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UltravioletPhotography

Anybody have some alabaster? WB in fluor photos?


Andrea B.

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I thought 'white' was equal amounts of RGB, but then we add near UV & near IR, so how does that change things.....?

 

Those who enjoy UV and/or IR photography in any combination typically take "white" to be the reflection from a material such as Spectralon which diffusely reflects all light equally between about 300 - 1000 nm. (Look up the specs if you want a precise range there. "-) )

 

Using the term "white balance" for white-click processing on a neutral area of a UV photo is, of course, not really an accurate use of the term because there is no "colour temperature" for false colours. But the method works to produce a standardized outcome which is desireable in documentary work.

 

I've always been very careful to stress that white balancing a UV (or an IR) photo is simply for standardization across different gear and processing platforms. It is certainly not mandatory.


 

 

As a white reference for UVIVF, do the reference itself have to have fluorescence or can a controllable very weak white light source work?

 

My personal preference is to have a reference which itself fluoresces white. But if a very weak white light source works, then why not? I have not experimented with that. So if you make such an experiment, we will all enjoy reading about it!


 

 

I'm not sure if the accurate color of the fluorescence can be recreated on a computer screen regardless of the accurate calibration.

 

That statement can be generalized: not every reflected visible colour can be recreated on a computer screen. Monitors have become much more sophisticated in recent years and now have much wider gamuts (colour spaces), but every colour space leaves some colour out. We usually do not notice that some colour is "missing" from the typical JPG viewed on a browser. And not to forget, your camera also has a gamut which cannot produce every visible reflected colour.


 

 

I think it was Mark who mentioned that there was a particular color of red fluorescence he could see in real life but couldn't reproduce with his photos?

 

And I have also seen this: some olive oil which I was fluorescing under a Nichia 365 nm torch produced a very intense green which I could not match during processing. It was really pretty.


 

 

The other problem is white is different to different people.

 

Subjectively, perhaps. But -- when using a reflective standard, we don't have to rely on the subjectivity of our eyes & brain. What we are aiming for with reflective white standards of any type is to eventually produce on the monitor an RGB "colour" for which the values R=G=B. Like, (0,0,0), (128,128,128), etc.


 

 

But this is the issue: are we aiming to duplicate the colors that we see with our eyes when the objects fluoresce? Or are we trying to match a daylight white balance? These aren't the same. Or do we find a material that fluoresces in a way that looks "white" to humans and use that to white balance with?

 

My take on this is the same one that UV Innovations has produced. We want to standardize the outcome of a visible fluorescence photo by using a standard-which-fluoresces-white to set an in-camera white balance under the conditions in which we are shooting the fluorescence. Or, alternately, we standardize the outcome of a visible fluorescence photo by photographing the standard to use for a white-click area when converting the raw file.

 

My experience with using this standard-which-fluoresces-white is that it it does indeed produce a reasonable match of the fluorescence I'm seeing while shooting. But do remember that the UV Innovations standard-which-fluoresces-white is tuned to a particular set of filters and a particular area of UV light (365-370 nm, I think?) It will not be entirely accurate under other filters or UV ranges.


 

 

Can I make one more small speculative statement? I don't think our vision goes entirely to scotopic when sitting in the dark closet while photographing visible fluorescence. Because we are looking intently at the visible light from the fluorescing subject. What do you think?


 

 

Alternative opinions, corrections or even agreements(!) are welcomed for the preceding attempt at explaining the extension of the concept of white balance to UV or UVIVF to produce standardized outcomes.

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All very interesting & especially when all put together like this....

Equal amounts of RGB give white light, but so does over saturation of one colour on the receiver / sensor.

Because UVIVF is a low intensity light, anyway, I wonder if there is a 'colour' under the white at a lower threshold / intensity.

I am not properly set up yet, but I will like to find out...?

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Col, I'm not sure what you mean about over saturation of one colour producing "white"?

 

Because UVIVF is a low intensity light

Depends on what you are fluorescing. If you fluoresce those text highlighter markers, they can be quite bright. And fluoresced optically brightened paper is a very bright blue. Flowers, of course, not so much brightness.


 

 

Oh. That reminds me. Fluorescent text highlighters are good test subjects for getting your UVIVF "white balance" correct.

I just bought a pack of highlighters which has 10 colours. I've been meaning to test whether they are all fluorescent.

