Hello,
Doing some first experiments with x polarisation, I notice that the small strip of film that I stickytaped directly over the flash head appears to develop a yellowish haze/streak in the centre. I suspect that this is due to heat and that heat from flash destroys polarising film. So I should place it further from the head. Am I correct?
Also, does the way round that the film is placed over the flash source matter? My rough and ready 'look at the monitor through the film' test would appear not, but maybe x polarisation means it does.
Many thanks for looking.
Polarising film over flash goes yellow
Moderators: rjlittlefield, ChrisR, Chris S., Pau
Regular linear polarizing filme does not matter which way it goes.
Circular polarizing film does matter because it consists of a layer of linear polarizer and a 1/4 wave plate.
The yellow you are seeing in the center is consistent with heat damage of linear polarizers. These are manufactured with mechanical stresses in them to induce the anisotropy and the pulses of heat anneal them.
Moving the film farther from the flash lamp should prevent it from happening.
Circular polarizing film does matter because it consists of a layer of linear polarizer and a 1/4 wave plate.
The yellow you are seeing in the center is consistent with heat damage of linear polarizers. These are manufactured with mechanical stresses in them to induce the anisotropy and the pulses of heat anneal them.
Moving the film farther from the flash lamp should prevent it from happening.
Johan, I use X-pol a lot, and am noticing an effect similar to what you describe. The polarizing material I use on my flashes (an Edmund Optics high extinction product), isn't turning yellow, but is discoloring in the center after hundreds of flash cycles when taped directly on the flash. I'm not home right now to recheck the nature of this discoloration, but iirc, it's a lightening with a neutral color cast. I suspect that you and I may be seeing the same phenomenon, with the yellow vs neutral color difference being induced by differences in our materials. This could be in the polarizing layer itself, the plastic to which it is laminated, or both.
So far, I've just replaced the flashes' polarizing material when it discolors. Since I'm just using a small piece cut with scissors from a larger sheet, this isn't terribly expensive. So treating this material as consumable is OK for now. But it's on my "to do" list to figure out what's going on, and come up with a better way of dealing with it.
I don't know what causes the bleaching (as I call it) of the polarizing material. Heat, UV light, and visible light come to mind as obvious suspects. I don't know what the effect would be of moving the polarizing material farther from the flash, as you propose. My guess is that the bleached area would take longer to show up, and be larger than what we are seeing now--but that's just a guess. If you try this, I'd be interested in your results.
My first thought has been to put a UV filter between flash and polarizing film. Or, if that fails, try it with IR filter, with ventilation to carry away the heat that the IR filter absorbs.
With X-pol, orientation of the polarizing material does become important. So you will want to be able to rotate either the polarizing filter that your camera is looking through (aka the "analyzer"), or the polarizing material that the light shining on the subject is coming through. If your light polarizing material and analyzer both have high extinction factors, you should see a big difference as you rotate either against the other. For convenience, I find it easiest to be able to rotate both--but this is not strictly necessary. Rotating just one is fine.
Cheers,
--Chris
So far, I've just replaced the flashes' polarizing material when it discolors. Since I'm just using a small piece cut with scissors from a larger sheet, this isn't terribly expensive. So treating this material as consumable is OK for now. But it's on my "to do" list to figure out what's going on, and come up with a better way of dealing with it.
I don't know what causes the bleaching (as I call it) of the polarizing material. Heat, UV light, and visible light come to mind as obvious suspects. I don't know what the effect would be of moving the polarizing material farther from the flash, as you propose. My guess is that the bleached area would take longer to show up, and be larger than what we are seeing now--but that's just a guess. If you try this, I'd be interested in your results.
My first thought has been to put a UV filter between flash and polarizing film. Or, if that fails, try it with IR filter, with ventilation to carry away the heat that the IR filter absorbs.
With X-pol, orientation of the polarizing material does become important. So you will want to be able to rotate either the polarizing filter that your camera is looking through (aka the "analyzer"), or the polarizing material that the light shining on the subject is coming through. If your light polarizing material and analyzer both have high extinction factors, you should see a big difference as you rotate either against the other. For convenience, I find it easiest to be able to rotate both--but this is not strictly necessary. Rotating just one is fine.
Cheers,
--Chris