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Bismuth tri-iodide (BiI 3) is an intermediate band gap semiconductor with potential for room temperature gamma-ray detection applications. Remarkably, very different band gap characteristics and values of BiI 3 have been reported in literature, which may be attributed to its complicated layered structure with strongly bound BiI 6 octahedra held together by weak van der Waals interactions. Here, to resolve this discrepancy, the band gap of BiI 3 was characterized through optical and computational methods and differences among previously reported values are discussed. Unpolarized transmittance and reflectance spectra in the visible to near ultraviolet (UV-Vis) range at room temperature yielded an indirect band gap of 1.67±0.09 eV, while spectroscopic ellipsometry detected a direct band gap at 1.96±0.05 eV and higher energy critical point features. The discrepancy between the UV-Vis and ellipsometry results …