Photoelasticity

[blackfox]

2nd attempt using i.pad for illumination and a different ornament for practise gave slightly better results .TBH its not something that will push me down a different path but fascinating to have a play with . best results so far below the question over what type of polarising filter is best remains though

[Bikie John]

I responded to Dewi9 's comment about needing a linear polariser to say that, based on my understanding of the theory, I thought it would work with a circular polariser. Then blackfox's comment about it not working made me doubt that. As we all know, in theory there is no difference between theory and practice but in practice there is, so I just tried it. Nothing artistic.

Here is the filter, which looks pretty unequivocally like a circular polariser:



Here is its box, which sort of confirms it:



Just to have something to photograph I suspended the filter box in front of my desktop screen and shot it with the filter on the 12-40 f/2.8, and got the stress patterns in the clear plastic bit of the lid exactly as expected:



So - it "works", with this setup at least.

I tried looking through the filter at the desktop screen and rotating the filter, and sure enough it get significantly darker or lighter depending on the orientation. So for comparison I also looked at the screen on a Samsung tablet, and the effect is there but less pronounced. I wonder if blackfox's iPad is not providing polarised light - perhaps Apple have been "clever" and put a quarter-wave retarder on it to remove the polarisation.

One way to check would be to look at the screen through the filter (the right way round - as if it were on a camera) and twiddle it and see if the brightness changes significantly. If it does, it is polarised and I don't understand why it didn't work. If it doesn't, the iPad's light is not polarised, which would explain the failure.

John

[Luv2shoot]

Blck Fox, I understand that you won't be creating many more of images like this, but I got to say, that one is really cool. Evidently, the tall grasses that make up the foreground are made out of plastic too. This is so interesting and quite eye-catching. I think IAN should use it in the banner.

John, Yours looks like a UFO caught on a radar screen. I thought about using my plastic filter screen box too but could not remove the paper label from inside.
Thank you BOTH for joining me in this experiment. Maybe I am easily impressed but I was excited the first time it worked for me.

[Bikie John]

linus1884 us said in an earlier comment:

Yes you do need a linear polariser https://www.youtube.com/watch?v=8rtts66THp0

This is a bit misleading. The video matches my understanding - he says in the video that the filter on the camera can be a linear or circular polariser, it makes little difference which you use. What he says is that if you are using a polarising filter to create a polarised light source, that is the filter than needs to be linear. In the setup Lorell describes, there is no need to do this at all because the light from the screen is already polarised.

In fact I suspect (but won't know until I can work out how to try it) that you could use a circular polariser for this as well, as long as you use it the "wrong way round", with the side that normally faces the camera facing towards the light source.

John