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UV MineralsDescribing fluorescence

28th Oct 2017 18:27 UTCAlysson Rowan Expert

One of the problems with describing fluorescence is that mere words fail to convey much more than a vague idea of what the colour you're describing actually looks like.

That brilliant, yellowish green, for example, which is subtly different between mineral A and mineral B at a location - how do you describe the difference. Is one more intensely coloured? Is on more yellowish? Is there a hint of red or orange in there? A touch of blue, perhaps?

We have tools available with which to specify or discover a colour in an image - it is the same as is used to describe the colours on a web page (RGB Colour triplets). Admittedly, it is not ideal - there are colours that cannot be described adequately using three 8-bit (i.e. 256 levels) channels, but it is what is readily available.

But how do we predict the colour that we will be looking for? How can we identify a mineral from its coloured fluorescent emissions?

In answer to THAT problem, I have built a simple tool (in an Excel spreadsheet) that will mix a series of wavelengths of light of different relative intensities, and yield Hexadecimal (HTML) colour codes, Decimal codes and a colour swatch. (as well as producing the intermediate HTML colour codes).

If anyone wants to try this out, the spreadsheet is on my Google Drive:
Calculated fluorescence colour triplets.xls

or on my Academia.edu page:

Alysson Rowan on academia.edu

You will need to turn macros on in order to get the functionality.

At some point I will probably re-write this to accept a text file and to generate a text or HTML file.

30th Nov 2017 16:51 UTCOwen Lewis

Alysson Rowan Wrote:

-------------------------------------------------------

> But how do we predict the colour that we will be

> looking for? How can we identify a mineral from

> its coloured fluorescent emissions?

>

> In answer to THAT problem, I have built a simple

> tool (in an Excel spreadsheet) that will mix a

> series of wavelengths of light of different

> relative intensities, and yield Hexadecimal (HTML)

> colour codes, Decimal codes and a colour swatch.

> (as well as producing the intermediate HTML colour

> codes).

Sorry Alyson but I think you are chasing a moonbeam (or perhaps a rainbow might be more appropriate). UV fluorescence in minerals is frequently caused not by the mineral itself but by some trace impurity trapped in the crystal lattice or else in trace level substitution for one of the essential elements for the minerals make-up.

A good simple example is ruby, aka red corundum, aka red Al₂O₃ . In its pure state, Al₂O³ is colourless and does not fluoresce. However, if close to 1% of the Al atoms are substituted by Cr, such a specimen will have a strong red colour and also UV fluoresce a strong red. Both the colouration and the fluorescence are related to the presence in the host mineral of the right chromophore element and in the right quantity. Drop the Cr substitution level below 1% and , as it reduces, the red fades through ever-weakening shades of pink until, eventually an observing eye can no longer detect any red at all. Go the other way and raise the Cr content above 1% and the attractiveness of the red reduces, until, at around 4% substitution, all red is gone and one is left with an unattractive brown specimen.

Change the chromophore to Fe+Ti in the correct ratio and with the correct ion charge states and you will have an attractive blue corundum stone (var. sapphire) that does not fluoresce at all. Change the chromophore to Fe only and the corundum will be either yellow or green, dependent on the ionic charge of the Fe.

It gets worse. Fe is the most common of a number of elements that, if present in corundum along with Cr, will act to prevent ('quench' is the term used) the UV fluoresence from a neighbouring Cr-substituted corundum molecule in the crystal lattice. Given the wide-spread presence of iron oxides in the Earth's crust, it is a commonplace to find rubies that only fluoresce weakly or even do not fluoresce at all, despite the correct-level presence of Cr that produces the correct red colouration. These have a less attractive red because of the loss of visual pizzazz given by the strong UV fluorescence that can be present in daylight.

In conclusion:

- There is no way that fluoresence can ID correctly all samples of corundum (or anything else).

- Nor is fluorescence a reliable test (stand-alone) even for var. ruby, as the common presence in the lattice of a quenching agent (e.g. Fe) makes a test for UV fluorescence generally unreliable.

- Even where conditions are perfect and a ruby (e.g; from Mogok) fluoresces to look like a hot coal, such a result, stand-alone can't distinguish a Mogok ruby from a Mogok red spine, nor (with a bit more of a stretch) from unquenched Mogok chrysoberyl var. alexandrite.

Before spending time on developing a software driven colour grading system, you might want to:

- Study closely the fifteen causes of colour, of which the phenomenon of fluorescence is one.

