Dear Forumers, As a non-English speaker, I am confused about absorbance and absorptance. I found some definitions on the Internet, but I am not sure if understand them well. Oxford dictionary says the following (Oxford Dictionary of Physics, 4th ED. ISBN0-19-860759-8) for abroptance: "Symbol &alpha. The ratio of the radiant or luminous flux absorbed by a body to the flux falling on it. Formerly called abroptivity, the absorptance of a black body is by definition 1." alpha = Ia/Io, where Ia is the absorbed light, and Io is the incident light. I found what is absorbance in the Wiki, but, telling the truth, I did not understand, even I am a bit confused about transmittance as well. (Transmittance should be the transmitted light/incident light, but then, what is absorbance? From the def. equation, it seems that absorbance is only the negative logarythm of transmittance.) Many thanks, if you can help, or suggest literature! Kind regards, TtM.