Hot Mirror / IR Filter

christian mann · April 2, 2011

is there a difference between a hot mirror and a IR filter? if so when would i use a IR and a hot mirror ? is it not both to protect the chip from infrared light ?

as far as i know there are combined IR ND filters but only straight hot mirrors - any reason for why there is no ND hot mirror ?

Jonathan Bowerbank · April 4, 2011

Basically the same thing, and they serve the same purpose, I think it just depends on whether a specific brand chooses to call theirs an IR or Hot Mirror. Different brands have different characteristics in regards to how much IR they remove, so it's worth testing between say Tiffen or Schneider, to name a couple.

Having IR/ND combos has had its problems on a couple shoots I've been on. If you want to stack IR/ND's, there can be reflection issues between the filters, so it's best to just have an IR out in front, then a couple stages behind it for regular ND filters. Besides, with the newer MX and Alexa chips, IR contamination really isn't a huge issue until you go over 1.2 ND or so, as I've been told by DIT folks. So having the lighter grades of ND equipped with an IR overlay is overkill at this point.

John Sprung · April 4, 2011

The difference would be that an IR filter absorbs the IR, while the hot mirror reflects it away, and passes the visible light. For camera filters, it doesn't make a difference. It matters where you have so much light that the heat of the unwanted IR is significant, like in digital projectors. You need to not burn up the chips.

-- J.S.

Jonathan Bowerbank · April 5, 2011

Can someone explain how an IR filter is able to "absorb" IR light? I tried looking for specific explanations, but no luck. Just that they have special "technology" that absorbs it. Is this just a fancy word for a different way of reflecting away IR? Where it's not simply cutting it like a Schneider TruCut, but perhaps using micro particles in the filter that simply scatter the IR light?

Anybody, anybody?

John Sprung · April 5, 2011

Absorbtion works in IR just like in visible light. A green filter absorbs the red and blue wavelengths, and passes the green. Dyes are big complicated molecules, and some of the bonds in those molecules capture certain wavelengths, and turn the energy into motion on the molecular level, which is heat. Of course it's only a significant amount of heat if you have a huge amount of light, like in digital projectors. For camera filters, it's too little to matter. But the gels that we use on lights do fade and/or burn after a while.

The one sorta goofy thing is that what we call an IR filter absorbs IR and passes the visible light, while what we call a green filter passes the green and absorbs the rest. So, the names are the other way around....

-- J.S.

Jonathan Bowerbank · April 7, 2011

So it just uses the same principle as with any other element in the world that absorbs light and reflects, which makes it the color see. Hence the reason for white balancing when an IR filter is down (because of the green cast), whereas with a Hot Mirror it's not AS necessary it seems.