Why absorption/transmission spectroscopic data is normalized?

A typical data processing step for acquired absorption/transmission data is normalizing, i.e. stretching the curve such that it is bounded between 0 and 100%.


However, the absorption/transmission curve changes with the density and thickness of the sample. Moreover, absorption is a non linear phenomena (for example [Beer–Lambert law](https://en.wikipedia.org/wiki/Beer%E2%80%93Lambert_law)), so I would expect that the **relative** height of the various peak would not remain constant across several samples of the same material.


Why, therefore, is it justified to normalize acquired spectroscopic data? Why not present data with original abs./trans. values, while providing details about the sample itself (for example thickness)?


In addition, how can spectroscopy be quantitative if the relative height of the peaks changes with illumination/thickness/density/etc.?




There is confusion here about absorption at one wavelength and the absorption spectrum. At *each* wavelength the Beer-Lamber law applies $$I\_{trans\_{\lambda}}=I\_{0\_{\lambda}}\exp(-\epsilon\_{\lambda} c l)$$ for a compound with extinction coefficient $\epsilon$ at wavelength $\lambda$ and concentration *c* and path length *l*. 


When an absorption spectrum is produced it is normal to show it as optical density vs wavelength, i.e. $OD\_{\lambda}=\log(I/I\_0)\_{\lambda}=-\epsilon\_{\lambda} c l $ and in this case the *ratio* of peaks for the same compound is constant no matter what the concentration. (Naturally assuming that it does not absorb so much that the instrument fails).


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You normalize as the emmision strength of machines varies and is not controlable and the precision of spectroscopy is not such that you can justify such high resolution. Franky absolute data would not be very informative. Further You have a good observation that thickness can affect results. This is explained by the fact that absorbance is not constant across all wavelengths.


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