This paper presents a method for calculating the linear optical properties of solids using the all-electron full-potential linearized augmented planewave (LAPW) method. The approach is based on the random-phase approximation (RPA) and considers both interband and intraband contributions to the dielectric tensor. The theoretical framework includes the derivation of the dielectric tensor, symmetry considerations, and the relationship between optical constants. The momentum matrix elements are evaluated in detail for the LAPW basis, and the interband and intraband contributions to the dielectric tensor are discussed. The results are presented for the metals aluminum and gold. For aluminum, the optical spectra are found to be highly sensitive to the Brillouin zone sampling. The interband contribution to the imaginary part of the dielectric function is shown to be significantly affected by the number of k points used in the calculation. The plasma frequency is determined to be 12.6 eV, which is in agreement with experimental data when using a dense k-point mesh. The results are cross-checked using sum rules, which confirm the accuracy of the calculations. For gold, the influence of relativistic effects on the dielectric function is investigated. It is shown that the scalar-relativistic effect is much more important than spin-orbit coupling. The paper also discusses the interpretability of the Kohn-Sham eigenstates in terms of excited states. The paper concludes that the LAPW method provides a reliable way to calculate the optical properties of solids, particularly for metals. The method is extended to include localized basis functions, and the results are validated against experimental data. The study highlights the importance of accurate Brillouin zone sampling and the role of relativistic effects in determining the optical properties of materials. The results demonstrate the sensitivity of optical spectra to the choice of basis set and the importance of using high-quality calculations to accurately predict the behavior of materials under light excitation.This paper presents a method for calculating the linear optical properties of solids using the all-electron full-potential linearized augmented planewave (LAPW) method. The approach is based on the random-phase approximation (RPA) and considers both interband and intraband contributions to the dielectric tensor. The theoretical framework includes the derivation of the dielectric tensor, symmetry considerations, and the relationship between optical constants. The momentum matrix elements are evaluated in detail for the LAPW basis, and the interband and intraband contributions to the dielectric tensor are discussed. The results are presented for the metals aluminum and gold. For aluminum, the optical spectra are found to be highly sensitive to the Brillouin zone sampling. The interband contribution to the imaginary part of the dielectric function is shown to be significantly affected by the number of k points used in the calculation. The plasma frequency is determined to be 12.6 eV, which is in agreement with experimental data when using a dense k-point mesh. The results are cross-checked using sum rules, which confirm the accuracy of the calculations. For gold, the influence of relativistic effects on the dielectric function is investigated. It is shown that the scalar-relativistic effect is much more important than spin-orbit coupling. The paper also discusses the interpretability of the Kohn-Sham eigenstates in terms of excited states. The paper concludes that the LAPW method provides a reliable way to calculate the optical properties of solids, particularly for metals. The method is extended to include localized basis functions, and the results are validated against experimental data. The study highlights the importance of accurate Brillouin zone sampling and the role of relativistic effects in determining the optical properties of materials. The results demonstrate the sensitivity of optical spectra to the choice of basis set and the importance of using high-quality calculations to accurately predict the behavior of materials under light excitation.