Low-cost color chart for UV-A

[Kai]

For various purposes I was looking for a (low-cost) color card for the UV-A range. The question arises because I don't have beautiful flowers all year round.

 

So I looked for materials that appear colored in the UV image. Many plastic samples appear purple, so they prefer to reflect longer-wave UV light. Some textiles appear yellowish or greenish, especially those that appear yellow and orange in the VIS light. Apparently they prefer to reflect shorter-wave UV.

I glued a few samples to a piece of plywood along with a piece of PTFE and a piece of black leather (seems to be black in all wavelengths).

 

Here are a few photos (unfortunately not in the sunlight, as the cloud cover is currently too thick):

 

1. Canon 500D-FS, enlarger lens, Osram Lumilux deLuxe 950 and white balance against the good old Kodak Graycard.

 

 

 

 

2. Canon 500D-FS, enlarger lens, Baader-U with a QB21, Xenon lamp (Eprom-Eraser ZAX Quick-E II) and white balance against sunlight. First image native, second image color-enhanced.

 

 

 

 

 

 

3. Canon 500D-FS, enlarger lens, Baader-U with a QB21, 370 nm LED with UG-1. First picture with white balance against sunlight - everything appears yellow here. Second picture with white balance against PTFE - everything appears nearly achromatic here. Sure, the light source is also pretty "monochromatic". But a slight color differentiation can still be seen on closer inspection... 

 

 

 

 

Note: Nice how you can see the glue residue in the UV. 
 

Question to all
Who has discovered materials that only reflect in deep UV-A (appear as pure green)?
Klaus Schmitt reported on a Zinnia species (https://photographyoftheinvisibleworld.blogspot.com/2015/07/mexican-zinnia-zinnia-haagenea-in-deep.html). But I am interested in a permanent substance (mineral, pigment, polymer, ...).
 

.

[colinbm]

The X-Rite Color Checker is a good standard.

[Kai]

3 minutes ago, colinbm said:

The X-Rite Color Checker is a good standard.

Is there also one for "UV colors"? Which range?
Do you have sample pictures or could you post a link? 

[colinbm]

Andrea & other have posted multiple pictures of the X-Rite Color Checker in UVA.

[Andy Perrin]

Colin, you are missing the intention here. The standard color checker is only good for visible light. He is looking for a UV equivalent that shows all the false colors.
 

Unfortunately this has been attempted many times and there are no good shortwave reflecting substances except for flowers (which may not be due to a pigment at all — it could be structural color).

 

Cadmium found some pastels in different UV false colors, but not much different from what you have above:

https://www.ultravioletphotography.com/content/index.php?/topic/3403-plastic-fantastic/&do=findComment&comment=28814

 

In fact, read that whole topic.

 

[Doug A]

Kai, I find this subject very interesting. Hope a working test card is found.

Thanks,

Doug A

[Andrea B.]

No, the X-Rite Color Checker is not good for a UV standard because it only produces false-blues. You need somethng which produces also false yellows.

 

The X-Rite Color Checker can be seen often in my UV test photos or Birna's along with some other UV test subjects (sunflower, rudbeckia, white PTFE or white Spectralon), but that is only because we are making a series under different filters, visible, UV and/or IR. And we don't want to change the test set-up even though the Color Checker is not useful in UV.

[Stefano]

UV-lavender materials: most materials, including most plastics, a lot of flowers, and probably yellowish-white materials are good candidates;

 

UV-yellow materials: some yellow flowers (dandelions, sunflowers, etc), a few red flowers ([url=“https://www.ultravioletphotography.com/content/index.php?/topic/4575-another-visible-red-uv-yellow-flower”]example[/url]), some yellow fabrics and even some black fabrics (I found one, I probably should photograph it sooner or later). Also, UV-pass filters (such as UG1, UG11, etc.) are UV-yellow.

 

UV-green: bandpass filters and rubies (read this topic: https://www.ultravioletphotography.com/content/index.php?/topic/2155-uv-green-object)

 

The first link didn’t become text. It still works.

 

 

 

 

 

[Andrea B.]

 

 

I recall vaguely some discussion of a car's red rear tail lights and of a manila envelope? Anybody remember whether those were relevant to producing specific UV false colors?

[Stefano]

Andrea, I think you mean the last topic I linked.

[Andrea B.]

Yessir, I do. We posted at exactly the same time, so I didn't see your post.

Thanks for digging that out !

 

Problem is that lit up car tail lights are kind of a difficult thing to use easily for balancing UV false colors.

The synthetic ruby & sapphire would be easier, but I suspect they might a bit expensive even though synthetic?

