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UltravioletPhotography

White balancing in the UV using Culture Hustle Black 3.0 paint


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As some of you know, I'm a big fan of Culture Hustles black paint as it shows good absorption in the UV, and I've used it for a number of my builds.

 

They recently released a white paint, and I got to wondering whether it could be mixed with black to create white balance targets for UV. As it turned out, it wasn't great for that as its reflectance in the UV was very different to the visible. However I also tried using the Black 3.0 paint by itself, and the Black 3.0 paint mixed with magnesium oxide powder to make a grey target. These were compared against a Spectralon 10% diffuse reflectance standard. I've written it all up here;

 

https://jmcscientificconsulting.com/uv-white-balancing-photos-with-culture-hustle-black-3-0-paint/

 

End result, the Black 3.0 paint and the Black 3.0 paint mixed with magnesium oxide worked very well and gave almost identical results to the Spectralon target for white balancing in the UV. Could be useful if you're looking for a darker target than the usual PTFE ones.

 

Link to the latest version of the Black 3.0 paint: https://culturehustle.com/products/black-3-0-the-worlds-blackest-black-acrylic-paint-150ml

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Thanks Jonathan

Have I understood the setting of WB balance correctly...?

As I understand it, any neutral object, as in having no colour, is suitable for taking a WB..?

That is black, grey or white ?

Cheers

Col

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Hi Col, essentially yes, as long as it is not under of over exposed. So you can used a white target such as PTFE, but the exposure time needs to be short so you haven't just blown out all the channels. Likewise you can use a black target, but then the exposure time needs to be longer so you're not just getting noise in all the channels. I normally take a separate white balance shot with the same lighting so can adjust the exposure accordingly, but some like to include the target in with their subject at the same time, which is where it can get tricky with trying to use PTFE alone as it is so reflective.

 

What you're looking for is something which has similar (or ideally the same) reflectance across the range of wavelengths you are imaging. For instance, the grey tiles in normal photo targets are no good for UV as they vary in reflectance as a function of wavelength below 400nm.

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Thanks Jonathan,

OK, so it is 'reflectance' that is the problem, we need it to be matte ?

I have used a 'ruffed up' teflon block before in the past, is that now not good enough today.

Why isn't this, Black 3.0 paint, good enough.....is it not matte enough ?

Col

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Ruffing up teflon will be fine, but it might still be too high reflectance to include with the subject at the same exposure. So you may need to do a separate image to get it exposed correctly. With the Black 3.0, it may be that it is too dark to include with your subject. As with the white one though, you could do a second image at a longer exposure and use that to get the white balance settings.

 

Ideally you want matt finish, so you're not just getting specular reflection.

 

Hope that helps.

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Andy Perrin
This seems quite useful. Did you try adding different amounts of magnesium oxide to make something like a Color Checker?
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Andy, I had hoped to do that, but the black paint does a very good job of darkening the mixture. I added as much magnesium oxide as I could without the whole mixture setting like concrete, and it was still very dark grey (about 10% reflectance). Perhaps the paint could be let down with a bit of water, and more magnesium oxide added, but I haven't tried that. I think if I were to be heading towards that route, I'd go back to my mixtures of magnesium oxide, plaster of Paris and carbon black.
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Andy, the Black 3.0 paint, they also make a White 2.0 paint, would these not be easier to mic together for different greys ?
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Col, no I tried their white paint. Mixing the white and grey paints may work for grey targets for visible usage, but it doesn't for the UV. The white paint absorbs more UV than visible, and its reflectance spectrum varies across the UV, making it unsuitable for use as a white balance target.
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Get work Jonathan.

I know it not accurate, but artistically I like the 50:50 image best.

Many may have guessed that. I normally like more warm, yellow images. I have been pushing UVB there for a while. Don't like the raw green.

I have some 2.0 black that I have been using for a while. Glad to see not out dated as may be needed for under coat or for really shinny things. I need to paint some of my tubes. There are parts still quit shinny. Black 2.0 worked very well on my filled down aluminum c-mount rings that I mounted my elements into.

 

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