Short-wave Infrared imaging (SWIR) is an advanced technique, used for producing images based on radiation in the region of the electromagnetic spectrum invisible to the naked eye.
All objects with a temperature above absolute zero (0K) will exhibit thermal emissions that are detectable from Mid- and long-wave infrared (MWIR/LWIR) spectral bands, making it possible for thermographic cameras to gather images based on localized temperature variations.
Short-wave infrared (SWIR) imaging is distinct in that the radiation of interest is nearer to the visible spectrum but will still permit temperature sensing, usually over 100 °C.
SWIR cameras have occupied a unique spectroscopic niche in scientific and industrial markets by extending into wavebands that are invisible to both MWIR/LWIR thermal imaging and conventional optics.
Some of the basic principles and applications of SWIR imaging are looked at in more detail in this article, in addition to how SWIR cameras have occupied a unique spectroscopic niche in scientific and industrial markets.
Understanding SWIR Imaging: The Basics
IR radiation extends from the red edge of the visible spectrum of light across a wavelength range of between 700 – 1,000,000 nanometers (nm). There are four distinct wavebands within that spectrum: near-infrared (NIR), MWIR, SWIR, and LWIR.
How objects reflect or emit IR radiation is indicative of a number of physicochemical and thermal properties. For example, based on their distinct chemistries, materials possess characteristic SWIR absorption/reflection characteristics which permit SWIR imaging to detect specific materials remotely.
SWIR imaging is not dependent on the inherent emissivity of objects, unlike MWIR/LWIR, so it is not usually characterized as thermal energy. SWIR imaging is also uninhibited by ambient working conditions that render visible light-based optics unsuitable.
There are multiple advantages to these properties which are increasingly exploited across a large market cross-section utilizing cameras based on cooled indium gallium arsenide (InGaAs) focal plane arrays.
SWIR refers to the portion of IR radiation adjacent to NIR, inhabiting a nominal wavelength range of between 1,400 and 3,000 nm. As there is no definitive standard of where each waveband separates, these wavelength ranges are described as approximations. Some classifications even think of SWIR as an extension of the NIR band.
InGaAs focal plane cameras are made up of an epitaxy grown InGaAs semiconductor bump bonded to a 2-dimensional readout integrated circuit (ROIC). Due to a wider bandgap, InGaAs absorbs IR light invisible to silicon and converts incident light into electrons digitized by the ROIC and the camera electronics.
Compared to silicon-based devices, InGaAs materials have a relatively high dark current. Thus, cooling is vital for the reduction of dark noise and the enhancement of the low light level capabilities of InGaAs focal plane arrays for SWIR imaging.
Key Applications of SWIR Imaging
SWIR imaging with cooled InGaAs focal plane arrays provides extremely high contrast with unprecedented resolution, enabling process engineers and researchers to visualize what was invisible previously.
It is easy to penetrate opaque materials non-invasively or to distinguish between regions that are chromatically similar using a SWIR camera. Some applications of SWIR imaging include:
- Industrial sorting
- Solar cell inspection
- Active imaging
- Range finding
- Enhanced vision assistance in misty conditions
- Free space optical communication
- Hyperspectral imaging
- Small animal imaging
- Laser beam profiling
This information has been sourced, reviewed and adapted from materials provided by Photonic Science.
For more information on this source, please visit Photonic Science.