Investigation on surface inspection using polarizing image sensors

Surface inspection in industrial automated processes is very often challenging. Especially the detection of transparent liquid materials such as water represent a major challenge for standard imaging systems. One approach to overcome the limitation of these imaging systems lies in the exploitation of the polarization effect. This effect surely can only be applied if the contaminants have polarizing features but can help to use invisible characteristics of light for quality inspection tasks. In this work investigations on surfaces which are contaminated with water will be presented. Therefore, an imaging system using an RGB dome light illumination was set up in combination with a four-channel polarizing camera. The dome light, which is equipped with three different LED wavelengths, will be mixed so that the illumination which hits the sample is completely unpolarized. So, any effect on the surface which leads to a polarizing effect can be observed. The system delivers a four-channel image with different polarization angles that have to be processed. Therefore, an algorithm realizes a demosaicing which separates the four different polarized pixels into individual images. Based on this, the stokes equation which allows the calculation of the degree of polarization and the angle of polarization has to be processed for the image presentation. To achieve a better visualization of the degree of polarization an HSV-transformation based on the polarization parameters was also realized.