I don't get this. Everywhere I look, the units of transmittance of light are dimensionless. I get that it's a ratio of incident to transmitted radiation, t(f) = I(f)/T(f), and these two have the same units. However, light intensity is always properly defined as the power per metre squared of radiation in a given spectral range, f + df, then the intensity is ∫I(f)df. If you want an intensity at a single frequency, you'll never find it because you will never detect a photon at EXACTLY f. This is the same as probability distributions. The pdf of a distribution is not a probability itself, it must be integrated over probability space. In the same way, the spectral intensity is not an intensity until it is also integrated. So that leaves me thinking, what on earth is the transmittance? In my reasoning we should still define it in terms of a spectral range, and say the transmittance between f and df is t(f)df. Then the units are still inverse frequency because t is the ratio (dimensionless) and df has dimensions of frequency. Anyone follow? Would love a yes/no answer!