Movie heat vision has trained most of us to expect infrared cameras to show glowing orange bodies against a cold blue background. That is thermal imaging, which picks up the heat objects give off. A less famous branch of infrared imaging behaves much more like an ordinary camera, and some of what it shows is stranger.
Short-wave infrared, usually shortened to SWIR, lies beyond the red end of what human eyes can see, past the near infrared. Cameras built for it use sensors made of indium gallium arsenide, or InGaAs, and most work between about 900 and 1,700 nanometers, with extended versions reaching past 2,500. Like visible light, the SWIR light these cameras record is mostly reflected light, so the pictures look like black-and-white photographs. The surprises are in which things change color.
Water turns black
Water absorbs SWIR light strongly, and its absorption jumps around 1,450 nanometers. In a SWIR image, wet patches go dark and a glass of water can look as if it were full of ink. The same effect makes moisture inside materials visible, which is one reason these cameras are used to inspect products on industrial lines.
Silicon becomes see-through
Silicon, the material computer chips are made of, is opaque to your eyes. At SWIR wavelengths it lets light through, so a silicon wafer looks closer to a window. Chip makers use these cameras to inspect wafers for defects inside the material, something a visible-light camera cannot reach.
Why your phone camera can’t do this
The same property explains why SWIR needs special sensors. Silicon is transparent at these wavelengths because the light does not carry enough energy to be absorbed by it, and a sensor that cannot absorb light cannot turn it into a signal. The silicon chips in phones and ordinary cameras stop responding a little past 1,000 nanometers. Indium gallium arsenide keeps absorbing well beyond that, which is why InGaAs became the standard sensor material for short-wave infrared, and why these cameras still cost far more than a visible-light camera with the same resolution.
Paintings show their sketches
Art historians use a related technique called infrared reflectography. Many paint layers let infrared light pass, while the carbon-based materials that artists used to sketch their compositions absorb it. Point a SWIR camera at an old painting and the underdrawing can appear, along with the changes the artist made while painting, which conservators call pentimenti. Infrared reflectograms of the Mona Lisa are among the best-known examples.
Haze and camouflage
Fine particles in haze scatter short wavelengths of light more than long ones, so a SWIR camera can show detail in a hazy landscape that looks washed out to the naked eye. Defense and border-security users value SWIR for a related reason: some materials that blend into their surroundings in visible light stand out clearly in short-wave infrared. More of these cameras are now flown on drones for exactly that kind of work.
What it cannot do
SWIR does not show body heat. That is the job of thermal cameras working at the longer mid-wave and long-wave infrared wavelengths, the ones behind the glowing figures in the movies. It also cannot see through walls, whatever the ads for “X-ray specs” in the back of old comic books promised.
What it does is reveal properties of materials that visible light hides. Scientists use SWIR cameras for spectroscopy, reading the composition of a material from the wavelengths it absorbs, and astronomers use them to image faint objects that shine in the infrared. In medicine, the same detectors are used in optical coherence tomography, which builds cross-sectional images of tissue, and in SWIR fluorescence imaging. Many of these cameras cool their sensors with thermoelectric coolers so the image stays clean during long exposures.
If you want to go one level deeper, the scientific instrument distributor Merkel Technologies has a readable primer on InGaAs cameras that walks through how the sensors work and where they are used, from wafer inspection to drones.
The next time a film switches its hero to infrared vision, it will almost certainly be the thermal kind. A SWIR version would make for a stranger scene: a glass of water gone black, a computer chip you can see through, and an old painting quietly showing the sketch underneath.
Amanda Dudley is a lecturer and writer with a Ph.D. in History from Stanford University. After earning her doctorate in 2001, she decided to pursue a fulfilling career in the educational sector. So far, she has made giant strides by working as an essay writer for EssayUSA, where she delivers high-quality academic papers to students who need them.
