Avoiding reflections...?

[cmoody]

Hello all

 

Here's a technical question...I'm slowly getting through photographing bird specimens in visible and uv spectrum. However, a problem I am going to run in to soon is iridescent and glossy plumage.

 

I was wondering if any of you have made set-ups to reduce or eliminate hard reflections in glossy subjects?

 

I'm thinking along the lines of either a lightbox or reflectors to diffuse the light...but then this poses the question of suitable materials that will not absorb the UV from the CFL lightsource we are using.

 

The best idea I've had so far is to use some kind of white PVC sheet...angle them towards the subject, and have the lighting bouncing off of them?

[nfoto]

In principle, there are two roads to solving your problem. Either make the incident light as diffuse as possible, or use cross-polarisers on light as well as on the lens.

 

The first probably is the easiest for UV. Try making a light box covered with [lots of] crumbled aluminium foil on the inside. Aluminium reflects UV well over the entire range of interest. Not so sure about what white PVC or other plastics will contribute or remove, other than they probably soak up a portion of your precious UV light.

[cmoody]

My logic with the PVC(Or PTFE?) was that it was my understanding it's a poor-mans reflectance standard, being very similar to labsphere standards. A lot more manageable and reproducible to have a flat sheet of reflecting material without any gloss to it, rather than crumpling foil, which I suppose may never be possible to reproduce exactly due to the crumpling?

[JCDowdy]

It is PTFE you are thinking of, not PVC. PTFE (virgin Teflon) may be had in sheets of sufficient size to use as you suggest. I have in fact done this and it does seem to work fine as far as I can tell, although your UV white balance might be slightly different.

 

An alternate approach might be to make a UV diffuser from a sheet of UV-transparent frosted fused silica or UV-transparent PMMA acrylic. This was discussed recently somewhere on this forum. Some CD/DVD cases are made of suitably UV-A transparent acrylic, take a UV photo of, or through, the CD/DVD case to see. Cut to shape and sanded they may suffice.

 

Of course nothing precludes the combination of these approaches, as well as Bjørn's crumpled Al foil, to create something tailored to your application.

[Alaun]

brushed aluminium plates come to mind or better a surface which got a caustc treatment:

 

So I did a very simple test with my Apple MacBook, using its backside as reflector for one of my LED torches and directed the reflected light on to some cloth (which then turns -due to washing agents- the UV into white visible light) and it turned out I get a very nice distribution of the light.

 

Werner

[Andrea B.]

Wouldn't a UV Polarizer help?

Edmund Optics sells them. Probably other optics stores also. They are expensive.

http://www.edmundopt...polarizers/3684

 

Edit: I was tired and garbled my next question. So here it is again.

 

Teflon/PTFE is not diffuse as a reflector, is it?

But it can act as a diffuser if you shine the light through it?

Is that what you mean??

 

****************************

 

While trying to find the answer myself, I happened upon this:

Ultraviolet characterization of integrating spheres

http://citeseerx.ist.psu.edu/showciting?cid=24228355

 

Significant absorption and fluorescence features were observed in the UV region and attributed to the contamination in the integrating sphere. We demonstrate that integrating spheres are easily contaminated by environmental pollutants such as polycyclic aromatic hydrocarbons emitted from engine exhaust.

[enricosavazzi]

[...]

Teflon/PTFE is not diffuse as a reflector, is it?

But it can act as a diffuser if you shine the light through it?

Is that what you mean??

PTFE is translucent, so it diffuses also when used as a reflector. Part of the light penetrates below its surface, is scattered, and re-emerges in a different direction. There is also some specular reflection from the surface, which can be diffused by sanding the surface. For this use, the PTFE sheet should be rather thick (5-10 mm) so that losses from transmission are minor.

 

When used as a transmission diffuser, a thin sheet will work best. Even a sub-mm thick sheets diffuses very effectively. A low thickness reduces transmission losses and removes some of the color bias. Some thick PTFE sheets I have do look yellowish in transmission in my experience, but I don't know whether this is an intrinsic property of the material or is due to aging or contamination.

[DaveO]

You have probably found this yourself, but here is a link to my shots of a peacock feather which shows how the iridescence can be changed by angle of incidence.

http://www.ultravioletphotography.com/content/index.php/topic/775-peacock-feather/

 

Dave

[Andrea B.]

OK, so you can sand PTFE to give it more diffusion as a reflector.

But it's not ever going to be Lambertian.

 

So -- how important is it for the reflection light to be Lambertian-ly diffuse within a light box

if one needs to photograph UV-iridescent subjects such as some of the desert flowers we encountered ?

 

Probably not all that important for generic "field work" like the flowers in situ.

But for scientific documentation?

[nfoto]

Large flat sheets of reflective material can create problematic reflections if the subject itself has even areas or is very reflective on its own. The light box should take this into account. Most reflectors for studio flashes have a hammered uneven surface to add softening of the light.

 

UV polarisers will likely not help enough unless the incident light is also polarised. Besides, price might be prohibitive for general use.

[Andrea B.]

Thanks, Bjørn.

