Realization of the National Institute of Standards and Technology detector-based spectral irradiance scale

A detector-based spectral irradiance scale has been realized at the National Institute of Standards and Technology (NIST). Unlike the previous NIST spectral irradiance scales, the new scale is generated with filter radiometers calibrated for absolute spectral power responsivity traceable to the NIST high-accuracy cryogenic radiometer instead of with the gold freezing-point blackbody. The calibrated filter radiometers are then used to establish the radiance temperature of a high-temperature blackbody (HTBB) operating near 3,000 K The spectral irradiance of the HTBB is then determined with knowledge of the geometric factors and is used to assign the spectral irradiances of a group of 1,000-W free-electron laser lamps. The detector-based spectral irradiance scale results in the reduction of the uncertainties from the previous source-based spectral irradiance scale by at least a factor of 2 in the ultraviolet and visible wavelength regions. The new detector-based spectral irradiance scale also leads to a reduction in the uncertainties in the shortwave infrared wavelength region by at least a factor of 2-10, depending on the wavelength. Following the establishment of the spectral irradiance scale in the early 1960s, the detector-based spectral irradiance scale represents a fundamental change in the way that the NIST spectral irradiance scale is realized.     

Long-term temporal stability of the National Institute of Standards and Technology spectral irradiance scale determined with absolute filter radiometers

The temporal stability of the National Institute of Standards and Technology (NIST) spectral irradiance scale as measured with broadband filter radiometers calibrated for absolute spectral irradiance responsivity is described. The working standard free-electron laser (FEL) lamps and the check standard FEL lamps have been monitored with radiometers in the ultraviolet and the visible wavelength regions. The measurements made with these two radiometers reveal that the NIST spectral irradiance scale as compared with an absolute thermodynamic scale has not changed by more than 1.5% in the visible from 1993 to 1999. Similar measurements in the ultraviolet reveal that the corresponding change is less than 1.5% from 1995 to 1999. Furthermore, a check of the spectral irradiance scale by six different filter radiometers calibrated for absolute spectral irradiance responsivity based on the high-accuracy cryogenic radiometer shows that the agreement between the present scale and the detector-based scale is better than 1.3% throughout the visible to the near-infrared wavelength region. These results validate the assigned spectral irradiance of the widely disseminated NIST or NIST-traceable standard sources.     

Thermodynamic Radiation Thermometry Using Radiometers Calibrated for Radiance Responsivity

Using radiometry, thermodynamic temperatures can be determined by a variety of experimental techniques. Radiometers without imaging optics can be calibrated for spectral power or spectral irradiance responsivity, and radiometers with imaging optics can be calibrated for radiance responsivity. These separate approaches can have different uncertainty components with different uncertainty values. At NIST, thermodynamic radiation thermometry is performed using radiation thermometers calibrated for radiance responsivity using laser-irradiated integrating sphere sources (ISS). The radiance of the ISS is determined using Si-trap detectors whose spectral power responsivity is traceable to the electrical substitution cryogenic radiometer. The radiometric basis of the NIST approach is discussed. The uncertainty budget for the measurements as well as the characterizations to determine the component uncertainty values is listed.     

New PTB Setup for the Absolute Calibration of the Spectral Responsivity of Radiation Thermometers

The paper describes the new experimental setup assembled at the PTB for the absolute spectral responsivity measurement of radiation thermometers. The concept of this setup is to measure the relative spectral responsivity of the radiation thermometer using the conventional monochromator-based spectral comparator facility also used for the calibration of filter radiometers. The absolute spectral responsivity is subsequently measured at one wavelength, supplied by the radiation of a diode laser, using the new setup. The radiation of the diode laser is guided with an optical fiber into an integrating sphere source that is equipped with an aperture of absolutely known area. The spectral radiance of this integrating sphere source is determined via the spectral irradiance measured by a trap detector with an absolutely calibrated spectral responsivity traceable to the primary detector standard of the PTB, the cryogenic radiometer. First results of the spectral responsivity calibration of the radiation thermometer LP3 are presented, and a provisional uncertainty budget of the absolute spectral responsivity is given.     

