Optical properties of birefringent porous-silicon layers are studied within the density functional theory. Starting from a (110)-oriented supercell of 32 silicon atoms, columns of atoms in directions  and  are removed and the dangling bonds are saturated with hydrogen atoms. The results show an in-plane anisotropy in the dielectric function and in the refractive index (n). The difference Δn defined as n
 is compared with experimental data and a good agreement is observed. Also, the possibility in determining the morphology of pores by using polarized lights is analyzed.