1. A non-transitory, computer-readable medium, comprising: code for causing a computer to receive a spectral response of an unfinished, multi-layer semiconductor wafer across a spectral range; code for causing the computer to determine a plurality of parameter values of an optical dispersion model of one or more layers of the multi-layer semiconductor wafer based at least in part on the spectral response, wherein the optical dispersion model includes a Cody-Lorentz model having a first derivative of a dielectric function with respect to energy that is continuous at the Urbach transition energy of the Cody-Lorentz model, wherein the rate of attenuation of the Urbach function, E u , is defined as E u = E 1 ∂ E 1 / ∂ E t = E 1 D , wherein D = E 0 4 - E t 4 ( E 0 2 - E t 2 ) 2 + Γ 2 ⁢ E t 2 + E p 2 ( E t - E g ) 2 + E p 2 · E t E t - E g ; and wherein E t , is the demarcation energy, E 0 , is the resonant energy of a Lorentz function, E g , is the band gap energy, E p , is the transition energy, and Γ, is the width of the Lorentz function; and code for causing the computer to store the plurality of parameter values of the optical dispersion model in a memory.