Infra-Red Filtration for Digital Cameras
So far, in previous posts, I’ve covered the utility of Polarizers, neutral density, and soft effect filters as still quite useful in the digital realm. The last filter type I’ll cover in the Tech Corner (for now) is Infra-Red or IR for short.
What We Can and Can’t See
The visible spectrum contains all of the light we can, as humans, “see” and includes all of the colors of the rainbow. Outside of our retina’s response capability lies wavelengths both shorter, and longer than we can see. Ultraviolet (UV for short) contains wavelengths shorter than the color violet, or 400nm. IR contains wavelengths longer than the color red, approx 700 nm wavelengths).
There are applications where imaging subjects illuminated by frequencies outside of our visible spectrum might be desirable or even mandatory.
Forensic applications of image evidence photography benefit from that which we cannot see with our eyes yet are very revealing when illuminated and photographed with UV or IR. I’ve personally been involved in selling digital systems where, for instance, the Secret Service used IR imaging to examine altered signatures on checks and “funny money” counterfeits. The longer wavelengths penetrate paper and other translucent surfaces deeper than visible light, revealing what the eye cannot see.
In the days of Old
With the use of film (silver halide based) as our recording medium, the sensitivity and recording capability were heavily biased towards the shorter wavelengths, such as blue. In fact, the typical film is highly reactive to UV and is just one of the reasons that a UV filter was highly recommended when photographing outdoors when the sun would be blasting significant amounts of UV. Typically UV would be effectively scattered by water vapor in the atmosphere and would inhibit imaging long-distance clarity.
Films, while UV reactive, were relatively insensitive to longer wavelengths. As such, it took many years after the initial progress of creating film itself to even image red light significantly. In fact, orthochromatic films, which are completely blind to red, were still used until recently in darkroom applications when red filters would create masks. The “quick mask” in Photoshop is still a visual echo of this tool.
IR was a real struggle for film. Several film stocks were manufactured over the years, including Kodak’s Kodak High-Speed Infrared film (35mm only), as well as the Konica 750nmIR film made in both 35mm and 120 rolls. The ASA (now called ISO) ratings were very low, on the order of 12, so exposures were very long, and the film was rather grainy. There were others as well (like Agfa, Ilford), though these are discontinued. While it’s still possible to obtain IR film, there are a host of challenges to use them, including the demand to load the film in total darkness, due to their lack of anti-halation coatings.
Back to the Future!
Enter the digital realm! Silicon is wildly reactive to IR and relatively insensitive to UV. Virtually every digital camera sensor needs to be shielded with a dedicated IR blocking filter to achieve accurate color or it faces being swamped by the effect of IR even through the color filters.
However, if the goal is to image IR, silicon is vastly superior to anything created in the film days. There is so much reaction to IR within most sensors and even WITH the IR blocker in place, it’s still possible to place a Kodak Wratten 87C filter, or B+W 093 (opaque to all visible light and black to our eyes) over the lens of almost all digital cameras and still be able to create an image in that spectrum. More on this later…
But, if the built-in IR filter is removed from a camera system (typically installed on top of the sensor), then it becomes an ideal device to perfectly image in wavelengths longer than our eyes perceive. It can enable us to see in those images what would be “the dark”. Perhaps even more importantly, it allows us to see how everything in the world reacts to light outside of our visible range, and in the process create “otherworldly” looking images from ordinary subjects.
Imaging in IR using film can take the form of color or black & white, but imaging in digital is almost exclusively a black & white experience. While it’s perfectly possible to design a camera from scratch using separation filters that create false colors (looking like RGB images, which is what color IR film does), and filtering for different ranges within the realm of IR, almost every image captured by a digital camera will be displayed as a black & white image.
The spooky magic of IR imaging is that things that appear completely black to our eyes might be clear or white within IR. Many trees and foliage types typically look mid-tone grey in standard black and white conversions, but appear nearly white in IR images, almost as if self-illuminated.
It’s not only for scientific and forensic purposes that we can enjoy peering into the parts of the spectrum in which we normally cannot view.
As mentioned, it is still possible to create images within the IR spectrum without modifying a camera at all. Even though the installed IR blocking filter is very effective in most current DSLRs, there is a little bit of IR leak that will allow a faint image to be created when using an IR filter over the lens. This method is best used with a camera that has a “live view” capability, as an IR pass only is opaque to our eyes, and doesn’t allow viewing through the conventional viewfinder.
While I do not currently own a converted camera, my experiments with this method have convinced me that I really want one!
The images presented below are taken with a conventional DSLR using a B+W 093 77mm filter over the lens, which passes no visible light whatsoever. Its transmission is only 1% at 800 nm, increasing to 88% at 900 nm. Because of the neutral density created by having both an IR only pass over the lens, and an IR block over the sensor, my tests indicate that using this method to photograph in IR, around 14 stops of compensation are required!!! Fortunately, most digital cameras function satisfactorily at ISOs in the 2000-4000 range, which in combination with a long exposure locked down on a tripod, allows for the possibility of midday/sunny IR photography. It’s like photographing in the 1800s (except we have both live view and instant review!!)
It’s also important to note that longer wavelengths refract less than shorter ones (that’s what makes a prism splay color.), Thus IR will focus at a different focus setting on the lens than visible light. In the olden days, lenses were given an IR mark for this compensation, but if you can focus using live view (on screen), then compensation is unnecessary.
What’s really interesting is viewing the relationships between luminance values when imaging in the long wavelengths. It differs than viewing a visible light color captured image then converted to B&W. Foliage typically reflects IR and appears nearly white. Scenics are somewhat ghostly as a result. Some things that would appear black in a conventional photo will be light, and vice versa
IR and People
Human skin and hair appear quite different as well when viewed in IR. It can make a photo of people look quite different than what you would normally see in a portrait.
©2007 Ken Sklute
Eye and Mind Opening
It’s not expensive to begin your experimentation in alternate spectra photography, especially if all you are adding is a filter to your already existing camera. And, of course, if you’re patient enough to lock your camera on a tripod and endure 20-second exposures and dim live view focusing. But, the mind and vision-altering results might be well worth the time!
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