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Equal amounts of RGB give white light, but so does over saturation of one colour on the receiver / sensor.

...Yes, kind of. When you look at a bright monochromatic light source, or when you do that with a camera, you see white (everything "blows out") because the light is so intense that it actually oversaturates ALL channels (in a camera) or ALL cones (in your eyes). For example, in a camera sensor, as you know, there is the CFA. Those filters are not perfect, and let in a small quantity of unwanted wavelengths (for example, the blue channel filter will be slightly transparent to red and green light). If those filters were perfect, you would just see a strong, saturated color. Using red light as an example, your camera will probably see red at first, then orange and yellow (the green channel is the first one to be affected) and then white.
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Stefano, if you use a monochromatic light to illuminate your scene, you will always get a BW picture, when you do a white balance (assuming you do not have fluorescence)
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Stefano, if you use a monochromatic light to illuminate your scene, you will always get a BW picture, when you do a white balance (assuming you do not have fluorescence)

Yes, I agree. You wouldn't have color information. I used that example to explain the concept Col talked about. I was thinking to say that any light which isn't white can become white if the sensor is saturated, but we are having a hard time defining "white" in the first place, so it would have been misleading.
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Actually you have a color information, when your sensor is white balanced to „normal“ white light.

Yes, you have information about what color it is, but once you WB that image, it becomes B&W (as you said), and you will not be able to distinguish any hues.
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enricosavazzi

I can easily start to see my cyan LED as white at a high-enough brightness. I'm not talking about staring at the LED, but about illuminating white objects in a ~30 cm wide area, concentrating there all the (maybe) 3 W of radiation emitted by this 8-9 W LED. I really see it as white at some point. The same worked with a green LED, it looked like a mint white after minutes staring at the output concentrated in a ~15-20 cm wide circle.

 

Regarding vision in the dark, rods peak in the blue-cyan region. I once played with a little blue LED (the epoxy type, but smaller than the 3 mm ones), and I saw blue near it. In the distance, its light appeared white, colorless. I could still see shadows, I could see my fingers, but the color blue disappeared.

From my darkroom work decades ago, I remember working several times with orthochromatic film under a red safety light. Within minutes my eyes adapted and the red looked white to my eyes. A red plastic tray of developer solution on the table looked just light gray, and a green tray a darker gray. Our vision system is a poor judge of color unless there is something else of another color (or a white standard) simultaneously in the field of vision to compare colors with, and performs even worse when the illumination source is not full-spectrum.

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From my darkroom work decades ago, I remember working several times with orthochromatic film under a red safety light. Within minutes my eyes adapted and the red looked white to my eyes. A red plastic tray of developer solution on the table looked just light gray, and a green tray a darker gray. Our vision system is a poor judge of color unless there is something else of another color (or a white standard) simultaneously in the field of vision to compare colors with, and performs even worse when the illumination source is not full-spectrum.

Also, think about the Bezold-Brücke shift. We can't even perceive the same color at different intensities. For me, at higher intensities, violet becomes bluish, green becomes also bluish (towards cyan), and red becomes orangish. Also, the only pure color we can perceive (in normal conditions) is red. My 660 nm deep red LED is almost too red, it's like looking through red wine, it's that kind of red. It almost appears purplish, like blackberry juice. My ~736 nm far red LED is even "stronger".
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I have some samples of white pigments I use, and there is a sample of gypsum - №4.

(All pigment powders mixed in chicken yolk + vinegar.) On the samples: half on the left - 1 layer of paint, on the right - two layers.

 

This photo with MTE 301 filtered 340.

Complete darkness. UV block - ЖС4 (420) glass on the lens.

 

I do the white balance the way: in NX-D, in the gray area on the cotton watercolor paper (Canson), which is always the same color.

I use this paper for targets, and as background, stable light gray color.

And color of luminescence on this is very close as I see with my eyes. Almost exactly the same.

 

Such a balance for pigments is the best I've found so far.. Of course I would like to improve it yet,

but I don’t know which way.

I do not have a 'special' target, so I selected this paper as a target, from many materials I checked.

 

post-242-0-10169900-1583255611.jpg

 

This paper also works well in reflected UV.

 

post-242-0-24985500-1583256015.jpg

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I use this paper for targets, and as background, stable light gray color.

 

Am I understanding correctly that Canson watercolor paper is useful as both a UV-induced visible fluorescence standard as well as a reflected UV standard??

