# Towards a Multi-Perspective Time of Flight Laser Ranging Device Based on Mirrors and Prisms
## 4 Discussion
From that point on, traces for configurations with the mirror set closer than 150 mm from the ranging device continue the same way as in Figure 8a or Figure 8b, but the remaining traces trend downwards. The effect diminishes as the distance between the mirror and the ranging device increases. Measurements with the mirror 40 cm or further from the ranging device seem effectively unaffected by the  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Distance to**} & \multicolumn{5}{c}{**Maximum Measurement Standard Deviation (mm)**} \\ \cline{2-6}  & **First Surface** & **First Surface** & **Regular Mirror** & **Regular Mirror** & **without Mirror** \\  & **Mirror (Clean)** & **Mirror (Dusted)** & **(Clean)** & **(Dusted)** & **without Mirror** \\ \hline / & / & / & / & / & 3.0 \\ 5 & 2.8 & 2.7 & 2.7 & 3.7 & / \\ 10 & 2.7 & 2.7 & 2.6 & 2.9 & / \\ 15 & 3.0 & 3.4 & 2.9 & 3.7 & / \\ 20 & 2.8 & 4.5 & 2.8 & 3.3 & / \\ 25 & 3.0 & 3.5 & 3.3 & 2.9 & / \\ 30 & 2.  
phenomenon. An explanation for the effect can be found by examining the reflected pulse's shape, as shown in Figure 9. When ranging at an infinity, no return signal should be observed, but when ranging at an infinity while redirecting the FOV with a dusted mirror, some weak return signal can be detected (blue trace in Figure 9). This is a reflection off the dust on the mirror's surface. It is below the detection threshold on its own, but when ranging on a target is carried out with such a setup, reflections off the target and dust combine into a single pulse (orange trace in Figure 9), resulting in a measurement error. It comprises two effects, moving the ranging result towards lower measured time, and pulse widening. In a direct time of flight ranging system with TOT-based walk error compensation, those effects work in opposing directions. The first lowers the compensated ranging result, and the other increases it. This study finds that moving the measurement towards lower time has more influence on the ranging, which results in a negative measurement error. As the target moves further away, the desired reflection grows dimmer, but the reflection off the dust remains unchanged. This means that the error will grow in magnitude, which can be observed in error plots in Figure 8b,d. The effect will be the most pronounced when the mirror is positioned around the end of the partially dead zone, as there the most parasitic reflection will be coupled into the receiver. As observed in Figure 8b,d, the amount of light from the parasitic reflection is negligible when the mirror is positioned at 10 cm from the ranging device or closer. With increasing distance, the parasitic reflection's intensity decreases alongside its influence on the measurement. Therefore, ranging with a dusted mirror 40 cm from the target or further is hardly affected by the parasitic reflection. The data about the effects of mirrors on ranging characteristics, presented in Figures 7 and 8, cannot be directly compared to other researches. In spite of that, some relatable data are found by taking multiple separate researches into account simultaneously. It has been found in [18] that redirecting Microsoft Kinect 2.0's FOV with mirrors results in an RMS error ranging between 4 mm and 8 mm for raw data, which was reduced to roughly 2.5 mm to 7 mm after the enhancement algorithm was applied. The data have to be read from the provided graph, as no numerical data were provided. It can be compared to the results of [27], where a camera's raw performance was evaluated. The mean measurement error has been found to be within 10 mm on the entire advertised measurement range of the Microsoft Kinect 2.0 (4.5 m). Even though the data cannot be compared directly because the distance to the object in the first study was not disclosed, it can be observed that the performance of the camera with reflected FOV was close to the study with direct measurements. This confirms our findings that clean mirrors do not have a large influence on the camera's performance.