A new solid-state source has been developed to emulate the solar spectrum for absolute radiometric calibration of advanced multispectral and hyperspectral imagers. The source is used in conjunction with quartz tungsten-halogen lamps (QTH) on an integrating sphere to provide a solar spectrum matching the AM1.5 spectrum from 385nm to 2500nm. The performance criteria required a source with “smooth” spectral shape to minimize spectral mismatch errors in the calibration as well as minimizing the differences between power in different bands to allow sensor arrays to be calibrated with equivalent integration times. A method has also been developed to quantify the “smoothness” of the calibration spectra and the expected error in the resulting spectral response calibration for measurement of various source spectra.
In partnership with Raytheon Intelligence and Space, Labsphere Inc. has been developing a technology demonstration system for a new type of on-board absolute radiometric calibration source. The Improved Radiometric Calibration of Imaging Systems (IRIS) addresses the need for reduced risk, cost, size, and mass for next generation Earth Observation (EO) satellites through paired onboard and vicarious calibration methods. In particular, the IRIS High-performance Integrated Flat Illuminator (HIFI) is a compact, combined VISNIR and SWIR (0.4 – 2.3μm), and MWIR-LWIR (3-14μm) Jones radiance source. Funded by the NASA Earth Science Technology Office (Grant to Raytheon #80NSSC20K1676), the IRIS Technology Demonstration Unit currently under test successfully meets significant program specifications for radiance, stability, adjustability, uniformity, and polarization. Development is ongoing to further improve system performance and achieve space flight qualification. This type of new technology additionally may provide a path to on-board calibration for small satellite architectures.
The IEEE Standards Association Project 4001 (P4001) has worked since 2018 to create a standard that covers a range of unmet needs in the field of hyperspectral imaging. Now in its final stages of review, the P4001 standard is the result of the collective work of a large group comprising a significant fraction of the hyperspectral community. The formal scope of the P4001 standard is hyperspectral imaging in the reflective domain (0.25 to 2.5 micrometer wavelength) using cameras which record at least 30 bands, based on four of the most common camera architectures. A main part of the standard is a set of characteristics covering the spatial, spectral, radiometric, temporal, coregistration, stray light, and imperfection aspects of camera performance. The set of characteristics covers several aspects where current practices have been inadequate including resolution, light collection, and coregistration. The characteristics are defined as physical quantities that are thus not tied to particular test procedures. However, the P4001 standard will incorporate a set of recommended testing procedures based on commonly available test equipment. The P4001 standard also defines a set of camera-related metadata aiming to provide information that can support extraction of reliable and accurate information from images, including information about noise, distortions, and uncertainties. Three notional use cases are defined in the standard representing machine vision, laboratory, and geoscience applications. The standard mandates different sets of characteristics and metadata for P4001-compliant specifications and image data respectively. The P4001 standard will undergo formal reviews during 2023 and is expected to be published in its first version at the end of the year.
Labsphere has created automated vicarious calibration sites using convex mirror technology in the new FLARE (Field Line-of-sight Automated Radiance Exposure) Network. FLARE has been operational for over two years, with network expansion and performance validation against industry standards and common methods for calibration and validation (cal/val) of 350-2500nm optical Earth Observation Systems (EOS). The FLARE point sources provide absolute and traceable data, creating a new tool in harmonization of satellites with ground sampling distances (GSD) of 0.3m to 60m. This paper provides an overview of the FLARE system and presents findings and improvements in operational hardware and software performance. Once commissioned, all FLARE nodes have been repeatedly targeting Landsat 8, Landsat 9 (starting 2022), and Sentinel 2A/B. This has produced a multi-year archive of radiometric and spatial calibration imagery. Landsat and Sentinel are the premier reference programs for Earth Observation performance and utilize both on-board calibration equipment and on-ground reference sites such as RadCalNet and PICS. This work compares the results of the FLARE technique to current official radiometric coefficients and spatial performance metrics for these satellites. Discussion will center on new insights gleaned from the archive analysis and FLARE’s contribution to the community’s capability for data fusion, instrument harmonization, and the potential to support the concept of Analysis Ready Data (ARD) for easier data use and information extraction. Finally, the future progression of FLARE sites, capabilities, and activities will be outlined.
