# Normalized absorbance/transmittance definition [closed]

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 $$A = -\ln{\frac{I}{I_{0}}}$$

The spectra then look like this:

[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.

• Upvoted, but it is not clear why the OP first posted at physics SE, accepted an answer and then came here. Modern double beam-in-time instruments are rather good, so double beam-in-space spectrometers are not necessarily preferred for source noise performance.
– Ed V
Dec 24 '21 at 14:32
• Oh thanks so much, it works.
– user76255
Dec 24 '21 at 14:55
• @EdV, I think the OP was confused how to normalize the experimental data. I did not know it was posted twice. Dec 24 '21 at 15:04
• Yes, that is what I figured. But the HCN IR spectrum was apparently used to calibrate a laser system, based on what I find when searching. In the old days, a pulsed laser might have been used to pump a tunable dye laser and then pulse-to-pulse fluctuations in the pump laser’s output, typically a few percent, had to be normalized out. Missing and runt pulses were especially annoying and experimental data had to be corrected for those. I used to use a window comparator to ensure collection of data from non-defective laser pulses.
– Ed V
Dec 24 '21 at 15:34