Infrared Filter Radiometers for Thermodynamic Temperature Determination below 660 °C

At the Physikalisch-Technische Bundesanstalt (PTB), absolutely-calibrated filter radiometers based on silicon photodiodes are routinely used for thermodynamic temperature determinations of blackbodies in the range from the zinc fixed point (FP) (419 °C) up to 3,000 °C. To extend the temperature range down to the tin FP (232 °C), we have designed two new filter radiometers based on indium gallium arsenide (InGaAs) photodiodes with center wavelengths at 1,300 nm and 1,550 nm. For the absolute calibration of the spectral irradiance responsivity of the new InGaAs filter radiometers, the spectral responsivity measurement in the near-infrared (NIR) spectral range has been significantly improved. With a newly developed tuneable laser and monochromator-based cryogenic radiometer facility, the relative standard uncertainty of the NIR spectral responsivity has been reduced from 0.17 % to about 0.03 %. By using the calibrated InGaAs filter radiometer in conjunction with the large-area double sodium heat pipe of the PTB, the first results for the difference between the thermodynamic temperature T and the ITS-90 temperature T ₉₀ in the temperature range from the zinc FP up to the aluminum FP (660 °C) are presented. The values for T – T ₉₀ determined with the new InGaAs filter radiometers are consistent within their relative standard uncertainty of about 30 mK at 419 °C to about 60 mK at 660 °C. References to commercial products are for identification purposes only and constitute neither endorsement nor representation that the item identified is the best available for the stated purpose.     

SSE- and Noise-Optimized InGaAs Radiation Thermometer

For measurements of radiance temperatures in the range from 150°C to 1,000°C, low uncertainties in the temperature measurements can be achieved by using near-infrared InGaAs radiation thermometers. The design and construction of the NIST near-infrared radiation thermometer (NIRT) that is optimized for low size-of-source effect (SSE) and noise-equivalent temperatures are described. The NIRT utilizes a 50 mm diameter achromatic objective lens with low scatter that images a 4.5 mm diameter spot at a distance of 50 cm from the objective in an on-axis design. A Lyot stop is implemented in the design with the aperture stop placed after the field stop resulting in a collection f/12. A 3 mm diameter InGaAs detector is cooled to − 70°C using a four-stage thermoelectric cooler to obtain high-shunt resistance for linear, low-noise operation at high transimpedance amplifier gains. For thermal and structural stability, the optical components are placed on four, 15 mm diameter graphite-epoxy rods making the optical throughput stable. Optical ray tracing with a commercial program is used to determine the Strehl ratio and other imaging parameters. A possible approach for a detector-based temperature scale in this range which could result in 10 mK (k = 2) thermodynamic temperature uncertainties at the In-point is discussed.     

Facility for spectral irradiance and radiance responsivity calibrations using uniform sources

Detectors have historically been calibrated for spectral power responsivity at the National Institute of Standards and Technology by using a lamp-monochromator system to tune the wavelength of the excitation source. Silicon detectors can be calibrated in the visible spectral region with combined standard uncertainties at the 0.1% level. However, uncertainties increase dramatically when measuring an instrument's spectral irradiance or radiance responsivity. We describe what we believe to be a new laser-based facility for spectral irradiance and radiance responsivity calibrations using uniform sources (SIRCUS) that was developed to calibrate instruments directly in irradiance or radiance mode with uncertainties approaching or exceeding those available for spectral power responsivity calibrations. In SIRCUS, the emission from high-power, tunable lasers is introduced into an integrating sphere using optical fibers, producing uniform, quasi-Lambertian, high-radiant-flux sources. Reference standard irradiance detectors, calibrated directly against national primary standards for spectral power responsivity and aperture area measurement, are used to determine the irradiance at a reference plane. Knowing the measurement geometry, the source radiance can be readily determined as well. The radiometric properties of the SIRCUS source coupled with state-of-the-art transfer standard radiometers whose responses are directly traceable to primary national radiometric scales result in typical combined standard uncertainties in irradiance and radiance responsivity calibrations of less than 0.1%. The details of the facility and its effect on primary national radiometric scales are discussed.     

