A system determines a value, such as a thickness, surface roughness, material concentration, and/or critical dimension, of a layer on a wafer based on normalized signals and reflected total intensities. A light source directs a beam at a surface of the wafer. A sensor receives the reflected beam and provides at least a pair of polarization channels. The signals from the polarization channels are received by a controller, which normalizes a difference between a pair of the signals to generate the normalized result. The value of the wafer is determined through analyzing the signal with a modeling of the system.