抄録
We have developed a road surface condition detection system using near-infrared light, which enables remote measurement. The developed system can detect the difference of road surface conditions as dry, water, ice, and snow.
In this case, we focused on the light-absorption characteristics of water. To remove the affect of the thickness of the water layer, we have proposed a measured metrics as absorption ratio at two wavelengths. The absorption spectra were measured by using a water layer of 1mm thickness. The experimental results showed that the absorption peak shifts to longer wavelength when the water state changes into ice state. A ratio of absorption coefficients at 1300nm and 1410nm is constant even when the layer thickness changes from 0.2mm to 0.8mm. This result indicates that the proposed absorption ratio can be used as a criterion to detect the water state or the ice state.
We have also proposed a reflected-light separation method by using polarization characteristics of light. There are two reflected light components. One is the reflected light from the water surface and the other is one from the road surface. The reflected light from the water surface keeps the polarization state of the input. However, the polarization state from the road surface becomes random state because of the rough surface. To remove the surface reflection of the water layer, orthogonal polarization detection of input and output pair is effective. This is confirmed by the experiment.
We made a prototype of the above-described detection principle and it was tested by the experiment. As a result, the feasibility of determining road surface conditions such as dry, water and ice is confirmed by simultaneous measurement of light-absorption characteristics and reflection characteristics.