Absolute Radiation Thermometry in the NIR

A near infrared (NIR) radiation thermometer (RT) for temperature measurements in the range from 773 K up to 1235 K was characterized and calibrated in terms of the “Mise en Pratique for the definition of the Kelvin” (MeP-K) by measuring its absolute spectral radiance responsivity. Using Planck’s law of thermal radiation allows the direct measurement of the thermodynamic temperature independently of any ITS-90 fixed-point. To determine the absolute spectral radiance responsivity of the radiation thermometer in the NIR spectral region, an existing PTB monochromator-based calibration setup was upgraded with a supercontinuum laser system (0.45 \(\upmu \hbox {m}\) to 2.4 \(\upmu \hbox {m}\)) resulting in a significantly improved signal-to-noise ratio. The RT was characterized with respect to its nonlinearity, size-of-source effect, distance effect, and the consistency of its individual temperature measuring ranges. To further improve the calibration setup, a new tool for the aperture alignment and distance measurement was developed. Furthermore, the diffraction correction as well as the impedance correction of the current-to-voltage converter is considered. The calibration scheme and the corresponding uncertainty budget of the absolute spectral responsivity are presented. A relative standard uncertainty of 0.1 % \((k=1)\) for the absolute spectral radiance responsivity was achieved. The absolute radiometric calibration was validated at four temperature values with respect to the ITS-90 via a variable temperature heatpipe blackbody (773 K ...1235 K) and at a gold fixed-point blackbody radiator (1337.33 K).     

Construction and Characterization of a Large Aperture Blackbody for Infrared Radiometer Calibration

A large aperture blackbody (LABB) with a diameter of 1 m has been successfully constructed for calibrating radiation thermometers and infrared radiometers with a wide field of view in the temperature range between 10 °C and 90 °C. The blackbody is a 1 m long cylindro-conical cavity with a diameter of 1.1 m. Its conical bottom has an apex angle of 120°. To achieve good temperature stability and uniformity, the cavity is integrated to a water-bath to which the pressurized water is supplied from a reservoir. To reduce the convection heat loss from the cavity to the ambient, the cavity is purged of the dried air that passes through a coiled tube immersed in the reservoir. For an uncertainty evaluation of the LABB, its temperature stability was measured by using a reference radiation thermometer (RRT) and a platinum resistance thermometer (PRT), and its radiance temperature distributions on the aperture plane were measured by using a thermal camera. Measuring the spectral emissivity of the coating material, the effective emissivity of the blackbody was calculated to be 0.9955 from 1 ??m to 15 ??m. The expanded uncertainty of the radiance temperature scale was evaluated based on the PRT readings, which vary from 0.3 °C to 0.5 °C (k = 2) in the temperature range. The temperature scale is validated by comparing with the RRT of which the temperature scale is realized by a multiple fixed-point calibration.     

Developments for a New Spectral Irradiance Scale at the National Institute of Standards and Technology

Recent developments for a new spectral irradiance scale realization at the National Institute of Standards and Technology have been targeted to reduce the present relative expanded uncertainties of 0.67 % to 4.34 % (coverage factor of k = 2 and thus a 2 standard deviation estimate) in the spectral irradiance scale to 0.17 % for the range from 350 nm to 1100 nm. To accomplish this goal, a suite of filter radiometers calibrated using NIST’s high accuracy cryogenic radiometer have been used to measure the temperature of a high-temperature black-body. A comparison of the filter radiometer calibrations with the spectral irradiance scale along with an evaluation of the black-body calibration technique have been performed. With the aid of a monochromator, the calibrated filter radiometers will then be utilized to calibrate primary and secondary spectral irradiance standard lamps at NIST.     