[Andy Perrin]

I wonder what would happen if you crushed a UV yellow absorption filter?

[Stefano]

I’m guessing a UV-yellow powder but whiter because of the added scattering. That could be interesting.

[ulf]

Maybe a stack with a matted PTFE disk and a UV-yellow absorption filer would be interesting too.

That could be done non destructive.

As the light would pass back and forth though the filter, less thickness is needed.

[Kai]

Thank you all for kind answers :)

@Colin:
Thanks for the clarification;)
I'll test my old colored pencils as soon as possible. But it may be that the binder darkens everything.
I'm sure I will find a piece of geothite in the vastness of my cellar. I'm curious!

@Doug A
I think I've found a good approach. I post the days ...

@Stefano
Red flowers (Anagallis arvensis?) with yellow UV color:
Why not - my orange textile samples also have a yellow shimmer.

Green lights:
I would suggest that it is NIR. My Baader-U also likes to show a slight green shimmer in green plants. If I combine a QB21, the green shimmer is gone.

Now I just did a test with a Philips Tungsten bulb (150 W, red glass). Left side with Baader-U only, right side with Baaer-U plus 2mm QB21:

[Guest]

Has anyone an art store nearby with a sample board of pigment paints using a UV enabled camera and portable flash. 

 

https://goldenpaints.com/products/colors/heavy-body

[colinbm]

That would be like inventing another X-Rite Color-Checker ?

[Kai]

Well, the X-Rite Checker is only for the VIS area.
As far as I understood flowers are currently used here in the forum as the color standard for false color comparisons.
Since these flowers are not available anytime and anywhere, it should be interesting to make/define a "UV color checker" based on common available pigments and materials.

So far I have found materials that appear yellow or blue/purple (white balance against PTFE). However, the yellow materials I found all appear paler than the reference flowers. Apparently these reflect a little more narrowly than the pigments/materials?

 

Has there been any experience with dried flowers?

 

But now I've also found an object that works on the basis of interference colors. I will introduce you today...

[Kai]

@BlazerOne: Great idea!
However, it is important to note which binding agent was used for color cards: none (pastel chalk), gum arabic (watercolor paint), vegetable oils, acrylates, casein, ... 

[Guest]

Kai, I look forward to your reveal. 

 

I have read elsewhere that cadmium red and zinc white are used as calibration for those who photograph historical art work. Such as to identify which pigments were used to make a color.
 

All I want to do is replicate what I see in flowers as a fractional standard as false color for lens testing. I don’t need full spectrum just something relative to my lighting setup. Therefore, if I can have the flower and the item in the same frame and they reflect the same false color that should be good enough for me. 

We all know that minerals fluorescence but do they all have defined UV reflective properties? 

[Guest]

4 minutes ago, Kai said:

@BlazerOne: Great idea!
However, it is important to note which binding agent was used for color cards: none (pastel chalk), gum arabic (watercolor paint), vegetable oils, acrylates, casein, ... 

 

I understand. Many art stores have boards for each type. Keep in mind some raw pigments in powder form are highly toxic. 

[Guest]

24 minutes ago, Kai said:

 

Has there been any experience with dried flowers?

 

I wish I had thought about testing that earlier. They would need to be pressed and stored in between paper or else they shrivel  up. Some type of UV clear glass cover maybe. 
 

Maybe someone like Birna can confirm this. Dried pressed flowers are sold on eBay. That would be an easy inexpensive solution.

[colinbm]

3 minutes ago, Blazer0ne said:

I wish I had thought about testing that earlier. They would need to be pressed and stored in between paper or else they shrivel  up. Some type of UV clear glass

cover maybe. 

Just stored in paper, they can be exposed for photography them placed between paper again afterwards.

[Kai]

@Colin:
Some days ago we mentioned Goethit. Now I found a piece but it looks very very dark in sunlight (WB PTFE, range 330-400 nm, colors strongly enhanced). Unfortunately I cannot see any yellow :(

[Kai]

On 10/18/2021 at 10:16 AM, Blazer0ne said:

Has anyone an art store nearby with a sample board of pigment paints using a UV enabled camera and portable flash. 

 

https://goldenpaints.com/products/colors/heavy-body

I contacted an art paint manufacturer but haven't received a response yet.

 

The test card of my Schmincke Horadam watercolors mainly shows UV-violet colors ... 

Jaune Brillant (2) appears reddish violet.

Naples yellow (3) appears violet.
Indian yellow (6) - which looks orange - shows a hint of yellow. 
Cerulean Blue (23) appears dark blue violet.

Canon EOS 500DFS, diffuse sunlight, WB PTFE, Baader-U plus QB21, 330-400 nm