(1) is used to transform wavelength into energy as a new horizontal axis. The Tauc equation (eqn. ( )) and Kubelka Munk equation (eqn. ( )) transform the absorbance and the reflectance into a new vertical axis, respectively . The parameters of these equations are calculated as eqn. (4)eqn. (8). The bandgap energy is determined by the tangent of the plot with Ox. It is useful for single-component photocatalysts and heterojunctions .