Absolute Cryogenic Radiometer Research Articles

Measurements of fibre optic power using photodiodes with and without an integrating sphere

We have studied two kinds of reference detectors to be used for calibration of fibre optic power meters. The first type consists of an InGaAs photodiode mounted directly on an adapter using a standard FC connector. In the second type, a similar photodiode is fitted into a 5 cm diameter integrating sphere. The plain photodiodes are used for low power levels and the sphere detector for high power levels because of the low throughput (∼0.7%) of the sphere. The detectors, with their FC connectors removed, are calibrated using collimated laser beams, using as the reference a spectrally flat pyroelectric radiometer traceable to a cryogenic absolute radiometer. The spatial non-uniformities of the plain photodiodes are of the order of 0.3% throughout the active area of 5 mm diameter. The angular responsivity varies less than ±0.1% within the angle of ±8°, which covers 99% of the radiation emitted from the fibre end. The angular non-uniformity of the sphere detector is less than 0.1%. The results indicate that a plain photodiode can be used as a reference detector of fibre optic power. The calibration uncertainty at HUT is presently of the order of 1% (k = 2).

Primary realization of a spectral irradiance scale employing monochromator-based cryogenic radiometry between 200 nm and 20 µm

Here we report on an alternative approach for making spectral irradiance measurements of radiation sources traceable, employing our monochromator-based absolute cryogenic radiometer (ACR) facility to its full extent. The method makes use of the continuously tunable absolute radiant flux emerging from the ACR facility to characterize and calibrate the spectral irradiance responsivity of a second adjacent double-monochromator system. This system will be part of a new facility at NMi-VSL, called SIR, that will be used to measure spectral irradiance distributions of radiation sources from the ultraviolet to the far infrared. Following this traceability route, a fully characterized high-temperature Planckian radiator is not needed. Proof-of-principle measurements in the visible part of the spectrum show very encouraging results. Although the new facility is under construction, the theoretical background and a schematic description will be presented in this paper.

Ultraviolet calibration of detectors with respect to a cryogenic radiometer

A facility has been designed to perform a variety of detector characterizations in the air-ultraviolet in the spectral range 230 nm to 400 nm. The facility allows the measurement of spatial uniformity, linearity and the spectral responsivity of different detectors, irradiated by a He-Cd laser or by a monochromator and Xe discharge lamp. With this technique, a set of Si photodiodes, Si detectors in trap configuration, and ultraviolet (UV) radiometers built with SiC detectors in an integrating sphere, have been calibrated and their absolute responsivity at the He-Cd laser UV line has been determined with respect to an absolute cryogenic radiometer.

Intercomparison of lamp and detector‐based UV‐irradiance scales for solar UV radiometry

The high uncertainty of calibration is the most serious factor limiting accurate measurements of the solar UV, needed for the assessment of global UV trends. In this study, the calibration of FEL‐type standard lamps traceable to three primary standard laboratories were compared with a transfer uncertainty of ±1.4% (2σ). In comparison with the reference lamp, the spectral UV irradiance scales agreed within 1.5%. However, the difference between a new lamp and the present reference was 2%, which is close to the limits of total uncertainty. It was interesting to observe that one of the scales, based on a cryogenic absolute radiometer, was in good agreement with the scales based on blackbody radiation sources. Examination of the long‐term stability of the lamp‐based UV scale in Finland showed a significant decrease of 6% in the scale when the standard lamp was changed to a lamp directly traceable to the primary standard of the National Institute of Standards and Technology (NIST). Previously, the lamp was traceable to the NIST through the secondary standard of Optronic Laboratories Inc. The main obstacle in improving the global conformity of UV scales is the instability of halogen standard lamps. On the basis of the present study, it is strongly recommended to use detector stabilization of the standard lamps and to investigate the use of portable detector standards in the intercomparisons between primary standards laboratories.

Thermal-electrical modeling of absolute cryogenic radiometers

This study employs finite-element (FE) software to model the nonlinear thermal-electrical dynamic response of absolute cryogenic radiometers (ACRs). The high-accuracy cryogenic radiometer (HACR) and laser-optimized cryogenic radiometer (LOCR) have been analyzed to determine the input geometric and thermal parameters. Comparisons between the FE model and experimental results for the HACR and LOCR show an agreement in temperature within 2%. A proportional-integral-derivative (PID) temperature-control algorithm has been developed for both the receiver and the heat sink. The optimized control parameters are related to the thermal conductances and time constants of the radiometers. The LOCR receiver has the advantage of a reduction in measurement time over the HACR receiver; whereas the HACR heat sink offers the advantage of better temperature stability over the LOCR heat sink. The time constant of the HACR receiver can be significantly reduced without degrading the performance. The temperature stability of the LOCR heat sink can be improved by a factor of two by using a larger heat-sink heat capacity, C HS≥80 mJ/K. This research will help improve future design of ACRs.

