Often, we see in the scientific Raman contributions the unfortunate option of using arbitrary units (a.u.) for the intensity on the Raman diagram ordinate. Arbitrary units mean at least total arbitrariness – that is never the case! The use of (a.u.) should be avoided because that is simply wrong!
Background: Some reviewers or editors decline contributions in which the writer does not use such incorrect terms.
Funny explanation: Each tachometer in a car worldwide shows km/h, miles/hour, and so on. There are more or less significant differences in the absolute values. Nobody came to the idea of using arbitrary units (a.u) here.
Raman is similar to fluorescence in that the detector signal is "raw" - for fluorescence just the counts from the PMT, Raman usually uses a CCD detector. Unlike transmission or absorbance spectra where you can compare the sample to a reference and generate A or %T that is reproducible, the raw detector signal is very system dependent. What laser power? How is the detector set up? etc etc. So the units have no relevance to reproducibility - and many people just use "arbitrary units" rather than the raw detector units.
It is better to use only “Intensity.” Arbitrary units mean, strictly speaking, a chaotically organized ordinate. If I plot the measured intensities in an azimuthal plot, then have the intensities measured with the same instrument under the same conditions, a precise meaning. With another instrument, the intensities are (in the absolute sense) different and show the same picture.
Rainer Thomas
Can you please define the term "chaotically organized ordinate" or better, give a reference to its definition? I have never heard of this term, especially not in any spectroscopic textbook, and apart from this page here Google yields zero results for this term in this exact phrasing.
Just writing "intensity" without any unit or specification of (non-)linearity is disencouraged by most journals I know. It would be interesting to me which editor of what journal actually rejected this.
The term „arbitrary unit, “ in short (a.u.), is often used in the Raman spectroscopic work (see, for example, Spectroscopic Methods in Mineralogy, edited by A. Beran and E. Libowitzky (2004), European Mineralogical Union – Notes in Mineralogy, Vol. 6, 661 p.).
Arbitrary mean in German: willkürlich, beliebig, eigenmächtig, eigenwillig and rarer: frei wählbar, unbegründet.
Such a unit should not be used in science. If I am more hair-splitting, then it would stand on the ordinate, for example, the five between two and three (that would be a chaotically organized ordinate).
However, that is not an exception: the wrong unit “wt%” is generally used in geological and petrological literature, and it is often also used in the mineralogical literature, although most countries have signed the IUPAC 1988 convention.
I'd argue that arbitrary, in the unit context, is more related to the mathematical "undetermined/not assigned" point of view. So, when you can't define the unit rigorously because there are underlying variables/constants that you won't take into account, that's when a.u. makes sense.
In short: F(x) depends on x but you can't/won't define the exact dependence (in term of units). Then you use a.u. in the sense that the unit is arbitrary defined as the output units of your equipment (be it dependent on unknown sources of errors or not) and you won't try to explain how and why.
For each research area there are different discussions on what is acceptable or not as arbitrary. As scientists, we may discuss, change names and employ stricter definitions. It's all up to us (and to our will to argue with peer-reviewers).
Yeah, naming stuff is sometimes done in a sluggish manner and then you don't get rid of the term anymore because it sticks too well.
In musicology, people talk about the "new German School" of romanticism despite the fact that out of the original initiators only one guy (Richard Wagner) was actually German, but the name got stuck and now nobody bothers to rename it to something accurate. So you have that in all sorts of scientific branches.
Coming back to physics: if nowadays somebody would have to create a name for eigenstates with the same energy, "degenerate" probably wouldn't get passed anymore. Probably we also wouldn't call two different thermodynamic potentials "free energy" with two scientist's names by the side (OK, in German we actually don't do that, but I hope you get the point I'm trying to make). So, back in the day when people decided to name "scale for which we don't assign an actual unit but somehow want to state it's linear" as "arbitrary" (or willkürlich) they may not have thought through all semantics, but it's more or less industry standard in spectroscopy and it means the thing I put in quotation marks at the beginning of this sentence and not "somebody counts like King Arthur after opening the holy hand granade of Antiochia while assorting this scale".
You are correct on most points. However, in Raman spectroscopy, it is better to use counts per second or so and not arbitrary units (a.u.). Maybe simple “Intensity” is better than (a.u.).
However, I will not justify this further. In science, there are many inconsistencies used. I will not play Don Quijote and Sancho Panza (possibly not a reasonable comparison). Such discussions are futile.
Professor Hanson above is the most correct. You cannot define the units for intensity precisely for Raman spectroscopy - even on the same system day-to-day given the variations in the experiments (same laser? same power? same focus? same alignment? etc.) This is why being quantitative in Raman is difficult, if not impossible, at best. The axis is best labled "Intensity (a.u.)" to reflect this issue. Note that it may not even be a linear scale when comparing different peaks in the same spectrum due to cross-section variations. Then there are intensity polarization effects due to the spectrometer (are you using a "scambler?) Frankly, in this case, "arbitary units" is the most correct way to label the axis.
Your arguments are all correct. However, the word “arbitrary” means some others. It also means random. And that is not the case. Call the ordinate simple “Intensity” if you want.
You can also use Raman spectroscopy for quantitative determinations. There are many examples in the scientific literature.
Sorry Rainer, but Raman spectra are semi-quantitative at best. In the case of Raman spectroscopy "arbitary" is well understood in the literature (due to the many issues I addressed previously.)
Compared to quantitative measuerments (titrations, gravimetric analysis, or infrared absorption) Raman measurements have too many variables to control and no easy way to calibrate or have an internal standard. Even sampling volume is hard to control due to focusing and slit variations of the spectrometer. Then there is wavelength determination (unlike FTIR with the white light center burst and laser spacing make frequency determination very accurate) which ofer +/- 4 cm-1.
Sorry, I have worked 32 years with different Raman systems and can never follow your very pessimistic opinion regarding Raman spectroscopy. However, I see the discussion lead to nothing at last; therefore, I will end the discussion here.
It is only a remark to your opinion that “arbitrary” is well understood in the literature: well understood, OK. However, it means, at least, it must not also be correct, in analogy to “wt%.” Using wt% is absolutely wrong and is still widespread. The meaning of arbitrary is standing, for example, for choosing anything. I think the producer of Raman spectrometers would not be happy with such a definition.
I have worked with Raman spectroscopy for over 45 years. I have applied it to molecular crystals, polymers, LB and monolayer films and lately to the detection of biomarkers using SERS. I have used double and triple monochrometers, single monochrometers with notch filters, a wide range of continuous and pulsed lasers etc. I have used microRaman systems etc. My view is not pessimistic but realistic. Raman has limitations that need to be recognized - if you don't it can bite you.
Also, I personally know and have worked with a number of producers of Raman systems and I can tell you I have never heard any of them take issue with the use of "Intensity (a.u.)" used on the response axis.
I have recently started using Raman spectroscopy for the preliminary identification of minerals, and I need some guidance for a discussion. Here is my observation: when I increase the laser intensity or make the beam diameter smaller, the counts are higher (intensity is greater) for the same mineral. However, the relative intensity of different peaks at specific wavenumbers is more valuable to me, as they serve as fingerprints for the mineral. What units should I use when representing intensity? In many articles and in the RUFF database, there is no mention of units for the intensity (ordinate)
Dear Sourav Bhattacharjee,
I would use for the ordinate only “Intensity.” The use of the unit “a.u.” is very strange. Arbitrary can be something or all.
For example, Hemley et al. (1986) and (1994) use for their Raman diagrams only “Intensity.”
Best regards,
Rainer