[JCDowdy]

Here is an earlier paper cited by the 2007 Shaw article.

 

"Spectralon is a diffuse reflectance material that is both highly Lambertian and highly reflective (>0.94 from 250 to 2500 nm). Spectralon, which is the product of Labsphere, is composed of pure polytetrafluoroethylene (PTFE or Teflon) polymer resin that is compressed into a hard porous white material in a proprietary procedure."

 

I think Spectralon is a type of sintered granular PTFE. Virgin PTFE solid sheet will not be as highly lambertian but as Enrico points out the translucence imparts a degree of diffusion.

[colinbm]

(>0.94 from 250 to 2500nm)

[JCDowdy]

(>0.94 from 250 to 2500nm)

Thanks -typo

[DaveO]

PTFE doesn't melt, (it decomposes before melting) it has to be sintered (heated and compressed at the same time) so the degree of pressure applied will determine how glossy the resulting material turns out.

 

As an old Fluorine Chemist from way back.

 

Dave

[cmoody]

I'm starting to think that the best(easiest) solution will be to just make sure the lighting is as standardised as possible.

 

If the subjects are always lit from the same angle/distance, and the camera is always shooting from the same angle/position, at least in theory someone could come along at a later date and reproduce the shot. It might have some blown out highlights in it, but at least they'll get them too.

 

I thought about polarisers but we'd then need different light sources given the large light units we're using.

 

Andrea, yes, my thoughts with the PTFE were originally to bounce the light off of them. I'd give them a light sand with some ultrafine sandpaper so get rid of any gloss (and surface contaminants). I might also try the PMMA acrylic over the light sources. It might not eliminate the problem,but it might help reduce it.

 

Thank you all for the suggestions so far. I am glad it's raised an interesting point of discussion!

[rfcurry]

I don't know if you have this product in the UK - http://www.staples.c.../product_506832

There are many brands and sizes of this inexpensive foam core poster board to be found at office supply stores. The white is very highly UV-reflective. The matte finish gives a diffuse, not specular reflection. If you scribe the back, you can bend the boards easily to form an octagonal shape, giving you more reflective surfaces than a simple four sided box.

 

Regards,

Reed

 

P.S. - you can also add some diffuse UV-reflection by spraying a surface with white dry Teflon spray - http://www.itwpf.com.au/rocol/product.aspx?productid=86

[Andrea B.]

John, Dave - thanks for the additional info.

 

Reed - Great suggestion about the Teflon spray and the poster board. Thank you !!!!

I'm always looking for something to use as a background for the flower documentation.

 

*********

 

Cmoody writes: If the subjects are always lit from the same angle/distance, and the camera is always shooting from the same angle/position, at least in theory someone could come along at a later date and reproduce the shot. It might have some blown out highlights in it, but at least they'll get them too.

 

The repeatibility of the experiment is so very important. Good for you for thinking about that.

 

It might be important for the UV-iridescent or UV-glossy birds/feathers to be photographed from two (or more) angles to illustrate the effect of the iridescence or glossiness ??

[rfcurry]

I need to back off on the Teflon spray recommendation. I have not tried that particular brand, but it might be worth trying as it claims to be white. The Dupont Teflon Non-Stick Dry-film has a binder that seems to counteract any UV-reflectivity of the Teflon.

[Shane]

So -- how important is it for the reflection light to be Lambertian-ly diffuse within a light box

if one needs to photograph UV-iridescent subjects such as some of the desert flowers we encountered ?

 

If the iridescence is due to a diffraction type structure then you should use direct lighting not diffuse, the latter "Lambertian source" can result in no iridescence being visible.

If the iridescence is due to a thin-film interference structure then either type of light source can be used but direct lighting will work better.

[Shane]

If the subjects are always lit from the same angle/distance, and the camera is always shooting from the same angle/position, at least in theory someone could come along at a later date and reproduce the shot.

 

Again this depends on whether the iridescence is due to diffraction or thin-film type structures. Assuming the lighting has the same spectral characteristics then the experiment may be repeatable for thin-film type structures but, not necessarily in the case of diffraction type structures in which the subject (diffraction structure) may be required to be oriented the same also.

[Shane]

I should have added that in the case of feathers I seem to remember that most iridescence is due to diffraction structure.

[Shane]

Found this interesting reference by coincidence this morning when "cleaning" the folders for misplaced files.

 

"Floral iridescence, produced by diffractive optics, acts as a cue for animal pollinators"

 

Heather Whitney et al

[Andrea B.]

Yes, it is fascinating. We've talked about floral iridescence from conical cells here and there in the forum. Bjørn has an interesting UV video of floral iridescence posted somewhere. It appears as though the flower is flashing signals in UV as the breeze moves it.

[DaveO]

While idly searching on the subject of PTFE I came across this paper (done for NASA) which shows that Spectralon (at least in 1993) contained a contaminant that caused it to discolour under UV irradiation although past experience with PTFE had shown it was very stable under UV. The impurity could be removed by baking out in a vacuum.

 

http://hep.ucsb.edu/people/hnn/n/uvcontampfte.pdf