A unit of light transmission for the protective glasses used in welding. If T is the fraction of visible light transmitted, the shade number is 1 + 7(-log10 T)/3. For example, if 1% of the light is transmitted, the shade number is 4
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}