A necessary condition for accumulating fundamental climate data records is the use of observation instruments whose stability and accuracy are sufficiently high for climate monitoring purposes; the number of instruments and their distribution in space should be sufficient for measurements with no spatial or temporal gaps. The continuous acquirement of data over time intervals of several decades can only be possible under the condition of simultaneous application of instruments produced by different manufacturers and installed on different platforms belonging to one or several countries. The design of standard sources for pre-flight calibrations and in-flight monitoring of instruments has to meet the most stringent requirements for the accuracy of absolute radiometric measurements and stability of all instruments. This means that the radiometric scales should be stable, accurate, and uniform. Current technologies cannot ensure the high requirements for stability and compatibility of radiometric scales: 0.1% per decade within the 0.3 - 3 µm band and 0.01 K per decade within the 3 - 25 µm band. It is suggested that these tasks can be aided through the use of the pure metals or eutectic alloy phase transition phenomenon that always occur under the same temperature. Such devices can be used for pre-flight calibrations and for on-board monitoring of the stability of radiometric instruments. Results of previous studies of blackbody models based upon the phase transition phenomenon are quite promising. A study of the phase transition of some materials in small cells was conducted for future application in onboard monitoring devices and its results are positive and allow us to begin preparations for similar experiments in space.
The demands of modern radiation thermometry and radiometry are being satisfied by a large variety of high-precision unique BB sources (both fixed-point and variable temperature) designed for a wide range of temperature from 100 K to 3500 K. The paper contains a detailed review of low-, medium- and high-temperature precision blackbodies developed at VNIIOFI as the basis of the spectral radiance and irradiance calibration devices in the rank of National standards. The blackbodies include: 1) variable-temperature (100K..1000K) research-grade extended-area (up to 100 mm) models intended to perform radiometric calibrations by comparison with a primary standard source, as well as can be used as the sources for high-accuracy IR calibration of space-borne and other systems not requiring a vacuum environment; 2) low-temperature fixed-point blackbodies on the basis of phase transitions of pure metals such as In and Ga sources, and the metal-metal eutectics operating within the medium-temperature range (300K to 400K); these are used for pyrometric measurements, IR-radiometry, preflight and (future aspects) in-flight calibration of space borne IR instruments; 3) high-temperature wide aperture variable-temperature blackbodies (1800K to 3500K) such as BB3500MP, BB3500YY designed and fabricated, along with fixed-point cells working above the ITS-90 temperatures on the basis of phase transitions of metal-carbon eutectic alloys (Re-C, TiC-C, ZrC-C, HfC-C), which possess unique reproducibility of 0.1% or less.
The state-of-the-art electro-optical sensors being designed for today's space-based environmental applications require a complete characterization and thorough calibration. This is especially true for sensors designed to assess global climate change, which require very small uncertainties. This paper describes a system-level approach that addresses each phase of calibration, from planning to on-orbit operations. This approach encourages early planning and continuity of effort throughout the lifetime of the project (pre- and post-flight) to promote an optimum calibration approach that will minimize uncertainty for the intended application. This paper also discusses considerations for component level characterization, ground calibration and standards, in-flight calibration sources and trending, and in-flight validation assessment.
The full potential of current remote sensor technology is limited by the inability to correct biases once an exo-atmospheric remote sensor becomes operational. Even when the calibration is traced to the International System of Units, SI, and the instrument is performing within the operational envelope wherein it is calibrated, the problem exists and a Space Metrology Program is a potential solution to the problem. This paper discusses such a program, suggests a feasibility study to address the issues and recommends a plan of action.
Any operational instrument has a bias and reducing the magnitude of the bias can only be accomplished when an adequately accurate standard is accessible by the instrument while the instrument is in its operational environment. Currently the radiometric flux from the sun, the moon and the stars is inadequately accurate SI to provide a standard that is consistent with the remote sensor state-of-the-art technology. The result is data that is less accurate than it could be often leading to confusing and conflicting conclusions drawn from that data. Planned remote sensors such as those required to meet future program needs (e.g. the United States National Polar-Orbiting Operational Environmental Satellite System (NPOESS) and the proposed international Global Earth Observation Program) are going to need the higher accuracy radiometric standards to maintain their accuracy once they become operational. To resolve the problem, a set of standard radiometers on the International Space Station is suggested against which other exo-atmospheric radiometric instruments can be calibrated. A feasibility study for this program is planned.
