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UltravioletPhotography

A Calibrated Gray Scale for Forensic Ultraviolet Photography


colinbm

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I have read this paper on making & using grey scale patches for UV photogtaphy

http://www.researchg...let_photography

 

TABLE 1—Materials used to make a gray scale that can be used for

reflected UVA photography. Patch 1 is the lightest patch shown in Fig. 2.

The gray scale reflects similar levels of UVA and visible light, see Fig. 1A.

Patch MgO (%) Plaster (%) Carbon (%)

__1___70______26________4

__2___60______33________7

__3___60______31________9

__4___40______30_______30

__5___20______30_______50

 

I now have the ingredients for the grey scale patches.

I am not sure how these are combined ??

Are these mixing ratios by weight or volume ?

Are these mixed wet & left to set OR are they just mixed dry & compressed into a holder please ?

Cheers

Col

PS sorry the table got squashed by the text program :(

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Colin,

 

You are beating me to the post! I have also been contemplating this DIY UV Gray Scale. It would certainly be more affordable than a set of spectralon reflectance standards - even the uncalibrated ones. Unfortunately the spectroreflectometer one needs to self calibrate the resultant disks might make the acquisition of selected spectralon targets more cost effective in the long run.

 

I have the paper you reference and the citation to the recipe is to an earlier paper:

Dyer, Adrian G. "Reflection of near-ultraviolet radiation from flowers of Australian native plants."
Australian journal of botany
44.4 (1996): 473-488.

"
A set
of 13 grey scales was produced by mixing varying quantities of magnesium oxide, plaster (Selleys Spak
Filler) and carbon black (see Penny 1983). The mixed powders were pressed into 11 mm diameter circular holders whilst wet, allowed to dry completely, and then tested in a Varian DMS 100 spectrophotometer fitted with a Varian diffuse reflectance accessory to measure radiations reflected from the grey scales for wavelengths of 280-800 nm."

 

Which in turn cites an earlier paper:

 

Penny, J. H. J. "Nectar guide colour contrast: a possible relationship with pollination strategy."
New phytologist
95.4 (1983): 707-721.

A pdf of this paper is posted free:

 

 

This paper does not give as detailed recipe as Dyer's 1996 above, but the introduction should make for very good historical background for those here interested in the earliest UV flower photography.

 

- John

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Thanks very much John

It is to be accomplished as soon as I get back home for shopping :unsure:

Cheers

Col

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I have tried to make the lighter of the grey scales....70% MgO, 26% Plaster & 4% carbon, but it is very dark, darker than a normal gray WB target ?

I only used 4% carbon, by volume. If I had done it by weight it would have been much darker :)

Col

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Interesting, Dyer's 2004 J Forensic Sci. figure 1.A shows that 70:26:4 mixture produced a flat 0.5 UV spectral reflectance. I cannot think of any reason it shouldn't be likewise flat in the visible so as to appear darker than typical 18-20% gray WB target.

 

Several variables are not described in the papers, such as the particle size of the MgO or carbon black. I think the particle size of the carbon might be critical given that total particle surface area would vary . The recipe does not give us the source of the MgO and carbon reagent, at minimum one would want a supplier name and specification if there is more than one grade. The recipe also does not give us the amount of water to add to the dry ingredients or mixing procedure.

 

I Goggled "Selleys Spak Filler" it is hardly a reagent grade product and is available as a powder, premixed paste, indoor/outdoor and even cellulose reinforced. Who knows what else might be in it! What to use then, simple Plaster of Paris? Perhaps a dental or medical grade or some other high purity grade plaster might be preferred.

 

If your aim is to fabricate a UV WB gray target then you could simply reduce the proportion of carbon until you produce something comparable in appearance to a standard grey target in the VIS. You could easily then photographically compare your results to those obtained using PTFE or Spectralon WB targets.

 

If you are trying to produce an actual reference scale, it would need to be calibrated by measurements with a calibrated UV spectroreflectometer, as was reported by Dyer et al. Without such measurements there is no way to know if the recipe works, and as you have shown, results may vary.

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I used to be involved with R&D in Rubber and Plastics so got to know a bit about carbon black. The Penny reference was to "soot", I can state categorically that lamp black, the technological name for "soot", went out with button-up boots. Carbon black these days is 'furnace grade', made by burning atomised heavy oil under rigorously controlled conditions to give a product with a vast range of particle sizes and 'structure' which relates to agglomeration of smaller particles. Carbon black used as a pigment in ink is generally the smallest particles and has a habit of getting everywhere when you open the bottle!

