This work both numerically and experimentally investigates the reflectance from the smooth side of a semi-transparent 508-µm-thick window with metallic gratings attached on the other side. The incident light is oblique and linearly polarized, and its wavelength ranges from 2.5 to 25 µm. Three gratings with various levels of profile complexity and periods are investigated. Influences from the intrinsic material absorption, light refraction, total internal reflection, interference, and diffraction on both the specular and hemispherical reflectance are studied. Moreover, the applicability of effective medium theory to our samples is verified.