Tungsten is an important material for energy harvesting applications due to its high chemical and thermal stability; however, the difficulty in obtaining single-crystal films leads to a large variation in the optical properties. The present work focuses on the radiative and optical properties of thin tungsten films at wavelengths from 1 to 20 μm (wavenumbers from 10,000 to 500 cm−1), considering microstructural variations. Four films of a nominal thickness of 70 nm were deposited on silicon substrates using DC magnetron sputtering, and the effect of pre- and post-deposition treatments was investigated. Several analytical instruments were used to characterize the crystalline phases and microstructures, including X-ray diffraction, Rutherford backscattering, X-ray photoelectron spectroscopy, and scanning electron microscopy. The transmittance and reflectance of the film–substrate composites were measured at room temperature using a Fourier-transform infrared spectrometer. The dielectric function of each sample was obtained by fitting the measured radiative properties using the Drude–Lorentz dispersion model. The difference in the radiative properties between samples was analyzed and related to the crystalline phases and density.