Improved technology and emerging interferometric techniques have allowed the use of uncooled microbolometers in the long-wave infrared (LWIR; 8 to 14 μm) for hyperspectral imaging (HSI). The midwave infrared (MWIR; 3 to 5 μm) presents several advantages with respect to the LWIR for Earth and planetary science. For example, important atmospheric trace gases on Earth such as CO2 and CH4 are not masked by other atmospheric constituents in the MWIR. However, HSI in the MWIR is more challenging at ambient Earth temperatures because less radiance is available to measure. We describe how hyperspectral images in the MWIR can be acquired with an instrument using an uncooled microbolometer married to a Sagnac interferometer. Standard characterization tests are used to benchmark the performance of the microbolometer instrument with a cryogenically cooled photon detector with the same optical design. At a spectral resolution of 100 cm − 1 (17 bands between 3 and 5 μm), we measured a signal-to-noise ratio (SNR) of 100 at 30°C with the microbolometer instrument and an SNR of 50 at 50 cm − 1 (33 bands). Results from this work show that coupling microbolometers with interferometers allows for quality measurements with adequate SNR for high-temperature science applications.
Gases released from a volcano, such as sulfur dioxide (SO2) and carbon dioxide (CO2), present hazards to the environment and local populations as well as providing a means to monitor volcanic activity and study pre-eruptive signatures. In the Mid-Wave InfraRed (MWIR) from 3 to 5 microns both the aforementioned volcanic gases exhibit characteristic absorptions. Remote sensing in the MWIR, however, is challenging due to the limited amount of signal available to measure. This presents technical challenges on achieving high signal-to-noise ratios; therefore, acquiring adequate data in the MWIR has been difficult without cryogenically cooling the instrument. However, ecent improvements to microbolometer technology and emerging interferometric techniques have allowed us to acquire good thermal infrared data without the need for cooling. By utilizing the advantages of an imaging interferometer paired with an uncooled microbolometer, we demonstrate the use of a MWIR compact, hyperspectral imager for volcanic gas detection. The instrument, the Miniaturized Infrared Detector of Atmospheric Species (MIDAS), is representative of an instrument that could feasibly be flown on a small satellite in low earth orbit for the detection and monitoring of volcanic gases. Recently MIDAS was deployed to Kilauea’s Halema’uma’u pit crater which during the deployment had an active lava lake that was continuously releasing volcanic gases. Sources like the Kilauea lava lake provide high background temperatures that aid MWIR measurements of volcanic gases. We present hyperspectral analysis of volcanic gases from the Kilauea lava lake using data from the MIDAS instrument and line by line radiative transfer analysis. Brightness temperature maps of the lake surface show values consistent with direct thermocouple measurements and point radiometer measurements. Here we present resolved images of spectral radiance, brightness temperature, and CO2 concentrations. The map of CO2 is relatively uniform, but show subtle variation at the 2553 - 3313+/- 167 ppm level.
Hyperspectral imaging (HSI) in the Mid-Wave InfraRed (MWIR, 3-5 microns) can provide information on a variety of science applications from determining the chemical composition of lava lakes on Jupiter’s moon Io, to investigating the amount of carbon liberated into the Earth’s atmosphere during a wildfire. The limited signal available in the MWIR presents technical challenges to achieving high signal-to-noise ratios, and therefore it is typically necessary to cryogenically cool MWIR instruments. With recent improvements in microbolometer technology and emerging interferometric techniques, we have shown that uncooled microbolometers coupled with a Sagnac interferometer can achieve high signal-to-noise ratios for long-wave infrared HSI. To explore if this technique can be applied to the MWIR, this project, with funding from NASA, has built the Miniaturized Infrared Detector of Atmospheric Species (MIDAS). Standard characterization tests are used to compare MIDAS against a cryogenically cooled photon detector to evaluate the MIDAS instruments’ ability to quantify gas concentrations. Atmospheric radiative transfer codes are in development to explore the limitations of MIDAS and identify the range of science objectives that MIDAS will most likely excel at. We will simulate science applications with gas cells filled with varying gas concentrations and varying source temperatures to verify our results from lab characterization and our atmospheric modeling code.
