UVA and UVB polariser - Moxtek UVD260A

[JMC]

Andy, yes fully cross polarised is too much. It makes things look very flat. For pictorial purposes partially polarised would be better. No Buttercups here yet, but will try later on.

 

Stefano. The light source has a polariser fitted to it, and then there is another one on the lens set at 90 degrees to the one on the light source. The aim is to eliminate specular reflection, and allow only diffuse reflection through to be imaged. The cost is that it robs a lot of light (about 4 stops in this case) which is course for UV is not ideal.

[Stefano]

So your light source should appear black (with ideal polarizers), but you can see diffuse reflection a bit.

[JMC]

So your light source should appear black (with ideal polarizers), but you can see diffuse reflection a bit.

 

Yes. It's the same effect as if you hold up 2 polarisers to a light. If you turn them so one is aligned at 90 degrees to the other, they block the direct light and appear black.

 

By the way, I'm using linear polarisers here. Not sure if it is exactly the same with circular polarisers.

[dabateman]

Yes this will work with circular polarizers, but they have to be correctly aligned. Not all are. You can check them up to a light. If out of alignment, then you get spectral variation, see blue, green and red through them as you turn them.

Quite fun to play with.

[Andrea B.]

Thank you Jonathan for these very interesting polarizer experiments. And also to everyone else for their contributions.

Quite a fascinating topic. I'm very happy to see these experiments being made available to our readers.

 

*****

 

I am not completely convinced that I would want to use polarization in my documentary flower work because it seems to remove too much of the detail? I am not saying never, mind you. Just that I would need to see lots of floral examples with and without polarization to determine more fully the pros and cons of using it.

 

There are cases where a UV floral signature features a use of specular reflection and/or iridescence as part of the flower's "signaling" to pollinators. Parts of that can be captured in an unpolarized UV still or UV video. A classic example of specular "flashing" is seen in the California Poppy which is very UV-dark but "flashes" an iridescence when it moves in the breezes, (IIRC, Birna has a video of that.) So use of a polarizer for such a flower would conceal that.

 

Of course, using a polarizer is not an either/or situation! The UV flower photographer could quite easily made exposures with and without polarization.

 

The above is just my thinking it through for my particular thing. For dermatology work, I certainly can see that the polarizer experiments are very promising. There is no need there at all to attempt capture of specular reflections.

 

Looking forward to seeing more of this.

[JMC]

Andrea, fully cross polarised images are fairly extreme. They remove specular reflections which can hide what's happening beneath the reflection, but leave the image very flat. So pictorially, they aren't the best. The cool thing is though, a slight twist on one of them in relation to the other, and you can reduce but not eliminate, strong reflections. This is where I think it'd be more interesting for imaging certain flowers.

[Andrea B.]

Thank you for the additional remark, Jonathan.

Yes, reduction but not total elimination could be very useful for some photo subjects.

[JMC]

Definitely I like the cross polarization effect for the very shiny flowers. Try a buttercup next! I bet going to full cross-polarized is too much and it may be better to trade a little more light for a little less specular reflection blocking. But I know you were in testing mode here, not art mode.

Just for you Andy (and of course any one else who's interested). We have Buttercups in the garden now. I've done a series of images using my UV modified Nikon d810, Rayfact 105mm UV lens, and Hamamatsu LC8 light source. One polariser on the light source, another on the lens. I've included a 20% diffuse reflectance standard in the image.

 

No polarisers at all (0.6s exposure)

 

Parallel polarised (one on light source, one on lens, 4s exposure)

 

45 degree cross polarised (4s exposure)

 

70 degree cross polarised (4s exposure)

 

80 degree cross polarised (4s exposure)

 

90 degree cross polarised (4s exposure)

 

By altering the angle between the two polarisers you can vary the amount of specular reflection in the image. By having them just off from 90 degrees to each other, the reflection can be dialled down without completely eliminating it.

 

The parallel polarised light looks very similar to non polarised, just with a lot longer exposure required.

[Andy Perrin]

Thank you, Jonathan! I think my favorite is the 70 degree one. Just a bit of specular reflection, not too much. If you do a closeup of the flower the with different angles, that might also be interesting?

[JMC]

Andy, below are crops from the images above, shown at actual resolution.

 

No polarisers (0.6s)

 

Parallel polarised (4s)

 

45 degree cross polarised (4s)

 

70 degree cross polarised (4s)

 

80 degree cross polarised (4s)

 

90 degree fully cross polarised (4s)

[dabateman]

I like the crops better. It shows the effect on the highlights. The 80 degrees looks best to me.

The 90 degrees has lost too much and looks a little flat.