Photographers use light to make art, while scientists, from medical researchers to wildlife biologists, use it to make discoveries. Meanwhile, military and surveillance cameras use light and heat to detect and remediate threats.
It turns out that Superman’s X-ray vision isn’t entirely a fantasy: some cameras can “see” what’s inside a plastic bottle or detect bruising on a piece of fruit that isn’t visible to the naked eye. These specially designed cameras use materials sensitive to radiation emitted or reflected in specific wavelengths within the infrared spectrum, but various types are available. Learn the differences between SWIR, MWIR, and LWIR cameras.
The Infrared Spectrum
Visible light exists along a very narrow portion of the electromagnetic spectrum. Energy gets measured in wavelengths extending from gamma rays with a wavelength of 10-15 micrometers all the way up to radio waves with wavelengths up to one millimeter. Visible light exists within the range of 0.4 to 0.7 micrometers.
Infrared cameras detect light or heat at wavelengths ranging from 0.7 to 2.5 micrometers (short-wave infrared light), three to five micrometers (middle-wave infrared light), or long-wave infrared light (eight to 14 micrometers).
Cameras made especially to detect these wavelengths include SWIR, MWIR, or LWIR cameras. The differences between SWIR, MWIR, and LWIR cameras revolve around what they’re made of and how they get used.
SWIR Cameras
The significant difference with short-wave infrared cameras (SWIR cameras) is that they capture light reflected or absorbed by an object. In contrast, MWIR and LWIR cameras detect heat emitted by the object (or animal).
SWIR cameras can “see” defects in fruits and vegetables, capture images of the night sky, or create artistic nighttime cityscape images. The pictures these cameras produce look a lot like regular black-and-white photographs. These cameras can “see” through fog, clouds, and hazy air pollution to create clear images.
Agriculture, electronics manufacturers, counterfeit currency detectors, and solar cell inspectors use SWIR cameras for quality control and to identify fraudulently printed paper money. They can also help art curators detect fakes or discover an artist’s original plan for a painting in drawings beneath the paint’s surface.
SWIR technology relies on Indium Allium Arsenide (InGaAs), Germanium (Ge), or Indium Gallium Germanium Phosphide (InGaAsP) to detect light reflected in wavelengths that silicon will not absorb.
While standard InGaAs cameras are typically sensitive from 0.9 to 1.7 µm, there are now cameras on the market with extended range. For example, Sony recently developed their SenSWIR technology by making sensors with high sensitivity from 0.4 to 1.7 µm, thus covering both visible, near-infrared, and SWIR wavelengths. This has many applications, especially for hyperspectral imaging. There are even range extenders now available for SWIRs that can take these cameras up to 2.2 µm, enabling other applications involving lasers at those wavelengths.
While linear line scan InGaAs sensors are generally available, it may surprise you to learn that area scan InGaAs sensors are governed by International Treaty and Arms Regulations (“ITAR”), which the United States government administers. This means that imports and exports of these devices are strictly regulated, and although they may be available for commercial purposes, the use of some types of SWIR lenses may require licensure.
MWIR Cameras
Middle-wave infrared cameras detect thermal radiation (heat) emitted at three to five microns (millimeters). They don’t work well through dust, smoke, air pollution, or fog, but they help detect gas leaks that the naked eye can’t see.
Drone-mounted MWIR cameras get sent to detect toxic gas leaks that aren’t visible or to check on machinery that uses hazardous gasses. Because they work better indoors where solar radiation doesn’t interfere with their thermal detection, MWIR cameras detect passenger body temperatures in airports and get used for other types of indoor surveillance. The hotter an object (person or animal) gets, the brighter it will appear to the MWIR camera.
Mid-wave infrared cameras excel at long-range surveillance applications. MWIR cameras have revolutionized airport perimeter security, vessel traffic monitoring, and critical infrastructure protection.
These cameras are also less susceptible to atmospheric conditions. To be detected by passive infrared cameras, infrared radiation must travel through the atmosphere to the detector. The range of detection can be limited by absorption, scattering, and refraction due to water vapor or CO2 in the atmosphere. This affects long-wave infrared light more severely than mid-wave radiation.
Thus, because MWIR radiation travels more easily through the atmosphere, MWIR cameras provide the longest detection ranges, seeing about 2.5 times farther than long-wave infrared (LWIR) cameras. Consequently, MWIR cameras can detect humans, vehicles, and objects even if they are many miles away.
MWIR cameras can operate in various climates, including those with high humidity, making them particularly well-suited to marine and coastal environments. Additionally, because they are typically smaller and lighter than LWIR cameras with similar optics, MWIR cameras are the obvious choice in applications with size, weight, and power (SWaP) requirements, such as airborne operations.
MWIR cameras may require integrated cooling capabilities to reduce “noise” created by ambient heat emitted by objects or people adjacent to machinery or objects getting measured.
MWIR camera detectors use Indium Antimonide (InSb), Lead Selenide (PbSb), or Mercury Cadmium Telluride (HgCdTe) to sense thermal emissions.
LWIR Cameras
Long-wave infrared cameras and technology get used in actual military operations. These cameras detect thermal emissions from people, animals, and vehicles when they emit heat at higher temperatures than the surrounding atmosphere.
LWIR cameras are less sensitive to thermal “noise” in the environment and thus are useful outdoors. They can “see” through foliage to detect heat emissions from enemy fighters, dangerous predators, or hidden vehicles. They can also detect roadway hazards in very dark conditions helping to avoid collisions with animals or vehicle damage by unexpected potholes or washed-out areas.
Home and building inspectors may also use LWIR cameras to locate areas with poor insulation or water damage. They can also detect damaged electronic parts that aren’t emitting the heat level they would if they were operating correctly.
Although the “band gap” between wavelengths in the LWIR range of eight to 14 micrometers can be very small, which would ordinarily make an instrument seeking emissions in this range susceptible to “thermal noise,” the materials used to detect heat in these cameras thwart that noise. LWIR cameras use microbolometers to detect heat.
Emitted heat strikes the microbolometer, usually made of vanadium oxide (Vox) or amorphous silicon (a-Si). The surface heats up (electrons become excited), and the camera measures the change in surface resistance in each pixel of the surface to create an image.
There are “cooled” versions of LWIR cameras used in research or medical applications that provide images with much greater detail. However, the cameras must get cooled to 77K or -321 degrees Fahrenheit, which requires specialized equipment and parts and can be prohibitively expensive. Most LWIR cameras are the “uncooled” type, which provides adequately detailed images for detecting the presence of people, animals, or vehicles.
How To Choose Your Infrared Camera
Whether you use an SWIR, MWIR, or LWIR camera depends largely on why you need to use it. They all do some things better than others. If you’re inspecting agriculture products, paper money, or works of art, SWIR might be the best choice. MWIR cameras are essential safety equipment for manufacturers and industries that use hazardous gasses or for gas utilities to detect hidden gas leaks.
Wildlife researchers may use LWIRs to track or record wildlife populations and movements, while military and government organizations may use them for security and defense purposes.
At Axiom Optics, our mission is to supply scientists, researchers, medical and pharmaceutical professionals, and companies with high-quality optical equipment that best serves their needs. Browse our collections and contact us with any questions about which equipment will best suit your operation, research, or project.
