Editing Absorbance (section)

==Shade number==
Some filters, notably [[welding]] glass, are rated by shade number (SN), which is 7/3 times the absorbance plus one:<ref>{{cite web |url=http://www.unc.edu/~rowlett/units/dictS.html |author=Russ Rowlett |title=How Many? A Dictionary of Units of Measurement |publisher=Unc.edu |date=2004-09-01 |access-date=2010-09-20 |archive-date=1998-12-03 |archive-url=https://web.archive.org/web/19981203072555/http://www.unc.edu/~rowlett/units/dictS.html |url-status=dead }}</ref>

<math display="block">\begin{align}
\mathrm{SN} &= \frac{7}{3} A + 1 \\
            &= \frac{7}{3}(-\log_{10} T) + 1\,.
\end{align}</math>

For example, if the filter has 0.1% transmittance (0.001 transmittance, which is 3 absorbance units), its shade number would be 8.