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UltravioletPhotography

Anybody have some alabaster? WB in fluor photos?


Andrea B.

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https://www.fluomin.org/

 

One thing led to another today and I fell down the rabbit hole for a long time.......

 

 

 

Soooo, along the way I read that alabaster fluoresces white under 365nm.

How white? It did not say.

 

But it occured to me that placing a hunk of alabaster (or other white fluorescing mineral) in a fluorescence photo might possibly serve as a white balance object. Trying to WB fluorescence photos has been an on-going topic here on UVP for the last few years. The WB standard for fluorescence is quite expensive, so now I'm hoping a chunk of some mineral would be useful and much cheaper.

 

Here is the place to make a list of white fluorescing minerals under 365nm.

I've never heard of most of these.

There may be colour casts to the fluorescence, so there would need to be testing.

http://www.fluomin.org/uk/searchbycoloruk.php

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New bone is said to fluoresce bright white. Save the bones from your chicken dinner????
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As we have said before, there is a degree of arbitrariness to what one calls "white" in an emission spectrum, unless we define it more rigorously. Perhaps one could do it in terms of color temperature (maybe something like 6500K?) as measured by a colorimeter. I wonder what the standard phosphor that you say is expensive does in this regard...
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I looked through a few of the pages at UV Innovations, but found nothing about how they define "white". I didn't have time to look at everything though. :cool:

I suppose one could argue that defining emitted "white" is unnecessary if the goal is standardization rather than color accuracy??

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The other problem is white is different to different people.

Here in North America we add a slight blue dye in our laundry to get whites "white".

In South America they add red dye to get whites "white".

I thought that was funny when I interviewed at a leading company in developing detergents.

Just look in a box of dry laundry detergent and you will see blue flakes.

 

So what is white? 255, 255, 255 or bluish?

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I think that "white" is sunlight in midday. That's what our eyes evolved with.

Hahahahah, oh Stefano, this is an ancient debate on the board. People usually start off thinking like you did, but actual trials using (for example) a white balance set under daytime sunlight lead to results that don't match what people actually see. There are many complicating factors, including that color perception under dim light gets messed up, and that people's vision adapts to the colors of the emitted light, and the fluorescence sometimes seems to be out of gamut for the device...it's a mess!

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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.

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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.....?

 

Ditto.

 

I guess that fluorescence is about the specific spectrum which is monochromatic? Using the fixed daylight white balance could be good enough?

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Ditto.

 

I guess that fluorescence is about the specific spectrum which is monochromatic? Using the fixed daylight white balance could be good enough?

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?

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The other problem is white is different to different people.

Here in North America we add a slight blue dye in our laundry to get whites "white".

In South America they add red dye to get whites "white".

I thought that was funny when I interviewed at a leading company in developing detergents.

Just look in a box of dry laundry detergent and you will see blue flakes.

 

So what is white? 255, 255, 255 or bluish?

 

Way back in the 1960s I was a chemistry student on a tour of a UK detergent manufacturer and one of our group asked about the blue flakes which were common even then in soap powder. We were told these flakes were just the same as the white ones but they had been dyed blue "because customers think they make the washed garments look whiter".

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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?

 

Assuming that the most accurate color of the fluorescence can be observed in the total darkness, there is no "ambient light". The problem would be that the WB of digital cameras are there to compensate for the coloration of the ambient light.

 

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

 

Can a computer monitor recreate the color of the monochrome spectrum in the first place?

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Can a computer monitor recreate the color of the monochrome spectrum in the first place?

I am not sure what is meant by "the monochrome spectrum" here. Most fluorescent photos that we take involve a wide variety of substances in the same picture each with their own fluorescence spectrum. The results are anything but monochrome.

 

That said, I suspect not all the light is within the gamut of computer monitors. 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?

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I am not sure what is meant by "the monochrome spectrum" here. Most fluorescent photos that we take involve a wide variety of substances in the same picture each with their own fluorescence spectrum. The results are anything but monochrome.

