RED Hot Mirror Shoot-Out
We test Hot Mirror filters from three major manufacturers to see which does what on the RED
By Art Adams | February 11, 2009
Not all infrared or Hot Mirror filters are made the same, and not all work the same way on different cameras. In this test we looked at filters from Schneider, Formatt and Tiffen to see how they perform on the RED ONE.
First, let's review what the issue is with HD cameras and IR:
The visible spectrum runs from approximately 400 nanometers to 700 nanometers in wavelength. Unfortunately, silicon is most sensitive to light energy above the visible spectrum: it's the perfect infrared imaging material. Without some sort of "Hot Mirror" filter (a filter that reflects heat, or IR) this invisible energy will overwhelm the visible light hitting the sensor and ruin our images. Most cameras are built with a Hot Mirror filter mounted to the front of the sensor, but some Hot Mirrors are better than others.
Under normal conditions there is a certain amount of what is called "far red" (red light beyond the visible spectrum, but not yet IR heat energy) captured by a sensor, but it is easily overwhelmed by the amount of visible light in the image so we don't normally see it:
Neutral density filters block visible light but not far red or infrared, so adding ND filters reduces the ratio of visible light to far red light hitting the sensor. At some point that ratio becomes low enough that far red becomes visible and is easily seen in reflective synthetic materials and some clothing dyes, which makes those materials appear a muddy maroon or, in some cases, bright blue.
Here's a test setup shot on a RED with no filtration:
Here's the same shot as above but with a Schneider ND 1.2 filter:
Believe it or not, the bright blue in the black jacket on the left of frame is IR contamination. In the past I've only seen IR turn dark objects maroon, and at first I had no idea what to make of this. A conversation with Adam Wilt confirmed that not only was this IR contamination, but that the blue hue was not uncommon. Apparently the blue filtration on most sensors is very good at isolating blue in the visible light spectrum but not always so good at blocking light in the non-visible part of the spectrum. Under certain conditions such as this test, where we're using a camera that is nominally daylight balanced in a tungsten environment with ND (not a common scenario), this small weak point in the sensor's blue filtration becomes apparent.
Until single-chip cameras became fashionable we rarely saw these issues. The prisms in three-chip cameras pass only very specific wavelengths of light and are very good at excluding far red and IR. Single chip cameras, though, are a completely different story, and those I've tested so far benefit from additional filtration in situations where the ratio of visible to far red light, or IR energy, becomes too low.
The filters we tested all worked in slightly different ways, and it's fascinating to see what works well and what doesn't, and why. What's more, these filters seem to be camera specific, so what works on one camera may not work on another.
All of the footage used for this test was shot in Redcode 28, 4k 16:9, and saved as 2k DPX files, which were then brought into Adobe Photoshop CS4 and white balanced by using the white, gray and black pickers, in the curves adjustment tool, using the following targets:
The white balancing was necessary as several of these filters imparts a subtle hue to the image, which is then easily removed in post.
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