## 2 Materials and Methods
### Advancing the Experimental Setup
In this work, we used a commercially available plastic mirror, consisting of a plastic body covered with a specular reflective foil, which was cut to the same size configuration as it is defined for Area 1, 0.7 cm \(\times\) 4.7 cm (see Table 2). This mirror element, which in the following is abbreviated as ME, was then placed at one edge of the train platform. Figure 3 shows the train equipped with the ME and a 3M green foil in the size configuration of Area 4. By utilizing the specular reflecting ME, we can anticipate two characteristics, which will fulfill two requirements. First, since the ME will reflect the complete spectrum of the impinging light, in contrast to the colored foils, the resulting reflections will be measurable for all three color channels, and since the reflectivity of a mirror is usually between 80% and 99%, the intensity of the reflected light will also be unique compared to the intensities of the individual colored foils. Second, as the ME is specular reflecting, we can also anticipate that only when the ME is close or directly under the VLS unit, the ME will reflect light back toward the photodiode. This will furthermore result in clear and steep flanks in  
Figure 3: Lego train equipped with the mirror element and a 3M green foil of size Area 4.  
the acquired data of the color channels over a short time interval. Figure 4a,b shows an exemplary zoomed-in view of the acquired outputs of the three color channels (depicted with their respective channel colors, y-axis) over time, given as sample numbers (x-axis) when the train passes under the VLS unit with the speed setting of Speed 3. Please note that in these figures, the data from the oscilloscope were already resampled, as described before, to emulate a sampling rate of 100 kHz. For these exemplary measurements, the shades of the laboratory room in which the setup was assembled were closed, blocking the sunlight, and also the ambient room lighting was turned off, leaving the LED of the VLS unit as the only active light source during these experiments. In order to make sure that there were no reflections from the utilized colored foils, we only placed the ME on the train (at the position shown in Figure 3). Figure 4a shows the output values for the movement direction Forward and Figure 4b for the movement direction Backward (see Figure 1). From the run of the curves shown in Figure 4, we can clearly observe that our anticipations regarding the utilized ME are correct since the expected clear and steep flanks for all three color channels are clearly deductible. Furthermore, also our second requirement, that the ME will not interfere with the reflections of the foils, is fulfilled, as we discuss in the following. As mentioned before, the train was only equipped with the ME and there were no retroreflective foils placed on the train. When the train is moving in the Forward direction (see Figure 4a), the course of events is as follows. At the beginning of the curve shown in Figure 4a, from sample number 0 to around sample number 80,000, the train is not under or close to the VLS unit; therefore, only the stable reflections from the surroundings are acquired. At sample number 100,000, the Lego train has entered the detection area of the VLS unit and the anticipated clear and steep flanks, due to the ME, can be seen. It is important to point out that, as verified by the measurements and intended in our solution approach, for a triggering mechanism without the need of placing the light barrier in the infrastructure, the flanks are present in all of the three color channels. Thus, we can perform the triggering for the acquisition based on a falling flank detection. As the train moves along with the given speed, a second clear and steep flank occurs (at sample number \(\sim\)102,000), when the ME moves out of the VLS unit.