The IEEE P4001 Hyperspectral Standard concluded the first draft of its initial standard as of December 2021. The document is under formal review now by IEEE and will be heading to general voting in 2022 with potential for publication in 2023. This talk will give an overview of the radiometric, spectral, spatial, data structures, terminology and verification testing that was done as the final push to finish the work.
The Empirical Line Method (ELM) is a widely applied technique of achieving absolute atmospheric correction assuming a linear relationship between the raw Digital Numbers (DNs) or at-sensor radiance and surface reflectance measurements collected in-situ. The ELM measures reference targets of known reflectance in an image. Labsphere has created an automated vicarious calibration system using the SPecular Array Radiometric Calibration (SPARC) mirror technology in the new Field Line-of-sight Automated Radiance Exposure (FLARE) network. In the FLARE system the known reflectance targets are convex mirrors - because of that it is titled Mirror based Empirical Line Method (MELM). In this context, the objective of this work is to present the initial results of the MELM using one the FLARE network system. The FLARE system evaluated in this work is the Alpha Node located at Arlington, SD. Initially, the data collected in 2020 and 2021 with the Alpha FLARE concomitant with the OLI sensor overpass on-board the Landsat-8 satellite were used in the assessment. In summary, the surface reflectance image product available to download for OLI sensor were compared directly with the surface reflectance image resulting from the MELM method. The preliminary results showed the mean absolute error data between the surface reflectance from the OLI Level-2 product image and the surface reflectance from the MELM was lower than 0.01 for the Blue, Green, Red and SWIR-1 bands; lower than 0.03 for the for the NIR and SWIR-2 spectral bands; and around 0.05 for Coastal Aerosol band (all in reflectance units). These results suggest the MELM technique using FLARE has great potential for reflectance surface evaluation of orbital sensors.
The P4001 standard defines characteristics for hyperspectral camera performance. The standard will also include guidelines for measurement of these characteristics. The ambition to give a complete set of performance characteristics tends to require an extensive set of tests. An important aspect of the work is therefore to devise test protocols that are time efficient and have moderate requirements on the test equipment. Work is underway to define tests for radiometric performance, co-registration, spatial and spectral resolution, as well as stray light. The complete set of tests can be carried out using four test setups. These test methods, according to the current draft, will be outlined.
Hyperspectral imaging has over the last thirty years developed into a power analytical tool for the determination of chemical and other properties. As a result, there has been strong development in both the design of spectral cameras and in the applications for which they are used. This has led to a diversity in the way fundamental instrument performances are characterized, reported, and understood. As a result, this makes it difficult to compare instruments for applicationspecific needs, or for commercial market needs. In 2018, the IEEE P4001 group was formed to facilitate the development of a standard to unify the use of terminology, spectral camera characterization methods, and the meta-data structures that are needed to represent spectral camera performance. This talk provides an update on the work to date, and the significant progress made towards the first draft of the standard.
Hyperspectral images are ultimately analyzed by users for specific applications. Metadata supplied with the images are critical to enable the user to extract full information from the data. An important part of IEEE P4001 Hyperspectral Standard is therefore to define metadata, which will be physics-informed and related to the camera characteristics. With metadata such defined, it will for example be possible for a user to estimate signaldependent noise in the images. Different applications will require different levels of metadata. P4001 therefore defines notional use cases for machine vision, laboratory, and Geospatial Imaging, in order of increasing need for metadata. P4001 will define required and optional metadata for these use cases.
The IEEE P4001 group is developing a new standard for hyperspectral imaging. A central part of the work is to define a standardized way to specify hyperspectral cameras. P4001 aims to define a complete description of the camera as a "black box", i.e. without referencing camera internals such as f-number. The aim is to use characteristics that are welldefined physical quantities, and which capture all important aspects of performance, in some cases with a tradeoff to avoid the need for excessive testing equipment or effort. The characteristics apply to the most widely used camera architectures based on dispersive spectrometers or tunable filters.
The goal of the P4001 Terminology Subgroup was to collate and define physics-based terminology in order to establish methods for testing and calibration that accurately convey product specifications. Written definitions and well-defined equations (where possible) that objectively and unambiguously define these terms is critical for success.
