The paper presents an experimental study on the optical conductivity of single-crystal graphene samples on a SiO₂ substrate, covering photon energies between 0.2 and 1.2 eV. For photon energies above 0.5 eV, the graphene exhibited a spectrally flat optical absorbance of (2.3 ± 0.2)%, which is consistent with the predicted universal value of πα or a sheet conductivity of πe²/2h, based on the model of non-interacting massless Dirac Fermions. However, at lower photon energies (0.2 eV), significant spectral variations and sample-to-sample differences were observed, indicating a breakdown of the universal behavior. These deviations are attributed to the effects of finite temperature, doping, and intraband transitions. The study confirms the universality of the optical conductivity at higher photon energies but highlights the limitations imposed by finite temperature and doping at lower energies. The results are supported by theoretical calculations and are discussed in the context of the impact of many-body effects and band structure deviations.The paper presents an experimental study on the optical conductivity of single-crystal graphene samples on a SiO₂ substrate, covering photon energies between 0.2 and 1.2 eV. For photon energies above 0.5 eV, the graphene exhibited a spectrally flat optical absorbance of (2.3 ± 0.2)%, which is consistent with the predicted universal value of πα or a sheet conductivity of πe²/2h, based on the model of non-interacting massless Dirac Fermions. However, at lower photon energies (0.2 eV), significant spectral variations and sample-to-sample differences were observed, indicating a breakdown of the universal behavior. These deviations are attributed to the effects of finite temperature, doping, and intraband transitions. The study confirms the universality of the optical conductivity at higher photon energies but highlights the limitations imposed by finite temperature and doping at lower energies. The results are supported by theoretical calculations and are discussed in the context of the impact of many-body effects and band structure deviations.