polarizer behind Hot Mirror

[M Munoz]

Can I place a polarizer between the lens and a hot mirror?
When light is polarized, does any light get bounced back toward the mirror?

 

[Birns and Sawyer]

As long as you have the magenta or reflective side facing out, it shouldn't matter what you put behind it.

The filters are designed to avoid reflection problems.

 

[Jim McKinney]

Though it's counterintuitive to me, I remember someone from Tiffen at NAB last year insisting that it doesn't matter which way the mirror faces, in order for it to work.

 

[SF Geek]

It still works, you just have more flare/reflection issues

 

[Jeff Kilgroe]

Uh... The guy at NAB was wrong. You need the mirror facing OUT. The mirror should be the front-most filter. Any light passing through filters and into the lens will meet resistance at every filter. If you have a mirror filter, it's a mirror because light is being reflected. Any reflected light must travel back the way it came (well, off the appropriate reflective angle) and can cause internal reflections and glare between filter elements. A hot mirror works because it is a mirror. By flipping it around, mirror pointing inward toward the lens, you are greatly reducing it's effective IR blocking ability.

A polarizer can be used with a hot mirror, and should be placed behind the mirror. Its effect is greatly diminished when placed behind the mirror, vs. being out in front as you would normally use it. But if you place it in front, you run a high risk of glare or internal reflection, in fact you're almost guaranteed to have problems. IMO, I'd really like to see polarized versions of these hot mirror filters, makes sense to me. But they're somewhat of an odd product brought on by the needs of digital acquisition systems. I think over time, they won't be as necessary as the ability of sensors to deal with non-visible wavelengths is improved.