Normalized absorbance/transmittance definition [closed]








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In the absorption spectroscopy we can calculate transmittance $T$ of a given sample by comparing how the intensity of the incident beam $I\_{0}$ is decreased with the distance (Lambert-Beer Law).


$A = -\ln{\frac{I}{I\_{0}}}=\alpha L$


where $A$ is absorbence, $T = 1 -A$, $\alpha$ is an absorption coefficient and $L$ is the distance.


But I encountered something called Normalized transmittence I don't really understand how can I get it from measured data. Normalized how? It appears in scientific articles but I couldn't find any good source explaining it really.


I read that I should divide my measured $I$ by reference intensity (it can fluctuate so it is measured along the way too). So the normalized absorbance would be $A = -\ln{\frac{I}{I\_{0}^2}}$ or just by definition $A = -\ln{\frac{I}{I\_{0}}} $


Can anyone help me please?


The spectra then look like this:
[![enter image description here](https://i.stack.imgur.com/uDmfv.png)](https://i.stack.imgur.com/uDmfv.png)


[https://www.researchgate.net/figure/Hydrogen-cyanide-H-13-C-14-N-2-3-rotational-vibrational-band-spectrum-obtained-by\_fig4\_242279131]




Recall the more familiar definition of light transmittance


$T = \frac{I}{I\_o}$, many instruments quote percent transmittance as


$\%T = 100\frac{I}{I\_o}$


Normalized transmittance is nothing but the maximum measured transmittance set to unity. This is easy to accomplish. Suppose you have 2000 data points (y-values) in your spectrum collected as a function of wavelength. Locate the *maximum* value of transmittance, say it was 95% or 0.95 in your data set (depending what was collected experimentally). Now divide the entire y-values with 95.65% or 0.9565. With that your maximum, transmittance will be exactly equal to 1 and the rest of the values will be less than 1. This process is called normalization.


It is just for convenience when you are comparing a lot of spectra.


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