binary encoding method is also called the absolute method since the reference base is physicallymarked on the graduated glass circle. Light from a light source passes through the encoded glass cir¬cle, which is marked with the number of the graduation in a binary code. Each corresponding digitbinary code is represented by a transparent or an opaque element.of theThe glass circle contains either coded rings or is marked by an absolute code, which can be read by an arrayof CCD5 sensors. In Figure 2.15, a glass circle with coded rings (left) and an absolute code (right)seen. The required resolution can be achieved by interpolation algorithms.[Ingensand, 1998] and [Schlemmer, 1996].can beFurther details can be found inFigure 2.15: Binary encoding with absolute orientation according to [Ingensand, 1998]. Coded rings (left) andabsolute code (right).2.3 Deflection SystemScanning the environment requires the deflection of the laser beam in two directions, horizontally andvertically, respectively. Due to constant angle increments the result is a grid of points defining a pointcloud. The horizontal deflection step width is often forced to equal the vertical deflection steppoints are arranged in vertical profiles and shifted by a discrete horizontal angle increment.width. TheThe deflection can be separated into a primary deflection and in a secondarydeflection for two-mirrorsystems,cf. Section 2.3.1. Each mirror rotates about one specific axis to deflect the laser beam in a 2D plane.A common practice is to perform a primary deflection in the vertical direction to generate a vertical profile.The secondary deflection then shifts the laser beam horizontally to acquire the next vertical profile adjacentto the previous profile. In the case of a one-mirror system, the laser beam is deflected by a rotating mirror,which is moving about two perpendicular axes, cf. Section 2.3.2.The dimensions of the environment thatcan be scanned depends on the principle used for deflectingthe laser beam. This field of view is eitherlimited by a section, i.e. camera scanner, or covers a full view, i.e. panorama scanner, cf. Figure 2.16.There are different techniques for deflecting the laser beam. A detailed discussion is given in [Kern, 2003],where it is distinguished between a deflection using plane mirrors, polygon mirrors, and prisms. Anotherpossibility is the distinction regarding the motion of the mirrors. They can either oscillate or simply rotate.5CCD: Charged Coupled Device