Thermal hyperspectral imagers and their applications

Thermal hyperspectral imagers provide information that conventional spectral imagers cannot. A broader range of materials can be detected, mapped, and sorted by thermal hyperspectral imagers in the mid-wavelength infrared (MWIR) and/or long-wavelength infrared (LWIR) spectral range than by imaging systems that operate in the visible and near-infrared (VNIR) and short-wavelength infrared (SWIR) spectral regions. Previously commercially available thermal imagers were limited to Fourier Transform and chromotomographic imaging spectrometers, which have considerable limitations, i.e., only stationary targets can be imaged from stationary platforms.¹

Push-broom instruments are the only means of producing spectrally and spatially accurate data when the target and/or the camera platform is in motion. These instruments are therefore ideal for use in aerial and ground-based remote sensing, as well as for industrial situations. In commercial and defense remote sensing, targets are typically at ambient temperature and therefore a high signal-to-noise ratio is required to reliably measure their thermal emission spectra. In industrial applications, where targets are hotter or can be illuminated, more cost efficient, lower sensitivity instruments may be utilized.

Specim's AisaOWL, LWIR-HS, and MWIR imagers exploit a combination of reflective and refractive optical elements with a transmission grating to achieve high performance in a compact device.² This approach has not been used previously in any commercial or research instrument. The Specim AisaOWL employs a cryo-cooled MCT (mercury cadmium telluride) array, temperature-stabilized instrument optics, and its instrument radiation is suppressed by special filtering and corrected by BMC (background-monitoring-on-chip). It delivers a signal-to-noise-ratio greater than 500:1 for targets in ambient temperature, noise equivalent spectral radiance (NESR) of 18mW/(m²sr μm) for a 300K target, and fulfills military vibration and shock tests for airborne use. The AisaOWL is an order of magnitude smaller than any competing instrument, yet has significantly better performance. The Specim LWIR-HS and MWIR are designed for industrial applications with illuminated or higher temperature targets: the former employs an uncooled microbolometer array and the latter uses a cryogenically cooled InSb camera. The specifications of the instruments are summarized in Table 1.

Despite the cold ambient temperature of −10^°C, the outdoor scan performed using the AisaOWL imager (see Figure 1) contains a high level of detail. Different targets can be distinguished based on differences in materials and temperature. Such experiments demonstrate that gas releases containing, for example, methane or traces of 1,1,1-2-tetrafluoroethane can be detected. Thermal hyperspectral imaging may provide a new rapid cost-effective remote sensing technique for monitoring industrial sites or natural gas distribution networks.

Figure 1. An outdoor gas measurement experiment with the AisaOWL in an ambient temperature of - 10^°C. The person on the left holds a can of compressed gas. As the gas, which includes the propellant 1,1,1-2-tetrafluoroethane with a distinctive spectral signature in the LWIR, is released, it spreads towards the right. Against a warm background, the propellant gas is detected based on its absorption peaks in the radiance spectra (red spectrum); against a cold background, the propellant gas is detected based on its emission peaks (green spectrum). Value is spectral radiance in W/m²sr μm; the wavelength region shown is from 7600 to 12,400 nm.

Hyperspectral imagery is an efficient tool for geological mapping over large, remote, and/or inaccessible parts of the Earth. The AisaOWL can deliver high quality, geo-referenced spectral radiance data. Minerals that cannot always be mapped uniquely with VNIR/SWIR hyperspectral data, such as quartz, feldspar, and chalcedony, can be identified with LWIR hyperspectral data, as demonstrated with the non-commercial airborne imager SEBASS.³ Through a combination of SWIR and LWIR spectral imaging, most minerals of commercial interest can be readily identified in geological samples. LWIR is necessary for the detection of feldspar, silica, calcite, garnet, and olivine minerals. The performance of the LWIR-HS and AisaOWL (LWIR-C) imagers are compared in Figure 2 for such an application. The lower-cost LWIR-HS provides valuable information for minerals that do not have a response in the SWIR, whereas the AisaOWL, with its high spectral resolution, can also differentiate between similar minerals, such as quartz and feldspar. The AisaOWL could also be used to distinguish plagioclase feldspars, which would be of significant value for commercial mining operations.

Figure 2. A set of geological samples scanned with the LWIR-HS and AisaOWL (LWIR-C) imagers. The AisaOWL has higher spectral resolution, leading to more accurate mineral identification. In the zoomed AisaOWL image, the quartz and feldspar spectra are clearly distinguishable.

NIR and SWIR systems are increasingly used to sort polymers in recycling operations where they can reliably distinguish clear and colored materials, but fail to separate black and dark grey plastics because the black carbon coating absorbs radiation at all SWIR wavelengths and therefore prevents detection of the base material's spectral signatures. With the Specim MWIR imager, however, automated black plastic sorting is possible.

A novel highly-sensitive LWIR push-broom hyperspectral imager in compact form can be employed for making ambient and airborne measurements. The instrument's high performance is adequate for most advanced defense and remote sensing applications. The MWIR and LWIR hyperspectral imagers can provide valuable information for several geological and industrial applications.