Structural Colours

[Steve Axford]

I have recently been posting images of Scarlet Honeyeaters and have had some problems with not over-saturating the reds. It made me think that this is most likely due to a mix of pigment and structural colour for the reds. This would create the problem I am having where the red is much brighter than it should be if it were merely a pigment. Artists (painters) have struggled with this problem for centuries, as no pigment based colour can represent the brightness of a structural colour. The classic problem is how to represent a peacocks tail or the feathers on a pheasant when they glow in the sun. No pigment can show this as the effect is as if the colour is reflecting more than 100% of the incident light. This is obviously not possible with a pigment, but it is possible with a structural colour.

Has anyone else thought about this or had any bright ideas. One way is to avoid sunshine - and it is possibly the only way.

[Steve Axford]

No takers? Pity - here's a link to Wiki http://en.wikipedia.org/wiki/Structural_coloration

[ameerat42]

Steve, I'm a taker. Just had to bone up and marshal (holler for a) some thoughts. (And I've read the article in the link. Very interesting.)


I have recently been posting images of Scarlet Honeyeaters and have had some problems with not over-saturating the reds. It made me think that this is most likely due to a mix of pigment and structural colour for the reds. This would create the problem I am having where the red is much brighter than it should be if it were merely a pigment.

When I first saw the images you refer to I thought it was a processing problem. I was surprised, because your images don't normally (or ever) show this.


Artists (painters) have struggled with this problem for centuries, as no pigment based colour can represent the brightness of a structural colour. The classic problem is how to represent a peacocks tail or the feathers on a pheasant when they glow in the sun.

OK.


No pigment can show this as the effect is as if the colour is reflecting more than 100% of the incident light. This is obviously not possible with a pigment, but it is possible with a structural colour.

I have enhanced the salient text in your quote to make it clear. Now, are you saying that pigment a structuaral colour reflection can reflect MORE light than is incident on the surface,
or, are you still invoking the "as if" condition?


Has anyone else thought about this or had any bright ideas. One way is to avoid sunshine - and it is possibly the only way.

I am now thinking about this.
Am.

[Steve Axford]

"Now, are you saying that pigment a structuaral colour reflection can reflect MORE light than is incident on the surface"
Yes, a structural colour can reflect more than the incident light at that angle. It does this by collecting the light at many angles and only reflecting it at one (or a few angles).

If you take a pigment (like normal paint), it will absorb some colours and reflect what is left. This will always be less than 100% of the incident colour. Take chlorophyll in leaves as an example. The chlorophyll absorbs in the red and blue end of the spectrum, leaving mainly green to be reflected, or transmitted back. That green will be up to, but never more, than 100% of the green that is incident at any particular angle.

A structural colour works by a crystal surface creating a diffraction grating effect (probably best to read the article here as I am no great teacher). This can reflect a particular frequency of light in only one direction. It can do more than this. It can reflect different colours at different angles (irridescence) and it can amplify the light in one direction by collecting it from many. This can be done with individual colours (like the Scarlet Honeyeater) or white or many colours (like a peacock's tail). Butterfly wings are another example. Often a single colour will flash at a specific angle, so as the butterfly moves its wings, so the colour will flash - and that flash will be greater than 100% of the incident light at that angle because at other angles there will be nothing (black).

Another way of making things brighter than 100% is to use fluorescence. This is where UV is converted into visible light and thus the reflection really is "whiter than white", as the old detergent add used to say. This can apply to nature, but is most common in man made fabrics and plastics. Try shining a UV torch around at night. You'll find lots of plastics, golf balls, etc, etc.

Anyway, all the things like this make it very hard to represent some things photographically. I suspect that when we see something like the flash of colour from a butterfly's wing that our brains somehow average the images across time so that we can get an impression of what happened. This is hard to do with a computer screen - perhaps a hologram would work.

[ameerat42]

Of your first sentence above, I have not been able to find the reference to that phenomenon in the article you linked to.
DO you have a reference?
Am.

[Steve Axford]

Try this one http://animals.howstuffworks.com/insects/butterfly-colors.htm
I'm surprised that Wicki doesn't make this clear.

[arthurking83]

While I understand some of the topics you've raised re structural colours, I think the solution to your specific problem(ie. this red bird) is a bit more simple to fix than working on it from such a technical aspect.

Some of the points raised in some the reading available is a bit over my cranial capacity .. so I can't offer solutions on how to minimise the problem for 'ya at that level.

BUT! .. having seen the images of the red honeyeater birds .. I think that similar problems exist in other areas of nature too.
Now I can't remember names of flowers, let alone point them out as examples, but on a few occasions I've photographed closeups of some red flowers, the reflective nature of the flower was so high that it produced the same results your getting with the red honey eater.

