# Testing and Validation of Automotive Point-Cloud Sensors in Adverse Weather Conditions
## 1 Introduction
In CEREMA's fog chamber, they have a wide variety of adjustable parameters (fog's particle size, meteorological visibility, rain's particle size, and intensity). In self-built chambers, the fog density is primarily controlled by the number of fog layers. Tests performed in fog chambers are always static, and neither the sensors nor the targets were moving at all. The targets used are usually \"natural\" targets, i.e., vehicles, mannequin puppets, but calibrated targets were occasionally used [6; 7; 8]. [PERSON] et al. [7] built their own rain simulator that consisted of rain layers that are individually controlled. The rain intensity is adjusted by varying the activated rain layers. By increasing the number of active rain layers, they find the point where the sensor is no longer able to differentiate the target from the rain. This is the basis of their benchmark methodology: The number of rain layers with a specific rain intensity defines the performance of the sensor tested. The rain simulator and methodology can also be used to validate theoretical models. Basic radar, LiDAR, and camera sensors were tested to show the influence of rain on their performance. As a target, they used the Euro NCAP vehicle target, which is a standardized test object. [PERSON] et al. [8] continued their work by creating a fog simulator. Its principle is similar to their rain simulator as it consists of individually controlled fog layers. Sensor disturbance is, again, increased by activating more and more layers. They did not use any glycol or glycerin-based fluid to create the fog, thus ensuring the natural absorption and scattering properties of real fog. The principle for detecting the performance of sensors is also similar to their rain simulator: When the sensor is no longer able to differentiate its target from the fog, its limit is reached. The number of active fog layers serves as the assessment criterion. In their study, [PERSON] et al. [9] concentrated on how fog affects the visibility of a safety sensor for a robot. Hence, the working environment is not in traffic, but the sensors used are the same as in the traffic environment. In their artificial fog chamber, the density of fog and brightness of the chamber are controlled to meet a desired target value. The laser device measures the spectral transmission. The visibility of Velodyne VLP-16 was investigated visually and their result was that its performance decreases the denser the fog is. [PERSON] et al. [10] presented a test setup with a sensor set that consisted of a low-THz imaging radar, a LiDAR, and a stereo optical camera. Their aim is to highlight the need for low-THz radar as a part of automotive sensor setup especially in weather conditions where optical systems fail. They built their own fog chamber inside a large tent in order to maintain a dense fog. The fog was created with a commercial smoke machine. As targets, they had two dummies as pedestrians, a metallic trolley, and a reference target. Visibility was measured with a Secchi disc. They recorded images at three fog density levels. Visually examining the results yielded that fog does not affect the performance of radar much, whereas the visibility of LiDARs and cameras is decreased when fog density increases. [PERSON] et al. [6] had four LiDARs from Velodyne and Ibeo evaluated in CEREMA's climate chamber, where it is possible to produce two different fog droplet distributions. The density of fog is continuously controlled, thus keeping the fog stable. The test scene consisted of various different targets, e.g., reflector posts, pedestrian mannequins, a traffic sign, and a car. LiDAR sensors were located at their pre-selected positions on a test vehicle, which was placed at the beginning of the chamber. Velodyne sensors were mounted on top of the car and Ibeo sensors at its bumper. The static scene was recorded with different fog densities and droplet distributions with all sensors. These measurements were visually investigated and resulted in the general conclusion that fog reduces the maximum viewing distance.