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.
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.
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.
CCD based spectrometers are commonly used to characterize the optical performance of LEDs. All CCD based
spectrometers exhibit varying amounts of stray light. This situation is exacerbated by using tungsten-halogen based
sources for calibration. NIST has come up with methods to characterize the stray light associated with CCD based
spectrometers using multiple lasers spanning the visible and near IR spectrum. At Sphereoptics we have developed
methods to map out and correct for stray light using a double monochromator and software. We have been able to
mathematically remove the effects of stray light on many common LED characterization measurements such as
spectral flux for instance. These effects were found to be quite substantial especially in the UV part of the spectrum.
In this paper we will present our findings on the nature of stray light found in a B&W BRC112E UV spectrometer as
well as our progress in correcting the errors it causes.
A vacuum compatible integrating sphere was built to operate inside a thermal vacuum chamber. This paper presents the
design and test results for a 1.65 meter diameter vacuum compatible integrating sphere with a 1.0 meter diameter exit
port and approximately 10kW of internal tungsten lamps. Liquid nitrogen is used as cooling medium to remove the heat
generated by these lamps. There are no moving parts inside the vacuum chamber.
The radiance is monitored with two filter-wheel detectors, one TE-cooled silicon and one TE-cooled germanium, as well
as a TE-cooled silicon array spectrometer. All three detectors are located outside the thermal vacuum chamber and view
the sphere radiance through fiber optic cables.
The system was tested inside a thermal vacuum chamber at NASA Goddard Space Flight Center before commissioning
in the 5.5 meter thermal vacuum chamber at Space Applications Centre in Ahmedabad, India. Results of tests of radiance
uniformity, radiance levels, and radiance stability are presented. Comparisons of the filter radiometers with the array
spectrometer are also presented.
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