Polarizing filter - degree of polarization vs polarizing sunglasses?

Question

I thought a polarizing filter only lets through light of one polarity.

Thus if I have 2 polarizing sunglasses and look through both superimposed and rotate one it should completely block light at 90 degrees rotation. I've tried this and it works.

However, I have two of Massa lens polarizing filters and they only slightly reduce the light when thus superimposed and rotated. I complained to Massa and they claimed two superimposed filters rotated 90 degrees should block the light.

Looking on YouTube I saw that viewing a filter against a computer monitor, it will block the light completely when rotated if it is a good filter – e.g. Hoya filter was shown to do exactly this.

Does this mean that the Massa filters are defective? If not, are many so-called polarizing filters (other than Hoya) this bad at polarizing compared to polarizing sunglasses or can I get ones that polarize 100%?

I checked a similar topic but it does not look at the degress of polarizing affected: How do I choose a polarizer?

Answer 1

The bulk of polarizing filters these days for photographic purposes are so-called "circular polarising filters", in brief CPL. They consist essentially of a proper linear polariser and a "quarterwave plate" that "scrambles" the polarised light in order to make it mostly unpolarised again. The reason is that parts of camera optics particularly in SLR and DSLR, like beam splitters, phase-based autofocus, separate exposure meters, analog TTL flash metering will fail to work reliably with polarised light.

The quarterwave plate is not perfect but good enough for those purposes. As a result, CPL filters have a "polarised" and an "unpolarised" side in a manner of speaking, with the polarised side turned to the scene and the unpolarised turned to the camera.

For any purposes intended to try blocking light, you thus need to place CPL filters with their front sides facing each other or with the front side turned to (say) a laptop screen.

If you instead use the backside, the polarised light will first get depolarised by the quarterwave plate and then half of it gets blocked by the polariser plate. Since "depolarisation" is not perfect except for a single wavelength of light, you'll get various hues by rotating the filter then, but no blockage. Orienting it the other way should give you the expected blockage on certain angles.