When you work with spectra, recorded with a detector or a Fourier Transfer Spectrometer, I hear that quantitative intensities is hard thing to measure and that you deal with relative intensities or relations. A few questions about this: Why is it hard to measure quantitative intensities and why deal with relative? Don't you lose information? Can you do concentration and temperature measurements even if you 'normalize' the spectrum with for example the highest intensity in the spectra interval? Can you compare simulated spectra (that you calculate) and measured spectra, for example by normalizing the spectra as mention above? Are they comparable then? What if you don't know the unit of the measured spectra, can you still extract this information? Does there exist any procedure of how to compare spectra's (measured and simulated?) Reading report about FTS I have encountered the term, instrumental line shape (ILS). What is this? Is this the same as the term, a spectrometers response function? If a spectra has already been processed by using a measured BB (blackbody) and calculated BB to obtain the systems response function and then use this to obtain the measured *true* spectra, does the ILS need to be accounted for again?