Polarisation question - sun vs flash

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LordV
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Polarisation question - sun vs flash

Post by LordV »

A rather odd question - think we know that you can use a CPL filter under the right conditions to cut down bright reflections from shiny objects because the sunlight gets polarised as it reflects off the subject (well think that's what I was told at school). We also know that using a single CPL filter does not really have much effect on flash reflections - you actually have to use a polariser on the flash head aswell.
But why the difference - is it something to do with the sun's rays being nearly parallel whilst I guess the flash beam is fairly scattered - (well that's what we try to do with a diffuser) or is it to do with light angles, or does sunlight get polarised as it travels down through the atmosphere ?

The question came up on another forum and I realised I didn't really know the answer.

Brian v.
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DaveW
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Post by DaveW »


Pau
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Post by Pau »

As stated in the Dawe's second link, direct sunlight isn't polarized, and because specular reflection doesn't polarize it, a single polarizer has no effect cutting off this reflections. You would need to put a polarizer between the sun and the subject and another in crossed position between the subjet and the lens (or sensor), like in studio setups. Most times this approach will not be very practical in the field.
Pau

Ulf W
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Post by Ulf W »

Hi,

Pau, I do not agree with the second part of

"direct sunlight isn't polarized, and because specular reflection doesn't
polarize it, a single polarizer has no effect cutting off this reflections."

Most if not all specular reflections do polarize the light to some degree.
The ammount of polarisation depends of the angle between the reflecting
surface and the light to be reflected. At a special angle, called the Brewster
angle, all light reflected is polarized.

If the angle is not favorable for the flash light, you can make the light
polarized before the reflection to make it possible to cancel the unwanted
reflections.


/Ulf

Pau
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Post by Pau »

Ulf, you are right, so I was wrong :oops: . Always is good time to learn.
(Although in any case for practical macro in the field searching that angle in subjets with curved surfaces don't seem a practical aproach)
Pau

Harold Gough
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Re: Polarisation question - sun vs flash

Post by Harold Gough »

LordV wrote:you can use a CPL filter under the right conditions to cut down bright reflections from shiny objects because the sunlight gets polarised as it reflects off the subject .
Not only shiny objects: it is a well-known photographic technique to use a polarizer to intensify, i.e. increase saturation, of colours of subjects such a sunlit walls of painted fishermen's cottages.

A shiny surface such as foil will polarize artificial light. The reflector of most flashguns will have reflective surfaces in more than one plane, and the planes of polarization will match these, such that whatever the position of a single polarizer, it can cut out only a proportion of the polarized light.

Harold
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elf
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Post by elf »

Rik is probably sleeping in so I'll post this :)
http://www.photomacrography.net/forum/v ... php?t=3945

rjlittlefield
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Re: Polarisation question - sun vs flash

Post by rjlittlefield »

Not really sleeping in, just off doing other things and had not finalized the following post...

Some of this has been covered above, but to pull it all together:
LordV wrote:A rather odd question - think we know that you can use a CPL filter under the right conditions to cut down bright reflections from shiny objects because the sunlight gets polarised as it reflects off the subject (well think that's what I was told at school). We also know that using a single CPL filter does not really have much effect on flash reflections - you actually have to use a polariser on the flash head aswell.
But why the difference - is it something to do with the sun's rays being nearly parallel whilst I guess the flash beam is fairly scattered - (well that's what we try to do with a diffuser) or is it to do with light angles, or does sunlight get polarised as it travels down through the atmosphere ?
Sunlight does get polarized as it reflects off the surface of most materials, but the % of polarization depends strongly on the angle. To have a large %, the angle must be close to Brewster's angle, which means that the sun must be behind the subject. This is why Polaroid sunglasses work so well to kill reflections from water -- under typical viewing conditions the objects being reflected are behind the water and the angle is close to Brewster's.

A polarizing filter would work equally well on flash illumination at the same angle, but that would be a very odd setup.

Direct sunlight does not get polarized as it goes through the atmosphere, but indirect sunlight from blue sky does. This is why a polarizing filter can darken the sky so much at certain angles. (The % polarization is maximum at right angles to the sun.) Under some conditions, the subject will produce a specular reflection of this blue sky, in which case a polarizing filter can also reduce that reflection even at angles far away from Brewster's.

See the whole thread HERE for some discussion of these effects as they relate to macro photography.

--Rik

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Re: Polarisation question - sun vs flash

Post by rjlittlefield »

Harold Gough wrote:A shiny surface such as foil will polarize artificial light.
What kind of foil is this? Metallic foils will not polarize light under any conditions. They just retain the polarization of the incident light. Plastic foils will partially polarize reflections, the same as water and many other materials, but again the % of polarization depends on how close you are to Brewster's angle, and it is usually far short of complete polarization.