 

I would also guess that MgO particle size is very important in this case and that commercial grade will have changed over the years, so it is impossible to know how modern materials compare with those in previous papers. I would think titanium dioxide would be the first thing you went to these days for a white pigment, not MgO.

 

When in doubt "suck it and see" but not literally in this case.

 

Dave

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Yes Dave "suck it & see" can leave a horrible taste in your mouth :)

Although Charcoal & Magnesium powders are good for your digestion.

I have used pharmacy grade charcoal powder in this case, as I had a box of it from my yacht building days, when it was used to mix into the epoxy resin filler to make it black.

I have always been amazed how little charcoal powder was needed to turn a white epoxy mix stark black !

Cheers

Col

 

PS I will stick (no pun intended :P ) to the Spectralon WB target, I was just trying to get the grey scale like Dr Dyer suggested.

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I suspect that grey scales are no longer relevant, the papers above photographed their subjects on black & white film and then printed the results on photographic paper so the grey scale was used to keep the whole process calibrated in view of all the variables that could exist with exposure and development of film and then the same with prints.
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Keep in mind that a gamma curve is applied to linear digital data so that the resulting foto approximates human vision. Therefore, when a set of reflectance standards is photographed, the reflectance values do not translate to linear "brightness" values.

 

I have a calibrated set of diffuse reflectance standards at 99, 75, 50, 25 and 2% which hold through UV, Visible and IR light. (Don't remember the extent of the range off the top of my head just now.) I'm not sure how useful this set of 5 is for general UV photography - such as what we show here on UVP - because we must also apply the camera's gamma curve to our UV photos in order to have a photo which can be meaningfully viewed by humans. "-)

 

I think it might be useful for the very "precise" (hmmm, read "picky" here?) UV photographer - to have a diffuse white and a diffuse very bright grey reflectance standard for setting white balance (either in-camera or in-editor) in generic UVIR work. The "diffuse" quality of a good reflectance standard helps prevent specular blowouts due to light angle although you can still overexpose it, of course, when making a photo for use in an editor/converter.

 

However, many of us have successfully used white Teflon for setting white balance in UV and that certainly works well enough. And Teflon is considerably less expensive than calibrated, diffuse Spectralon. It's also easier to blowout to the point of unrecoverability, so be careful when exposing your Teflon photo for editor work. In strong light I've on occasion had in-camera WB go awry over shiny Teflon, but a tiny shift of position fixes that readily.

 

Also note that Bjørn and/or I have seen that the black square on a CC Passport works pretty well for an initial WB of a UV photo in an editor if you have an editor/converter with a good click-white tool. The black square would no good, however, for in-camera WB.

 

The ultimate cheap trick is to shoot with an in-camera Incandescent WB and white-click on a magenta area of the photo. "-) That's how I originally made and whitebalanced all my D200 UV photos in 2007 & 2008.

 

******

Does Titanium DiOxide retain its white reflectance in UV??

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I just discovered that the reflectance spectrum of TiO2 does a nosedive at around 400 nm so, it wouldn't be any use at all as a UV standard, which may also explain why the color patches on the ColorChecker charts look so peculiar in UV if the visual whiteness was based on TiO2 pigment. Oh, well, yet another of my lead balloons. :)

 

Dave

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Well, Dave, I guess we all just naturally tend to think Visibly - :) -

easily neglecting the idea that reflectance of pigments might not always carry over to UV.

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Thanks very much Dave.

No lead balloons, just more knowledge, another feather in the hat.

Just explains why some white surfaces have better performance in WB.

Cheers

Col

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The spectral behaviour of TiO2 may help explain why white-painted buildings often appear quite UV dark (or medium UV blue).
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TiO2 is a sunscreen active ingredient, as is ZnO. The ABS is however rather flat in the near UV, "broadspectrum" in sun protection parlance, but no where near the high extinction coefficient of chemical sunscreens.

 

I do think the Dyer recipe may have utility if an updated formula can be shown to produce a flat enough spectral reflectance. One could then cast a picture frame sized UV WB target to hang on the wall for studio use. Brushing with a stiff bristle before it is fully set or sanding with a coarse grit should produce a reasonably diffuse reflecting surface. For the price a 12" x 12" Spectralon and one can afford a lot of plaster! I have a box of plaster somewhere, now to source some carbon........

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I you look for some powder, Bariumsulfat (Bariumsulphat) might be the right choice.

Werner

 

Yes, Barium sulfate has crossed my mind. BaSO4 is used to coat integrating spheres but it will not reach as far down into the UV as the gold standard compressed PTFE powder or Spectralon.

