A method of measuring a physical characteristic of a patterned substrate comprises determining a wavelength where a first reflectance from a patterned substrate equals a second reflectance from the patterned substrate. The first and second reflectances are generated from substrate regions having different pattern densities. A physical characteristic value that is associated with the determined wavelength is identified. The value identification may be done by looking up the determined wavelength in a database, for example by referring to a graph.