UV colors through thin-film interference - investigation of peacock feathers and a kind of fixed oil spot on a wet road

[Kai]

When looking for UV-colored materials, I also examined bird feathers.
Here is the not very interesting (because only black and violet) result with a peacock feather:
 

But through this feather I got the idea to look at thin clear coat films that were fixed on black cardboard and appear beautifully colorful in the visible, in the UV-A.
These films do not show the original colors of the rainbow, since the color differentiation is not a consequence of the wavelength dependence of the refractive index (as with the prism or roughly with the rainbow). Rather, the interference on thin layers in these films is the reason for the colors. This results in mixed colors. Some of them are not part of the solar spectrum (e.g. purple, brown or beige/gold). On the other hand, pure red is missing. When red interferes constructively, a little violet or blue always interferes at the same time.

 

Here are some pictures. Taken under diffuse sunlight.

Canon EOS 500 D-FS camera, 50 mm enlarger lens. The UV-A images were taken using a combination of Baader-U plus 2mm QB21 to ensure that green is real UV green and certainly not NIR :)

 

 

The patterns are similar, of course the colors are not.

It should be noted that the films are generally thicker in the middle than at the edge. This is due to the manufacturing process. A drop of clear nail coat drips onto a surface of water and spreads to a more or less round spot. The solvent evaporates and the thin film is created. This is fixed onto cardboard. Since the cardboard is black, only the reflected (and interfered) colors can be seen afterwards.

The observed color is dependent on the viewing angle (corresponding to the different path length).
It is noteworthy that the UV colors of the 2nd order appear significantly purer than the UV colors of the first order (outermost colors). This is understandable if you look at the expected wavelength distributions. In the figure, the corresponding curves are calculated for layer thicknesses that would lead to a reflection maximum at 355 nm. 

 

 

 

 

In the following figure, the VIS part is compared with the classic thin-film interference colors (and the division into orders) and the UV-A part with the spectral colors (enhanced). In the UV part you can see that the purple in the VIS (red plus violet) corresponds to an almost black in UV (UV green plus UV violet). 

 

 

 

 

And at least here are a few comparisons of fixed thin films, in the VIS and in the UV-A (approx. 330-400 nm). In addition to the native UV images, color-enhanced images can also be seen:

 

 

 

 

 

 

 

 

[Stefano]

Pretty patterns. Now I wonder how they look like in IR.

[Kai]

Thank you, Stefano.
 

IR pattern: Depending on the wavelength range - i.e. different depending on the camera-filter combination. The distance between the orders should become larger.

You could also take a picture in sunlight and without a filter (UVA to NIR).

 

With monochrome lighting (laser pointer) you can only see dark and light areas.
In this way I once took three pictures in violet, green and red. If the images were combined, the result is the familiar image corresponding to the exposure to white light.
Illuminating with a laser pointer was not that easy. To avoid speckles, the laser and the diffusion lens had to be moved during the long time exposure. The pictures therefore unfortunately show some artifacts, the composite looks correspondingly bad.

But the similarity to the picture that was taken in white light is unmistakable, I hope ;) 

 

 

[Andrea B.]

Wow, Kai, that is interesting stuff.

Have you tried this with anything else beside the clear nail polish?

I'm thinking about the swirly colors seen when motor oil spreads in rain puddles. Could you float a drop of oil on the surface of water in a glass and produce a thin interference film? Granted, that  would not be as easy to photograph as it would never get dry and fixed.

 

I was wondering what white balance you used in the camera to produce the violet and green? And I'm also wondering what the raw non-white-balanced colors were? Do you have a converter which will give you that result?

[colinbm]

Very clever work Kai
I particularly like the split & butted images showing the two spectrums adjacent to one another.

Are you aware that the Peacock feathers are not real colours but are 'Structural Colours'.

[Andy Perrin]

Colin - he must be aware, because that was his inspiration for this experiment.

[colinbm]

In the Peacock feather in the enlarged crop it can be see that the dark black is very fine pitched & as the colours change to blue, cyan, orange & yellow the pitch is courser.

Kai, you mention that brown is not in the rainbow, actually it is dark orange.

 

[Kai]

On 10/18/2021 at 11:46 PM, Andrea B. said:

Wow, Kai, that is interesting stuff.

Have you tried this with anything else beside the clear nail polish?

I'm thinking about the swirly colors seen when motor oil spreads in rain puddles. Could you float a drop of oil on the surface of water in a glass and produce a thin interference film? Granted, that  would not be as easy to photograph as it would never get dry and fixed.

 

I was wondering what white balance you used in the camera to produce the violet and green? And I'm also wondering what the raw non-white-balanced colors were? Do you have a converter which will give you that result?