- Review carefully the many colour grading systems already out there and in serious use. No one wants just another 'also-ran' widget; they want a better widget. So what will make *your* widget stand out ahead of all the others?

Gemmologists, many of whom make their living from the identification and grading of fluorescent and other coloured minerals, do not ever consider testing for UV LW/SW fluorescence as more than a supportive test that may confirm a specimen ID that has been primarily determined by other and much more reliable testing methods.

1st Dec 2017 15:14 UTCAlysson Rowan Expert

Thanks for that Owen,

I have already studied colour sufficient unto what I need (indeed, until I am truely sick of it).

I know that fluorescence is not of itself much use for mineral identification, but it is useful as a diagnostic for distinguishing between some species at a particular site - as I have found in studying uranium secondaries locally. There are a small number of minerals there that have a fluorescence atypical of the uranyl ion (blue instead of green) - very useful in that locality.

I am not trying to come up with something that is yet another colour grading system or some magic analytical tool - what I am trying to do is to come up with a fairly simple way of visualising a colour (in this case, fluorescence) in a specimen when all I have is "green, slightly brighter than mineral (a) but with more blue" accompanied by the peaks from a spectrometer. I want to be able to get an idea of what that description actually looks like as a colour swatch alongside one for the the other mineral mentioned in the description.

If this means that I also have a way of describing a colour to others by way of an RGB colour triplet, then so much the better.

It is not meant as a diagnostic tool - and there is no way I'd expect a gemmologist to want it (their toolset is way beyond this), it is a simple visualisation tool that can be used in order to aid identification under particular conditions. Something that can be run in order to get colour swatches that reflect the figures for that mineral under those conditions.

------

As far as the Mogok ruby is concerned (I have never seen one in person under UV) - how hot the coal? Are we talking a rich red, fiery red, orange-red, red-orange, grading into yellow?

Give me a colour triplet and I can visualise that hot coal, rather than the random one I may have in mind - it may not be terribly accurate (these phenomena have a nasty habit of being well outside the colour gamut of a standard monitor, being emission sources), but it is better than a vague guess.

And that should tell you exactly what the particular value of the tool is (especially given that it is free, gratis and without cost).

1st Dec 2017 16:13 UTCReiner Mielke Expert

Hello Alysson,

I think what you are doing is very useful for a given locality. Some way of accurately describing the color of fluorescence is badly needed.

2nd Dec 2017 10:39 UTCAlysson Rowan Expert

Thanks, Reiner.

2nd Dec 2017 11:03 UTCJolyon Ralph Founder

I believe the only correct way is to use a digital spectrometer and to convert that reading, as Alysson suggests, into a displayable sRGB web colour on screen.

You'll need a digital spectrometer (>$1000) and a regularly-calibrated high quality monitor on your computer. But there's no other sensible way to do it.

Let's hope that digital spectrometer prices come down. It's basically just a camera sensor in a box with a diffraction grating, no reason they couldn't be built MUCH cheaper.

Jolyon

2nd Dec 2017 11:05 UTCJolyon Ralph Founder

I'm guessing the digital spectrometer could also be used to calibrate your screen.

Perhaps this is an all-in-one solution we should be looking to develop.

2nd Dec 2017 16:41 UTCOwen Lewis

And thank you Alysson for a stimulating discussion.

I suspect (but cannot prove) that the human mind is excited by (and attracted to) fluorescent colours, over and above the excitement experienced by colouration from some non-fluorescing object. This conjecture could explain the otherwise irrational preference of many for gems with a fluorescing chromophore over others that are equally good but do not demonstrate fluorescence.

If you were to succeed in the task you set yourself I think you would have take several important steps in converting the conjecture above into a proof. For you to be able to codify the difference between colour resulting from fluorescence and colour which has some different cause, it seems to me that you must first be able to differentiate, reliably and repeatedly between colour created by fluorescence and that created by any other means.

It seems to me that colour does not exist in any absolute sense. Rather, it is an analogue used by the mind to make relatively crude differentiations between electro-magnetic radiation in one narrow waveband and all the others that fall with the larger (but still small) waveband we call the optical spectrum that is detectable by the eye. We know for sure that that colour perception is not common to all sighted life-forms. We also have to teach our children the difference between (say) blue and yellow.

