Investigation of TLS Intensity Data and Distance Measurement Errors from Target Specular Reflections

Abstract

Terrestrial laser scanners (TLSs) can provide accurate and high-resolution data by measuring the distances (ranges) between the scanned points and the scanner center using time-of-flight or phase-shift-based methods. Distance measurement accuracy is of vital importance in TLSs and mainly influenced by instrument mechanism, atmospheric conditions, scanning geometry, and target surface properties. In general, existing commercial TLSs can achieve millimeter precision. However, significant errors (centimeter and even decimeter levels) beyond the instruments' nominal accuracy exist in distance observations for targets with highly reflective surfaces whose specular reflections are dominant because these reflections can increase the backscattered laser signal power considerably and cause further disorder in the echo detection and recognition by TLS photodetectors. Apart from distance, the intensity value derived from the backscattered signal and influenced by the same factors as that of the distance measurement errors is recorded by TLSs. A certain link exists between the two instrumental observations. In this study, the anomalous distance measurement errors caused by target specular reflections are explored. The different planar reflective targets scanned by a Faro Focus 3D 120 terrestrial scanner are used to experimentally investigate the relationship between the original intensity values and the distance measurement errors. Results imply that the distance measurement errors caused by specular reflections are not as erratic as they ostensibly seem. On the contrary, distance measurement errors are strongly related to the original intensity values. A polynomial can be established to empirically model the relationship between the original intensity data and the distance measurement errors. With use of the original intensity to compensate for the measured distance observations, the point cloud data accuracy can be improved by approximately 55.52%.

Measuring Surface Moisture on a Sandy Beach based on Corrected Intensity Data of a Mobile Terrestrial LiDAR

“…The radiometric correction is necessary to eliminate the impact of the distance and incidence angle on the intensity data and to convert the raw intensity into a corrected value that is proportional or equal to the target's reflectance [39,45,46,[49][50][51][52][53][54][55][56][57][58][59][60][61]. However, regarding the distance effect, due to the possible existence of automatic reducers for the near-distance backscattered signals and amplifiers for weak backscattered signals [46,62], only the part of the intensity data within a specific distance follows the theoretical LiDAR formula [39,48,61,63] and can be corrected effectively through 1/R 2 [33,34]. On the other hand, the effect of the incidence angle is mainly related to the target surface properties and surface irregularities [35,39,64,65] and the cosine law is the most common method to rectify the effect of the incidence angles on the backscattered intensity [45].…”

Comparison of Time-of-Flight and Phase-Shift TLS Intensity Data for the Diagnostics Measurements of Buildings

“…where c is the velocity of light along the path from sensor to target, and ∆t is the time interval between the emitted and received laser signal. The quality of the distance measurement is directly related to the accuracy of the time measurement and the accuracy in detecting the backscattered signal [60]. Thus, atmospheric corrections must be made to improve the measurement precision and accuracy.…”

Monitoring earthen archaeological heritage using multi-temporal terrestrial laser scanning and surface change detection

“…TLS point clouds also record information about surface color and the intensity of the laser signal it reflects [47], providing additional information on the texture and material of the scanned object.…”