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Polarisation question - sun vs flash
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rjlittlefield
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Joined: 01 Aug 2006
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Location: Richland, Washington State, USA

PostPosted: Sun Oct 10, 2010 5:32 pm    Post subject: Reply with quote

DQE wrote:
I'm trying to think of specific unique cases of interest, and jewel wasps come to mind, although they may have highly dimpled surfaces. I can also visualize small, very shiny, metallic-appearing wasps but can't recall their names at the moment.

In other words, how much do the widely varying surfaces of **nominally smooth-surfaced** bugs affect the use of on-camera polarizers to reduce and manage excessive surface reflections of flash-exposed photos? Along the same line of thought, are there unique bug's surfaces that involve thin-film interference effects or real metal surfaces, thereby reducing the benefits of on-camera polarization?

As far as I know, all of the "metallic" finishes found on insects are due to many-layer interference effects. See HERE in Google Books for section 25.2 Physical Colors, starting on page 657 of "The insects: structure and function" by R. F. Chapman. An interesting snippet begins on page 659:
Quote:
The brightness of the reflected color increases with the number of reflecting surfaces. In Morpho species, there are only eight to 12 horizontal mullions, and similar numbers of layers are present in the cuticles of buprestid beetles. Only a small percentage of light, less than 40%, is reflected. By contrast, the metallic gold or silver cuticles of pupae of danaid butterflies have over 200 reflecting layers and almost 80% of the incident light over a broad band of wavelengths is reflected. Amongst scarab beetles, reflectivity is enhanced by large amounts of uric acid in the reflecting layers.

Your particular choice of the jewel wasp makes a good test subject, because I still have available the specimen pictured HERE. I tested it just now with polarizing filters, and discovered that its brightly colored cuticle has a very interesting mix of characteristics. When illuminated by non-polarized glancing light near Brewster's angle, the reflections become highly polarized as one would expect for an organic material. However, when illuminated from the front, the cuticle acts more like blue metal. If the front illumination is non-polarized , the shiny blue cuticle adds no obvious polarization of its own, and when illuminated by polarized light, it preserves that polarization. This means that using crossed polarizers to kill specular reflections is a complete disaster. When arranged to eliminate the unpleasant reflections of my tiny fiber optic bundle, crossed polarizers made the whole wasp go black!

--Rik
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DQE



Joined: 08 Jul 2008
Posts: 1653
Location: near Portland, Maine, USA

PostPosted: Sun Oct 10, 2010 7:20 pm    Post subject: Reply with quote

rjlittlefield wrote:
DQE wrote:
I'm trying to think of specific unique cases of interest, and jewel wasps come to mind, although they may have highly dimpled surfaces. I can also visualize small, very shiny, metallic-appearing wasps but can't recall their names at the moment.

In other words, how much do the widely varying surfaces of **nominally smooth-surfaced** bugs affect the use of on-camera polarizers to reduce and manage excessive surface reflections of flash-exposed photos? Along the same line of thought, are there unique bug's surfaces that involve thin-film interference effects or real metal surfaces, thereby reducing the benefits of on-camera polarization?

As far as I know, all of the "metallic" finishes found on insects are due to many-layer interference effects. See HERE in Google Books for section 25.2 Physical Colors, starting on page 657 of "The insects: structure and function" by R. F. Chapman. An interesting snippet begins on page 659:
Quote:
The brightness of the reflected color increases with the number of reflecting surfaces. In Morpho species, there are only eight to 12 horizontal mullions, and similar numbers of layers are present in the cuticles of buprestid beetles. Only a small percentage of light, less than 40%, is reflected. By contrast, the metallic gold or silver cuticles of pupae of danaid butterflies have over 200 reflecting layers and almost 80% of the incident light over a broad band of wavelengths is reflected. Amongst scarab beetles, reflectivity is enhanced by large amounts of uric acid in the reflecting layers.

Your particular choice of the jewel wasp makes a good test subject, because I still have available the specimen pictured HERE. I tested it just now with polarizing filters, and discovered that its brightly colored cuticle has a very interesting mix of characteristics. When illuminated by non-polarized glancing light near Brewster's angle, the reflections become highly polarized as one would expect for an organic material. However, when illuminated from the front, the cuticle acts more like blue metal. If the front illumination is non-polarized , the shiny blue cuticle adds no obvious polarization of its own, and when illuminated by polarized light, it preserves that polarization. This means that using crossed polarizers to kill specular reflections is a complete disaster. When arranged to eliminate the unpleasant reflections of my tiny fiber optic bundle, crossed polarizers made the whole wasp go black!

--Rik


Fascinating!

With the complexities of multi-layer optics, polarization effects may be challenging to explain in detail.

Are *any* of the metallic appearing insects actually coated in a metallic substance? It's hard to imagine that such a wide variety of metallic colors are in any instance made of real metals. Yet they look so convincingly metallic when viewed in person or in macro photos.

Thanks for the extra info and links to previous posts. Not sure I could have reliably searched for them.
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rjlittlefield
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Joined: 01 Aug 2006
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Location: Richland, Washington State, USA

PostPosted: Sun Oct 10, 2010 11:03 pm    Post subject: Reply with quote

DQE wrote:
Are *any* of the metallic appearing insects actually coated in a metallic substance? It's hard to imagine that such a wide variety of metallic colors are in any instance made of real metals. Yet they look so convincingly metallic when viewed in person or in macro photos.

If I recall correctly from synthetic image generation, the appearance that we recognize as "metal" is characterized by having specular reflections whose color is the same as bulk reflections. This contrasts with non-metallic appearance in which the specular reflections are "white" (no color added). I'm thinking that the multilayer structures manage to color-filter the light at the same time they're reflecting it, and voilĂ  -- "metallic".

Quote:
Thanks for the extra info and links to previous posts. Not sure I could have reliably searched for them.

You're welcome, but I can't take much credit. One of those links was my own post, and the other I reconstructed based on email sent to me by an entomologist whom I had asked about an extraordinarily reflective fly face.

--Rik
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Harold Gough



Joined: 09 Mar 2008
Posts: 5787
Location: Reading, Berkshire, England

PostPosted: Sun Oct 10, 2010 11:17 pm    Post subject: Reply with quote

This topic about filtration has, coincidentally, taught me a lesson about filtering e-mail messages. I found that notifications of replies were not appearing in my Inbox. Then I found them in my Ebay folder. The word "question" in the subject heading had been the trigger, according to my message rules, to divert them. Rolling Eyes

Harold
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