A new design and its characterizations for use in thermal-infrared radiation thermometer and sensors are described. This new design uses an optical configuration where critical optical elements such as the field stop, Lyot stop, collimating lens, and detector, are placed inside a thermally stabilized assembly that is controlled using thermo-electric coolers and feedback from thermistors. The optical components have been incorporated into an operational system and calibrated using both variable-temperature fluid-bath and heat-pipe blackbodies from -45°C to 75°C. The radiation thermometer has been calibrated using modified Planck function and these blackbodies. This new design has been characterized to determine noise-equivalent temperatures and long-term stability. This new design, without the need for cryogenic cooling, demonstrates sub-millikelvin temperature resolution with the possibility of weeks-long and extended-length stable operations while measuring room-temperature objects with few millikelvin stability.
The paper describes efforts to establish traceable measurements of radiance temperature on laser-induced heated metal surfaces on the NIST Additive Manufacturing Metrology Testbed (AMMT). Knowledge of radiance temperature with a well understood uncertainty budget is a necessary initial step towards an ultimate project goal of traceable emittance and true surface temperature across the heat affected zone, which is a key objective in additive manufacturing research, and the subject of another paper at this conference.
Reliable measurements of radiance temperature with an imaging system require (1) calibration of its responsivity at select radiance levels, (2) establishing a calibration equation that interpolates between these levels, (3) dealing with finite spectral bandpass and spatial non-uniformity of the sensor responsivity, and (4) ability for compensate effects of imperfect optical imaging and readout electronics on spatial distribution of the target.
The developed system includes an integrating sphere-based calibration source, a pyrometer for its calibration against external blackbody, and an imaging system co-axially aligned with the heating laser, each of which using identical narrow band filters. This paper describes the evaluation of an 850 nm band, with additional wavebands planned for the future. This paper presents experimental results, description of measurement equation and processing algorithm, as well as a framework for establishing an uncertainty budget, including current estimates and future performance goals.
KEYWORDS: Calibration, Solar cells, Spectrographs, Silicon, Personal digital assistants, Spectral calibration, Indium gallium arsenide, Lamps, Solar energy, Free electron lasers
The spectroradiometric characterization of the NIST indoor pulsed solar simulator is described. The solar simulator has
a flash duration of 36.4 ms and is designed for solar panels having a maximum size of 2.0 m by 1.6 m. As per industry
standards, the performance of the solar simulator is evaluated on the basis of three criteria: spatial uniformity, temporal
stability, and spectral irradiance. Results from evaluating the NIST solar simulator on all three criteria is reported, but a
greater focus is given to the spectral characterization. Reported spectral irradiance measurements were made using a
high-speed, diode-array spectroradiometer that was calibrated using NIST standards. An uncertainty analysis of the
spectral irradiance measurements is developed, and the extent that the calibrated spectroradiometer can be used to
improve solar module measurements is explored.
Short-wave infrared detectors and regular-glass optics are used to construct a calibrator for infrared collimators. The
advantages of using short-wave infrared detectors with thermo-electric cooling instead of cryogenically-cooled infrared
detectors are shown. Diffraction-limited imaging is obtained using off-the-shelf achromats for rejection of stray
radiation and for collection of the thermal radiation. The design of a prototype calibrator is shown and the noise-equivalent
irradiances (NEI) are determined using a separately calibrated, off-axis infrared collimator. The measured
NEI of 7 fW/cm2 demonstrates at least several orders of magnitude better performance than existing infrared calibrators.
The calibration of infrared (IR) radiometers, thermal imagers and electro-optical systems relies on use of extended area blackbodies (BB) operating in the ambient environment. "Flat plate" designs, typically using a thermoelectric heat pump backed with an air- or liquid-cooled radiator, allow one to adequately meet the requirements of geometrical size and temperature span. The tradeoff comes in the form of limited temperature uniformity and lower emissivity that such an approach can provide given the limitations in achievable thermal conductivity of the plate and reflectance of the black paint, respectively.
The availability of spectrally resolved radiance temperature data for infrared calibrators has become especially vital in the last few years with the widespread use of multi- and hyper-spectral electro-optical systems that enable better detection and identification of targets.
In an effort to increase the measurement accuracy of IR spectral radiance of near-ambient BB calibrators, NIST has recently built a dedicated capability which is a part of its new AIRI (Advanced Infrared Radiometry and Imaging) facility. The Tunable Filter Comparator (TFC) is a key new element in this setup, allowing us to perform a precise comparison of the unit under test (UUT) with two reference blackbodies of known temperatures and emissivity.
