Optical-constant models are functions which can simulate the optical constants of a material in a wide spectral range; they depend on few free parameters that must be determined for each specific material. Examples are the Cauchy and Sellmeier models, the Drude model, and the Lorentz oscillator. Other popular models, the Tauc-Lorentz and the Cody-Lorentz models, have been revisited to improve both the mathematical properties of these optical-constant models and their capacity to fit the optical constants of materials in a wider spectral range.
The dielectric function ε ̃(E)=ε1 (E)+iε2(E) for B films. Experimental data by Fernández-Perea et al. is fitted with Cody-Lorentz model after Ferlauto et al., and with the modified Cody-Lorentz model through a complex analytic function. ε ̃(E)=[N(E)]2, with N(E)=n(E)+ik(E).