SWIR, MWIR, And LWIR: One Use Case For Each

By John Oncea, Editor

SWIR, MWIR, and LWIR describe different ranges of the electromagnetic spectrum, primarily in the context of infrared (IR) radiation. All three correspond to a specific range of wavelengths within the IR spectrum, and they are commonly used in various applications including remote sensing, thermal imaging, and spectroscopy.

The choice between Short-Wavelength Infrared (SWIR), Mid-Wavelength Infrared (MWIR), and Long-Wavelength Infrared (LWIR) depends on the specific application, budget constraints, atmospheric conditions, and the desired features such as image quality and temperature sensitivity. Each region of the infrared spectrum has its strengths and limitations, making them suitable for different scenarios.

Here, we take a look at the differences and similarities between the SWIR, MWIR, and LWIR, as well as one specific use case for each of them.

Wavelength 101

SWIR, MWIR, and LWIR are all regions within the infrared spectrum, each with distinct characteristics, applications, and technologies. SWIR refers to the portion of the infrared spectrum that covers relatively shorter wavelengths and typically ranges from approximately 1,000 nanometers (nm) to 3,000 nm, or 1 to 3 micrometers (μm). SWIR imaging and spectroscopy are often used in applications like material identification, moisture detection, and certain medical imaging techniques.

MWIR encompasses wavelengths ranging from about 3,000 nm to 8,000 nm, or 3 to 8 μm. MWIR radiation is commonly used in thermal imaging for detecting heat differences, as well as in applications like missile detection and imaging through atmospheric haze.

LWIR covers longer wavelengths, typically ranging from around 8,000 nm to 15,000 nm, or 8 to 15 μm. LWIR radiation is often associated with thermal imaging and thermography, where it's used to detect the naturally emitted heat from objects. This range is particularly useful for applications like night vision, temperature measurement, and thermal analysis.

SWIR cameras are often more expensive due to the use of specialized materials and technologies while MWIR and LWIR technologies have become more accessible over time, leading to a wider range of cost options. Other characteristics include:

• Emission Sources
  • SWIR: Mainly relies on reflected sunlight or thermal radiation from warm objects.
  • MWIR: Primarily emanates from heated objects, often in industrial processes and military applications.
  • LWIR: Originates from objects with moderate to low temperatures, including room-temperature objects and cooler surroundings.
• Atmospheric Interference
  • SWIR: Experiences some interference due to water vapor and atmospheric scattering, but less compared to MWIR and LWIR.
  • MWIR: Suffers moderate interference from atmospheric conditions, especially water vapor and aerosols.
  • LWIR: Highly affected by atmospheric moisture, carbon dioxide, and other molecules, which can attenuate the signal.
• Detector Technologies
  • SWIR: Often employs InGaAs (Indium Gallium Arsenide) sensors.
  • MWIR: Uses materials like HgCdTe (Mercury Cadmium Telluride) or InSb (Indium Antimonide) for detectors.
  • LWIR: Uses materials like microbolometers, which are thermally sensitive materials that change their electrical resistance with temperature.
• Imaging Quality
  • MWIR and LWIR are more commonly used for thermal imaging due to their ability to detect heat signatures from objects and environments.
  • Generally, LWIR cameras tend to provide lower spatial resolution compared to visible and near-infrared cameras due to longer wavelengths.

SWIR is used in various applications such as surveillance, agriculture, food sorting, and some medical imaging, and MWIR is often applied in military applications (night vision and target detection), industrial process monitoring (high-temperature processes), and environmental monitoring. LWIR is widely used in thermal cameras for security, surveillance, firefighting, medical imaging (especially for detecting temperature differences in the body), and industrial inspection.

Next-Gen SWIR Emitting Probes

SIREN is a German and French research project developing contrast agents for SWIR imaging. The project focuses on inorganic nanomaterials, such as gold nanoclusters and semiconductor quantum dots, to develop new SWIR contrast agents and advanced image analysis. The project brings together experts in quantitative optical spectroscopy, photo-physics, probe design, and optical imaging focusing on inorganic nanomaterials such as gold nanoclusters and semiconductor quantum dots.

The Bundesanstalt für Materialforschung und -prüfung (BAM) writes, “Biomedical applications of optical imaging have been extensively developed over the last years. Challenges for sensitive bioimaging with fluorescence techniques are a high penetration depth, high contrast, and high spatial resolution, which are hampered by light scattering, absorption, and tissue autofluorescence. Therefore, new contrast agents with an emission in the short-wavelength infrared (SWIR) (900-1700 nm) are required, which have a high absorption and a high luminescence quantum yield.