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I use this paper for targets, and as background, stable light gray color.

 

Am I understanding correctly that Canson watercolor paper is useful as both a UV-induced visible fluorescence standard as well as a reflected UV standard??

 

Andria, I think the idea here was because you wanted a sample of alabaster, and the gypsum test sample #4 is related to that (?).

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I became/am curious about the actual paper shown. It seems to be neutral in the UViVF photo.

 

But, yes, the gypsum is also interesting as related to alabaster. The gypsum doesn't seem to be completely neutral.

 

Added later: Or vice versa. See next.

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Andy Perrin

I became/am curious about the actual paper shown. It seems to be neutral in the UViVF photo.

 

But, yes, the gypsum is also interesting as related to alabaster. The gypsum doesn't seem to be completely neutral.

Doesn't that depend on which thing the white balance was done on? The gypsum could the the neutral one and the paper tinted?

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Yes, of course.

My interest is this:

If we could measure by how much either the paper or the gypsum is "off" as compared to Spectralon, then maybe we could develop a useful white balance adjustment for everyone to use who cannot afford the UV Innovations fluorescence standard. When I was experimenting earlier with the UV Innovations targets, I tried to coorelate the WB results to one of the in-camera settings like Daylight or a K number, but neither could be used. Too far off.

 

The paper would probably be easier to use than gypsum powder or solutions.

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Am I understanding correctly that Canson watercolor paper is useful as both a UV-induced visible fluorescence standard as well as a reflected UV standard??

 

I can not say if this is useful as any standard.. May be.

I'm doing balance this way - for UVIVF, on this 'target' - paper.

It works pretty well for me, but I don’t have a 'professional target', so I can’t compare how well.

 

For reflected UV - PTFE this is the best target, of course. But these paper samples are very close, and can be used as a background: for photo of luminescence, for reflected UV, IR and IR luminescence. The background will be +_ the same everywhere.

 

Test UVIVF:

Left - RAW, right - white balance NX_D on paper No. 4.

If you see how this situation can be improved, please say..

 

post-242-0-99126300-1583428868.jpg

 

The big sheet and the round target - PTFE (thanks to Cadmium).

My paper samples:

 

1 - "For oil and acrylic" paper FABRIANO.

2 - Оrdinary Russian standard for watercolor.

3 - Сanson Arches France for watercolor.

4 - LANA "Aguarelle Grain fin".

5 - Canson (Grain).

6 - Canson Montval.

 

As a background for paints and for the target for balance - I use paper sample № 5 usually. The gypsum sample on the first test was also painted on paper № 5

Other samples of paper are slightly different, you can see on this test a few.

I think this watercolor paper is called - 'cotton cold pressing', but I do not know for sure.

 

 

Vis - reflected UV (S8612+U360):

 

post-242-0-70194300-1583429013.jpg

 

 

Paper № 4: Vis->UV->UVIVF

 

post-242-0-86060900-1583429266.jpg

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P.S. I found a local source for that paper, so I'll give it a try. Why not? :grin:

 

Part of the problem with fluorescence targets is that so far the only existing ones are "tuned" to a particular peak wavelength (around 365 nm).

 

The good thing about reflective targets such as Spectralon and to a slightly lesser extent PTFE is that they are stable (or almost stable) across a very broad range of light. I'm thinking we won't be finding anything similar for UViVis fluor. But that's probably OK as most of us are using 365nm illumination for UViVF.

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Ninjin -- THANK YOU for posting this information. It is all very interesting and useful. Even if the paper is not the "perfect" target for white balance, it appears to be quite useful as a background. I will try to test some paper against Spectralon and post what I get.

If I can measure, in some way, the difference between Spectralon and some paper, this will be useful to all of us I hope.

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Colin, I hope this helps answers those questions.

Here is my test using Canson Watercolor Paper, Spectralon (white 99%, gray 75%, and black 02%), PTFE 1mm thick, and WhiBal gray card.

 

UVIVF (with and without Jupiter Logic Uranium Marble), and Visual.

UVIVF shots were WB on paper. Visual was WB on SRS-99.

post-87-0-96081600-1583470840.jpg

 

Reflected UV. White Balanced on SRS-99.

post-87-0-65646600-1583470848.jpg

 

Canson Warecolor Paper:

post-87-0-53668800-1583470857.jpg

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