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UltravioletPhotography

enricosavazzi

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enricosavazzi

Some time ago, I started wondering about whether it was practical to have a set of "standard", cheap and easily obtainable materials that reflect in the commonly seen UV false colors. This set of materials would be very useful as an easier-to-use alternative to bandpass filter strips like the "Sparticle" (these filters are expensive and made in small series, so future availability is questionable) and to specific flowers (available only in particular regions and seasons).

 

This is only a starting attempts, and I am the first to object to my initial choice of test materials: plastics are very often mixed with additives, UV blockers, plasticizers and a host of other chemicals, usually undocumented. Many of the common "plastics" have specific names, but can actually be a mixture of different chemical species, once more, usually undocumented. Nylon is an example.

 

Nonetheless, I/we have to start somewhere. In this test I left out PTFE, since we already know it is a good reflector of UV with a relatively flat spectral distribution, so not useful as a false-color target. Here is a set of five plastic sheets in VIS, all except the bottom one chosen because they are white/whitish and, according to the seller, natural (i.e. not colored):

 

post-60-0-61768900-1563892256.jpg

 

From top to bottom:

  • "Nylon"
  • Unplasticized polyvinyl chloride (uPVC)
  • Polypropylene (PP)
  • High-density polyethylene (HDPE)
  • Light-grey colored plastic box, most likely polypropylene (PVC) with added colorants, plasticizers and possibly UV absorbers

and the same in UV with Sony A7 II, Coastalopt 60 mm Apo, Baader U, Bowens 1500 Pro studio flash with non-coated Bowens tube, custom WB set with this equipment in sunlight.

 

post-60-0-73526300-1563892657.jpg

 

They all are fairly "white" in UVA, with the exception of uPVC and the (likely) PVC box, which have the same blue false color. Not much to show as a result, except that PVC and its uPVC variant seem to be a reasonable candidate for a false-blue target (the exact blue or violetish-blue shade depends in large part also on the CWB used). This could be a start.

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I would guess that the uPVC has a load of optical brighteners in it, hence the strong reflection at the top end of the UVA band (and the blue colour). Be interesting to see what it looks like under fluorescence too - should be very bright.
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enricosavazzi

And just because I have them handy, VIS-transparent (non-tinted) polycarbonate goggles, which I use as UV protection. Obviously not useful as a false-color target, unless we want to include also UV-black.

 

post-60-0-54059600-1563894051.jpg

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enricosavazzi

I would guess that the uPVC has a load of optical brighteners in it, hence the strong reflection at the top end of the UVA band (and the blue colour). Be interesting to see what it looks like under fluorescence too - should be very bright.

Yes, the uPVC sheet fluoresces strongly blue, but the PVC box (bottom row) does not fluoresce at all. Since both materials have much the same false color in UVA regardless of VIS color and VIS fluorescence, this is encouraging for use as a false-blue target.

 

None of the other materials display a significant 365nm-excited fluorescence.

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Yes, the uPVC sheet fluoresces strongly blue, but the PVC box (bottom row) does not fluoresce at all. Since both materials have much the same false color in UVA regardless of VIS color and VIS fluorescence, this is encouraging for use as a false-blue target.

 

None of the other materials display a significant 365nm-excited fluorescence.

 

So both reflect strongly at the long end of the UVA region, but with different fluorescence. Interesting, that suggest different optical brighteners or fillers in the different PVC samples. They do tend to have al manner of things put in them, such as UV absorbers and antioxidants to prevent yellowing when out in the sun. Years ago I worked on a project where the manufacturer of a PVC window were seeing yellow and pink colours in the PVC which was out in the sun, and they wanted to know what was causing it. Turned out there was a lead compound in there which broke down under UV and formed colloidal lead. The size of the colloid determined the colour.

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enricosavazzi

[...] The size of the colloid determined the colour.

This is how quantum dots work, too. A collection of quantum dots of 3-4 right sizes would make an excellent set of color targets for UV false-colors, without any need to look for specific materials (quantum dot color depends on their size alone), but I don't even know whether UV quantum dots are available, or have any technological use yet. Let alone that we know practically nothing so far about quantum dot toxicity and the like.

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Andy Perrin

We don't know the long-term stability of quantum dots, either, which probably depends on both their material and environment.

--

 

I've had this thought before (and probably it is not original to me either), Enrico, and I believe I discussed it once with Cadmium, who tested a massive number of powders back here:

https://www.ultravio...dpost__p__25119

 

The problem is that there are no good false yellow powders, much less anything more shortwave.

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enricosavazzi

[...]

The problem is that there are no good false yellow powders, much less anything more shortwave.

True, but this does not mean such a thing does not exist. For example, false yellow through a Baader U filter is easily obtained by placing one of the several U340-based filters on top of a UV-white reflector.

 

post-60-0-08542500-1563906409.jpg

 

Do we know what makes the U340 transmit with a peak around 350 nm? This would be a good starting point.

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Andy Perrin
True, but this does not mean such a thing does not exist.

Yep. But sometimes pigments of certain colors can be extremely hard to find, even in the visible spectrum. You probably heard this one already, but just in case (and for anyone else who has not), there is the story of blue pigments.

"There are only very few blue pigments known to mankind now. For example, most of the blue you see in nature, like a blue sky, has got no pigment. It's just a scattering of the light. You think about the bluebird or peacock or morpho butterflies, they don't have an atom of pigment in their beings or in their feathers. You can't make blue easily."

On what it was like to discover a new color

"It was something. I also didn't think so much about it before I discovered this blue. I always thought all the colors had been discovered because we see all the colors in nature. For 200 years, many companies, many laboratories have been trying to create a new blue which is nontoxic. Cobalt really is quite durable, but it's got cobalt, which is carcinogenic. So it was really unbelievable."

