In optics and spectroscopy, transmittance's the fraction of incident light at a specified wavelength that passes through a sample. Specifically, visible transmittance (VT)'s this fraction for visible light. » mathcal, where alpha's the attenuation coefficient and x's the path length. Note that the term "transmission" refers to the physical process of light passing through a sample, whereas transmittance refers to the mathematical quantity.… (More on Transmittance)