Surreal Images from an Invisible Dimension

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Propelled largely by the digital revolution, the expansion of this genre of photography is made possible because the silicon-based sensors in modern digital cameras have the capacity to record electromagnetic radiation outside the visible spectrum.

Reflected Near Infrared (NIR) radiation can now be photographed digitally relatively easily and cheaply.

Infrared photography is not new; film-based infrared images have been produced for many decades. However, numerous drawbacks have limited its accessibility. With a film camera the photographer may have to shoot blind and because focal lengths change when visible light is blocked she/he may also have to guess at focus. In addition, infrared film is expensive and is generally not reliably processed by commercial labs; most are not set-up to deal with the special, one-up requirements. Some of these films have highly specialized uses, for example, in forensic, military, or forestry applications.

By contrast in the digital world NIR images are visible on the screens of small cameras with “live preview” giving the photographer greater control over the process.

Strange Yet Familiar Images

Images of the world as seen in NIR are at once eerily strange and yet hauntingly familiar. In bright sunlight reflected NIR produces strikingly different tonal values of common natural objects compared with the same images produced under visible light. For example, chlorophyll producing vegetation fluoresce in the NIR spectrum, hence leafy plants appear white, red, or even turquoise depending on the sensor and the visible light-blocking filter.

Unusal things can happen with manmade objects too. For example, some opaque objects like bars of soap reportedly become transparent under IR, with some cameras. In bright sunlight the same thing may even happen with certain textiles depending on the sensor and whether the material is dry or wet.

One camera manufacturer has modified a particular model with an especially sensitive sensor so that IR images cannot be shot in bright daylight on beaches to prevent this so-called “X-ray effect.”

False Color Schemes

Many color IR images are printed in strange colors. These are really false color schemes which add to the mystique and surreal quality of IR images. The colors are really artifacts buried in a camera's hardware and built-in software. It has no direct connection to the objects themselves or the reflected NIR itself. Colors aren't even defined in the NIR! Some digital IR photographers, me included, sometimes manipulate these color artifacts for aesthetic reasons in post-processing.

Infrared Images Aren't for Everyone

Infrared pictures aren't for everyone. As accomplished film IR photographer Josh Putnam once wrote, "... people either love [IR photography] or just don't get it, but the ones who love it really love it." That seems to be even truer among photographers than viewers. IR photographers commonly find their IR images generate greater interest than more traditional images. Many people appreciate IR's fresh view of things, but it's not just a matter of novelty. IR images have a rich beauty all their own.

The Physics of Infrared Radiation

For some people the science of light is difficult to comprehend. Light is a form of electromagnetic radiation which travels through space in waves having different lengths. Infrared photographs are produced by using portions of the spectrum of light that are invisible to the human eye, which contains sensors called rods and cones. These biological sensors detect light and dark as well as colors based on wavelength sensitivity. When activated these sensors transmit the information to the brain via the optic nerve. The rods and cones are only sensitive to electromagnetic radiation in a limited range. NIR is just outside and on the upper end of that range. X-Rays are among the shortest waves; terahertz radiation and microwaves are the longest waves.

Sunlight, including the invisible bands of light, spans a midrange and can be separated by wavelength through prisms. In order from shortest to longest, the visible light bands (violet, indigo, blue, green, yellow, orange, and red) measure 400 nanometers (nm) to 700nm in length. (Nanometer = one-billionth of a meter.) An easy way to remember the bands of visible light is with the grade-school reverse moniker "ROY G BIV." Sunlight is made up of the spectrum including ultraviolet light, visible light, and infrared.

What Is Sunlight?

Ultraviolet occupies about six percent of the spectrum with wavelengths from 10nm to 400nm. Visible light occupies about 46 per cent of the spectrum. The remaining 47 per cent is occupied by infrared light in wavelengths from 750 nm to 100 µm (µm is the scientific symbol for a micron, which is equal to one millionth of a meter.) While the human eye cannot detect "dark" light, it is incredibly sensitive within the visible spectrum, more so than most film and digital cameras. This has led painter Ron Donoughe, for example, to assert that "cameras lie." (Donoughe admits, however, that good digital photographs viewed on a computer monitor can come closer to what the trained human eye sees.) Just as visible light is divided into bands, so is infrared radiation. The International Commission on Illumination (CIE) recommended the division of optical infrared radiation into the following three bands

• IR-A: 700 nm-1.4 µm

• IR-B: 1.4 µm -3 µm

• IR-C: 3 µm -1 mm (millimeter=one-thousandth of a meter)

IR-A is the same as what I earlier referred to as Near Infrared (NIR) and is used in fiber Optic communications, remote control devices, and in infrared photography.

IR-B is also called Short Wavelength Infrared (SWIR) and is used in long distance communications. IR-A and IR-B are sometimes called "reflected infrared."

IR-C is sometimes subdivided into Mid Wavelength (MWIR), Long Wavelength (LWIR) and Far Infrared (FIR). MWIR and LWIR are sometimes called "thermal infrared." LWIR is used for Thermal imaging. Silicon sensors detect only NIR wavelengths; entirely different media are required for thermal imaging which is very expensive and is mainly used in medical and military applications.

Colors and Color Distinctions Disappear

Most common Infrared photographs capture Reflected Infrared Radiation, not heat. While colors and color distinctions disappear, what is captured is the intensity of reflected infrared waves. Although infrared images are produced using both color and black and white media, the colors that are produced are more a function of the contrast between mid-tone and shadow areas. How digital cameras have been programmed to interpret those tonal differences can have a big impact on the finished image.