Initial work has focused on collating terms for review. In order to align with the efforts of the Characterization & Testing Subgroup, the terms are divided into four categories:
Spectral Terminology
Spatial Terminology
Signal/Radiometric Terminology
System and Operational Terminology.
In 2018 the IEEE P4001 working was formed from industry specialists to facilitate consistent use of terminology, characterisation methods and data structures. This talk is a progress report to inform the hyperspectral community of the status of the work to date, the interconnection with other standards and outline the roadmap towards completion of the first draft of the standard for voting in early 2022.
Labsphere has created automated vicarious calibration sites using the SPecular Array Radiometric Calibration (SPARC) mirror technology in the new Field Line-of-sight Automated Radiance Exposure (FLARE) network. A short introduction to FLARE and SPARC will be given showing how arduous field ground calibrations can now be done remotely through FLARE nodes via an internet portal. Preliminary results of the performance of the system’s absolute radiometric and spatial calibration capability were published in 2020, demonstrating validation and uncertainty against current methods of remote calibration and spatial and geometric performance against edge and line targets. This paper will describe FLARE’s impact to ongoing evaluation and maturation of automated analysis processes for all data processing levels for space satellite and UAV imagers.
Hyperspectral imaging is an innovative and exciting technology that holds incredible diagnostic, scientific and categorization power. Current industry innovation is a testament to the creative power and imagination of the diverse community seeking to optimize this technology. However, fundamental instrument performance is not consistently well characterized, well understood or well represented to suit distinct application endeavors or commercial market expectations. Establishing a common language, technical specification, testing criteria, task-specific recommendations and common data formats are essential to allowing this technology to achieve its true altruistic and economic market potential. In 2018 the IEEE P4001 was formed to facilitate consistent use of terminology, characterization methods and data structures. This talk is a progress report to inform the hyperspectral community of the status of the work to date, the interconnection with other standards and outline the roadmap.
The SPecular Array Radiometric Calibration (SPARC) methodology uses convex mirrors to relay an image of the sun to a satellite, airborne sensor, or other Earth Observation platform. The signal created by SPARC can be used to derive absolute, traceable calibration coefficients of Earth remote sensing systems in the solar reflective spectrum. This technology has been incorporated into an automated, on-demand commercial calibration network called FLARE (Field Line-of-site Automated Radiance Exposure). The first station, or node, has been successfully commissioned and tested with several government and commercial satellites. Radiometric performance is being validated against existing calibration factors for Sentinel 2A and diffuse target methodologies. A radiometric uncertainty budget indicates conservative 1-sigma uncertainties that are comparable to or below existing vicarious cal/val methods for the VIS-NIR wavelengths. In addition to radiometric performance, SPARC and FLARE can be utilized for characterization of a sensor’s spatial performance. Line and Point Spread Functions, and resulting Modulation Transfer Functions, derived with SPARC mirrors are virtually identical to those measured with traditional diffuse edge targets. Ongoing development of the FLARE network includes improved radiometric calibration, web portal scheduling and data access, and planned expansion of the network to Railroad Valley Playa and Mauna Loa, Hawaii.
Spectralon® is a high reflectance excellent Lambertian diffuser used to reflect sunlight for use as a calibrator for on-orbit and ground instruments. Radiometric calibration of the reflective bands in the 0.4 to 2.5μm wavelength range is performed by measuring the sunlight reflected from Spectralon® . Reflected sunlight is directly proportional to the Bidirectional Reflectance Distribution Function (BRDF) of the Spectralon® . On-orbit exposure to sunlight results in solarization due to solar UV. Previously, the rate / amount of solarization has varied as observed from on orbit measurements as well as laboratory UV exposure testing of samples. A method for determining whether a particular batch of Spectralon® has low solarization has been developed. This method relies on hemispherical reflectance measurements in the 0.25- 0.5 μm wavelength range before and after Spectralon® bake out. This method is reliable for as-made Spectralon® , not for contamination verification after shipment. We have also determined that additional Spectralon® bake outs do not change the as-made Spectralon® solarization rate. Knowledge of possible Spectralon solarization is important prior to its shipment to customers and eventual deployment in satellite and ground-based instrument calibration.
