# Light Transmission in Fog: The Influence of Wavelength on the Extinction Coefficient
## 3 Materials and Methods
### Methodology  
The measurement protocol was set up in Cerema's PAVIN BP platform, presented in the previous section. It used the following equipment:  
* a Spectral Evolution PSR+3500 spectroradiometer measuring radiance for each wavelength from 360-2450 nm. Radiance measurements were grouped into 10-nm packets;
* a continuous spectrum light source. The light source chosen was a 400-W halogen lamp whose characteristics were measured. This source was stable over time, and its relative spectrum was measured and is shown in Figure 4. The halogen source was similar to a black body whose temperature was estimated at 2905 K by optimization; this value was used in the MODTRAN model;
* a PALAS WELAS 2100 PSA measuring the DSD of fog over the range of 0.3-17 \(\upmu\)m. This PSA identified the number of drops in 60 classes in this range;
* a Degreane Horizon TR30 transmissometer measuring the MOR throughout the measurements. Throughout the tests, the sensors were all placed in the same horizontal plane, 1.20 m above the ground. The timestamps of all sensors were synchronized. The light source was placed in front of the spectroradiometer at a distance \(D\) (see Figure 6). The spectroradiometer was positioned to target the light source. To prevent any condensation on the optics of the spectroradiometer, the latter was heated. The values of \(D\) varied from 2.5 m to 27.5 m in 2.5-m steps. The PSA was placed near the light source/spectroradiometer axis in order to limit errors due to the imperfect homogeneity of the fog. It was then calibrated. All measurements were made in the dark to avoid intrusive light. For each distance \(D\):  
* a reference measurement was made with the spectroradiometer with no fog (for an MOR greater than 1000 m, according to the WMO standard [10]);
* the fog was produced in the facility until it reached an MOR of 10 m (very dense fog). The minimum value of 10 m was chosen because this was the limit of the transmissometer measurement;
* the production of fog was stopped, and measurements were then made continuously as the fog dissipated from 10 m-1000 m. The measurements included that of the MOR using the transmissometer, the radiance received by the spectroradiometer, and the DSD measured by the PSA;  
Figure 6: Protocol used to measure the extinction coefficient on the PAVIN BP platform. * to obtain enough data, a second complete dissipation was reproduced using the same protocol. After making measurements for all distances \(D\) with the first reproducible fog type on the platform (radiation fog, with a 0.5-\(\upmu\)m centered radius mode), measurements were made with the second fog type (advection fog, with two radius modes centered at 0.5 and 5 \(\upmu\)m) for each distance \(D\). During measurement, the actual DSD of the fog was recorded with one-second steps. The DSD was then used as input for the two models. Figure 7 presents the DSD recorded during the measurement for each MOR. The MOR was here given by the transmissometer; the MOR calculated from DSD data might be different. In particular, the DSD was truncated because the device used during the experiment did not allow particles above 17 \(\upmu\)m to be detected, as shown in Figure 7. However, the largest drops had a strong impact on visibility estimation. In Figure 7, the DSD is presented in log scale, which is more common in the meteorological domain. This explains why the standard deviation represented was visually asymmetrical. Figure 7 shows that the standard deviation was high. There were several reasons for this. Firstly, measurements were made with one-second steps, without temporal smoothing. This choice was made so as to be able to measure DSD during fog dissipation, which may take place quickly, especially for high MOR. Secondly, DSD measurement was difficult because it was based on the continuous suction of a small sample of fog. This measurement generally made sense on data smoothed out over several tens of seconds.