Humans are not all equally gifted with colour perception, I'm a man. With that certain statement comes the knowledge that almost all the female half of humanity was born with a greater ability to differentiate accurately one shade of colour from another. And not only was I born a man but have with the passage of time become old, assuring that however sharp my colour sense may once have been (yet still inferior to that of almost all females) I am presently left with only some fraction of the colour perception I once had. So forgive me it I prefer to discuss variations in EMR frequency, wavelength, or wave number rather than the colour shades of, say, hot coal.

2nd Dec 2017 16:47 UTCGary Weinstein

Also, please keep in mind that everyone sees colors differently. This is why the experts can't make a colored gemstone color chart like the diamond grading chart. I had a guy tell me he could not tell the Esperite from the Willemite. And many men are color blind to varying degrees.

2nd Dec 2017 17:18 UTCGregg Little 🌟

Just a anecdotal story to follow up what Owen articulated about males and the concept of, and perception of colour.

I taught a geology student who was colour blind and could only see shades of brown. He was exceptionally good at visual colour descriptions?! How he achieved this was by attributing textures to the colours. This just further confirms the bizarre ways the human mind handles the concept of colour.

It would seem that the codification of colour is as problematic as the observer's perception. As Owen notes, we may ultimately be left with the only option of descriptions by EMR frequency, wavelength, or wave number. At the least there is consistency there.

3rd Dec 2017 01:33 UTCAlfredo Petrov Manager

Another factor to consider is that fluorescent colors can look quite different depending on amount and type of visible light contamination - ie. quality of the filter, and whether or not one is in a completely dark room or there is some daylight present. Getting everyone to describe their observations under the same ideal controlled conditions might be a hopeless task. ;((

4th Dec 2017 12:07 UTCAlysson Rowan Expert

Owen - we are fascinated by fluorescence for at least one reason beyond colour in general. In this case, it is because the amount of light being seen is far in excess of the amount of light we can see impinging on it - it is an object that is too bright for the conditions (and it isn't fire, which our brains comprehend implicitly).

Jolyon - a surprisingly good spectometer can be made using a transmission grating and an HD webcam - certainly good enough to see the Fraunhoffer lines in daylight, and to see that the Na-D line is a doublet (without being fully resolved) which is better than the laboratory spectrometers I learned on.

Gregg - codification of colour between media (in this case, actual material and some kind of recording) is a pain because we have the ability to process out specific aspects of a colour (in the same way that we can listen to a particular instrument while listening to an orchestral piece) - thus RGB (video) and CMYK (printed) colours can never appear quite right. Even using the Pantone system has the limitation of the pigments selected.

On a general note regarding colour perception:

We all percieve external stimuli using a neural network which is unique to the individual and we sense those stimuli using sensors that are also unique to the individual. If I map my visual response to another person's brain, they will experience ... stuff. It will be a non-sensical jumble of meaningless signals.

As a part of our development, we learn to apply labels to the referents that we experience - thus the colour stimulus received from reference objectis labelled with some name that we can then apply to a similar stimulus from another object.

We are all born colour blind - only the rod receptors in our eyes function fully. We then develop a red sense, followed shortly by green and then (aged about 2 years), blue. My earliest memories are of my bedroom - which appeared to be painted entirely in earth-tones until my blue sense developed.

Beyond the age of two, a few of us (perhaps as many as 2.5%) go on to develop a 4th (and possibly a 5th) type of cone receptor, giving those few an improved colour sense.

ALL of us can see into the infra-red spectrum, and putting on a pair of IR goggles (it takes a while for the eyes to adapt) is a strange and wonderous experience on a summer's day (black tarmac appears bright, for example.)

Knowing how the eye senses colour allows us to trick our eyes into seeing a continuum of colour by use of three stimuli (red, green and blue - either by addition (video) or subtraction (print)).

With video, the colour gamut is limited in resolution (typically 256 steps of brightness for each colour channel); the hues which may be represented - indeed, we only achieve about 45% of the total range of visible colours using sRGB; and the dynamic range (deep shadow and bright light sources are lost).

Again, we can trick our minds into seeing an approximation of the real scene, but until we have a better way of displaying colour, the best we can achieve is a vague approximation of most of the colours visible to the human eye.

7th Jan 2018 08:25 UTCcharles kraft

I agree with Jolyon on the use of a spectrophotometer, however my cheap ($1,000) one was not sensitive enough. I just now tried something, a color detector plug in for my cell phone. I like the one by MOBIALIA, it gives RGB as well as some fanciful names for the colors. I'll try it with some other tablets, see if there is any consistency. For just one used looking for data for their own purposes and not to exchange, it looks good to me. It might be useful for data exchange if it proves consistent among other phones.

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