A set of nonlinear equations about the band gap and the indexnof material property with the absorptance, reflectance and transmittance were produced based on Tauc relation. Optimizing fitting the experimental spectroscopy data, such as absorption, reflection and transmission, the band gapand the indexnrelated to the material property could be obtained accurately and reliably. Meanwhile, the experimental data were used as more as possible, and the artificial errors at pre-determining the indexnwere avoided. The lowest correlation coefficient and the largest average relative error between the experimental and the calculated data are 0.9588 and 2.7% in all considered cases respectively. The best results show the band gap obtained from this method is more accurate, easier and faster than traditional extrapolation. Hence, this work would promote the precision and reliability for predicting the band gap and index of materials naturally.