 

That said, I suspect not all the light is within the gamut of computer monitors. 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?

 

With the monochrome spectrum, I meant the bright line spectrum or the emission line spectrum, like H alpha.

 

But the UVIVF seems to deal with wider spectrum range than the monochrome spectrum. So, the white balancing might work?

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I am not sure what is meant by "the monochrome spectrum" here. Most fluorescent photos that we take involve a wide variety of substances in the same picture each with their own fluorescence spectrum. The results are anything but monochrome.

 

That said, I suspect not all the light is within the gamut of computer monitors. 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?

 

Yes you have to be careful with the visible cut off filter. I saw the same when I was using a S8612 and 2A to get my visible only fluorescence. Switching to a Sigma SD15 hot mirror fixed that problem, as it has hard straight cuts at 405/410nm to 680nm.

My new much cheaper SvBony UV/IR cut filter also cuts off all UV, but leaks a little into the IR, to give that better red look that we see.

We see some IR, and I that is the red we are cutting out.

 

I like to dial in my Kelvin as a shoot to get the closest WB. That seems to work and Olympus cameras allow for 2000 to 14000. I will always look for that range before getting a newer camera.

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I am confident that it would be rather easy to design a tuneable light source for white balance for UVIVF-exposures.

There are LEDs (RGB) with wavelength-peaks in the same range as the much wider R-, G- and B-peaks of common bayer matrixes.

 

I would suggest two or three controls.

  1. Intensity balance between red and blue LED.
  2. Intensity control of green LED (independent of (1)).
  3. General intensity setting of all three LEDs

If such a LED cluster is placed reasonably far away from a diffusor the light will be mixed evenly.

 

As the device would be used just temporarily it can be battery operated.

 

The device would just be for creating a small patch glowing with a controlled white light, not for illumination of other objects.

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I am not sure what is meant by "the monochrome spectrum" here. Most fluorescent photos that we take involve a wide variety of substances in the same picture each with their own fluorescence spectrum. The results are anything but monochrome.

 

That said, I suspect not all the light is within the gamut of computer monitors. 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?

When I tried to find the approx. RGB coordinates of a 600 nm orange LED and a ~500 nm cyan one, I noticed (especially with the orange) that the color given off by the LED had at least twice the saturation of the orange on my LCD computer monitor. Also, deep red (660 nm) and far red (736 nm) are RED, the reddest red you will ever see. On my phone, 255, 0, 0 looks orangish-red in comparison.
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I am confident that it would be rather easy to design a tuneable light source for white balance for UVIVF-exposures.

There are LEDs (RGB) with wavelength-peaks in the same range as the much wider R-, G- and B-peaks of common bayer matrixes.

 

I would suggest two or three controls.

  1. Intensity balance between red and blue LED.
  2. Intensity control of green LED (independent of (1)).
  3. General intensity setting of all three LEDs

If such a LED cluster is placed reasonably far away from a diffusor the light will be mixed evenly.

 

As the device would be used just temporarily it can be battery operated.

 

The device would just be for creating a small patch glowing with a controlled white light, not for illumination of other objects.

So, like, you could shine it on some PTFE or a color-checker and snap a photo for white balance afterwards?

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So, like, you could shine it on some PTFE or a color-checker and snap a photo for white balance afterwards?

Either that or shine it through some suitable diffusing material. I think that would be more efficient, but either method will work.

You do not need a neutral reflective surface to make it work.

 

As the LEDs have a narrow peak and the bayer dyes different sensitivity bumps have slopes the system will be slightly sensitive to wavelength shifts.

If the LEDs run at reasonably low stable current their peaks will be relatively stable too, as they will not get much heated.

 

I can happily build something like this, but lack a suitable setup to test it in, as I do not have any space available for a UVIVF shoot.

My apartment is filled to the brim. It almost looks like Birna's famous car trunk filled with photo stuff.

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