Why Is the Sky Blue? What Causes Sunsets? And Where Did the Haze Go?

Particles in the atmosphere cause light to scatter. Shorter wavelengths scatter more than longer wavelengths do. The gases in the atmosphere scatter the shorter wavelengths in the ultraviolet to blue end of the specturm more than the red-yellow end of the spectrum. This is the reason the sky appears blue when the sun is highest in the sky. When the sun is lower in the sky, light has to pass trough more atmosphere to reach the viewer. More of the specturm is scattered including the longer red-yellow-orange end. This is known as Rayleigh scattering, and is the reason we see “sunsets.”

Larger particles in the atmosphere, such as water vapor and pollutants, cause even greater scattering of light. The result is haze which limits visual contrast in more distant objects. Every competent painter or photographer understands that objects in the foreground must be represented with stronger values than those in the background. Most photographers use ultraviolet-blocking filters to counteract some atmospheric haze which is especially troublesome when longer lenses are used. This is an aerial form of the Tyndall Effect, which is seen when flour, for example, is suspended in water.

Because the longer wavelengths of infrared radiation are less prone to scattering caused either by the gases in the atmosphere or by water vapor and pollutants, distant objects and close objects appear to have more similar tonal values in IR photography.

Haze provides an important visual cue for perceiving distance. The absence of this important, though largely unconscious, cue contributes to the surreal feeling produced by IR images. Other cues like perspective and depth of field are unaffected in IR.

Daylight Must Be Blocked

To capture infrared images visible and ultraviolet light must be blocked with a dark filter called an Infrared Pass Filter. These filters may also block more or less of the IR-A band depending on the opacity of the filter. Many variables can therefore affect the final product: the sensitivity of the film or digital sensor, the opacity of the filter, and the final processing. Some IR Pass Filters, like the Hoya R72, do allow small amounts of visible light from the high end of the red band to pass through (from about 680nm to 700nm). Depending on the sensor and on how the "white balance" is set images may have a magenta, blue, purple, or orange caste. For example, when shot using the "incandescent white balance," images may take on a blue caste, because that white balance setting is designed to correct for the strong reddish tone of incandescent light.

Infrared photography with film cameras involves the use of special infrared film. Opaque filters must be installed in front of the lens. This is the reason IR photographers using single lens reflex cameras (SLR) must essentially shoot blind. (This is largely true for both film and digital SLRs, although beginning with the 2009 models many DSLRs now have live preview. This feature makes hi definition video possible with an SLR, but also enhances a DSLR's usefulness for IR.)

To limit interference with the visible spectrum, most digital cameras produced within the last decade block much of the infrared spectrum with a transparent filter over the camera's sensor. This filter is called both a “hot mirror” or “IR cut filter.” Just as sensitivity of silicon sensors to NIR varies, so does the impact of the hot mirrors. Infrared photography is still possible with some digital cameras by blocking visible light with an opaque filter and shooting very long exposures. A tripod or other stabilizer is necessary. Handheld IR photography is made possible by removing the Hot Mirror and installing an opaque filter to block visible light. Once the IR Cut Filter is removed, the camera can no longer be used for visible light photography because IR distorts color. For best results, the opaque filter should be mounted in front of the lens to give the user a choice of filters, which can produce quite different results.

While some cameras with IR capability are produced and sold, many people decide to convert a digital camera to IR by employing a specialty service to remove the IR filter and even entirely replace the main sensor. The usual fee ranges from $250 to $400, over the cost of the camer itself. In addition to removing the hot mirror and replacing the main sensor, these services usually re-program white balance and adjust focal length settings. For those requiring an IR DLSR this is the only realistic option. For the technologically savvy individual with a little daring, it is possible to do it oneself for next to nothing on a small point and shoot with IR sensitivity. (WARNING: digital cameras contain capacitors which can produce a serious electrical shock. Children, and most adults, should never open a digital camera.) More post-processing is required with this alternative because of the color artifacts. In my experience, focal length differences between the visible and IR spectrums, have do not have a appreciable impact with point and shoot digital equipment.

My Infrared Cameras

I chose to convert an old point-and-shoot that had been sitting unused in a drawer for a very long time. The camera is a 3 MP Fuji FinePix A330. After dismantling the camera and removing the Hot Mirror I installed a homemade IR Pass Filter. Because of the construction of this particular camera, I concluded there was no reasonable way to install a pass filter in front of the lens or to use a commercially produced opaque filter without a special apparatus. Instead, I cut a piece of developed, but unexposed EktaChrome slide film in place of the IR Pass Filter over the sensor where the IR Cut Filter had been and closed up the camera. (And yes, the capacitor bit me, even after many months unused in a drawer.)

Many IR photographers have found that lower mega pixel cameras actually work better for IR photography. Higher mega pixel counts mean higher color definition. Since there are no colors in the IR band a high mega pixel count tends to create digital noise.

I also use an unmodified 5MP Canon A540 with a Hoya R72 filter mounted in front of the lens. This particular camera has significant IR sensitivity, but because some IR is blocked it cannot be used handheld for IR photography. IR images with a false color scheme have a strikingly different appearance with this setup. One big advantage of this camera is that I've been able to download a nondestructive hack to the camera's BIOS enabling me to shoot in RAW. This capability means greater flexibility in post processing.

Visit “Monochromatic Experiments with Digital Infrared”

Visit “Experiments with Digital Infrared and False Color”