Short-wave infrared (SWIR) light is radiation with wavelengths spanning the range of 0.9–1.7 μm. This light can permeate through materials deemed impenetrable to visible light, making it perfect for industrial processing applications and quality control measures, machine vision applications, surveillance, security, and the management of natural resources. Water vapor, fog, and silicon are a few examples of substances that appear transparent when viewed with SWIR.
With SWIR light, firefighters can see through smoke and ash to keep teams safe and identify rescue targets. Image Credit: Avantier Inc.
More Special Uses of SWIR
Materials' physicochemical and thermal properties are often revealed in the way they reflect or emit infrared light. This means that SWIR light can, in some circumstances, be used to identify specific materials remotely. Distinct SWIR bands can also be used to establish a substance's water and lipid concentrations. The capacity to gauge hydration gives SWIR an important role in the fields of agriculture and medicine as well as in applications across various industries.
The color image of the apple was captured using conventional visible light techniques. In contrast, the black-and-white image was taken using SWIR imaging. Notice that the bruise, which was invisible to the naked eye, is now immediately obvious in the SWIR image due to the higher moisture content contained in that area of the pear.
Image Credit: Avantier Inc.
Shortwave Infrared Cameras
Natural radiators of SWIR include ambient starlight and night glow, the background radiance that the atmosphere emits. Night glow appears five to seven times brighter than starlight, but you never see this illumination: being SWIR, it is invisible to the human eye. Therefore, night-time imaging devices should be optimized for the SWIR spectrum, as lenses typically developed for visible light will generate poor-resolution images with optical aberrations.
But how is it possible to just optimize an imaging device for SWIR light? Since SWIR can pass through silicon, silicon-based cameras cannot detect SWIR. Instead, SWIR cameras typically use sensors based on indium gallium arsenide, a low-cost, practical material that can absorb the SWIR radiation and transform it into a readable charge.
InGaAs sensors can be extremely sensitive, with the capacity to detect scattered photons of SWIR light. They can potentially run high frame rates and produce images with low noise profiles. It is possible to detect weak signals, which can then be imaged with a long exposure time. InGaAs imaging devices are well-suited for use in machine vision applications and laser beam profiling, as well as in various medical and research applications.
Where other options, such as mercury cadmium Telluride (HgCdTe) and indium antimonide (InSb), require the extensive use of cooling technologies, InGaAs do not. Moreover, where InGaAs image sensors are regulated by international treaties on arms regulations, linear line-scan InGaAs sensors are available commercially without restriction.
SWIR image captured with InGaAs technology. Image Credit: Avantier Inc.
Quantum dot sensors can be another consideration when it comes to SWIR imaging. These sensors use semiconducting nanoparticles with unique photoluminescent and electroluminescent properties—known as colloidal quantum dots—on silicon wafers. Though their performance is not that of InGaAs sensors, they are more economical to produce. The colloidal quantum dots may be manufactured using lead sulfide, indium arsenide, or other alternatives.
How does SWIR compare with thermal images?
Thermal imaging devices detect radiated heat in the IR range. Conversely, SWIR imaging uses SWIR light reflected off an object similar to the way conventional photography cameras use the visible light reflected off of objects. In effect, a researcher using a thermal imaging device will be able to identify the presence of a warm object against a cold object on a dark night, whereas an individual equipped with a SWIR camera is far more likely to be able to identify exactly what the object is.
How can SWIR be used for remote sensing?
Since some visible light-blocking substances can transmit SWIR, short-wave infrared cameras can usually determine the internal structure of objects without the need to break them open.
Can SWIR be used to inspect the inside of pharmaceutical bottles?
Yes, since SWIR light is not impeded by the opaque plastic used in pharmaceutical bottles, it can be applied to view or determine the fill levels of the powder or pills contained inside such containers.
Can SWIR be used for night vision in an enclosed place?
SWIR imaging depends on the presence of SWIR light. Where there is no atmospheric night glow, for example, in a tunnel or underground room, an object or area can be ‘lit up’ with SWIR radiation using invisible SWIR lasers or LEDs.
This information has been sourced, reviewed and adapted from materials provided by Avantier Inc.
For more information on this source, please visit Avantier Inc.