Hyper-spectral imaging is an innovative and exciting technology that holds incredible diagnostic, scientific and categorization power. However, fundamental instrument performance is not consistently well characterized, well understood or well represented to suit distinct application endeavors or commercial market expectations. Establishing a common language, technical specification, testing criteria, task-specific recommendations and common data formats are essential to allowing this technology to achieve its true altruistic and economic market potential. In 2018 the IEEE P4001 was formed to facilitate consistent use of terminology, characterization methods and data structures. This talk is a progress report to inform the hyper-spectral community of the status of the work-to-date, the interconnection with other standards and outline the road map for future work until publication of the standards in 2022.
Hyperspectral imaging is an innovative and exciting technology that holds incredible diagnostic, scientific and categorization power. Current industry innovation is a testament to the creative power and imagination of the diverse community seeking to optimize this technology. However, fundamental instrument performance is not consistently well characterized, well understood or well represented to suit distinct application endeavors or commercial market expectations. Establishing a common language, technical specification, testing criteria, task-specific recommendations and common data formats are essential to allowing this technology to achieve its true altruistic and economic market potential. In 2018 the IEEE P4001 was formed to facilitate consistent use of terminology, characterization methods and data structures. This talk is a progress report to inform the hyperspectral community of the status of the work to date, the interconnection with other standards and outline the roadmap.
Uniformity from Lambertian optical sources such as integrating spheres is often trusted as absolute at levels of 98% (+/- 1%) or greater levels. In the progression of today’s sensors and imaging system technology that 98% uniformity level is good, but not good enough to truly optimize pixel-to-pixel and sensor image response. The demands from industry are often for “perfect” uniformity (100%) which is not physically possible, however, perfectly understood non-uniformity is possible. A barrier to this concept is that the definition and measurement equipment of uniformity measurements often need to be very specific to the optical prescription of the unit under test. Additionally, the resulting data are often a relativistic data set, assigned to an arbitrary reference, but not actually given an expression of uncertainty with a coverage factor. This paper discusses several optical measurement methods and numerical methods that can be used to quantify and express uniformity so that it has meaning to the optical systems that will be tested, and ultimately, that can be related to the Guide to the Expression of Uncertainty in Measurement (GUM) to provide an estimated uncertainty. The resulting measurements can then be used to realize very accurate flat field image corrections and sensor characterizations.
Sensor fusion and novel “multi-image” systems that have several different spectral ranges are proliferating in tactical and commercial applications. Calibrating these devices requires a variety of sources from quartz-tungsten halogen to blackbodies to more selectable band sources such as LEDs. Usually these sources are used independently in discrete spectral regions, but real reflective and emissive targets often have signatures that make combining these sources necessary if one is to emulate these real spectrums for testing in either image (collimator) or flood (sphere) configurations. A novel approach to combine LED and broadband emitters has been developed to effect stable, calibrated, traceable sources that can match real target spectral signatures.
Many existing and emerging remote sensing applications in the UV, Visible, NIR, SWIR, MWIR and LWIR regions are challenging the conventional thinking of radiance and temperature calibration techniques. While the relationship between blackbody temperature and optical radiation is well understood, often there is an “invisible” dividing line between treatments of these values as either optical radiance or temperature. It is difficult to perform seamless temperature and radiance calibrations across the point of 2.5um. Spectrum above 2.5um is typically related in temperature terms and below 2.5um may be either spoken of in terms of temperature or optical radiance. There is also a natural unit “convergence” issue at 2.5um, due to the crossover of significant levels of emissivity, reflectance and temperature at this point. NMI traceability in the spectral region of 2.5-14.0um can also be a problem especially for spectral radiance. This paper will outline a possible turn-key test bench solution that provides traceable solutions for both temperature and radiance value in these regimes. The intent of this paper is to offer a possible solution and challenge the infrastructure that exists today over the 0.3-14um range in order to obtain a valid spectral radiance or temperature value, or both, to support emerging sensor fusion technology.
