Transmittance refers to the degree to which light or another type of energy passes through a material. However, when we consider antonyms for the word "transmittances," we are looking for terms that are the exact opposite of this phenomenon. Therefore, some of the most relevant antonyms for transmittances include opacity, darkness, opaqueness, obscurity, and impenetrability. These words convey the idea of blocking or hindering the flow of energy, thereby preventing it from passing through a particular material. While transmittances are essential for various purposes, including optics, communication, and energy transfer, their antonyms play an equally important role in the prevention or containment of harmful substances, noises, or radiation.