Where is the mark on the polarizer supposed to be?

Started 3 months ago | Questions thread
petrochemist Senior Member • Posts: 2,651
Some technical info on polarization.

ThrillaMozilla wrote:

Gerry Winterbourne wrote:

No filter removes all polarised light.

In the right orientation they do remove virtually all polarized light. Photographic polarizing filters are almost perfect in that respect. (Footnote: someone once tested a horrible, cheap filter that transmitted a few percent of polarized light, but that was an exception.)

However, most light that you see in nature is not 100% polarized.

Virtually all is IMO a rather unhelpful term. In some cases 0.002% transmission could be significant, in many the few percent transmitted by that cheap filter will not matter.

Polarizers will indeed remove the bulk of visual light that is polarized in the opposite orientation, but very few are effective with Near infrared which many digital cameras have some sensitivity to. You can buy polarizers that are effective over the entire wavelength range silicon sensor can record, sadly they are quite expensive so I haven't got any.

I don't know of any natural source where light is 100% polarized & I think lasers may be the only man made sources that actually achieve 100% (even the very best polarizers leak a few ppm of the polarization they are designed to block). Most natural sources have photons of random polarization but fortunately this can be treated as vectors the proportion in the diracton of the polarizer being blocked & the remainder passed. Refection etc will only polarize a portion of the reflected light with the maximum portion when the light is incident at the materials brewster angle (63 degrees for water IIRC).

A theoretically ideal polarizer would block 100% of the light that matches it's polarization and transmit 100% of that of the perpendicular polarization. Where light is of an intermediate polarization it. In real life filters don't manage this. There are filters available now in both 'high extinction' & 'high transmission' variants. I believe this is a case of choosing between blocking a little of the light the filter should pass to ensure none of the light it should block gets through (high extinction) versus only blocking most of the polarized light it should, while passing all that of the opposite polarization. Normal filters will be somewhere in between these extremes.

There are also some odd old polarizers that are colour dependent, some just blocking polarized light in a specific colour band & passing other colours irrespective of polarization and a few others (probably made up of two of these) that let through red light polarized one way & blue light of the opposite polarization. I don't think these are made any more, certainly all those I've collected are at least 30 years old.

Using two crossed  normal polarisers gives a fairly good indication of how much light of each polarization is removed it not hard to get down to around 0.5% transmission in the visual with this approach. With all of my polarisers this raises to around 50% transmission by about 750nm - easily seen with several of my unmodified cameras and not dissimilar to a R72 filter on my modified bodies. To achieve the effect of crossed polarizers the front one must either be a linear polarizer or it must be turned around to be face to face with the rear one. Circular polarizers have a quarter wave plate added behind the polarizer to throwing out the polarization of the emergent light.

BTW things can get weird if you have 3 linear polarizers. Use 2 at 90 degrees to each other & nearly all the light is blocked. When the 3rd in an intermediate alignment is added between these (without touching either so they remain crossed) light transmission increases!

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