Optical band gap determination in nanostructured semiconductor thin films is to some extent difficult. Procedure of the extrapolating the linear part of the graphs in optical band gap determination in Tauc’s model for nanostructures has shortage because the edges of the tail states complicate the definition of the accurate optical band gap (due to the surface atoms). A new revised method (named as DITM: Derivation of Ineffective Thickness Method) is proposed for the exact determination of the optical band gap in addition to the treatment of optical transitions in nanostructure semiconductors. In Tauc model and ineffective thickness method (ITM), one has to determine the kind of optical transition before the determination of the optical band gap, but in DITM, not any supposition of the treatment of optical transition. DITM method was employed on CdSe thin films semiconductor alloy in order to confirm the validity of this new method.
