<p>In this study, the crystal structure, electrical structure, and optical properties of single-doped Ca₂InNbO₆ double perovskites with different concentrations of rare earth ions (Dy, Er, Pr, and Yb) were investigated via the first-principles plane‒wave pseudopotential method, which is based on density functional theory. The results show that Ca₂InNbO₆ exhibits an indirect optical transition with a bandgap width of 3.471 eV. The valence band is affected mainly by the O-<i>2p</i> orbital, and the conduction band is affected mainly by the Nb-<i>4 d</i> orbital. Doping with rare earth ions reduces the bandgap width of Ca₂InNbO₆, accompanied by an increase in the energy level density. In addition, owing to the contribution of the rare earth ion <i>4f</i> orbital, many impurity levels approach the Fermi level, which reduces the energy required for the valence electron transition to the conduction band and improves the conductivity of the material. An investigation of the optical characteristics revealed that the undoped system has a low absorption capacity in the photon energy range of 0–3 eV. The Yb-doped system results in a redshift of the Ca₂InNbO₆ material, widens the edge of the absorption band, and a new absorption peak appears in the low-energy region. The results show that Dy single-doping has a maximum peak value in the ultraviolet range, which significantly improves the absorption capacity of Ca₂InNbO₆ and can be used to prepare efficient luminous materials.</p>