However, BaSO4 likely will have acceptably flat reflectance within the spectral range of these cameras. Some issues I recall are fragility and humidity with spheres needing to be re-coated more frequently.

 

There are comercially available spray on BaSO4 premixed coatings one can load into an air brush and coat a surface available from the usual suppliers:

http://www.labsphere...ce-coating.aspx

http://www.edmundopt...ce-coating/1325

http://www.aviantech...reflectance.php

 

I cannot imagine why one could not add in a pinch of carbon black and be able to make a serviceable UV-gray WB target.

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The reason TiO2 is used in paints is its ability to absorb UV, thus protecting the finish from the deleterious ultraviolet. However, it also creates free radicals, which attack the finish anyway. It was also added to skim milk for "color" until recently, when a cheaper compound was found.

 

French-press fumed MgO was a standard at one time. Fumed MgO is simply magnesium burned in a glass container - collecting the microparticles of MgO on the glass.

 

http://uvrdefensetech.com/images/Page_134.jpg

 

http://uvrdefensetech.com/images/Page_135.jpg

 

Some pages from my book.

 

Regards,

Reed

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Thanks very much Reed for this info.

I looked last night for some graphs like these, without any luck.

It is interesting that the current model mosquito traps are using UV lamps shining on to a TiO2 surface that generates CO2 that attracts the lil' suckers to their death.

Your book looks most interesting too. Is there an Australian outlet ?

Cheers

Col

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Yes, POD, the only way to fly. Amazon, B&N, and others buy direct from the printer and it is shipped straight to them within two days. The printer is owned by the world's largest book distributor and will print in the US, England, or Australia, whichever is closest. There are even automated kiosks at air terminals where a machine prints and binds the book while you wait.

 

I heartily recommend my printer, LSI.

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Reed,

I must confess, I did not know you wrote a fishing book, I must have had my blinders on, you also just sold another copy!

My boss just happens to be getting back into fly fishing, we even took photos of some flies! :lol:

I remember trout UV vision from way back in photobiology. You probably already have this but just in case: http://onlinelibrary.wiley.com/doi/10.1111/j.1751-1097.1997.tb08552.x/pdf

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John,

 

Thanks.

In writing about UV vision in trout I noticed that the scientific community was finally beginning to accept that other organisms could see in wavelengths <400nm. I'll start a thread about that as it might be of interest to others.

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  • 3 years later...

A bit of a thread resurrection here. For the work I'm currently doing I've been wanting a set of reflectance standards for UVA which ideally varied between 2 and 20% reflectance. The Spectralon ones are great, but expensive, and I would have to get custom ones made to get enough in the range between 2 and 20% to be useful. I came across Dyers work while reading up on the area, but as has been commented above, details were lacking. I also came across other work by Evelyn Ayre's work which had also used the Magnesium Oxide/plaster of paris/carbon black recipe, but again there were some questions I had from that work which weren't addressed in her paper. As a result I reached out to her for clarification on a few points, which she was very helpful with.

 

Now the fun part. I wanted a real 'proof of concept' experiment. Could I make something using Magnesium Oxide, plaster of Paris and Carbon black, that would behave as expected across UV and visible? Would it make something which was relatively optically neutral and offer similar or ideally the same reflectance across a wide range? As it was a real proof of concept I spent the princely sum of £20 buying some MgO (animal food supplement), plaster of Paris and carbon black (pigment for paint), and a coffee grinder to mix them in all from eBay. To see if it would even work I 'eyeballed' a mixture of the three ingredients to make something which looked 'dark grey' to me, and mixed it in the coffee grinder before adding water to make a suspension and casting into a small jar lid (about 1.5cm across). This is what it looked like once dry. Note the air bubbles which came to the top and popped while it dried, which was a bit of learning for me.

 

post-148-0-09534500-1497602723.jpg

 

Once this was set after about 24 hours I ran it on a Perkin Elmer Lambda 650S UV-Vis spectrometer (150mm integrating sphere) to measure surface reflection between 250nm and 800nm (1s collection time per nm). The graph is shown below, both the full range and the 300nm to 400nm portion.

 

post-148-0-28758000-1497602807.jpgpost-148-0-22048600-1497602820.jpg

 

Overall, it looks reasonably flat in terms of reflectance, only varying between 6 and 6.3% from 400 down to 300nm. I will be making a range of these with different ratios of carbon black to magnesium oxide to see whether they will be useful as reflectance samples. I have an idea about getting rid of the air bubbles during mixing buy using vibration. Yes, I haven't used the the most 'reagent grade' materials here, and no I wouldn't be comfortable making these without being able to test them on a UV Vis, but overall it looks to have potential as a route.

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