The feathers of the red honey eaters have a highly reflective surface and coupled with the fact that they are red, which is always the first colour to over saturate in most exposures, I think the two simplest ways to combat the issue is with either.

1/. use a polariser

2/. choose a less red whitebalance setting.

While I expect that the polariser is not going to be a cheap option for you in such circumstances(I'm assuming the 300/2.8 lens was used) .. it really would be the best option, as it helps to achieve the best data capture for the raw file.

After about 20 shots of this particular shiny red flower, and not getting a good(balanced) exposure on it .. I mounted the CPL I have for my 105VR macro lens and presto!
In one shot, the red of the flower didn't blow out massively(being both red and highly reflective) and of course the shadows didn't dip too far into darkness either.
(FWIW: I think the name of the flower may have been bromeliad .. but this is a wild guess on my part based simply on the fact that I know of this flower's name for some reason).

While you may be reluctant to play around with a WB setting that may not be totally true(to life) .. sometimes it can be the only way to overcome the problem.
My use of the term "less red" doesn't mean to simply bump up the hue(or tint) slider(magenta-green axis) .. nor the warmth slider(magenta-green axis) but a combination of both should help.

Even tho a push to the most extreme green end of the hue slider will make the image a lot more green, the green channel in the histogram is not as affected by each degree of movement as will be the red channel.
The warmth slider will obviously affect both the red and blue channels equally(if not then almost) .. so again, it probably doesn't need all that much adjustment.

I've had to use this processing option myself on numerous occasions to keep the red highlight in check, mainly in sunset images with the sun near the horizon. The images may have lost a small amount of redness to them, but the alternative would have been to allow the red channel to totally blow out

But ...

I reckon that the polariser is the first option to chase up, even tho it's an expensive piece of gear(if my assumption of the lens you want to use is correct) .. and I reckon about $300-ish would probably be an indicative price(note that the Nikon version is about double that .. so look at it from the point of view that the Canon version is actually a bargain!
if you're using a more normal lens(ie. as in a normal sized front thread size) .. the cost wouldn't be a problem.

Looking at the bird images, you can see white highlights on the red feathers .. indicating a highly reflective surface even tho it's coloured red. A polariser should give you at least 2 stops of 'better light' on this area of the subject, while not affecting the darker shades quite as much.

As for the notion of more light being reflected back from a subject than the power of the incident light .. like I said, such topics are way over my head .. but I think of those car paint jobs that change colour as you move past it on either side.
My understanding of this effect is that small metallic flakes are the cause of this effect. As they stand on end, and reflect some colour one way and another the other way .. this is a simple description of structural colour that I think most of us have been exposed too(and for those of us curious enough to ask the obvious question of "HOW?").

My guess as to this type of iridescence in nature is maybe not so much about more light being reflected back than the incident light .. it may be better described as more efficient reflection of such light(ie. 99-100% rather than 75%).

[ameerat42]

I would agree with your last statement, AK. Nothing can reflect more than the incident light - and not even 100%.
That 2nd link, Steve, says "The multiple reflections compound one another and intensify colors." This is not a very precise statement
as it does not say exactly how this happens. I don't know the source of the article.
Am.

[swifty]

Interesting topic. I had not heard of structural colours.
Just thinking aloud but perhaps a filter for balancing out the red channel and using the uniWB type workflow might yield some improved results?

[Steve Axford]

While I understand some of the topics you've raised re structural colours, I think the solution to your specific problem(ie. this red bird) is a bit more simple to fix than working on it from such a technical aspect.

Did I really hear you say that, Arthur?

You are possibly right,but a couple of comments.
1. A polariser will do nothing. The light isn't polarised - as far as I am aware. I do have a drop in polariser for that lens, so I can try some day and who knows, it may do some interesting things,
2. I have got the photos so that they do look quite good, but I always want more (don't we all). Like most good bird photos, these will be best in diffuse light. I know with fungi - never take them in sunlight (well, almost never).
3. I think you would do well to read some more on structural colours, or even get a book on the subject. There is a very good one called "Seven deadly colours" by Andrew Parker. There will also be lots on the internet. I'm sure you can cope with the science as I'm sure you understand diffraction gratings and things like that. It isn't anything to do with bits of metal in paint - it is much more sophisticated than that.

I am amazed that nobody has heard of this thing before now. Colour is an amazing topic and one that I would have thought photographers would be right into. I'll leave you with the task - "Why is it so" that a butterfly's wings flash with colour that is much brighter than what we would expect if 100% of the colour was reflected uniformly? It is certainly true, so how does it happen?

