Photonics Media article SWIR cameras.
A whole new waveband of the electromagnetic spectrum has been opened up for exploitation over the last few years, the Short Wave Infrared (SWIR). The band has traditionally been invisible to all detectors and ranges from the edge of the near IR region at 900 nm up to 1700 nm.
Now, for a wide range of applications, Indium Gallium Arsenide (InGaAs) sensors are able to image within this waveband. For instance, bruising of fruit can be detected by imaging the sub-surface accumulation of water, and stimulated photoemission silicon devices can permit the in-line inspection of their internal structure during manufacture.
Surveillance applications are able to benefit from decreased atmospheric scattering because of mist in the SWIR band and can benefit from the SWIR band “night glow” of a clear night sky. Invisible to the majority of detectors, SWIR band laser illuminators can supply high-quality night vision when coupled with a SWIR camera.
So it is possible to take advantage of this new part of the electromagnetic spectrum in a number of ways. Spectroscopy is the first obvious use. A normal CCD color camera has a sensitivity of up to around 700 nm, where the IR cut filter cuts off the near-infrared that would otherwise ruin the color balance of scenes intended to look natural to the human eye.
Without the IR cut filter, a monochrome CCD will see up to about 1000 nm. On the other hand, an InGaAs SWIR sensor has sensitivity from about 900 nm up to 1700 nm and so, makes an ideal component of a multi-sensor system for hyperspectral imaging.
Cameras are now available with a visible extension down to 400 nm and with waveband extension up to 2200 nm, making an extremely wide spectral sponge possible within a single camera.
The remote detection and identification of chemical spillage and staining is a recent spectroscopic application. Materials reflect, absorb, and even fluoresce in unfamiliar ways in the SWIR spectrum.
Characterizing these effects can enable the non-contact identification of materials to be performed where previously physical samples would have to be taken for chemical analysis.
For instance, in the recent EU-funded Opticlean project supporting the pharmaceutical industry, the spectral signatures of chemical stains are measured under broadband illumination in 120 wavelength bands throughout the SWIR spectrum, utilizing filters and an interferometer in order to isolate the individual wavelength contributions.
The nature and extent of the staining can be identified by comparing this signature with those of pre-characterized samples.
Operating in the 3 to 5 and 8 to 12-micron wavebands, conventional thermal cameras are well known for their ability to detect body heat. SWIR cameras also detect temperature changes but at higher temperatures (around 100 °C and over).
This makes them an ideal choice for remote temperature mapping in industrial conditions, like in the UK center for nuclear fusion research, the JET tokamak. In this application, by monitoring temperature changes within delicate parts of the enclosing toroidal structure, SWIR cameras supply a safety system to check the proper confinement of the plasma.
SWIR cameras are also being utilized by the surveillance industry. Rayleigh scattering of sunlight in the atmosphere, which is responsible for red sunsets and blue sky, and which causes the blue haze overlaying distant scenes, is strongly reliant on the wavelength of the light. Red is scattered least, and blue the most.
The longer wavelengths of SWIR are scattered even less by the atmosphere even further into the red, and so they have an ability to penetrate misty, hazy, or smoky conditions, vital in a long-range surveillance system.
Furthermore, compared to conventional visible band color cameras, SWIR wavelengths are less influenced by heat haze and atmospheric turbulence. While traditional thermal cameras are good at detecting body heat, they are very poor at supplying details for identification. For example, facial features cannot be made out, nor can writing on signs.
On the other hand, a SWIR image can look similar to an image gathered in the visible spectrum. This makes a SWIR camera a powerful surveillance tool. At night SWIR imaging comes into its own, a phenomenon known as night glow results in a clear night sky being quite bright in the SWIR spectrum.
Therefore, a reasonable night vision is possible with a SWIR camera. This can be enhanced using near IR illumination, which is invisible to all conventional CCD and thermal imagers if above 1000 nm, and so makes for a simple high-quality covert imaging system.
Video delivery via GigE and Genicam compliance is vital here, as they simplify camera integration greatly, and supply reliable high frame rate image delivery in a networked surveillance system.
SWIR imaging is also utilized to examine the subsurface structure of paintings: the longer wavelengths penetrate the surface layers and can show original details that have been covered by later overpainting. This can assist with restoration and help with confirming the artwork’s authenticity.
Here, one of the key requirements for the imaging system is high intrascene dynamic range, so that both the low-intensity parts of the surface highlights and subsurface artwork can be imaged at the same time.
The image of an 18th-century Russian icon shows a good example: note how the SWIR image shows the epitrachelion under the hand.
This information has been sourced, reviewed and adapted from materials provided by Photonic Science.
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