Refractive Index Estimation Using Polarisation and Photometric Stereo
This paper describes a novel approach to the estimation of refractive indices of surfaces using polarisation information. We use a moments estimation method for computing the polarisation image components from intensity images obtained using multiple polariser angles. This yields estimates of the mean-intensity, polarisation and phase at each pixel location.The surface normals are estimated using the photometric stereo. Using the Fresnel theory at each pixel we estimate the refractive index of the surface from the zenith angle of the surface normal and the measured polarisation. The method has been applied to determine the variations in paintings, human skin refractive indices and also for inspecting fruit surfaces. To test the effectiveness of the method, we coat a variety of objects with a layer of transparent liquid of known refractive index. Experiments on naturally occurring surfaces (e.g. human skin and fruits) and manufactured objects such as a plastic balls and paintings illustrate the effectiveness of this method in estimating refractive indices.