The results of a comparative study of the effect of irradiation with helium ions on the optical properties of single-crystal molybdenum mirrors with crystallographic orientations <110> and <111> are presented. The irradiation regime corresponds to the conditions in plasma cleaning systems of the entrance mirrors from contamination in optical diagnostics of ITER while helium is using as a working gas. As a result of such irradiation, a change in specular reflection and diffuse scattering of the mirror occurs, which is practically independent of the initial structure of the surface layer of the mirror and the duration of the irradiation process. Authors explain the observed changes in the optical characteristics of the mirror by the formation of nanosized bubbles in the surface skin layer. Rayleigh scattering of incident radiation on these bubles decreases the intensity of specular reflection and increases diffuse scattering. A model of the formation and growth of nanosized bubbles and their effect on optical properties is proposed. The obtained results should be taken into account when analyzing experimental data in optical diagnostics of ITER after removing contaminants using helium, when choosing a working gas for mirror cleaning, and also for the formation of a nanoporous structure in a thin surface layer of metals.