Measuring with a spectroscopic ellipsometer (SE) in the 1.8–4.5 eV photon energy region we determined the complex dielectric function (&egr; = &egr;1+i&egr;2) of different kinds of amorphous silicon prepared by self‐implantation and thermal relaxation (500 °C, 3 h). These measurements show that the complex dielectric function (and thus the complex refractive index) of implanteda‐Si (i‐a‐Si) differs from that of relaxed (annealed)a‐Si (r‐a‐Si). Moreover, its &egr; differs from the &egr; of evaporateda‐Si (e‐a‐Si) found in the handbooks as &egr; fora‐Si. If we use this &egr; to evaluate SE measurements of ion implanted silicon then the fit is very poor. We deduced the optical band gap of these materials using the Davis–Mott plot based on the relation: (&egr;2E2)1/3∼ (E−Eg). The results are: 0.85 eV (i‐a‐Si), 1.12 eV (e‐a‐Si), 1.30 eV (r‐a‐Si). We attribute the optical change to annihilation of point defects.