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.
The calibration of IR sensors, thermal imagers, instrumentation for remote earth sensing, signature recognition, and low background spacebome radiom-eters requires development of low- and near-ambient-temperature reference standard sources capable of operating in vacuum at low or medium background conditions. The paper contains the characteristics of blackbodies (BB), developed at the All-Russian Research Institute for Optical and Physical Measurements (VNIIOFI). A novel Medium Background Facility (MBF) intended for calibrating IR sources has been developed at VNIIOFI. The MBF used for the calibration under vacuum conditions (10-3 Pa) and medium background temperatures includes a Ga Fixed-Point Blackbody (29.76 °C), Variable Temperature Blackbody (-60 °C ~ +80 °C) and Filter Radiometer. A brief description of the operating principles and specifications of the MBF is presented. The results of a comparison of the Variable Temperature Blackbody and Ga Fixed Point Blackbody in the Medium Background Facility are given. The measurement uncertainties radiance temperature associated with spectral bandwidth are discussed.
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 design of filters with specific spectral characteristics is a requirement not only for the design of filter radiometers, but also for many applications in optical measurements. The most general type of absorptive filters are composite subtractive-additive filters and the general problem of filter radiometer spectral response optimization using such filters is formulated. The algorithm and software realization of constrained optimization for various objective functions with arbitrary weight functions are described. Successive random search and Hooke-Jeeves methods are employed in the optimization and several goodness-of-fit criteria are used for evaluation of the results. Illustrative numerical examples are presented.
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.
The Monte Carlo method was applied to statistical modeling of radiometric properties of integrating spheres. According to employed model of reflectance, specular and diffuse components are approximated by polynomial function of incident angle cosine. Developed software allows to simulate the irradiance distribution over internal surfaces of sphere with relative uncertainty of 0.1%. Sphere geometries with arbitrary number of apertures and baffles of circular shapes are admitted. To illustrate capabilities of software, an irradiance distribution over internal surfaces of the integrating sphere is plotted for several cases of practical importance.
This work summarizes the progress in the development and testing of the telescope for a space imaging spectrometer 'OMEGA' of a new stereo-spectral-imaging system ARGUS developed in the frame of the International project for MARS 94/96 missions. The results of the telescope aberration calculation are given. Brief description of the facility for the main optical characteristics measurements are described. The telescope was tested at a special vacuum chamber at the temperature range from 180K to 300K. The results of the telescope investigation are given.
A new low-cost absolute cryogenic radiometer of the electrical substitution (ES) type optimized for performing black-body models calibration is described. Nitrogen is used as a cryogenic liquid for cooling of a radiometer receiving cavity up to temperature 80 K. This absolute ESR has been developed for measuring the irradiance in the range of 10-3 divided by 10-6 W/cm2 with the uncertainty of 0.1% for the upper level of irradiance range. The receiving cavity having 16 mm aperture, is fabricated from copper foil with thickness of 30 micrometers . A substitution winding of the receiving cavity is made of manganin wires. Measured heat conduction of the receiving cavity is 1.1 X 10-3 W/K. A single time constant of the receiving cavity is 80 sec. Brief description of the design, operating principles, and measurements results of the new cryogenic radiometer at nitrogen temperature are given.
A diffuse target illuminated by the sun radiation is used for on-board calibration of a new stereo-spectral-imaging system ARGUS developed in the frame of the International project for MARS 94/96 missions. The target manufacturing technique is described. The experimental procedure used for getting the spectral and angular responses of target reflectivity is presented. The diffuse target was tested for the spectral properties at angle illumination -70 degree(s) (relative to the normal of the target) and angle of viewing +20 degree(s). A brief description of the setup for measurements of the spectral and angular responses of the target reflectivity is given. Results of the diffuse target calibration in the spectral range from 320 nm to 5200 nm are presented.
