Polarizing Filter Mystery!

[Gabe]

Hi folks, I've got an unusual phenomena I've discovered involving a polarizing filter for photography. I thought a video would do a much better job of explaining the question than text would, so I made a quick video.

So obviously this filter can't be JUST two polarizing filters, but what do you think it's comprised of? The effects of polarization only happen when light is passing through the filter as if it were attached to a lens: you can only observe polarization effects when the side that attaches to the lens is facing you. It screws on to the front of the lens when it's used. The order the light goes through the element(s) of the filter must matter, so I'm guessing there's maybe a third glass element that randomizes the transmitting light's polarization? This doesn't make much sense though because it would have to be on the inside of the filter (between the filter and lens). I can't think of a use for an optic like this inside a filter in that position. I don't even know if glass can be made to do that in the first place. What do you think? I'll stew on it further.

 

[BowtieGuy]

Hi Gabe, for some reason, your video isn't working for me. Hopefully it's OK for others.

 

[Gabe]

Hi Gabe, for some reason, your video isn't working for me. Hopefully it's OK for others.

Uh oh, that's kinda the most important part. I'll set it to public, maybe that'll fix it

 

[Gazen]

It’s working now, at least for me

 

[Radim]

For me it is not working. Can't help since I do not understand what you mean. I have extensive knowledge of optics so far, so once I see it we can discuss it.

In general polarizing filters are composed of long molecules of polymer allowing just structurally given polarization to pass. It looks like form of diffraction grating, so maybe what you see is just some diffraction pattern.

 

[Gabe]

ugh that's frustrating, it must depend on what platform you're viewing on. Here's the direct link.

https://youtu.be/WrADyPDVYRg

 

[BowtieGuy]

Here you go Gabe, this should work for everybody (I hope). :yh:

 

[Gabe]

oh you're right, I totally forgot about how you just place the string of letters in the youtube brackets, not the whole link. Forgive me. I wonder why it still worked for me and Gazen though? That's a mystery for another day.

 

[Singlemode Laser]

One side of the filter seems to reflect some light. I think this is some anti-reflection coating on the polarizer which is applied on only one side. When coming from the other side light hits the uncoated area and gets reflected inside multiple times. This effect should get worse when you incline the angle of the filter. Have you tried this?

Singlemode

 

[Gabe]

Thanks for speculating singlemode. I think i may understand what you're getting at.

it seems like the angle has little to do with the effect. One thing I forgot to demonstrate in the first video which shows up in this second one is the fact that this effect ONLY happens with polarized light. Through both directions, the same amount of unpolarized light passes through no matter the filter's orientation. And through only one direction does this act as a regular polarizing filter: through the other direction it acts most like a tinted piece of glass.

 

[paul1598419]

That would be my guess as well. I have several polarizing filters that are not AR coated and the effect on my TV are as expected. I use them mostly with my He-Ne laser to find the exact polarity of those that are polarized.


E: I see the angle has no effect. There must be some way in which the crystals align to give polarity only one direction. It could be the angle of the crystal's orientation that makes this possible.

 

[Singlemode Laser]

Your polarizer contains a linear polarizer followed by a retardation wave plate with lambda/4 retardation. It creates circular polarization from linear one.

So your filter filters all light on one direction (depending on it's orientation) and only 1/2 in the other independent of orientation. (Only valid for linear polarizered light).

Singlemode

 

[paul1598419]

What would be the purpose of having a quarter wave plate incorporated in the linear polarized filter? I get how it works, just not why.

 

[Singlemode Laser]

Seems to be useful for modern cameras that use beam splitter to sample some part of the light to measure the needed exposure time and also the autofocus.

These splitters seems to be very sensitive to polarization. I am not an expert in cameras and photography though.

Singlemode

 

[Gabe]

What would be the purpose of having a quarter wave plate incorporated in the linear polarized filter? I get how it works, just not why.

I remember reading that most modern cameras that use through the lens exposure metering and autofocus use a semi-silvered mirror to divert some of the incoming light for the measurements. If the light is linearly polarized then I guess it doesn't act right when it gets reflected, similar to how unpolarized light gets polarized when it's reflected, and either the metering or the autofocus doesn't work. I've never actually learned about quarter wave plates and what circular polarization is. I assumed that these variable density filters were just two linear polarizers, but there must be a circular polarizing filter after the linear filter(s) so that the metering and autofocus can still work. I think I'm understanding this. Thanks so much guys, would give out more reps if I could.

 

[paul1598419]

Wave plates are made with birefringent materials. A quarter wave plate causes circular polarization, while a half wave plate turns the polarity of the light into a different direction. Birefringent minerals such as quartz, mica and calcite have two indices of refraction. An ordinary one and an extraordinary one. This depends not only on the material, but also the thickness of the wave plate and the wavelength of light in question. They have been used as quarter wave plates in laser cavities that once the light bounces off the HR mirror and returns through the quarter wave plate, becomes a different polarized light that can be picked off and sent out as coherent light. How this is used in cameras, I don't know as the wave plate is wavelength dependent.