In optics and spectroscopy,transmittance is the fraction of incident light (or other electromagnetic radiation) at a specified wavelength that passes through a sample. A related term is absorptance, or absorption factor, which is the fraction of radiation absorbed by a sample at a specified wavelength. Occasionally one also hears the terms visible transmittance (VT) and visible absorptance (VA), which are the respective fractions for the spectrum of light visible radiation.