Short-wave infrared (SWIR) is typically considered to span from 0.9 – 1.7 μm. The SWIR spectral region is a particularly challenging region to perform measurements as traditional silicon sensors can only detect wavelengths shorter than 1.1 μm.1
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For SWIR detection, the unsuitability of silicon means indium gallium arsenide (InGaAs) detectors are used instead. InGaAs, and derivatives such as indium gallium arsenide phosphide, are semiconductor materials with tunable bandgaps covering most of the SWIR region. The band gap can be tailored for the application by changing the ratio of indium to gallium in the material structure.
Although needing InGaAs detectors poses some specific challenges and significant expense relative to cheap, visible silicon-based cameras, precise imaging information can be obtained with SWIR radiation. SWIR can be used to ‘see through’ objects to the layers and information underneath. This has made SWIR imaging a powerful tool for non-destructive imaging for applications such as art history and restoration.
SWIR
The basic principles of SWIR imaging are very similar to visible imaging. Objects absorb or reflect light of certain wavelengths that can then be translated into an image by a camera. In the captured image, there are regions of contrast, where more or less light has reached the camera, and also wavelength information.
In visible imaging, the wavelength information manifests as different colors in the image. While SWIR images appear black and white, they still contain the contrast information from the object. Many objects that are opaque in the visible region of the electromagnetic spectrum and absorb all incident light will be transparent to SWIR radiation. This means visible and SWIR imaging are highly complementary as SWIR imaging can recover information on parts of the object that are ‘hidden’ behind other layers.
SWIR imaging differs from longer wavelength imaging techniques as thermal imaging methods usually rely on photons emitted from the sample. The hotter the temperature of the object, the more thermal radiation that is emitted and the brighter the image. SWIR imaging does not rely on thermal emission but instead on photons absorbed or reflected by the sample, but SWIR imaging is still an effective tool in low visible light conditions.
Applications
One of the most rapidly growing applications of SWIR imaging is in machine vision. In machine vision, a camera is used to record visual information interpreted by some image recognition algorithm to feed information to a computer, often as part of a feedback control loop.
The ability to ‘see’ on the inside and outside of objects with SWIR as well as recover some information on the chemical structure of species as part of hyperspectral imaging set ups.2 This can be used for object recognition or material identification as part of sorting processes for recycling or manufacturing process control with defect identification. Many semiconductor manufacturing processes use SWIR cameras for their machine vision solutions.
The non-destructive nature of SWIR imaging and the ability to see ‘hidden’ structures underneath has made SWIR imaging a powerful tool in art inspection.3 SWIR imaging can reveal the pencil sketches underneath layers of paint and is often a better choice of imaging technique than near-infrared imaging as many pigments also have better transparency in the SWIR region. The main disadvantage is the greater price of the detectors needed.
Future Outlook
Biological imaging is one future application where SWIR imaging has great potential but faces current technological challenges. For biological imaging, visible light is mostly absorbed by the skin, and so longer wavelengths are preferred for their greater penetration depth. There are several regions in the near infrared and at longer wavelengths known as the ‘biological window’ where there are no species present in tissue that have significant absorption in these regions, and so the light used has a greater penetration depth.4
The challenges SWIR imaging faces for biological imaging are developing emitters in this wavelength region with high quantum efficiencies.5 Strong emitters give rise to better contrast images which are important in complex media such as tissue where there may be many overlapping structures. Developments in new nanoparticles with improved water solubility and other new tagging molecules will help exploit SWIRs advantages in terms of good transparency for biological imaging.
Many of the key challenges in the more widespread adoption of SWIR imaging are around the detection technologies available. While the InGaAs detectors used do not need to be cryocooled, their use in military applications means there are some regulations around their use and development. InGaAs detectors are often relatively limited in pixel size and noisy compared to visible light CCD/CMOS equivalents.
New InGaAs detectors with improved performance are becoming available as well as smaller detectors for portable applications. With the benefits of SWIR for many machine vision applications, finding ways to increase the affordability of InGaAs detectors or develop new detection schemes will be increasingly of interest.
References and Further Reading
Young, E. T. (2013). Long-Wavelength Infrared Detectors. In T. D. Oswalt & I. S. McLean (Eds.), Planets, Stars and Stellar Systems: Volume 1: Telescopes and Instrumentation (pp. 565–585). Springer Netherlands. https://doi.org/10.1007/978-94-007-5621-2_14
Hashagen, J. (2015). Seeing Beyond the Visible in machine vision. Optik&Photonik, 3, 34–37. https://onlinelibrary.wiley.com/doi/epdf/10.1002/opph.201500021
Gavrilov, D., Maeva, E., Grube, O., Vodyanoy, I., & Maev, R. (2013). Experimental Comparative Study of the Applicability of Infrared Techniques for Non- destructive Evaluation of Paintings. Journal of the American Institute for Conservation, 52(1), 48–60. https://doi.org/10.1179/0197136012Z.0000000002
Hemmer, E., Benayas, A., Légaré, F., & Vetrone, F. (2016). Exploiting the biological windows: Current perspectives on fluorescent bioprobes emitting above 1000 nm. Nanoscale Horizons, 1(3), 168–184. https://doi.org/10.1039/c5nh00073d
Thimsen, E., Sadtler, B., & Berezin, M. Y. (2017). Shortwave-infrared (SWIR) emitters for biological imaging: A review of challenges and opportunities. Nanophotonics, 6(5), 1043–1054. https://doi.org/10.1515/nanoph-2017-0039
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