Hyperspectral imaging (HSI) is an exciting and rapidly expanding area of instruments and technology in passive remote sensing. Due to quickly changing applications, the instruments are evolving to suit new uses and there is a need for consistent definition, testing, characterization and calibration. This paper seeks to outline a broad prescription and recommendations for basic specification, testing and characterization that must be done on Visible Near Infra-Red grating-based sensors in order to provide calibrated absolute output and performance or at least relative performance that will suit the user’s task. The primary goal of this paper is to provide awareness of the issues with performance of this technology and make recommendations towards standards and protocols that could be used for further efforts in emerging procedures for national laboratory and standards groups.
Sintered PTFE is an extremely stable, near-perfect Lambertian reflecting diffuser and calibration standard material that has been used by national labs, space, aerospace and commercial sectors for over two decades. New uncertainty targets of 2% on-orbit absolute validation in the Earth Observing Systems community have challenged the industry to improve is characterization and knowledge of almost every aspect of radiometric performance (space and ground). Assuming “near perfect” reflectance for angular dependent measurements is no longer going to suffice for many program needs. The total hemispherical spectral reflectance provides a good mark of general performance; but, without the angular characterization of bidirectional reflectance distribution function (BRDF) measurements, critical data is missing from many applications and uncertainty budgets. Therefore, traceable BRDF measurement capability is needed to characterize sintered PTFE’s angular response and provide a full uncertainty profile to users. This paper presents preliminary comparison measurements of the BRDF of sintered PTFE from several laboratories to better quantify the BRDF of sintered PTFE, assess the BRDF measurement comparability between laboratories, and improve estimates of measurement uncertainties under laboratory conditions.
Integrating sphere (IS) based uniform sources are a primary tool for ground based calibration, characterization and testing of flight radiometric equipment. The idea of a Lambertian field of energy is a very useful tool in radiometric testing, but this concept is being checked in many ways by newly lowered uncertainty goals. At an uncertainty goal of 2% one needs to assess carefully uniformity in addition to calibration uncertainties, as even sources with a 0.5% uniformity are now substantial proportions of uncertainty budgets. The paper explores integrating sphere design options for achieving 99.5% and better uniformity of exit port radiance and spectral irradiance created by an integrating sphere. Uniformity in broad spectrum and spectral bands are explored. We discuss mapping techniques and results as a function of observed uniformity as well as laboratory testing results customized to match with customer’s instrumentation field of view. We will also discuss recommendations with basic commercial instrumentation, we have used to validate, inspect, and improve correlation of uniformity measurements with the intended application.
Integrating spheres for optical calibration of remote sensing cameras have traditionally been made with Quartz
Tungsten Halogen (QTH) lamps because of their stability. However, QTH lamps have the spectrum of a blackbody
at approximately 3000K, while remote sensing cameras are designed to view a sun-illuminated scene. This presents
a severe significant mismatch in the blue end of the spectrum. Attempts to compensate for this spectral mismatch
have primarily used Xenon lamps to augment the QTH lamps. However, Xenon lamps suffer from temporal
instability that is not desirable in many applications. This paper investigates the possibility of using RF-excited
plasma lamps to augment QTH lamps. These plasma lamps have a somewhat smoother spectrum than Xenon. Like
Xenon, they have more fluctuation than QTH lamps, but the fluctuations are slower and may be able to be tracked in
an actual OGSE light source. The paper presents measurements of spectra and stability. The spectrum is measured
from 320 nm to 2500 nm and the temporal stability from DC to 10 MHz. The RF-excited plasma lamps are quite
small, less than 10mm in diameter and about 15 mm in length. This makes them suitable for designing reasonably
sized reflective optics for directing their light into a small port on an integrating sphere. The concludes with a
roadmap for further testing.
Application-specific integrating sphere-based, integral veiling glare measurement systems are described. The sources use
the integral method for measuring the veiling glare (VG) index of various lens-based imaging systems. The calibration
source has provisions in the form of a collimating lens holder to simulate a situation where the black target and bright
surround are at a sufficiently great distance to give measurements of VG index which are the same as that which would
result if the distance where infinite. The design criteria for the integral VG test source are presented. Included is a
summary of the end-user specifications in regards to spectral radiance, levels of attenuation, irradiance stability, and
aperture uniformity and contrast. Spectral radiometric predictions and actual output levels are compared.
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