Recent advances in nanofabrication technology have facilitated the development of arrays of nanostructures in the classical or quantum confinement regime, e.g., single-walled carbon nanotube (SWCNT) arrays with long-range order across macroscopic dimensions. So far, an accurate generalized method of modeling radiative properties of these systems has yet to be realized. In this work, a multiscale computational approach combining first-principles methods based on density functional theory (DFT) and classical electrodynamics simulations based on the finite element method (FEM) is described and applied to the calculations of optical properties of macroscopic SWCNT arrays. The first-principles approach includes the use of the GW approximation and Bethe–Salpeter methods to account for excited electron states, and the accuracy of these approximations is assessed through evaluation of the absorption spectra of individual SWCNTs. The fundamental mechanisms for the unique characteristics of extremely low reflectance and high absorptance in the near-IR are delineated. Furthermore, opportunities to tune the optical properties of the macroscopic array are explored.