A decent polarising lens that cuts out dashboard reflections during the day, but also doesn't hinder night footage is a very worthwhile investment.
I don't want to tread on anyone's toes, but how is such a thing possible?
A normal CPL will reduce light transmission by 50%. That's because it's cutting out all light that is polarised in the same direction as light reflected off the windscreen.
Yes, that has a downside because twice the exposure time is needed, meaning twice as much motion blur. And if it's too dark you may get underexposure by up to 50%.
I don't see how you get around that. I can't help but suspect that such an alternative CPL will get around the problem by, for example, only cutting out half of the polarised light in one direction. That means you will get 75% of image brightness (better than 50%), BUT you will still get 50% of the reflection (much, much worse than near 0% reflection with a properly oriented, standard CPL.) I don't like the sound of that.
Now a standard CPL would, I imagine, have a certain transmission curve: light polarised at 0° will have 100% transmission; light polarised at 90° will have 0% transmission; light at 45° will have 50% transmission; I'd guess there's a linear relationship for values inbetween.
Do these alternative CPLs have a different transmission curve, for example somehow letting most light polarised at 0° to 80° through, yet blocking most light between 80° to 90°? I would love to think so, but does such technology exist?