Short-wave infrared (SWIR) vs Near infrared (NIR) and SWIR Imaging
Short-wave infrared (SWIR) defines a specific wavelength range over which optical and electronic components are designed and coated.
What is near infrared (NIR) and shortwave infrared (SWIR)? What are their advantages and suitable applications?
Typically we define near infrared (NIR) from 780 nm to 1400 nm and shortwave infrared (SWIR) from 1400 nm to 3000 nm. But it is also common to refer to the entire range from 780 nm up to 3000 nm as NIR or SWIR.
The NIR/SWIR region has multiple advantages, and some of the most well-known ones are discussed below.
Beyond visible
NIR and SWIR wavelengths aren’t visible to human eyes and have less energy than UV and visible wavelengths, but the light still interacts with objects. So in the case of NIR/SWIR imaging, we can capture images of objects and see aspects that we couldn’t see in the visual range.
In addition, longer wavelengths can be used to look through plastic packaging and silicon, providing another method for nondestructive testing.
Example applications:
- Automotive
- Surveillance
- Remote sensing
- Wafer inspection
- Packaging and filling inspection
Silicon wafer: (Left) Visible image, (Right) SWIR image.
Water content measurement
Water has a high absorption rate at 1450 nm and 1900 nm, making it easy to detect areas with high water density using NIR/SWIR.
Example application: Food quality sorting
Food sorting
Identifying contaminants (stones) in coffee beans
Thermal detection
SWIR has the capability to detect heat even though it’s not as sensitive as MWIR (midwave infrared) and LWIR (longwave infrared), which can detect low temperatures. Unlike MWIR and LWIR imaging that blur the contours of objects in an image, SWIR imaging produces clearly defined images with added temperature information, allowing the detection of hot spots in a scene or object.
Example application: Process control
Material differentiation
Between 780–2500 nm, substances such as plastic, organic compounds, and non-organic compounds can be easily differentiated, which is hard to do in the visible range. Hyperspectral SWIR imaging can provide detailed information about an object based on its infrared spectrum.
Example applications:
- Plastic sorting
- Agriculture
- Pharmaceutical inspection
Example of hyperspectral imaging
Different plastic types: (Left) Visible image, (Right) After hyperspectral imaging.
How are NIR and SWIR detected?
There are multiple methods and materials to detect NIR and SWIR including InGaAs (indium gallium arsenide), MCT (mercury cadmium telluride), and QDIP (quantum dot infrared photodetector). These three are discussed below.
InGaAs (indium gallium arsenide)
InGaAs is the most well-known material for NIR and SWIR detection. Like silicon photodetectors, InGaAs photodetectors are photovoltaic detectors with a p-n junction, but they have a smaller bandgap energy than silicon, so they detect a longer wavelength range. Standard InGaAs can detect light from 900 nm to 1700 nm, and extended InGaAs can go all the way to 2500 nm. InGaAs detectors are suitable for most NIR/SWIR applications due to their high sensitivity and high linearity.
ProsConsHigh sensitivityDark current is sensitive to temperatureHigh linearity
To see Q&As about InGaAs, click here.
MCT or HgCdTe (mercury cadmium telluride)
MCT detectors use the photoconductive effect, where the resistance value of the detector element decreases when exposed to light. MCTs have a wide bandwidth detection range, typically from 2 µm to 14 µm but can vary within 0.8 µm and 30 µm. They are suitable for wide bandwidth applications that have no RoHS requirement.
Pros
ConsHigh sensitivityHigh costWide bandwidth detectionLow linearity Restricted by RoHS directive Typically requires Stirling coolers
QDIP (quantum dot infrared photodetector)
Quantum dots are a new solution for certain NIR/SWIR applications. QDIPs use a layer of quantum dots on a silicon surface to create a three-dimensional quantum confinement active region, enabling the absorption of longer wavelengths. They are suitable for low cost and temperature-critical applications.
ProsConsLow costLow uniformityIndependent from temperature (or no temperature dependence)Low sensitivity
An article in EETimes Europe from August 24, 2022 argues that huge changes are happening in the consumer and automotive SWIR imaging industry. Some excerpts below.
https://www.eetimes.eu/how-smartphones-will-disrupt-the-swir-imaging-industry How Smartphones Will Disrupt the SWIR Imaging Industry
August 24, 2022 Axel Clouet and Eric Mounier
Sensing SWIR radiation requires imagers based on other materials, making them orders of magnitude more expensive than silicon-based imagers. Therefore, SWIR’s use today is limited to specific applications in defense, industry, or research.
… [A] pull from the consumer market is inspiring unprecedented changes in the SWIR industry, with the emergence of new technologies and the entrance of game-changing players who may enable market and technology disruption.
A newer technology, based on quantum dots (QDs), is emerging as a lower-cost alternative to InGaAs. … with a manufacturing process that is compatible with CMOS, allowing cost reductions by orders of magnitude.
QD technology is still emerging, with the first commercial products released in 2018 for the industry by SWIR Vision Systems.
SWIR’s technology development will be accelerated by the entrance of game-changing players: Sony released its first commercial SWIR imager in 2020, and in 2021, STMicroelectronics announced the development of SWIR imagers based on QDs. … [both are] leading companies in the consumer and automotive silicon-based imaging industry. Sony introduced a manufacturing method based on copper-to-copper bonding, inherited from its know-how in silicon-based imaging, to make InGaAs SWIR imagers. STMicroelectronics published initial results for its SWIR imagers based on QD technology … demonstrated high sensitivity, optimized at about 1.4 µm.
[Yole Intelligence] expect[s] the number of industrial cameras to increase significantly in the coming years, thanks to price decreases linked to QD technology penetration and the introduction of new manufacturing processes for InGaAs. These segments could represent a US$828 million market in 2027 at the camera level.
[Since] an artificial SWIR source needs to be used in combination with the imaging system. The SWIR source market should therefore benefit from the growth of the SWIR imaging market. SWIR edge-emitting diode lasers (EELs) are widely used today in the telecommunications market, … SWIR vertical-cavity surface-emitting lasers (VCSELs) should strongly benefit from the emerging consumer and automotive SWIR markets.
