Abstract: A method and apparatus for optically determining a physical parameter a pattern made up of features and disposed on an underlayer. The physical parameter can be, e.g., feature width, relative feature size, feature thickness, index of refraction n or extinction coefficient k and is determined from a response light generated upon illumination of the pattern and underlayer. The response light, e.g. light transmitted by or reflected from the pattern and from the underlayer is analyzed and broken down into response light fractions including an underlayer light fraction and a feature light fraction as well as any other background light fractions making up the response light. The physical parameter of the pattern is determined from the response light fractions and reference physical parameters) of the underlayer, which are either known a priori or determined.