An automated method to determine the band gap energy (
E
g
) of pure and mixed powder compounds using diffuse reflectance spectroscopy is presented. This method is based on a five-step algorithm that mimics the judgment made by an expert analyst in identifying the linear segments in Tauc plots and subsequent estimation of the
E
g
value. It is demonstrated that the method to estimate
E
g
by intersecting the straight-line fit of the Tauc segment with the photon energy axis is not appropriate for those samples containing more than one optical absorbing phase because systematic underestimation of the
E
g
value results. The automated method accounts for such cases by introducing a base line function. The robustness of the implemented algorithm was tested using three model systems, ZnO-Al
2
O
3
, ZnO-CoO and ZnO-CdO. The estimated
E
g
's using the automated method differ in less than 1% than those obtained by its manual counterpart.
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