Characterisation of optical detectors using high-accuracy instruments

The facilities of the Metrology Research Institute at the Helsinki University of Technology, and methods for characterisation of optical detectors for spectral radiant intensity and irradiance responsivity, are described. The instrumentation for such characterisations includes a reference spectrometer with a number of auxiliary set-ups, and equipment for the spectral irradiance measurements with a filter radiometer based on a trap detector. The methods of realising the spectral responsivity scales based on an absolute cryogenic radiometer in house are addressed. The procedures and results of characterisation of a multipoint measuring system of photosynthetically active radiation, by employing the available facilities, are briefly described. The absolute irradiance responsivity of the device is determined by using a photometric lamp, whose spectral irradiance has been measured with the filter radiometer. The combined standard uncertainty of this set of calibrations is 3.6% at the 1 σ level. The uncertainty is caused almost completely by the multipoint measuring system.

Ultraviolet radiometry with synchrotron radiation and cryogenic radiometry

The combination of a cryogenic radiometer and synchrotron radiation enables detector scale realization in spectral regions that are otherwise difficult to access. Cryogenic radiometry is the most accurate primary detector-based standard available to date, and synchrotron radiation gives a unique broadband and continuous spectrum that extends from x ray to far IR. We describe a new cryogenic radiometer-based UV radiometry facility at the Synchrotron Ultraviolet Radiation Facility II at the National Institute of Standards and Technology. The facility is designed to perform a variety of detector and optical materials characterizations. The facility combines a high-throughput, normal incidence monochromator with an absolute cryogenic radiometer optimized for UV measurements to provide absolute radiometric measurements in the spectral range from 125 nm to approximately 320 nm. We discuss results on photodetector characterizations, including absolute spectroradiometric calibration, spatial responsivity mapping, spectroreflectance, and internal quantum efficiency. In addition, such characterizations are used to study UV radiation damage in photodetectors that can shed light on the mechanism of the damage process. Examples are also given for UV optical materials characterization.

The new cryogenic vacuum chamber and black-body source for infrared calibrations at the NIST's FARCAL facility

The Facility for Advanced Radiometric Calibrations (FARCAL) at the National Institute of Standards and Technology (NIST) in Gaithersburg, MD, comprises the Medium Background Infrared (MBIR) facility and the Training for Calibration Expertise in Radiometry (TraCER), the latter being a classroom setting. The MBIR is a 120 cm by 180 cm vacuum chamber with an internal cold shroud and an integral roll-out table. The chamber will contain a black-body source and an absolute cryogenic radiometer (ACR). Both components are mounted on the roll-out table. Measurements will be performed in the MBIR facility that cannot be accomplished using existing facilities, including the NIST Low Background Infrared (LBIR) facility and the room-temperature non-vacuum radiometric facilities. Initially the MBIR facility will be used for 80 K medium background measurements and 300 K ambient background measurements.

New ultraviolet radiometry beamline at the Synchrotron Ultraviolet Radiation Facility at NIST

We have constructed a new ultraviolet (UV) radiometry facility at the Synchrotron Ultraviolet Radiation Facility (SURF II) at the National Institute of Standards and Technology (NIST). The facility combines a high-throughput normal-incidence monochromator with an absolute cryogenic radiometer (ACR) optimized for UV measurements to provide absolute detector-based radiometric calibrations in the spectral range 125 nm to approximately 320 nm. The system can be configured for spectroradiometric calibration of photodetectors with completely automated mapping of detector spatial and angular responsivity. The facility can easily be adapted for other spectroradiometric measurements including transmittance and reflectance and the spectral range can be extended to about 50 nm.

Measurement of thermophysical properties of polyimide and a black paint for future development of cryogenic radiometers

We have measured the thermal conductivity of polyimide rod at temperatures from 2.4 K to 8 K, and the specific heats of polyimide rod and a black paint at 2.4 K and 4.4 K. It is shown that the responsivity and time constant of cryogenic radiometers can be significantly improved by the use of a polyimide film as the heat link. This study will help future development of highly sensitive absolute cryogenic radiometers.

Improved Photometric Standards and Calibration Procedures at NIST.

NIST has recently established a detector-based luminous intensity unit (candela, cd), which is derived from the NIST absolute cryogenic radiometer. Subsequently, the luminous flux unit (lumen, lm) and the luminance unit (cd/m2) have been established based on the detector-based candela, and now all the NIST photometric units are tied to the cryogenic radiometer. The illuminance unit is realized and maintained on five standard photometers. The large dynamic range of the standard photometers eliminates the need for maintaining many working standard lamps of various wattages. The luminous intensities of lamps are determined from the illuminances measured with these photometers and the distances measured with a linear encoder system. Transfer photometers and illuminance meters are calibrated by direct comparison with the standard photometers with no distance measurements involved. The luminous flux unit is realized using an absolute integrating sphere method newly developed at NIST. The luminance unit is realized on an integrating sphere source, which is used for calibration of other luminance sources and luminance meters. These detector-based methods have made it possible to reduce the uncertainties of photometric calibrations and to provide more varieties of photometric calibration services at NIST.