Monochromator-Based Absolute Calibration of a Standard Radiation Thermometer

Centro Español de Metrología (CEM) is disseminating the International Temperature Scale (ITS-90), at high temperatures, by using the fixed points of Ag and Cu and a standard radiation thermometer. However, the future mise-en-pratique for the definition of the kelvin (MeP-K) will include the dissemination of the kelvin by primary methods and by indirect approximations capable of exceptionally low uncertainties or increased reliability. Primary radiometry is, at present, able to achieve uncertainties competitive with the ITS-90 above the silver point with one of the possible techniques the calibration for radiance responsivity of an imaging radiometer (radiance method). In order to carry out this calibration, IO-CSIC (Spanish Designated Institute for luminous intensity and luminous flux) has collaborated with CEM, allowing traceability to its cryogenic radiometer. A monochromator integrating sphere-based spectral comparator facility has been used to calibrate one of the CEM standard radiation thermometers. The absolute calibrated standard radiation thermometer has been used to determine the temperatures of the fixed points of Cu, Co–C, Pt–C, and Re–C. The results obtained are 1357.80 K, 1597.10 K, 2011.66 K, and 2747.64 K, respectively, with uncertainties ranging from 0.4 K to 1.1 K.     

Spectral Power and Irradiance Responsivity Calibration of InSb Working-Standard Radiometers

New, improved-performance InSb power-irradiance meters have been developed and characterized to maintain the National Institute of Standards and Technology (NIST) spectral responsivity scale between 2 and 5.1 mum. The InSb radiometers were calibrated against the transfer-standard cryogenic bolometer that is tied to the primary-standard cryogenic radiometer of the NIST. The InSb radiometers serve as easy-to-use working standards for routine spectral power and irradiance responsivity calibrations. The spectral irradiance responsivities were derived from the spectral power responsivities by use of the measured area of the apertures in front of the InSb detectors.     

Calibration of the Irradiance Responsivity of a Filter Radiometer for T Measurement at NIM

At the National Institute of Metrology of China (NIM), silicon photodiode-based narrow-band interference filter radiometers (FRs) have been designed for the radiometric determination of the thermodynamic temperature. The FR calibrations were performed on a new spectral comparator with a trap detector which was calibrated against the cryogenic radiometer at several discrete laser lines. The new spectral comparator is constructed from two grating monochromators assembled to give lower stray light and higher transmitted flux. Applying a transmittance measurement of the filter in the out-of-band region and careful control of the temperature, the irradiance responsivity of a 633 nm centered FR has been obtained over a dynamic range of nearly eight decades in the wavelength range from 450 nm to 1200 nm. The relative standard uncertainty of the responsivity is also analyzed and estimated to be less than 7 × 10^?4 at the 1 ?? level.     

Monochromator-Based Absolute Calibration of Radiation Thermometers

A monochromator integrating-sphere-based spectral comparator facility has been developed to calibrate standard radiation thermometers in terms of the absolute spectral radiance responsivity, traceable to the PTB cryogenic radiometer. The absolute responsivity calibration has been improved using a 75 W xenon lamp with a reflective mirror and imaging optics to a relative standard uncertainty at the peak wavelength of approximately 0.17 % (k = 1). Via a relative measurement of the out-of-band responsivity, the spectral responsivity of radiation thermometers can be fully characterized. To verify the calibration accuracy, the absolutely calibrated radiation thermometer is used to measure Au and Cu freezing-point temperatures and then to compare the obtained results with the values obtained by absolute methods, resulting in T ? T ₉₀ values of +52 mK and ?50 mK for the gold and copper fixed points, respectively.     