 

Anyway, my guess is that if a great false yellow pigment exists, it's going to be a semiconductor with the appropriate band gap. Looking through a list of bandgaps of various crystals might be a way to go.

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Other than flowers, this old box of pastels is one of the few things I ever photographed in UV that shows some light yellow.

Green might be even harder to find.

How long dose the yellow of a Rudbeckia last after it is dead, dried up, and ground up into a powder? :)

post-87-0-12687000-1563920884.jpg

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enricosavazzi

Other than flowers, this old box of pastels is one of the few things I ever photographed in UV that shows some light yellow.

[...]

Are those wax pastels, or chalk-based?

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enricosavazzi

I think the iron oxides for example are promising as selective reflectors at low UVA wavelengths. See for example the following reflectivity charts. Of course it would be ideal to have materials that display a narrow reflectivity peak somewhere in the UV, but this seems to be very unusual. The next best thing is a reflectivity curve that points sharply upwards at shorter UV wavelengths, instead of sharply downwards as typical of many (most?) materials. Iron oxides are generally stable and easy to get. Solid hematite is a common mineral, and polished pieces are easy to find on eBay and in "crystal healing" novelty shops. However, these diagrams seem to refer to powders rather than solid/crystal forms, so what we need for instance is hematite powder (red colored in VIS according to the second diagram).

 

post-60-0-00435400-1563954197.jpg

 

post-60-0-16377700-1563954220.jpg

 

This paper mentions the diagrams are Creative Commons licensed, so it should be fine to post them here. Source: https://www.research...r_Visible_Light

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enricosavazzi

There is also an interesting diagram of the reflectivity of aluminum alloys (which alloys it does not say) at:

https://blog.oceanoptics.com/reflection-aluminum-alloys-automotive-wheels

 

The first spectral diagram on the page shows high reflectivity peaks at roughly 320 and 365 nm. Ocean Optics should know what they talk about, so I tend to believe that these are genuine reflectivity diagrams, not biased by illumination with a mercury lamp (which happens to emit major lines near these two wavelengths). It remains to be seen which particular aluminum alloy is commonly usable in car wheels. Pure aluminum does not display such reflectivity peaks.

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Are those wax pastels, or chalk-based?

 

Enrico, I don't know. The seem like chalk, but I don't know anything about pastels.

I would guess that these are basically just pigments mixed with whatever kind of binder that might make them less crumbly.

They are old, like 1930 or older.

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Cadmium,

Is the peca 916 a mirror reflecting the image in front? Not sure why its looking like a brick wall, or is it a special x-ray filter.

Also you shouldn't put C4 in the middle bad things happen with that stuff.

 

But how did you set this up. What is purplish, is that the whit board. What is white? How is this lite?

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Andy Perrin
It is just a bunch of filter glass on a UV-white background, dabateman. Just like with stained glass (which this basically is), you can see partial reflections.
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The 916 has some reflection from the ceiling . I see some reflection in W4 also.

LP probably stands for linear polarizer.

 

The background is PTFE.

The purple/violet is the upper portion of UV, given those filters don't reach down far enough to be clear.

The yellow is the middle of UV-A, centered about 350/360nm, containing less upper UV (violet range).

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Ok I think I understand. You grabed a bunch of filters. Organised them on a PTFE sheet, which sits on top of white cardboard box outside in direct sunlight and took a picture.

So next would be to grab some left over glass pieces from filters you have made. Say 1cm x 1cm, glue them to a PTFE blacking about 2mm thick.

Place in order of, UG1, bg12, w4 and #12 and your set right?

A rainbow of glass on backing might be better though. Green, then yellow, then blue and then black. Like the order we see going down in energy.

Green is 300 to 355nm ish.

Yellow is 360 to 375nm ish

Blue is 380 to 390ish

 

Red comes back for me at 280nm, but its really really hard to image. And most likely seeing 290nm point of my 280bp10 filter.

300bp10 filter is very very green.

313bp25 is very green.

335bp10 is green

 

 

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enricosavazzi

By the way, I -kind of- found a reflective target that provides a little reflected UV-greenish yellow, but the problem is that, as a whole, it is much less reflective than other targets.

Left: "UV-greenish yellow" target

Right: UV-blue (or UV-violet, depending on CWB) target

 

Straight out of camera

post-60-0-15348300-1564680639.jpg

 

Enhanced color saturation in post-processing

post-60-0-92665200-1564680733.jpg

 

To make this target, I made an isopropylene extract of turmeric powder, wetted a kitchen towel (which is neutral UV-white) with the extract and let it dry. There is a literature about reflectance spectrometry of turmeric, which is used to detect adulterations, especially by the (rather frequent, and common in India) addition of large amounts of metanil yellow, which is carcinogenic among a dozen other toxic effects. Curcumin displays a broad reflectivity peak in the lower UVA and higher UVB wavelengths, which is the thing to look for. The turmeric-extract target is violently yellow-orange in VIS.

 

I don't know whether metanil yellow has a similar or better spectral behavior, but if any of you has this colorant in powder or solution form (it is used as a cytoplasmic stain in microscopy) it might be worth testing it.

 

And the histograms of the saturation-enhanced picture, which show the strongly enhanced UV-blue of the rightmost target (bottom) and the slightly enhanced UV-green/yellow of the curcumin extract (top).

 

post-60-0-72721800-1564681486.jpg

 

post-60-0-07710300-1564681506.jpg

 

One thing to attempt is a lower concentration of the extract, to try and see whether it provides a brighter UV-green. However, I am now packing my things for the next move, so I will not be able to continue this line of investigation for a few months.

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