A measurement methodology used in analytical chemistry to identify particular molecules (i.e. ethyl alcohol) and then quantify them (concentration) by detecting how much IR light at a particular wavelength is absorbed by the molecule (see pages 39 to 42 in the 2013 Training Aid). Note specifically Lambert-Beer law. Please note that there is a serious flaw in the Training Aid explanation of Lambert-Beer law that I was able to uncover in cross-examination of the author of this section - bandwidth of filters changes over time (see an Analytical Chemistry Text Book).