Infrared-based sensor system for contactless monitoring of wetness and ice

Abstract

In order to differentiate between a wet and a dry road surface, the water film height should be measured by using an infrared-based sensor system. By means of different wavelengths, it is also possible to distinguish between ice and water. In this article, a sensor system for the determination of the physical state of water on different surfaces using infrared LEDs and one photodiode is presented. This shall serve as a basis for the calculation of the road condition. Here, emitted and reflected signals of five different infrared emitters are acquired with an infrared receiver. In commercial sensors this method (operating with one emitter) is used to determine a distance. At a constant distance, different intensities of the reflected signal are measured depending on the colour and texture of the surface. Similar sensor systems work with a broadband light, usually a halogen lamp, and several photodiodes. This type of sensor system is more cost-intensive and not robust enough for the desired application. The transmission spectrum of water shows that light is absorbed at a wavelength of 900 nm and strongly at a wavelength of about 1.4–1.5 µm. From the absorption spectrum of ice, it can be concluded that the wavelengths up to 1.4 µm are absorbed less than the wavelength at 1.5 µm. Therefore, the intensity depends not only on the material surface, but also on the wavelength of the transmitting and receiving diode. In order to distinguish between ice and water, the intensities of the light at the wavelengths 1.45 and 1.55 µm are compared, which is shown in this article by the measurements. In ice, the infrared light is more strongly absorbed at 1.55 µm.