The report describes the major design features of the TFC comparator, the algorithm used for signal processing, and results of a performance evaluation of the TFC.
The TFC development has enabled us to achieve BB radiance temperature comparisons with a standard deviation of 5 to 15 mK at temperatures of 15-150 C across the 3 to 5 µm and 8 to 12 µm atmospheric band ranges with a relative spectral resolution of 2 to 3%.
New transfer standard pyrometers, named "RT900" and "RT1550," operating at 900 nm and 1550 nm, respectively, have been designed, characterized, and calibrated with defined fixed points of the International Temperature Scale 1990 (ITS-90) at the National Institute of Standards and Technology (NIST). The pyrometers are designed for radiance temperature measurements in the range between the freezing temperatures of Sn (231.928 °C) and Ag (961.78 °C). These instruments also incorporate design elements optimized for compactness and portability that allow them to be used to interpolate, maintain and disseminate radiance temperature scales as well as for inter-laboratory comparisons. The calibration of the RT900 at different fixed points demonstrate agreement to within 25 mK. The size of source effect (SSE) correction for a source with a 40-mm diameter has been measured to be as low as 0.01 %.
Realization of a radiometric temperature scale for near ambient temperatures with accuracy at the 20 to 50 mK level is crucial for a number of demanding military and commercial applications. In support of such measurements, radiation sources with high stability and spatial uniformity must be developed as reference and working standards. Traditionally, the temperature scale, maintained at the National Institute of Standards and Technology (NIST), relies on water bath and oil bath blackbodies in this temperature range. Recently, a water heat pipe blackbody was used at NIST as a spectral radiance source in a spectral emissivity measurement facility. Now a new, more versatile high emissivity water heat pipe blackbody was designed and characterized to be used as a reference radiance source for the radiometric temperature scale realization between 50 °C and 250 °C. Furthermore, it will serve as a reference source for the infrared spectral radiance measurements between 2.5 μm and 20 μm. The calculated spectral emissivity of the painted copper alloy cavity was verified by reflectance measurements using a CO2 laser at 10.6 μm wavelength. The spatial thermal uniformity and stability of the blackbody were characterized. Two independent realizations of the radiometric temperature scale were compared in order to verify the accuracy of the scale. Radiance temperature, calculated from the cavity temperature measured with a calibrated PRT contact thermometer and from the emissivity of the cavity, was compared to the radiance temperature, directly measured with a reference pyrometer, which was calibrated with a set of fixed point blackbodies. The difference was found to be within measurement uncertainties.
A new facility for the measurement of spectral emittance (emissivity) of materials that employs a set of blackbody sources is being built at NIST. This facility has also been used to investigate the capabilities of Fourier transform (FT) spectrometers to characterize the spectral emissivity of blackbody sources. The facility covers the spectral range of 1 μm to 20 μm and temperatures from 600 K to 1400 K. The principle of operation involves the spectral comparison of an unknown source with a group of variable temperature and fixed point reference sources by means of the FT spectrometer and filter radiometers. Sample surface temperature is measured by non-contact method using a sphere reflectometer. The current reflectometer setup allows measurements of opaque samples, but it is planned to include semitransparent materials at a later stage.
Baffles are often placed in integrating spheres to accommodate the non-ideal aspects of other sphere components. These include detectors, sources, sphere wall surface shape and coatings. Baffles intentionally prevent light interchange between these and other important sphere components and regions such as entrance/exit ports, sample, reference and detector field-of-view. The challenge for an integrating sphere designer is to position and construct baffles that achieve the primary goal of shadowing specific elements from each other, while at the same time minimizing all other “side” effects that the baffles may have. Perhaps the most important side effect is the additional signal loss for light arriving at or leaving the sample from or to the baffle due to its absorptance. This is especially true for coatings and spectral ranges where the wall reflectance is relatively low such as for BaSO4 above 1.5 mm and diffuse gold. A potential improvement that we have investigated in an infrared reflectometer sphere is the use of a specular coating that has significantly higher reflectance than any other available diffuse coating. In our case we have used specular gold versus the diffuse gold-coated plasma-sprayed metal coating that is on the sphere wall. Although this provides for lower loss of light reflected from the sample onto the baffle, the side effects must also be considered and reduced in the design. Specifically one needs to consider the mirroring that will take place in the sphere. In this paper we discuss the important design issues along with some integrating sphere characterization results that demonstrate improved sphere performance by use of specular baffles.