Thank you for your kind comment, Andrea!
I used two white balance references for comparison: PTFE and anodized aluminum.
Here are the original raw files.
My Canon camera is more green-violet than yellow-blue. Maybe the camera manufacturers use different dyes in their Bayer masks after all?
You can have a look at the raw files in your raw converter ... 

Movements in interference layers:

Yes, watching this movement is one of the most beautiful natural spectacles I've ever seen. What I like best is observing horizontal soap films under the microscope. The variation in shape with these pure colors is incredible. This can also look nice in the photo.

Here is a link to my 500px gallery with a selection of such photos. There are also macro shots of vertical, planar soap films (more streaky structures):
Soap Films, Oil Droplets and Bubbles curated by Kai / 500px

 

 

 

[colinbm]

Are you aware that Anodised Aluminiun is an oxide layer with coloured or clear waxes to seal the oxide & they may affect the UV reflection ?

[Kai]

16 hours ago, colinbm said:

In the Peacock feather in the enlarged crop it can be see that the dark black is very fine pitched & as the colours change to blue, cyan, orange & yellow the pitch is courser.

Kai, you mention that brown is not in the rainbow, actually it is dark orange.

 

Hello Colin, thanks for your hints.
Yes, orange changes to brown if the saturation and brightness are low. This is really surprising to see. 
But when I look at a spectrum through a grating or prism, the orange is always so bright that it looks orange. It is different with the interference colors. No matter how brightly I illuminate the films, the brown areas do not appear orange to me, but always beige or brown...

[Kai]

1 hour ago, colinbm said:

Are you aware that Anodised Aluminiun is an oxide layer with coloured or clear waxes to seal the oxide & they may affect the UV reflection ?

Yes. The aluminium plate was my first WB reference. Now I use PTFE as usual in this community.
But the differences are not that big, as far as I can see... 

[Andrea B.]

Kai, it is entirely possible that different Bayer dyes are used by different camera brands. I have indeed found some very slight differences in the white balance outcome from my converted Lumix S1R. I see more dark blue-green from the S1R along with the usual false blue/yellow. Of the various Canon images we've seen on UVP so far, they seem to be mostly the usual false blue/yellow also. That is not to say that different results won't ever be obtained.

 

My take on UV white balance is that we make it primarily to rid the file of the overwhelming red/magenta/orange/violet mess which obscures details in a UV photo. We also perform UV white balance in order to standardize the outcome of the botanical photos. Thus there is no "right" or "wrong" for UV false colors.

 

Thank you for uploading the raw files for me to look at in Raw Digger!!! I'll be back shortly with the results.

[Andrea B.]

Here is the Raw Composite of the PTFE file. No surprises here. We see the usual oranges and purples for which the red component is always the strongest.

 

Raw Composite from Raw Digger

No white balance applied.

 

 

 

And here is Raw Digger's white balance rendering of the PTFE file. I sampled the colors. A typical yellow is (115,115,87) A typical blue is (98,97,115).

 

That is to say, after white balance moderately bright, greyed-yellows and greyed-blues are obtained. A very slight shift of the white balance tool (or of the color wheel) with some additional saturation/contrast would push these colors to the greens & violets you have obtained.

 

For newcomers to this unusual topic of white balancing UV false colors, please remember that there are a *lot* of factors which can affect a white balance result --- camera, lens, filter of course, converter, white balance tool and others.

For the record, the output from Raw Digger has no "extra" saturation or contrast so these outputs don't represent any kind of final appearance.

 

We have had some discussions on UVP elsewhere in which color perception was discussed. In particular any yellow which has lower saturation can be perceived as a kind of olive green. Similarly greyed blues can appear lavender or dull violet. Stefano probably remembers the name of this effect! I've forgotten it.

 

RGB Render from Raw Digger

White balance based on camera characteristics.

Yeah, I know that looks green, but it isn't -- it's yellow.

[Andrea B.]

Here is the Aluminum set. Basically the same as the PTFE set.

 

Raw Composite from Raw Digger

No white balance applied.

 

 

 

RGB Render from Raw Digger

White balance based on camera characteristics.

 

[JCDowdy]

I must say, that second row tryptic is downright artistic!

[Kai]

2 hours ago, JCDowdy said:

I must say, that second row tryptic is downright artistic!

Thank you, JC :)

If you like them, you should do your own experiments!

It's as simple as cheap, and (at least in the beginning) really impressive. It's best to experiment with two or three nail polishes (clear coats) and different types of black cardboard.

The lighting must of course be large and diffuse ...

 

Concrete instructions can be found on the internet under "permanent rainbow", even if that is not entirely correct;)