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

'.4.1 Data Processing 75v=\ j—V2=(l,2)original signal'v2=(h^2lFigure 4.1: Geometrical interpretation of mixed pixels for two surfaces. The leftfigure shows a phase difference of lessthan n, the right figure shows a phase difference ofgreater than n, adaptedfrom [Hancock, 1999].completely avoided but can be minimized. In case of the direct time-of-flight principles,the measurementsystem can separate the incoming signals in first and last pulses. Thus, the problem of mixed pixels shouldnot be of interest if a first pulse and a last pulseis detectable.Figure 4.2 shows the presence of mixed pixels, which are either between two objectsthan the objects. In the figure, a sphere is placed in front of a wall. The mixed pixelssphere and between the sphereand the wall.or farther or closerare both in front of theFigure 4.2: Mixed pixels occurred near objects: A sphere is positioned in front of a wall. The mixed pixelsof the sphere are either in front of the sphere or between the sphereand the wall.seen at the4.1.3 Range/Intensity CrosstalkThe distance measurement system does not operate independently from the received intensityintensity value of the reflected laser beam influences not only the noise of the rangedistance value.Objects, which differ in reflectivity, measured at the same range,value. Thebut also the absoluteshould have identicaldetected ranges. Unfortunately, reflectivity causes different values for the distance to the objectsintensity of the laser beam varies. Thus, the less the intensity, the more biased the rangesince thedata. This is calledcrosstalk between range and intensity. This can be distinguished between electronic crosstalk, caused by theimplementation of detection electronics, and optical crosstalk, caused by the sensor opticsreflections.due to internalThe receiver electronic is limited to function only within a specific intensity range.Measurements derivedfrom intensity values outside the intensity range are noisy or erroneous, cf. Figure 3.44. Ideally, the operat¬ing range should be adjusted dynamically according to the intensity,as it is done in modern total stations.