*2.1. TLS Basic Principles*

TLS is a non-destructive testing (NDT) technique based on the emission of a laser beam of known direction, used to quickly measure the position of points on the surface of a surveyed object, in a reference system with the origin in the center of the instrument. Two principles are adopted to measure distances: time-of-flight (ToF) and phase shift. In our case, a TLS ToF is used. Figure 1 illustrates the operating principle of the ToF. A short pulse is emitted by the TLS through a photodiode. The distance is obtained through the ToF of the emitted impulse, which is the time interval Δt taken by the impulse to reach a target and go back. Then, the distance d from TLS to target is given by Equation (1):

$$d = \frac{c\Delta t}{2} \tag{1}$$

where c is the speed of light.

As for the direction of the laser beam, this is obtained by the scanning system, usually consisting of a multi-facet mirror that rotates around a horizontal axis for vertical scanning, while the entire instrument rotates around a vertical axis through a motor, like the alidade of a motorized TS. In the case of a line scanner configuration, rotation around the vertical axis is disabled. The maximum range can reach 6000 m, while the attainable precision for single points varies from 1 to 20 mm. Better values are obtained for shorter distances [23].

For our application, we made use of a TLS Riegl VZ-1000®, RIEGL Laser Measurement Systems GmbH, Horn, Austria, whose characteristics are summarized in Table 1.


**Table 1.** Characteristics of TLS Riegl VZ-1000®.

The Riegl VZ 1000® TLS is a versatile instrument. Along with the possibility to operate in line scan mode, it makes it possible to perform long-range surveys and, therefore, to obtain 3D models of objects and of the surrounding environment.

When line scan mode is selected, the laser beam describes a line repeatedly, being addressed only by the multi-faceted mirror (see Figure 1). Using the maximum scanning speed of 120 lines per second, it would be theoretically possible to measure vibration frequencies up to 60 Hz. Taking into account the precision of measurement, it is necessary to consider a strong noise in the time series of samples, which would make it very difficult to measure oscillations with a peak-to-peak difference less than 1 cm. This limit can be overcome if information on the geometry of the surveyed object is used.

**Figure 1.** Sketch of the TLS functioning in line scan mode. The distance d is computed in Equation (1).