[Steve Axford]

I would agree with your last statement, AK. Nothing can reflect more than the incident light - and not even 100%.
That 2nd link, Steve, says "The multiple reflections compound one another and intensify colors." This is not a very precise statement
as it does not say exactly how this happens. I don't know the source of the article.
Am.

Think about it Am. If you get light in at 100 different angles and it is only reflected on one then you do not have to create something out of nothing. Think of wave interference where the waves cancel out in most directions, but amplify in one. This is all hard physics, no jumbo jumbo.

- - - Updated - - -


Interesting topic. I had not heard of structural colours.
Just thinking aloud but perhaps a filter for balancing out the red channel and using the uniWB type workflow might yield some improved results?
Thanks swiftly. I seem to have it working ok. The structural colour issue may, or may not have anything to do with it, but it is a fascinating subject in its own right.

[ameerat42]

Steve. I am trying (ultimately) to get a handle on the original problem - the oversaturated reddish tones. I know about interferometry (at least) to the extent illustrated in the articles,
and I only wanted to make a point - perhaps pedantically - about the description involved. Apologies for the misunderstanding. However, for getting things straight on this forum,
we need to ditch the notion of greater than 100% reflection of light or colour. From your statement in the post above, I can see that you do too. So I see it as only differing on
some wording. The concept of "structural colour" which you have highlighted here is quite an interesting topic and quite suitable on a forum like this. It and the result you showed in the other thread
are my main interests.
Am.

[Steve Axford]

I think that greater than 100% reflection is a reasonable description! at least for the non-physicist. If you put in 10 units of blue at 100 different angles and you emit 1,000 units at one angle - isn't that greater than 100%?

Fluorescence also produces greater than 100% reflection of specific wavelengths because shorter wavelengths (UV) are converted to visible. Eg you put in 10 of visible and 10 of UV and you emit 20 of visible.

While I suspect that "reflect" isn't the accurate technical term, it is understandable and it is the best I can think of. Do you have a more appropriate word?

P.s. Anyway, it's provocative and it makes people ask questions.

[arthurking83]

Did I really hear you say that, Arthur?

You are possibly right,but a couple of comments.
1. A polariser will do nothing. The light isn't polarised - as far as I am aware. I do have a drop in polariser for that lens, so I can try some day and who knows, it may do some interesting things,
......

As I understand polarized light, broad daylight is always polarized in some way(ie. atmosphere).

Reflected light is polarized to some degree. The more reflective the surface, the more polarized the light it.

Again, not knowing the details of this bird's feathers .... if the red feathers are highly reflective, then the light coming back at you will be polarized(more than the shadow areas, with little to no polarization of what little reflected light is captured.

Your comment at the start fo the thread about tryign again in more diffuse light also has a strong amount of relevance too. Polarization of light and reflected light in overcast conditions is reduced greatly to almost zero in some instances.

In your Try Again thread (http://www.ausphotography.net.au/forum/showthread.php?134454-Try-again) the first and second image are what prompted my idea.
Both of those image look to suffer from what appears to be glare .. both the specular highlight washouts(top and back of bird's head feathers) as well as an oversaturated red glare from along the sides.
In the third image the red section of feathers that are in shadow are not blown out .. whereas just a cm or two further along and in direct sunlight the red feathers look oversaturated(or too contrasty) .. so detail in that red channel is lost.

The significant premise in the use of a polarizing filter is that it will significantly cut back on reflections(polarized light) .. and if the issue is strong reflected light .. well it sort of made sense to me to try it.

[Steve Axford]

I'll try it as it is quite easy to do. There is tons of light so losing 2 stops wont be an issue. Mind you, the bird has to cooperate.

[Steve Axford]

Nah, the polariser didn't seem to do anything except take 2 stops away and make the photography much more difficult. I tried a flash too, but that was even worse. While I now had plenty of light I had to wait for the flash recycle time and that meant I missed all the good bird shots. I seem to have worked it out now. I now use highlight priority and check the histogram very carefully before RAW processing. The colour is kept very neutral.

Just a note on structural colours (which is, after all, the subject of this thread). If the scarlet honeyeater's red was mainly due to a structural colour then polarisers, waiting for a cloudy day, flash etc would have no good effect. The wings of a dove often have structural colours (iridescence in this case) and you can only see them in bright light (preferable sunshine) and at certain angles. Change the conditions and the colour disappears. In the case of a pigeon, the colours aren't too bright for the camera, but changing the conditions won't dampen them as they are either on or off. Other things like fluorescence (eg a Sulphur Crested Cockatoo's crest is more yellow than yellow in the sun as it converts UV into yellow light, ie it fluoresces) can often be dampened as this is a simple effect of light on electrons in atoms. I can remember this from school physics.