The optical constants of a linear chain carbon (LCC) film in the spectral range from 248 to 1100 nm (1.13–5.0 eV with a step of 0.1 eV) were obtained by the Beatty ellipsometric method, and its thickness was determined. The LCC film was deposited on a pre-polished mirror surface of a massive steel sample using the technology of pulsed carbon plasma with ion stimulation. It was found that in the measured range of the spectrum the refractive index of the film varies from 2.35 to 2.73, and the value of k varies from a value close to zero to 1.02. The effective film thickness is determined to be d = 92 ± 2 Å. According to the type of conductivity, the LCC film is a dielectric (Eg= 4.1 eV). A simplified method for determining the band gap of nanosized semiconductor and dielectric films is proposed.