Absolute cryogenic radiometer for high-current photometric measurements

A cryogenic radiometer with electrical substitution has been developed for precise photometric measurements. The radiometer design and specifications are described. The radiometer has an accuracy of 0.02%. Its main use is to calibrate the spectral response of reference radiometers.

Monochromator-based cryogenic radiometry at the NRC

A facility is being set up at the National Research Council of Canada for the monochromator-based spectral calibration of transfer standard radiometers using a cryogenic absolute radiometer. General design features and target performance of the new apparatus are discussed. The sources of error inherent to detector calibrations performed using monochromator-based sources are analysed in some detail, with special attention devoted to bandwidth errors, polarization effects and the effect of the angular subtense of the beam. Some detector effects, such as spatial uniformity and temperature dependence, are also discussed. A status report is given on the development of the new cryogenic radiometer facility.

A method of realizing spectral-radiance and irradiance scales based on comparison of synchrotron and high-temperature black-body radiation

Improvements in synchrotron radiation and high-temperature black-body sources will provide a method of realizing scales of spectral radiance and irradiance with uncertainties comparable with those provided by absolute cryogenic radiometers. This paper discusses planned improvements in the synchrotron radiation facility at the National Institute of Standards and Technology (NIST), and how these improvements can be exploited to accurately measure the temperature of a high-temperature black body developed at the All-Russian Research Institute for Optophysical Measurements (VNIIOFI), thus allowing substantial improvement in radiometric scales.

Comparison of luminous-intensity scales based on trap detectors and incandescent lamps

New luminous-intensity scales have been developed in Finland and Sweden based on silicon trap detectors having responsivities compared with an absolute cryogenic radiometer at a wavelength of 543,516 nm. The low reflectance of the trap device makes it possible to use a V(λ) filter in conjunction with the trap where the filter is separately characterized in transmittance. The new scales were compared with the scale currently in use in Sweden, which is based on incandescent lamps calibrated by the Bureau International des Poids et Mesures (BIPM). The agreement of the scales is generally within ±1%. The Finnish scale was typically 0,2% to 0,6% below the BIPM-based scale, and the trap-based scale of Sweden is typically 0% to 0,5% below the BIPM-based scale when compared at a measurement distance of 4 m. Different sizes of aperture and different distances to the source showed variations up to ±1%. The transmittances of the V(λ) filters were measured separately by the two institutes and showed an agreement better than 0,1% in the colour-correction factor.

An absolute cryogenic radiometer for laser calibration and characterization of photodetectors

Absolute cryogenic electrical-substitution radiometers (ESRs) are widely used in many applications of absolute radiometry and are accepted today as being the most accurate instruments for radiant-power measurements. An absolute cryogenic electrical-substitution radiometer has been developed at the All-Russian Research Institute for Optophysical Measurements (VNIIOFI) as the radiometric standard of Russia. The design and characterization of the VNIIOFI absolute cryogenic radiometer currently used for the calibration of detectors in Russia is described. This radiometer uses laser radiation and a power level of 1,3 mW and achieves an uncertainty of 0,02%. The factors limiting the overall uncertainty of the VNIIOFI cryogenic radiometer are also analysed.

Realization of the candela from a partial filtering V(λ) detector traceable to a cryogenic radiometer

This paper describes the approach used to realize the candela from illuminance measurements made with a large-area, partial-filtering photometer. The methods followed to determine the absolute responsivity at 555 nm, traceable to an absolute cryogenic radiometer, and to measure the relative spectral responsivity are discussed, including the uncertainty values of the measured quantities. Finally, preliminary results obtained on comparing the photometer measurements with the previous luminous intensity scale are presented.

Optical power and transmittance measurements and their use in detector-based realization of the luminous intensity scale

Responsivity calibrations of trap detectors with a cryogenic absolute radiometer and a power-stabilized 543.51 6-nm He-Ne laser are described. The trap detectors are used as transfer standards of optical power at visible wavelengths because they have a predictable spectral responsivity. New results on transmittance measurements of PRC Krochmann <i>V</i>(λ) filters are presented, especially the angle and temperature dependence of the transmittance. The characterized trap detector and the <i>V</i>(λ) filter are used for a novel realization of the SI base unit of luminous intensity, the candela. Test measurements indicate a good agreement with previous photometric scales.

Radiometric method of constructing a thermodynamic temperature scale

A description is presented of a radiometric method of constructing a thermodynamic temperature scale in the range 300–1000 K. The method is based on the Stefan-Boltzmann radiation law. The experimental unit contains a vacuum radiator of the BBM (blackbody model) type with a regulable temperature, a cryogenic photometric chamber, and an absolute cryogenic radiometer with electrical conversion. A method of extrapolating temperature and the measurement error is examined. The total error of reproduction of thermodynamic temperature is no greater than 0.3 K.

極低温放射計の導入による測光・放射測定の高精度化

極低温放射計の導入による測光・放射測定の高精度化