Dear Nasrine Boughezala, this depends on the form of a sample. For thin film, in my opinion, the best way is to calculate absorption coefficient from transmittance (ideally, from both transmittance and the reflectance) and then use the Tauc plot.

Tomáš Halenkovič

Thank you very much for your interest

I made iron oxide in powder form, and since it is opaque only the absorption was calculated. My goal is to calculate Eg , so I ask about the appropriate method based on the absorption values.

Dear Nina,

From powder samples like pigments one has to record a diffuse reflection spectrum to determine the absorption spectrum in a qualitative way. In the attached ppts, you will find the details.

All the best for your research,

Thomas