We performed a study of temporal coherence of a commercial He-Ne laser in conditions of thermic instability through calculations based on interference pattern digital images helping by direct measurements of its output power and by temperature and polarization monitoring. This study is based on the variation of the complex degree of coherence with the optical path difference between two laser beams, obtained with the help of an experimental setup based on a Michelson interferometer with collimated light. We show the perturbative effect of the thermic instability on the modal composition and coherence of radiation. We had characterized the laser determining its modal structure and coherence length.