The method for on board calibration of the space imaging instruments using a diffusing target illuminated by the sun radiation is gaining acceptance. This raises a problem of measurements of the spectral and angular response of the target reflectivity in illumination conditions similar to the actual one on-board. This work summarizes the progress in the development and testing of the target for on-board radiometric calibration of the space video-spectrometers in stereo- spectral imaging system ARGUS. The spectral and angular response of the target reflectivity and the experimental procedure used for getting these characteristics are described. The target manufacturing technique is described. This target was tested for the spectral properties from 0.3 micrometers to 5.2 micrometers at angle of illumination -70 degree(s) (relative to the normal of the target) and angle of viewing +20 degree(s). The angular response was measured with three incident angles 70 degree(s), 75 degree(s) and 80 degree(s) (relative to the normal) for angles of viewing varying between 0 degree(s) and 60 degree(s) (relative to the normal). Brief description of the setup for measurements of the spectral and angular response of the target reflectivity are presented. The values of angular and spectral response of the reflectivity have been determined with the accuracy of 2 - 5% depending on the wavelength.
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.
A radiometric calibration of the space imaging spectrometers especially with a wide field of view is performed with a help on an integrating sphere as a radiance source. Spectral radiance of the integrating sphere is defined by a comparison with the absolute diffuse source. As a diffuse source for a calibration of the integrating sphere in units of radiance may be used a halon target irradiated by a halogen lamp. The absolute diffuse source based on the halon target/four halogen lamps configuration with a good radiance homogeneity is described. The diffuse screen with a diameter 180 mm, which is made of polytetrafluoroethylene (PTFE), is irradiated by 4 symmetrically located halogen lamps. The type of the lamp is KGM 24-150. The angle of incidence on the diffuse screen is 34°. Viewing is carried out through the output window along the direction, which is normal in relation to the diffuse screen. The diameter of the output window is 140 mm. A facility for relative radiance graduation and calibration of this diffuse source against a standard lamp traceable to All-Russian Research Institute of Optical and Physical Measurements (VNIIOFI) and the results of the absolute diffuse source radiometric calibration in the spectral range from 400 nm till 2500 nm are described. A spatial inhomogeneity not more than 2.5% along the whole output aperture. A level of radiance at 900 rim is 1.6.108W/m3sr. Described absolute diffuse source was created in according to a contract with the Space Research Institute of the Russian Academy of Sciences for the radiometric calibration of imaging cameras and spectrometer in stereo-spectralimaging system ARGUS for Mars 94/96 missions.
The quantum phenomena of parametric down conversion have been used for absolute measurement of the photodetector quantum efficiency of ultra-low-light radiation ranging from 10-12 to 10-15 W, which is equivalent to photon fluxes of 107 to 104 s-1. The process of parametric down conversion gives rise to highly correlated (in time and space) photon flux pairs. We describe the method, experimental investigation, and a new device involving the measurement of the coincidence rate of photodetection of these two correlated beams of light. The coincidence is lowered by a nonunit quantum efficiency of either detection. This fact was used for the definition of the absolute value of both photodetector's quantum efficiencies.
Victor Sapritsky, B. Carol Johnson, Robert Saunders, Lev Vlasov, Konstantin Sudarev, Boris Klevnoy, Vjcheslav Shapoval, Igor Dmitriev, Leonid Buchnev, Alexsandr Prochorov
This paper reviews the research and design of high temperature blackbody sources for the temperature interval from 2000 K to 3000 K. Sources with large apertures are addressed specifically, as these are well suited to the important problem of spectral irradiance scale realizations.
Primary radiometric standards of incoherent radiationin the 0.04-25 micron range, developed and maintained in the USSR, are described. The standards are based on a synchrotron radiation source, high- and low-temperature blackbodies and absolute radiometer. The basic characteristics of the standards are given and the prospects for the construction of the cryogenic blackbody and absolute radiometer to be used for low-intensity calibrations are discussed. The methods and calibration systems for aerospace instrument calibrations against the radiometric standards are mentioned.
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