--Rik

LordV
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Post by LordV »

Thanks for the comments all :).
So it's more to do with practical flash angles than anything else !
Brian V.
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canon20D,350D,40D,5Dmk2, sigma 105mm EX, Tamron 90mm, canon MPE-65

Harold Gough
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Re: Polarisation question - sun vs flash

Post by Harold Gough »

rjlittlefield wrote:
Harold Gough wrote:A shiny surface such as foil will polarize artificial light.
What kind of foil is this? Metallic foils will not polarize light under any conditions.
I recall a paper in which foil (presumably aluminium foil) was used in a very large enclosed room with bees. It was used as an artificial "sky", somewhat crinkled to give a "pattern" of polarized light, which the bees were than found to be able to use for navigation. I don't recall the details of the light source. Unfortunately, I no longer have access to the bee literature I was using when I noted then paper. Google searches indicate that at least one paper may be refering to this but, tantalisingly, only the abstract is accessible online and makes no mention of the polarization.

As an experiment, I have just held a preformed, rather corrugated, aluminium foil dish under indoor lights, at such an angle as to give lots of reflections. I then viewed the tray, through a polarizer filter, from about a right angle to the main path of light. With incandescent (bare bulb) light much of the relection was cut out but rotating the filter made no difference. With a fluorescent tube the same happened but, at some angles of the tray, more of some reflections were cut out by rotating the filter.

I tried the above experiment slightly differently. Viewing the reflection of the fluorescent tube in the shiny kitchen tiles behind it, I found that from a viewing angle of about 45 degrees the polarizer could be rotated to cut out the entire reflection.

The same was done with the reflection of a white plate illuminated by the tube, with the same result. I repeated the latter with an incandescent lamp and found that the reflection off the tiles was cut out by rotation of the polarizer from a similar angle.

I then viewed the white plate, illuminated by the incandescent lamp, in the corrugated foil dish. It was a bit tricky to get the best angle but the reflection (from parts of the aluminium dish) was again cut out by rotating the polarizer. Thus, it seems to me that polarized light in a particular plane is reflected selectively on mirror-like surfaces, including those of aluminium.

Finally, I placed some cooking foil under the fluorescent light. Rotating the polarizer at some angles of viewing reduced a small proportion of the reflections. In that the foil was flat, whereas the tiles and aluminium tray were vertical, it was not a good comparison but it did establish that polarized light in at least one plane was being reflected.

Based on the above, I still think what I said about flashgun reflectors will have some validity, even if my theory is inaccurate.

Harold
My images are a medium for sharing some of my experiences: they are not me.

scitch
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Post by scitch »

It is true that aluminum foil does not polarize light. In the article that I've mentioned before with a technique for calculating the degree of polarization of butterfly wings, they have a photo that compares the degree of pol of white paper, white electrical tape, black ink, black electrical tape, and aluminum foil. The results come out:
a) White paper: 0%
b) White electrical tape: 25%
c) Black ink: 45%
d) Black electrical tape: 70%
e) Al foil: 0%

For the experiment with the bees, it is highly likely that they were shining polarized light onto the aluminum foil. It will reflect polarized light, it will just not polarize non-polarized light.

Mike

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Post by rjlittlefield »

Harold, I've carefully read your description of the experiments several times, but I'm still not sure exactly what the setup was.

What I think you're telling me is that most of the reflections from the foil were not directly from the light source, but rather a reflection of another reflection,
such as light --> tiles --> foil --> camera.

In that case, bare aluminum foil would indeed preserve whatever polarization was introduced by the tiles.

I just now retested with aluminum foil (Reynolds Wrap). As expected, direct reflections from non-polarized light sources were not affected by a polarizing filter, while reflections of light that had already been polarized by other reflections were affected the same as if those reflections had not bounced again off the foil.

Your observations may differ, but if so it would be interesting to figure out why. Having a non-metallic coating on the foil, even if very thin, might make the foil become slightly polarizing, because then the overall reflection would be a combination of lots from the aluminum (no polarization added) plus a little from the coating (partially polarized depending on angle).

--Rik

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Post by DQE »

I tried to find the answer to the following question(s) in the posted links to this forum, to no avail.

What about "metallic appearing" bugs, including things like iridescent surfaces such as colors created by very thin surface coatings on the bug's exoskeleton? Some of these pseudo-metallic critters create the hardest to manage flash-exposed reflections. If their surfaces really are metallic or partly metallic in nature, this would complicate the use of on-camera polarized filters as an anti-reflection technique since reflections from real metallic surfaces are unaffected by on-camera polarizers. I keep dreaming about the dramatic benefit I obtain from using polarized sun glasses when viewing reflections from the windows of other cars on the thruway on a bright sunny day.

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?

Sorry for the lengthy wording - couldn't think of a reliable, simple way of phrasing my questions.
-Phil

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Harold Gough
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Post by Harold Gough »

rjlittlefield wrote:What I think you're telling me is that most of the reflections from the foil were not directly from the light source, but rather a reflection of another reflection,
such as light --> tiles --> foil --> camera.
For the foil, that is correct. For the first two experiments with aluminium dish it is direct light onto the dish, with no ceramic plate or tiles involved. The question there is whether the (presumed) aluminium of the dish has different characteristics than that of foil. If it is just preserving polarisation, then that must originate from the bare (no reflector) fluorescent tube. The proportion of reflections from the dish which responded to rotation of the polarizer was very small but the effect was repeatable.

Perhaps this foil question is a red herring, in that I doubt very much if that is the reflective material in flashguns.

Harold
My images are a medium for sharing some of my experiences: they are not me.

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