In this work, a theoretical transmittance model has been used to analyze the optical characteristics, namely, the refractive index, the absorption loss, and the thickness of Zn0.99-xCu0.01CoxO (x = 0.01-0.05, with an increment of 0.01) thin films based on their optical transmittance data. All the thin film samples are low loss and double-facet-coated substrate systems. Our model assumed the Cauchy's dispersion equation of three-term for refractive index and Lorentzian absorption profile for the extinction coefficient due to the low loss of the samples. The equations of refractive index and extinction coefficient, and the total transmittance for the thin films were derived and fitted to the experimental data in a least-squares method. The thicknesses of these sample films were found in the range of 338-355 nm. The effects of increasing cobalt concentration on the extinction coefficient have been observed. (C) 2019 Elsevier B.V. All rights reserved.