This question has come up a few times, there are some good answers at the following link:

https://www.researchgate.net/post/How_can_I_calculate_film_thickness_and_refractive_index_from_UV-vis_reflectance_and_or_transmittance_data_only_without_interference_fringes

Hope this helps.

Hello

If your film is fixed on a substrate (name of your uploaded file), you can use profilometric or interferometric techniques of thickness measurement. These methods can provide thickness distribution over relatively large areas. Also mechanical thickness meters are available, but they are not usable for very thin films.

Regards

Josef

A better method than UV-vis for measuring the RI and thickness of thin films is ellipsometry.  Confocal microscopy and SEM can also be useful.