The first step in unraveling the great mystery of what IR filters do, why you need one, and which one do you chose, has to start with an explanation of What IR is. Infrared light is a part of the electromagnetic spectrum just bellow what we call visible light. Visible light is a very narrow part of the electromagnetic spectrum that the human eye is sensitive to and allows us to see. This is measured in wavelength between 390-700 nm (nanometers). Essentially the longer the wave the warmer the light. Reds being in the bottom of that range and blues and violets at the top and a shorter wavelength. Infrared is actually split into three ranges described by their distance from the visible range. Near Infrared is between 800 to 2500 nm (0.8 – 2.5 µm) 1µ or micrometer is 1000 nanometers). Mid Infrared is 2500 – 25000 nm, and Far Infrared is 25000 – 1000000 nm (0.1cm). It is important to understand these numbers when we start to look at filtration, because every type of filter will block different parts of the spectrum. Then we can start to understand how they differ. To read more into light and electromagnetic spectrum, check out the Wiki article. The Chart bellow illustrates the various wave lengths. As a side note, and to geek out for a moment, the Å symbol is an Ångström. 1 Ångström is 1 ten billionth of a meter. Infrared light has a close relationship to heat. Essentially one creates the other. Heat is a collective energy of electrons bouncing off of each other at a molecular level. As the electrons’ movement increases they radiate more heat energy as well as light energy. Any mass warmer than absolute zero creates this energy. Infrared sensitive cameras are used to measure heat in a lot of military and industrial applications. Night vision looks at infrared spectrum and allows you to see into the dark where there is no visible light source. A contractor might use an infrared camera to see heat loss in your attic.
Digital sensors are made of silicon, and silicon is inherently sensitive to infrared energy. Camera manufacturers have to find ways to block infrared light from contaminating the image and only display light that is visible to the human eye to create a pleasing image. Not too long ago, digital sensors had a really hard time dealing with red light. Cameras had a very narrow red spectrum and a problem with bleeding and smearing in high saturated reds. The upside to better calibration by manufactures now, is that the red bleeding issues are a thing of the past. In most modern cameras, you have an optical low pass filter that protects the sensor and has filtration that blocks non-visible infrared light. Cameras are also very sensitive to visible light, so on a bright day we have to use Neutral Density filters to shade the sensor and cut the amount of visible light. These filters were designed for chemical film that was not sensitive to infrared, and were not designed to block infrared. As we increase the amount of ND filtration, the built in IR filtration quickly becomes insufficient as we allow a greater balance of infrared through. Arri and Red have both recommended extra IR filtration above ND.9.
Infrared Filters available on the market and the spectrum they block
Tiffen has been good enough to provide a nice chart showing the response of all of their filters.
(ND.9) is a standard Tiffen ND filter without IR filtration, T1 and T1/2 are Tiffen’s standard IR filters, (IRND.9) is Tiffen’s combo IR ND, (HM) is a Hot Mirror, (HMIRND.9) is IR ND plus Hot Mirror combo, (80CHM) is a Red One calibrated Hot Mirror. Hot Mirror refers to a metallic element that reflects heat. It must be used shiny side out. Tiffen IR ND filters tend to have a green shift as a result of cutting into visible red spectrum.
Schneider makes a series of filters called True Cut that cut the red spectrum at the values claimed in the respective titles by about 50%. True Cut 650, True Cut 680, True Cut 715, True Cut 750. They also make a line of IR ND filters called Platinum. These filters are designed to work with most digital cameras on the market and claim to hold a consistent cut to illumination across the full spectrum. However they do tend to have a magenta shift that would imply that they cut into the green spectrum at around 500 to 530nm.
Where the IR Cut happens will determine how much colour correction will have to be done vs IR contamination. Most filters will cut into visible light. The resulting effect is a colour shift to the image that is very easy to colour correct. The alternative contamination is much harder and more destructive to correct. Choosing the filter that will walk the line and leave the cleanest image can be a difficult choice, especially as manufactures find new ways to balance the filtration. Alternately some of the warmth and glow of skin tone is a result of a little bit of IR.
The missing piece of the puzzle in solving the great mystery of what filter to chose comes down to the closely guarded secrets of camera manufacturers. I have asked several manufactures about the spectrum of light that their cameras are sensitive to. Unfortunately this is like asking a magician to reveal his tricks. There are ways to test a cameras colour response, but I will leave that for another test.