## 2 Materials and Methods
### Advancing the Experimental Setup  
The experimental setup in our previous work consisted of the aforementioned VLS unit, incorporating the LED as the transmitter and the RGB photodiode as the receiver. The moving object, the Lego train, equipped with the retroreflective foils moves on the train tracks that resemble the shape of the number zero with the straight parts of this track layout, being 115 cm in length. The VLS unit is placed on a metallic bar facing downward toward the rails over one of the straight parts. The VLS unit is aligned with the tracks in a way that the LED and PD are over the center width of the rails. Identical to our previous work, we used the same flooring material under the experimental setup. Figure 1 shows a sketch of the new experimental setup, with the improvements of an increased distance between the VLS unit and the rails, which was 68 cm in our previous work, of 1.1 m, and that no light barrier alongside the track is necessary for triggering purposes, which is discussed in detail in this subsection. Furthermore, also the movement directions applied for the later discussed (see Section 2.4) movement direction determination are included in Figure 1.  
As mentioned earlier, we changed the LED type in our VLS unit to a CREE MC-E RGBW LED. With this modified experimental setup (different LED type, increased distance) and with the described applied power supply setting, this now results in an illuminance of \(\sim\)500 lux at the surface of the reflective foils on the train. Figure 2 shows the spectral power distribution of the impinging light on the surface of the train on the base of the applied LED. The spectral power distribution as well as the lux value were measured with a handheld MK350S PREMIUM spectrometer. Figure 1: Sketch of the improved experimental setup. Figure 2: Spectral power distribution of the light impinging on the surface of the train (only the LED of the VLS unit as a light source). In our previous experimental setup, a self-developed, so-called light barrier, consisting of an infrared LED and an associated infrared sensitive photodiode, was placed alongside the train tracks in order to trigger the data acquisition when the train moves through the light barrier. Although this light barrier is a straightforward and simple way to detect the triggering event, it also bears a certain drawback since additional components have to be placed in the infrastructure, in this case, alongside the track. In this work, we realized a solution approach to detect the triggering event without this light barrier, thus overcoming the issue of additional infrastructural effort. In order to establish a precise and unique triggering mechanism, we devised the following approach. The retroreflective foils placed on the train affect certain reflectivity parameters and, therefore, \"cause\" very different output values of the three color channels. Triggering the data acquisition utilizing the reflections of the foils is not a feasible solution since, for example, the green foil \"causes\" a precise and detectable peak value in the green channel, whilst, for example, the red foil does not cause such a peak value in the green channel. Therefore, according to this example, the triggering event might be detectable in the green channel for the green foils but not for the red foils. This already demonstrates that in order to establish a unique and generally applicable triggering mechanism, we have to use a different approach. Furthermore, the triggering event must not affect the acquisition of the reflected light of the foils. Following these two requirements for performing a unique and generally applicable triggering event that does not affect the foil determination, we devised a solution approach for which we incorporated a specular reflective element on the train. The most straightforward way to establish such an element is to use a mirror.