“In the project SIREN, BAM and IAB will design and synthesize bright SWIR emitters from gold nanoclusters (AuNCs) and heavy metal-free Ag2S quantum dots (QDs), and explore their optical properties, particularly their luminescence quantum yields and brightness. Signal-relevant spectroscopic key features will be compared with models (optical phantoms) and bioimaging studies. Also, tools and test materials will be developed to standardize SWIR luminescence measurements.”

MWIR: Keeping Things From Exploding

MWIR imaging cameras are the preferred choice for clear thermal imaging over 1km in defense, unmanned aircraft systems (UAS), counter-UAS, security, and long-range surveillance. They are also utilized by the oil and gas industry to detect and visualize emissions of hydrocarbon gases such as methane, propane, and butane, according to Sierra-Olympia Technologies.

The use of MWIR cameras in the oil and gas industry is crucial in ensuring the safety of workers. These cameras can quickly detect emissions and identify leaks, leading to increased operational efficiency and profitability. Additionally, MWIR-based hydrocarbon detectors have environmental benefits, as they help reduce the harmful effects of gas emissions on the atmosphere. This reduces the likelihood of fines from regulatory agencies like the EPA.

“Hand-held MWIR cameras can detect leaks within 1 to 25 meters (~3 to 82 feet), with a standard 25mm fixed-focal-length optic, and up to several dozen meters (~80 feet) with the use of telephoto lenses,” writes Sierra -Olympia Technologies. “When a larger inspection area is required, fixed-mounted MWIR cameras on motorized pan and tilt positioners can provide continuous monitoring in 360° for large radius coverage. If operators need to image transmission and distribution systems such as remote pipelines, MWIR cameras mounted on aerial platforms such as fixed-wing aircraft and/or helicopters can remotely detect leaks at up to 500 meters (1640 feet) line-of-sight with the appropriate lens.”

MWIR cameras use sensors that detect infrared radiation emitted by objects in the 3-5μm spectral range, instead of CCD or CMOS imagers that capture visible light. Photon-sensitive focal plane array (FPA) detectors made from indium gallium arsenide (InGaAs), lead selenide (PbSe), mercury cadmium telluride (MCT), and indium antimonide (InSb) have traditionally been used in these cameras. InSb-based mid-wave-infrared imagers are favored in optical gas imaging cameras due to their excellent price/performance ratio.

However, InSb MWIR FPA detectors have a major drawback: they require cooling to liquid nitrogen temperatures (down to 77K) to become photoconductive and suppress noise that would otherwise degrade performance. Consequently, the focal plane arrays of these detectors must be integrated into cryogenic coolers, increasing size, mass, power consumption, and cost.

Fighting Fire With LWIR

With wildfires burning across much of the U.S. and Canada, LWIR technology has been called upon to help fight them because of the technology’s ability to detect heat signatures. This makes it useful for locating and monitoring fires, even in conditions with limited visibility due to smoke, darkness, or other factors.

Following are eight ways LWIR technology is being applied to firefighting:

• Early Detection and Monitoring: LWIR cameras can detect heat sources associated with fires before they become visible to the naked eye or traditional cameras. This early detection allows firefighters to respond quickly, potentially preventing the fire from spreading further.
• Hotspot Detection: LWIR cameras can identify hotspots, even those hidden behind walls or structures, by detecting areas with elevated temperatures. This helps firefighters target their efforts more effectively and avoid potential flare-ups.
• Navigation and Situational Awareness: In low-visibility conditions caused by smoke or darkness, LWIR cameras can help firefighters navigate and maintain situational awareness. This is crucial for their safety and for efficiently directing firefighting operations.
• Search and Rescue: LWIR technology can assist in locating people who might be trapped in smoke-filled or dark environments. The heat emitted by a person’s body can be detected, helping search and rescue teams locate individuals more easily.
• Aircraft and Drones: Aircraft and drones equipped with LWIR cameras can provide aerial views of fire-affected areas, helping incident commanders assess the extent of the fire, track its progression, and allocate resources accordingly.
• Fire Behavior Analysis: LWIR cameras can monitor fire behavior, such as the spread rate and intensity of the flames. This information is crucial for making informed decisions about firefighting strategies and resource allocation.
• Hazardous Materials: LWIR technology also can be used to detect and monitor hazardous materials that might be involved in a fire, helping responders take appropriate safety precautions.
• Post-Fire Assessment: After a fire is extinguished, LWIR cameras can be used to conduct post-fire assessments to ensure that no hidden hotspots remain that could potentially reignite the fire.

It's important to note that while LWIR technology has valuable applications in firefighting, it's not a stand-alone solution. It complements other firefighting tools and strategies, such as traditional firefighting equipment, communication systems, and the expertise of trained firefighters. Additionally, LWIR technology can have limitations in certain conditions, such as when there's interference from other heat sources or when the fire generates excessive smoke, which might affect the accuracy of heat detection.