Abstract Dielectric function of disorder in single-crystalline silicon (c-Si) implanted by He with energy of 40 keV and fluences from 1 × 10 16 to 1 × 10 17 cm −2 were determined around the E 1 and E 2 critical points (CPs) by spectroscopic ellipsometry. The implanted material was modeled by an effective medium composition of c-Si and damaged Si. The dielectric function of damaged Si was calculated using the model dielectric function of Adachi to fit the E 1 and E 2 CP parameters of the MDF. The penetration depth of light in the photon energy range of 3–5 eV is less than 100 nm, which allows a simple layer structure of (surface oxide)/(surface amorphous layer)/(c-Si + damaged Si as a substrate). The oscillator energies and strengths decrease, while the broadening parameters increase with increasing fluence. Rutherford backscattering spectrometry was used for cross-checking of the surface disorder.