What is SWIR Technology?

What is Short-Wave Infrared Technology or SWIR?

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- GoPhotonics

Jan 17, 2023

Short Wave InfraRed is a light that comes under the electromagnetic spectrum at a wavelength range between 900 nm and 2500 nm. This range falls in between the near-IR and Mid IR wavelength bands. The light in this band is invisible to the human eye. This wavelength range can only be seen using specific sensors like Indium gallium arsenide (InGaAs) and Mercury Cadmium Telluride (MCT). These sensors are very expensive, application dependent, and have lower resolution and heavier weights when compared to other visible light or thermal infrared imagers.

SWIR imaging requires sensors and camera modules that can operate in the shortwave infrared spectrum, which goes beyond silicon's maximum limit which is 1 μm. The only wavelength technology that can take clear pictures through cloud cover is SWIR cameras. The primary sensor used in SWIR imaging is InGaAs. These sensors can cover a range as high as 2.5 μm and as low as 550 nm. They can detect both Near IR (NIR) and SWIR. No cryogenic cooling is needed for InGaAs sensors. Ambient starlight and background radiance are natural sources that generate or emit SWIR and excellent night time imaging is possible with them. The electromagnetic spectrum showing the SWIR wavelength band is depicted in Figure 1.

Figure 1: Electromagnetic spectrum depicting the SWIR wavelength range

In the same way, as photons from visible light are reflected or absorbed by objects, SWIR photons behave similarly, giving the significant contrast required for high-resolution imaging. That is, the wavelength within the SWIR band will interact with objects like visible light. The photons in this band are reflected or absorbed by objects and this allows high-resolution imaging with high contrast. The images obtained are in black and white but not in color. In the SWIR spectrum, molecular vibrations are the main reason for absorption. Using SWIR, many applications that are difficult or impossible to perform with visible light can be performed. Water vapour, fog, and other substances, including silicon, are transparent when imaging in SWIR. Also, SWIR makes it easy to distinguish between almost identical colors which are difficult to identify using visible light. 

SWIR Imaging Equipment

SWIR imaging sensors have become commercially available even though not as common as digital cameras or thermal imagers. Most of the SWIR imaging sensors are made of Indium Gallium Arsenide detector arrays. There are also other detectors such as Indium Antimonide (InSb), Germanium (Ge), and Mercury Cadmium Telluride (HgCdTe) detectors. InSb is available with a broad spectral response. But InGaAs arrays have been more practical because of their higher quantum efficiency and low dark current at room temperature. 

Advantages

  • Low light imaging: Due to their high sensitivity to light, SWIR cameras can capture and detect single photons with individual pixels of the focal plane array. Even on moonless or starless nights, they can see objects with a high level of detail.
  • Vision through fog and haze: This wavelength can penetrate through fog, haze, and other atmospheric conditions. Since they can see through these obstacles, superior images are produced.
  • Effective for Identification of high contrast: SWIR imaging technology can be used at night without additional illumination. SWIR wavelength has properties like reflection and contrast.
  • Vision through glass: SWIR can see through glass. In addition to allowing SWIR cameras to see through most windows, this also makes glass lenses and housings more accessible.

Disadvantages

  • Expensive: InGaAs and InSb focal plane arrays are really expensive which makes the SWIR cameras also expensive
  • International Traffic in Arms Regulations (ITAR) required: Due to their high performance, this technology is protected by the ITAR which places restrictions on where these products can be exported and who can buy them.
  • Preferred special lenses: For maximum detailing, specially developed lenses are preferred that can focus wavelength in the 0.9 - 1.7 µm region.

Applications 

SWIR cameras are used in a variety of applications in industry and research, ranging from inspection, quality control, identification, detection, surveillance, etc. They have applications in the inspection of food/products, solar cells, circuit boards, and counterfeit.

  • Inspection: While silicon electroluminescence occurs in the SWIR spectrum, silicon-based CCD or CMOS image sensors are unable to measure SWIR spectrum energy. Similar to how applying a voltage across an LED will light the LED, applying a voltage across silicon-based solar cells will illuminate the cells. This feature is useful for solar cell inspection.
  • Military Surveillance: Because of its longer wavelength, SWIR is less affected by the Rayleigh scattering effect and this enhances visibility. SWIR travels through largely unscattered, whereas small particles (haze, smoke, etc) scatter visible light. The main benefit of improved long-range visibility in surveillance applications over visible light cameras is the capacity to see through the haze.
  • Remote Sensing: The SWIR spectrum is famous for its sensitivity to moisture in remote sensing applications, which can be connected to significant metrics like leaf water content and other physiological states of crop canopies.

Other Applications:

  • Astronomy
  • Free space optical communication
  • Laser beam profiling
  • Spectroscopy
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