The large variety of high-precision unique blackbody sources: those operating at fixed temperatures provided by phase transitions of metals and metal-carbon eutectics, and variable-temperature ones had been designed in VNIIOFI for high-precision radiometry, radiation thermometry and spaceborne remote sensing within a 100 to 3500K temperature range. Paper reviews the blackbodies (BBs) ranged to low, middle and high temperatures, and describes spectral radiance and irradiance calibration facilities on the base of these BBs in IR and V-UV spectral ranges.
The latest investigations of high-temperature fix-points based on metal-carbon eutectics Re-C (2748K) demonstrated an excellent reproducibility of freezing plateau (up to 0.01% in terms of radiation temperature) between series of measurements/crucibles, and about 0.003% within a sample measurement session, i.e. better than 100mK. Further Re-C (spectral irradiance measurements) and TiC-C (3057° C) eutectics are being investigated for use as high-stable radiance/irradiance sources above the conventionally assigned values of temperatures of ITS-90.
KEYWORDS: Black bodies, Temperature metrology, Solar concentrators, Nitrogen, Calibration, Liquids, Reflectors, Reflectivity, Monte Carlo methods, Mirrors
Radiation temperature calibrations of IR radiometers and imaging systems, pre-launch characterization of spaceborne optical sensors require low and medium-background test facilities, equipped with reference blackbodies for full aperture calibration. Such extended area blackbodies have been recently developed and characterized by VNIIOFI and Vega International, Inc. Target technical specifications for the low temperature blackbody include 100 mm full aperture, plus or minus 12 degrees viewing angles, 0.999 effective spectral emissivity in 3 micrometer to 15 micrometer band, 100 K to 450 K temperature range, 50 mK temperature uniformity across aperture and, finally, 30 mK temperature setting/measurement accuracy. Monte Carlo technique and finite element method were employed for computer modeling of temperature distributions and effective emissivities of radiating cavities consisting of V-grooved flat bottom and particularly profiled reflector. The design features and technical specifications of blackbodies, developed for operation in high vacuum conditions in the temperature range from 100 K to 900 K, are presented. Results of investigation confirm applicability of the selected approach, though leaving space for improvement of blackbodies performance. Main directions of further research and development are discussed.
KEYWORDS: Black bodies, Temperature metrology, Calibration, Solar concentrators, Nitrogen, Reflectivity, Liquids, Monte Carlo methods, Reflectors, Control systems
Two extended area reference blackbodies, covering temperature range from 100 K to 900 K, have been designed and built by VNIIOFI and Vega International, Inc., for German Institute of Space Sensor Technology. The design objectives were to develop Plankian sources for high accuracy IR calibrations; provide, where possible, both high vacuum and atmospheric operation capabilities for easier achievable calibration traceability; and attain compact design to serve as a portable transfer standard. Target technical specifications for the low temperature blackbody BB100 include 100 mm full aperture, +/- 12 degree viewing angles, 0.999 effective spectral emissivity in 3 micrometers to 15 micrometers band, 100 K to 450 K temperature range, 50 mK temperature uniformity across aperture and, finally, 30 mK temperature setting/measurement accuracy. An employed design was selected after conceptual design study, which involved extensive thermophysical modeling and optimization. A finite element method has been applied to calculation of temperature distribution. Effective emissivity simulation was realized by the Monte Carlo method. The result of computer modeling of temperature distributions and effective emissivity are presented. Both target specifications and results of BB characterization are provided. Results of investigation confirm validity of selected approach and assumptions. Possibilities of further improvement of blackbodies performance are discussed.
The present state of the art of temperature blackbody (HTBB) sources development at the All-Russian Institute for Optical and Physical Measurements (VNIIOFI, Russia) and their characteristics are analyzed. The precision graphite blackbody BB22p, operating now at NIST, PTB, NPL and VNIIOFI, large area blackbody BB2000 and super high temperature pyrolitic graphite blackbody BB3200pg are described. Results of their theoretical and experimental investigation are given.
It's well known that just synchrotron radiation (SR) polarization characteristics determine the accuracy of the spectroradiometric scale based on SR sources. It's very effective way to use a SR source in "big bunch" mode to diminish the SR polarization characteristic influence on the total errors of secondary standards calibrated against the primary standard based on SR sources. The other way is connected with special SR source mode when SR ~-polarization component has the "plane angular distribution at the medium plane.
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