Thermodynamic Radiation Thermometry for the Next SI

The construction, the calibration, and the use of the NIST Thermodynamic Radiation Thermometer (TRT) to measure the temperature of the gold freezing temperature blackbody and a variable-temperature blackbody from 800 to 2,700°C are described. These temperature determinations are detector-based and derived from the electrical substitution radiometer and length units. The TRT is constructed using a cooled, near-infrared enhanced silicon detector with a room-temperature-stabilized five-position filter wheel. The characteristics of the TRT, such as the size-of-source effect and preamplifier linearity, are determined. The measured temperatures are compared with those obtained using the NIST Absolute Pyrometer 1 (AP1) and the current NIST standard radiation thermometer, the Photoelectric Pyrometer (PEP). After the performance assessments, the TRT will become the standard radiation thermometer for disseminating radiance temperature scales in the United States.     

Traceable radiometric calibration of semiconductor detectors and their application for thermodynamic temperature measurement

The non-contact measurement of temperature by using the emitted thermal radiation has been an innovative field of measurement science and fundamental physics for more than a hundred years. It saw the first highlight in Gustav Kirchhoff’s principle of a blackbody with ideal emission characteristics and culminated in Max Planck’s formulation of the law of thermal radiation, the so-called Planck’s law, forming the foundation of quantum physics. A boost in accuracy was the development of semiconductor detectors and the cryogenic electrical substitution radiometer in the late 1970s. Semiconductor detectors, namely photodiodes, deliver an electrical current proportional to the absorbed optical radiation. Due to the measurements of thermal radiation over a wide range of temperature and wavelength, thermodynamic temperature measurements with radiometric methods have set benchmarks to all, the electrical, dimensional and optical metrology. The paper describes the measurement of the spectral responsivity of semiconductor detectors traceable to the SI units and their application for thermodynamic temperature measurement by the absolute measurement of thermal radiation using filter radiometers with calibrated spectral irradiance responsivity.     

Comparative Study of Two InGaAs-Based Reference Radiation Thermometers

More than one decade ago, an InGaAs detector-based transfer standard infrared radiation thermometer working in the temperature range from \(150\,{^{\circ }}\hbox {C}\) to \(1100\,{^{\circ }}\hbox {C}\) was built at TUBITAK UME in the scope of collaboration with IMGC (INRIM since 2006). During this timescale, the radiation thermometer was used for the dissemination of the radiation temperature scale below the silver fixed-point temperature. Recently, a new radiation thermometer with the same design but with different spectral responsivity was constructed and employed in the laboratory. In this work, we present the comparative study of these thermometers. Furthermore, the paper describes the measurement results of the thermometer’s main characteristics such as the size-of-source effect, spectral responsivity, gain ratio, and linearity. Besides, both thermometers were calibrated at the freezing temperatures of indium, tin, zinc, aluminum, and copper reference fixed-point blackbodies. The main study is focused on the impact of the spectral responsivity of thermometers on the interpolation parameters of the Sakuma–Hattori equation. Furthermore, the calibration results and the uncertainty sources are discussed in this paper.     

Infrared spectral responsivity scale realization and validations

An InSb working standard radiometer, first calibrated at the National Institute of Standards and Technology (NIST) in 1999 against a cryogenic bolometer, was recently calibrated against a newly developed low-noise-equivalent-power pyroelectric transfer standard detector. The pyroelectric transfer standard, which can operate at the output of a monochromator, holds the newly realized NIST spectral power responsivity scale between 1.7 and 14?μm with an uncertainty of 1% (k=2). The InSb working standard was also measured at the National Physical Laboratory (NPL) of the United Kingdom in 1999. The less than 2% spectral power responsivity disagreements obtained on the InSb working standard (both from the 1999 NIST and NPL comparison and also against the pyroelectric standard) validate the three independently realized power responsivity scales and verify the long-term stability of the InSb working standard. The InSb working standard was also used in irradiance measurement mode to validate the previously determined spectral irradiance responsivity of four narrowband InSb radiometers that were applied to calibrate IR target simulators. The uncertainty of the present spectral irradiance responsivity scale held by the InSb working standard is 2.5% (k=2) in the 2 to 5.2?μm wavelength range.