15 replies to this topic

[DaveO]

I borrowed a feather from a friend who keeps peacocks just to see what they looked like in UV http://www.ultraviol...eacock-feather/




Visible Light: Pentax K-5 Full Spectrum Modification, Nikon Rayfact PF10545 MF-UV 105 mm f/4.5 lens, Metz 15 MS-1 flash, 1/180 s @ f/16 ISO 200, B+W UV/IR Cut Filter.

Image Reference: DO52427

Ultraviolet Light: Pentax K-5 Full Spectrum Modification, Nikon Rayfact PF10545 MF-UV 105 mm f/4.5 lens, Nikon SB-14 flash, 1/180s @ f/16 ISO 200, Baader UV-Pass Filter.

Image Reference: DO5430

Edited by DaveO, 11 April 2014 - 06:39.

[nfoto]

Did you try varying the incidence angle of the UV illumination? I would surmise the iridescence behaviour changed to some extent.

[DaveO]

Good idea. I'll go back and try that tomorrow

[Andrea B.]

Very interesting. The "eye" does stand out a bit in UV.

[DaveO]

Here's another with the SB-14 flash moved to another position

[nfoto]

Obviously the iridescence pattern changes with angle of incidence.

[Reed F. Curry]

The iridescence of bird feathers is due to structure not pigmentation, so angle of incidence does matter. Nevertheless, among the many different types of feathers I have shot in the UV, the iridescent sections inevitably reflected little UV.

Unlike shot silk, feather (and insect) iridescence is based upon refraction between transparent layers and air spaces. This was discovered by immersing feathers in water. Once wet the feathers were not iridescent.

Just some curious factoids.

[DaveO]

Does this tell us something about the false UV colors? The visible colors in this case are presumably due to interference effects as in diffraction gratings, where a specific wavelength (hence color) is seen at certain angles of incidence/reflection-refraction. The bright visible yellow-green around the perimeter of the visible pattern appears to become false-UV blue, which is a false-UV color that I see quite frequently in my UV images of Australian native wildflowers. On the other hand, the visible bright blue inner ring of feathers become very pale or almost non-colored in the UV images depending to some extent on the angle of the UV illumination used for the two UV images.

All this proves is that there are no "rules" that can be applied to a visible image to guess how it will appear in false-UV.

Dave

[enricosavazzi]

Perhaps it has already been mentioned, but see this paper:
http://adeline-loyau...Iridescence.pdf

[colinbm]

Thanks Enrico
I'll get a Peacock feather from a local park where they have some Peacock & take some shots with the Sigma camera.
Col

[DaveO]

Enrico,
The paper states "The train of the peacock.... is composed of iridescent structurally coloured eye spots not reflecting in the UV"

That statement was based on measurement of the UV-Vis reflection at a point near the base of the eye spot. !!!!
Dave

[JCDowdy]

I made this suggestion on another thread - UV video while rotating an iridescent specimen might be interesting.

[DaveO]

John,

The paper illustrates the point that measuring the UV spectrum of an isolated point on a feather or a flower petal may give an incorrect impression of the bigger picture.

Dave

[JCDowdy]

Yes, well how could it really?
With no disrespect to the authors, it reminds me of the story of The Blind Men and the Elephant.

[OlDoinyo]

Perhaps you could take three frames with different illumination angles and paste them into the R ,G, and B channels of a destination image. That would give you an (angulochromic?) image. Or you could extract the three hue channels after converting to HCL and paste those back into RGB; that might really give a measure of iridescence as opposed to specularity.

[Andrea B.]

Intesting idea Clark. I might have something I could try this with.