In porosity characterization option refractive index of the layer is determined as a mixture of a material selected from Layer Material database and Air, Water or Air+Water. You can select how the refractive index of the porous material  is calculated:  average weighted of permittivity, Bruggemann formula or average weighted of refractive indices.

This option is typically used for verification of characterization results.

For example, refractive index of very dense IBS TiO₂ film can be considered as  the refractive index of the bulk material (see picture on the right).  If it is assumed that the pores in e-beam evaporated film are filled with water, then the calculated packing of evaporated TiO₂ is equal about 85%. 

Find the details in our publications:

1. T. V. Amotchkina, M.K. Trubetskov, A.V. Tikhonravov, I.B. Angelov, V. Pervak, Reliable optical characterization of e-beam evaporated TiO₂ films deposited at different substrate temperatures, Appl. Opt., Vol. 53, Issue 4, pp. A8-A15 (2014)
2. A. V. Tikhonravov, M. K. Trubetskov, T. V. Amotchkina, A. A. Tikhonravov, D. Ristau, S. Günster, Reliable determination of wavelength dependence of thin film refractive index, Proc. SPIE. 5188, Advanced Characterization Techniques for Optics, Semiconductors, and Nanotechnologies 331 (2003).