Hyperspectral imaging (HSI) is a technique with a growing list of applications and potential users, as this technique combines the power of imaging with the chemical discrimination of spectroscopy. Because HSI divides light from the scene into narrow slices of wavelength, the technique is typically thought to require cryogenic arrays to achieve the ultimate sensitivity. However, within the last two decades microbolometer arrays have improved in sensitivity, pixel count and total array area. In Hawai’i we have shown that microbolometer arrays can provide sufficient sensitivities for a variety of infrared HSI applications. The ability of microbolometer arrays to operate at ambient-temperature make them attractive candidates for low power applications, including space-based instruments on small satellites. We have two NASA projects to determine the suitability of uncooled microbolometers for HSI systems with the aim of HSI measurements from smaller satellites than is possible with cryogenic instruments. The suitability of a detector is governed in part by its spectral response. Microbolometers have wide variations in spectral response by technology and vendor, as part of our NASA projects we are conducting a spectral response measurement campaign on five different microbolometer cameras. Three of the cameras are sensitive to the long-wave infrared from 7.5 to 14 microns (two FLIR cameras and a Sofradir camera), one to the mid-wave infrared from 3 to 5 microns (LumaSense camera), and the last is sensitive to both regions from 3 to 14 microns (INO camera). Results from this campaign will be presented.
Measurements of reflectance or emittance in tens of narrow, contiguous wavebands, allow for the derivation of laboratory quality spectra remotely, from which the chemical composition and physical properties of targets can be determined. Although spaceborne (e.g. EO-1 Hyperion) hyperspectral data in the 0.4-2.5 micron (VSWIR) region are available, the provision of equivalent data in the log-wave infrared has lagged behind, there being no currently operational high spatial resolution LWIR imaging spectrometer on orbit. TIRCIS (Thermal Infra-Red Compact Imaging Spectrometer), uses a Fabry-Perot interferometer, an uncooled microbolometer array, and push-broom scanning to acquire hyperspectral image data. Radiometric calibration is provided by blackbody targets while spectral calibration is achieved using monochromatic light sources. The instrument has a mass of <15 kg and dimensions of 53 cm × 25 cm ♦ 22 cm, and has been designed to be compatible with integration into a micro-satellite platform. (A precursor to this instrument was launched onboard a 55 kg microsatellite in October 2015). The optical design yields a 120 m ground sample size given an orbit of 500 km. Over the wavelength interval of 7.5 to 14 microns up to 50 spectral samples are possible. Measured signal-to-noise ratios range from peak values of 500:1 to 1500:1, for source temperature of 10 to 100°C.
TIRCIS (Thermal Infra-Red Compact Imaging Spectrometer), uses a Fabry-Perot interferometer, an uncooled
microbolometer array, and push-broom scanning to acquire hyperspectral image data. Radiometric calibration is
provided by blackbody targets while spectral calibration is achieved using monochromatic light sources. The instrument
has a mass of <10 kg and dimensions of 53 cm × 25 cm × 22 cm. The optical design yields a 120 m ground sample size
given an orbit of 500 km. Over the wavelength interval of 7.5 to 14 microns up to 90 spectral samples are possible. Our
performance model indicates signal-to-noise ratios of 400-800:1.
The mid-wave infrared is an especially informative wavelength range, permitting detection and characterization of a diverse range of materials and processes. The development of a new way to measure in this region, using a Sagnac interferometer spectrometer, has lead us to design the Miniaturized Infrared detector of Atmospheric Species (MIDAS). Instruments like MIDAS are attractive for space applications due to their low-mass and low-power consumption. An uncooled microbolometer and a cooled InSb photon detector version of MIDAS are currently set up for bench top characterization and preliminary science data collection.
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