Hot Mirrors - IR Polution
This is going to be a short post. I will talk way more in depth about IR pollution here soon. Infrared light is invisible to the naked eye.
For those who don't know, IR pollution is a phenomenon where infrared light is interpreted by the digital sensor as light input, generally, it makes black objects in the image red. Also, specific materials tend to reflect IR more potently than others. For example, synthetic fibers in clothing reflect more IR than cotton.
This does not happen on most cameras because placed right on the sensor is an IR filter. So you won't be having this issue with your iPhone. But, with cinema cameras, manufacturers usually place fewer filters on the sensor which increases both light and color transmission. "But Paul I haven't seen this happen with my cinema camera." Some cinema cameras do have IR filters (Sony, Canon, Panasonic generally do). It also allows a cinematographer to have ultimate control over their image. This problem also only really shows itself at high strengths of neutral density. Neutral density reduces visible light increasing the proportional amount of infrared light compared to visible light, making IR pollution more prominent even though it was always there. If you shoot without ND, you may never see this problem.
So, to fix this problem an IR filter is necessary. There are two ways to filter IR light: absorption and reflection. A "Hot Mirror" reflects infrared light, usually bellow 700nm wavelength, also known as the "near red" part of the spectrum. (Source, Tiffen http://www.tiffen.com/press_release_Hot_Mirror_IRND.htm)
This filter is very effective on cameras that have partial IR filtration on the sensor.The Blackmagic Cinema Camera, Arri Alexa, and Red Epic all have very minimal if any IR filtration and suffer from IR pollution. On these cameras, there is more pollution above the 700nm mark creating an image like this when using a hot mirror.
As you can see, it corrects some of the issues from the original image, but not all. This is where the absorption of 700nm+ light comes into play. Sadly, I do not own one of these filters yet. But with some power window work in resolve I will be able to correct specific issues like this one (synthetic material on velcro and edges). Also note that the grass is still a bit more red than normal.
Again, here is the effect a hot mirror has on a sensor with almost zero filtration.
Tiffen recommends using their Hot Mirror in combination with an IRND. To remove the additional IR bleed.
"The Hot Mirror (HM) filter can be used individually, stacked with our new IR ND Filters (IRND), or as a Hot Mirror IRND all in one combination filter (HMIRND). Tiffen IRND filters provide a true neutral density beyond the visible range into the near infra-red... IR ND filters and IR 85ND filters eliminate Infrared bleed above 700nm. This is particularly important as it now allows for the use of the heavier grades of Neutral Density filters needed for these hi definition cameras while, at the same time, maintaining complete color balance."