We have investigated the optical properties of thin films of topological insulators Bi2Te3, Bi2Se3 and their alloys Bi2(Te1−xSex)3 on BaF2 substrates by a combination of infrared ellipsometry and reflectivity in the energy range from 0.06 to 6.5 eV. For the onset of interband absorption in Bi2Se3, after the correction for the Burstein-Moss effect, we find the value of direct bandgap of 215±10 meV at 10 K. Our data supports the picture that Bi2Se3 has a direct band gap located at the Γ point in the Brillouin zone and that the valence band reaches up to the Dirac point and has the shape of a downward oriented paraboloid, i.e. without a camel-back structure. In Bi2Te3, the onset of strong direct interband absorption at 10 K is at a similar energy of about 200 meV, with a weaker additional feature at about 170 meV. Our data support the recent GW band structure calculations suggesting that the direct interband transition does not occur at the Γ point but near the Z-F line of the Brillouin zone. In the Bi2(Te1−xSex)3 alloy, the energy of the onset of direct interband transitions exhibits a maximum near x = 0.3 (i.e. the composition of Bi2Te2Se), suggesting that the crossover of the direct interband transitions between the two points in the Brillouin zone occurs close to this composition.