# Testing and Validation of Automotive Point-Cloud Sensors in Adverse Weather Conditions
## 2 Materials and Methods  
Two types of tests were performed: Indoors in the CEREMA fog chamber and outdoors in northern Finland and Sweden. The sets of sensors were nearly the same in both testing environments. A set of LiDARs used in tests consisted of Velodyne PUCK VLP-16, Ouster OS-164, Cepton HR80T and HR80W, and Ibeo Lux. In outdoor tests, Robosense RS-LiDAR-32 was also included, but it did not arrive in time for fog chamber tests. A selected set provided a wide range of LiDARs with different designs from various manufacturers. Their specifications are presented in more detail in Table 1. Velodyne, Ouster, and Robosense are of similar design with a cylinder-shaped casing and based on a constantly rotating structure. Cepton's design differs from this as it uses two mirrors and covers the field of view with a sine wave -like pattern. Other sensors use layers on top of one another to create a vertical view. Ouster operates on a different wavelength (850 nm) than the rest of the sensors, which use wavelengths around 905 nm. All sensors are said to work at sub-zero temperatures but Velodyne and Robosense only as low as -10 \({}^{\circ}\)C, which may not be enough for the snowy tests performed in a colder environment. Fog and rain tests were performed in the Clermont-Ferrand laboratory, where a fog chamber is located. It is a state-of-the-art fog chamber where it is possible to control and reproduce the fog's particle size, meteorological visibility, and rain's particle size and intensity [13]. Here, the meteorological visibility means the visibility distance, which is a practical parameter describing fog characteristics in relation to light transmission (see more detailed definition in [13]). LiDAR performance tests in foggy conditions were executed in late November 2018. LiDARs were mounted side-by-side facing the targets in the tunnel as shown in Figure 1. To reduce interference, only one LiDAR at a time was turned on and measuring. The sensors were used with their given settings and configuration and no additional adjustments were made. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Specifications** & **ibee Lux** & \begin{tabular}{c} **Velodyne** \\ **VLP-16** \\ \end{tabular}  & **Ouster OS-164** & \begin{tabular}{c} **Robosense** \\ **RS-LiDAR-32** \\ \end{tabular}  & **Cepton HR80T/W** \\ \hline Layers & 4 & 16 & 64 & 32 & \begin{tabular}{c} no (80 \(\times\) 80 points, \\ 160 \(\times\) 64 points) \\ horizontal and vertical \\ FOV: 15\({}^{\circ}\)\(\times\) 15\({}^{\circ}\), 60\({}^{\circ}\)\(\times\) 24\({}^{\circ}\) \\ \end{tabular}  & \ \begin{tabular}{c} **Velodyne** \\ **VLP-160 \(\times\) 64** \\ \end{tabular}  & \begin{tabular}{c} **Velodyne** \\ **VLP-160 \(\times\) 64** \\ \end{tabular}  \\ Vertical FOV & 3.2\({}^{\circ}\) & (+15\({}^{\circ}\)\(\times\)\(-\)15\({}^{\circ}\)) & (+15.8\({}^{\circ}\)\(\times\)\(-\)15.8\({}^{\circ}\)) & (+15\({}^{\circ}\)\(\times\)\(-\)25\({}^{\circ}\)) & 60\({}^{\circ}\)\(\times\) 24\({}  
The main targets were two pairs of optically calibrated plates with reflectivities of 90% and 5% (white and black, respectively). These are similar to the ones that are used by LiDAR manufacturers when they measure their sensor's performance. The targets were placed in pairs of white and black side by side. Larger ones (\(0.5\times 0.5\) m) were 1 m behind the smaller targets that were also lower so that they would not block the view to the larger targets. In the end, the larger ones were 0.5 m further than the indicated distance (e.g., target at 10 m means that smaller targets were at 9.5 m and larger ones at 10.5 m). These four targets were placed alternately at 10 m, 15 m, and 20 m. At first, a measurement with no fog was done to have a reference point for each LiDAR and distance. Fog densities used in the tests had visibilities of 10 m, 15m, 20 m, 25 m, 30 m, 40 m, and 50 m with smaller droplet size. Collected data from the fog chamber were then processed so that for each combination of LiDAR, target distance, and a target, a region of interest (ROI) was selected.