The optical properties of $ThF_4$ film are investigated and the results are used immediately in preparation of Germanium output-coupling mirror for $CO_2$ laser (10.6 $\mu$m). The complex spectral refractive index (n-ik) of the film is determined from the spectro-photometric data and computer numerical calculation. The numerical values of n and k are found to be 1.40-1.42 and 0.01-0.12, respectively, in the infra-red region of $\lambda$=4-12 um. The mechanical properties of the film, namely adhesion and hardness are tested out in single film of $ThF_4$ by using a Scotch tape adhesive and a steel needle. The paired film system of ZnS/$ThF_4$/glass is also tested and compared with the ZnS/$MgF_2$/glass film system. The scratch test on the ZnS/$ThF_4$/glass is found to be very stable and superior mechanically over the latter. The Germanium disk has surface reflectance of 3.8% (back surface is anti-reflection coated with $\lambda$/4ZnS) and when ZnS($\lambda$/4)/$ThF_4($\lambda$/4)$ film system is prepared on the surface of the same Ge substrate, the reflectance is increased to 65% which is agreeing within 3% with theoretically calculated value. In the present experiment, $ThF_4$ film is prepared by using both thermal evaporation technique and electron gun evaporation technique, but thermal evaporation process gives better film quality avoiding formation of $ThOF_2$.
