The upcoming IEEE standard P4001 outlines a set of parameters for characterizing the performance of hyperspectral cameras along with recommended measurement procedures. This study concentrates on validating a simplified approach to measure the across-track spatial resolution of a pushbroom hyperspectral camera with a sensor sampling factor of two or higher, in comparison to the scanning-based approach described in the standard. The findings indicate that the snapshot-based method produces values for the width of the sampling line spread function in the across-track direction that closely match those obtained through scanning with sub-pixel steps for hyperspectral cameras with a sensor sampling factor of two or higher.
Both MWIR and LWIR wavelength ranges have for long time mostly used for research activities. During recent years industrial use of these wavelength band ranges has also evolved and the same trend is expected to continue in future. Important application areas relate to mineral mapping and recognition, metal industry and black plastics sorting for instance. Key parameters in industrial use, in addition to good performance characteristics and data quality, are the cost and usability of the camera. The Specim hyperspectral MWIR and LWIR cameras employ push-broom imaging spectrograph, with transmission grating and on-axis optics. The cameras have thermally stabilized optics and cryogenically cooled MCT detector with Stirling cooler. The performance of the newly developed MWIR camera is targeted on reflectance measurements with illumination. The performance of hyperspectral camera enables reliable measurement of low reflectance level targets illuminated with moderate temperature heat source, and with less than 10% reflectance with 650°C illumination and frame rates of 380 frames /sec with 154 bands and 640 spatial samples. The SNR of 500 of the new LWIR hyperspectral camera suits emission measurements of normal room temperature targets but is applicable to reflectance measurement with illumination as well. High performance emission measurement with about 150 bands, 640 spatial samples and more than 300 frames per second can be achieved. The performance is verified with testing of several camera units and supported with simulation results. The performance characteristics of NESR and expected SNR with actual measurement parameters are presented.
Hyperspectral imaging is not only a very special type of imaging, but also a special type of spectroscopy. What is more, there are many different architectures of hyperspectral imagers. While this paper concentrates on push-broom architecture only, there are still several parameters for which a hyperspectral camera may be optimized in design. This optimization means that the best fitting use cases are in fact chosen simultaneously with the design decisions. It is not always self-evident how hyperspectral camera parameters affect the measured data and its interpretation. Hyperspectral cameras consist essentially of two separate optical parts and a camera. Each of those parts processes the information originating from the target. The selection of the optimization parameters is based on camera design principles, which may differ significantly from one manufacturer to another, or one instrument to another. Comparing these instruments is not easy, but it is often also unnecessary as not all the parameters are equally important in every practical use case. When a user chooses a hyperspectral camera, they should study the specifications of the performance parameters with the intended application in mind. Highlighting one of the parameters in the design results in a loss of performance in another. These parameter pairs include spectral sampling vs. light collection efficiency, number of pixels vs. frame rate, and optical sharpness vs. sampling artifacts in the image.
Several chemical compounds have their strongest spectral signatures in the thermal region. This paper presents three
push-broom thermal hyperspectral imagers. The first operates in MWIR (2.8-5 μm) with 35 nm spectral resolution. It
consists of uncooled imaging spectrograph and cryogenically cooled InSb camera, with spatial resolution of 320/640
pixels and image rate to 400 Hz. The second imager covers LWIR in 7.6-12 μm with 32 spectral bands. It employs an
uncooled microbolometer array and spectrograph. These imagers have been designed for chemical mapping in reflection
mode in industry and laboratory. An efficient line-illumination source has been developed, and it makes possible thermal
hyperspectral imaging in reflection with much higher signal and SNR than is obtained from room temperature emission.
Application demonstrations including sorting of dark plastics and mineralogical mapping of drill cores are presented.
The third imager utilizes a cryo-cooled MCT array with precisely temperature stabilized optics. The optics is not cooled,
but instrument radiation is suppressed by special filtering and corrected by BMC (Background-Monitoring-on-Chip)
method. The approach provides excellent sensitivity in an instrument which is portable and compact enough for
installation in UAVs. The imager has been verified in 7.6 to 12.3 μm to provide NESR of 18 mW/(m2 sr μm) at 10 μm
for 300 K target with 100 spectral bands and 384 spatial samples. It results in SNR of higher than 500. The performance
makes possible various applications from gas detection to mineral exploration and vegetation surveys. Results from
outdoor and airborne experiments are shown.
Two long-wave infrared (LWIR) hyperspectral imagers have been under extensive development. The first one utilizes a
microbolometer focal plane array (FPA) and the second one is based on an Mercury Cadmium Telluride (MCT) FPA.
Both imagers employ a pushbroom imaging spectrograph with a transmission grating and on-axis optics. The main target
has been to develop high performance instruments with good image quality and compact size for various industrial and
remote sensing application requirements. A big challenge in realizing these goals without considerable cooling of the
whole instrument is to control the instrument radiation. The challenge is much bigger in a hyperspectral instrument than
in a broadband camera, because the optical signal from the target is spread spectrally, but the instrument radiation is not
dispersed. Without any suppression, the instrument radiation can overwhelm the radiation from the target even by 1000
times.
The means to handle the instrument radiation in the MCT imager include precise instrument temperature stabilization
(but not cooling), efficient optical background suppression and the use of background-monitoring-on-chip (BMC)
method. This approach has made possible the implementation of a high performance, extremely compact spectral imager
in the 7.7 to 12.4 μm spectral range. The imager performance with 84 spectral bands and 384 spatial pixels has been
experimentally verified and an excellent NESR of 14 mW/(m2srμm) at 10 μm wavelength with a 300 K target has been
achieved. This results in SNR of more than 700.
The LWIR imager based on a microbolometer detector array, first time introduced in 2009, has been upgraded. The
sensitivity of the imager has improved drastically by a factor of 3 and SNR by about 15 %. It provides a rugged
hyperspectral camera for chemical imaging applications in reflection mode in laboratory and industry.
KEYWORDS: Long wavelength infrared, Cameras, Imaging systems, Sensors, Signal to noise ratio, Microbolometers, Calibration, Hyperspectral imaging, Black bodies, Temperature metrology
Performance studies and instrument designs for hyperspectral pushbroom imagers in thermal wavelength region are
introduced. The studies involve imaging systems based on both MCT and microbolometer detector. All the systems
employ pushbroom imaging spectrograph with transmission grating and on-axis optics. The aim of the work was to
design high performance instruments with good image quality and compact size for various application requirements.
A big challenge in realizing these goals without considerable cooling of the whole instrument is to control the instrument
radiation from all the surfaces of the instrument itself. This challenge is even bigger in hyperspectral instruments, where
the optical power from the target is spread spectrally over tens of pixels, but the instrument radiation is not dispersed.
Without any suppression, the instrument radiation can overwhelm the radiation from the target by 1000 times.
In the first imager design, BMC-technique (background monitoring on-chip), background suppression and temperature
stabilization have been combined with cryo-cooled MCT-detector. The performance of a very compact hyperspectral
imager with 84 spectral bands and 384 spatial samples has been studied and NESR of 18 mW/(m2srμm) at 10 μm
wavelength for 300 K target has been achieved. This leads to SNR of 580. These results are based on a simulation
model.
The second version of the imager with an uncooled microbolometer detector and optics in ambient temperature aims at
imaging targets at higher temperatures or with illumination. Heater rods with ellipsoidal reflectors can be used to
illuminate the swath line of the hyperspectral imager on a target or sample, like drill core in mineralogical analysis.
Performance characteristics for microbolometer version have been experimentally verified.
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