Absolute Spectral Responsivity Research Articles

An assessment of the ageing of the absolute spectral responsivity of trap detectors at krypton ion laser emission wavelengths

We have assessed the absolute spectral responsivities of three-element trap detectors calibrated against the National Physical Laboratory primary standard cryogenic radiometer at eight krypton ion laser emission wavelengths over an approximate period of 20 years. As expected, we observed that the absolute spectral responsivities decrease with time. Additionally, we found that as the wavelength decreases, the ageing rate increases. At 356.4 nm, an average responsivity decrease of around −1800 ppm yr−1 is reported. While at longer wavelengths, the slowest rate of ageing, approximately −40 ppm yr−1, occurs at 799.3 nm. Interestingly, the year-on-year changes exhibit a relatively linear trend, allowing us to predict the future performance of the trap detectors. It is important to note, however, that predicted performance cannot be guaranteed and regular recalibration against a primary standard is essential, especially if low calibration uncertainties are required.

JWST MIRI/MRS in-flight absolute flux calibration and tailored fringe correction for unresolved sources

Context. The Medium Resolution Spectrometer (MRS) is one of the four observing modes of JWST/MIRI. Using JWST in-flight data of unresolved (point) sources, we can derive the MRS absolute spectral response function (ASRF) starting from raw data. Spectral fringing, caused by coherent reflections inside the detector arrays, plays a critical role in the derivation and interpretation of the MRS ASRF. The fringe corrections implemented in the current pipeline are not optimal for non-extended sources, and a high density of molecular features particularly inhibits an accurate correction. Aims. In this paper, we present an alternative way to calibrate the MIRI/MRS data. Firstly, we derive a fringe correction that accounts for the dependence of the fringe properties on the MIRI/MRS pupil illumination and detector pixel sampling of the point spread function. Secondly, we derive the MRS ASRF using an absolute flux calibrator observed across the full 5–28 µm wavelength range of the MRS. Thirdly, we apply the new ASRF to the spectrum of a G dwarf and compare it with the output of the JWST/MIRI default data reduction pipeline. Finally, we examine the impact of the different fringe corrections on the detectability of molecular features in the G dwarf and K giant. Methods. The absolute flux calibrator HD 163466 (A-star) was used to derive tailored point source fringe flats at each of the default dither locations of the MRS. The fringe-corrected point source integrated spectrum of HD 163466 was used to derive the MRS ASRF using a theoretical model for the stellar continuum. A cross-correlation was run to quantify the uncertainty on the detection of CO, SiO, and OH in the K giant and CO in the G dwarf for different fringe corrections. Results. The point-source-tailored fringe correction and ASRF are found to perform at the same level as the current corrections, beating down the fringe contrast to the sub-percent level in the G dwarf in the longer wavelengths, whilst mitigating the alteration of real molecular features. The same tailored solutions can be applied to other MRS unresolved targets. Target acquisition is required to ensure the pointing is accurate enough to apply this method. A pointing repeatability issue in the MRS limits the effectiveness of the tailored fringe flats is at short wavelengths. Finally, resulting spectra require no scaling to make the sub-bands match, and a dichroic spectral leak at 12.2 µm is removed.

Absolute Radiometric Calibration of an Imaging Spectroradiometer Using a Laboratory Detector-Based Approach

The HyperSpectral Imager for Climate Science (HySICS) is the core instrument of the Climate Absolute Refractivity and Reflectance Observatory (CLARREO) Pathfinder (CPF) mission and is currently scheduled to be launched to the International Space Station (ISS) in 2023. HySICS is an Offner–Chrisp imaging spectrometer designed to meet an unprecedented radiometric uncertainty requirement of 0.3% (k = 1) over its entire spectral range of 350–2300 nm. The approach represents the need for significant improvement over the Radiometric Calibration (RadCal) of existing space-borne spectrometers. One strategy to demonstrate that HySICS achieves this level of accuracy is through an Independent Calibration (IndCal) effort that can provide an alternative referencing RadCal, which follows a traceability chain independent of the operational RadCal of ratioing approach. The IndCal relies on a pre-launch detector-based absolute RadCal of HySICS, using a tunable laser system as source, and the system planned for the HySICS absolute RadCal is the Goddard Laser for Absolute Measurement of Radiance (GLAMR). GLAMR was developed at NASA’s Goddard Space Flight Center and has been used to calibrate multiple operational remote sensing instruments, as well as the SOlar, Lunar Absolute Reflectance Imaging Spectroradiometer (SOLARIS), a calibration demonstration system developed for the CLARREO mission. In this work, the data of SOLARIS GLAMR RadCal conducted in 2019 are processed to derive the Absolute Spectral Response (ASR) functions and other key characterization parameters of SOLARIS detectors. The results are further analyzed with the goals to plan the HySICS GLAMR RadCal, in particular to optimize its configuration, to demonstrate the traceability route to the NIST standard, and to develop the error budget of the calibration approach. The SOLARIS calibration is also compared with other source- and detector-based calibrations to validate the absolute radiometric accuracy achieved.

Pyroelectric detector-based method for low uncertainty spectral irradiance and radiance responsivity calibrations in the infrared using tunable lasers.

The standard uncertainty of detector-based radiance and irradiance responsivity calibrations in the short-wave infrared (SWIR) traditionally has been limited to around 1% or higher by the poor spatial uniformity of detectors used to transfer the scale from radiant power. Pyroelectric detectors offer a solution that avoids the spatial uniformity uncertainty but also introduces additional complications due to alternating current (AC) measurement techniques. Herein, a new, to the best of our knowledge, method for low uncertainty irradiance responsivity calibrations in the SWIR is presented. An absolute spectral irradiance responsivity scale was placed on two pyroelectric detectors (PED) at wavelengths λ from 500 to 3400 nm. The total combined uncertainty (k=1) was ≈0.28% (>1000nm), 0.44% (900 nm), and 0.36% (≈950nm and <900nm) for PED #1 and 0.34% (>1000nm), 0.48% (900 nm), and 0.42% (≈950nm and <900nm) for PED #2. This was done by utilizing a demodulation technique to digitally analyze the time-dependent AC waveforms, which obviates the use of lock-in amplifiers and avoids associated additional uncertainty components.

Development of electrical substitution Fourier transform spectrometry for absolute optical power measurements.

We have demonstrated the first continuous-scan electrical substitution Fourier transform spectrometer (ES-FTS), which serves initially as an apparatus for absolute spectral responsivity calibrations of detectors over the wavelength range from 1.5 µm to 11 µm. We present data on the realization of a spectral detector-comparator system with high accuracy, high dynamic range, high spectral resolution and fast measurement in the infrared region, which is tied directly to an absolute power scale through electrical substitution. The ES-FTS apparatus employs a commercial Fourier transform spectrometer and a custom electrical substitution bolometer detector to enable spectrally-resolved absolute optical power measurements. A generalization of electrical substitution techniques enables determination of the voltage waveform that must be applied to the bolometer's electrical heater to cancel the optical signal from a Michelson interferometer in order to quantify the time-dependent optical power incident on the bolometer. The noise floor of the electrical substitution bolometer is on the order of 10 pW/Hz½ and its response is expected to be linear from the noise floor to 1 mW. A direct comparison between a pyroelectric standard detector and the ES-FTS has been performed, and experimental results reported here show great potential for this technique.

Development of a detector-based absolute spectral power responsivity scale in the spectral range of 300–1600 nm

The radiometry laboratory at the National Institute of Standards (NIS) has taken in his shoulder to develop and improve the spectral responsivity facilities to get high precision and more reliable optical detectors measurements. During the last few years, the calibration of the detectors was refined as well as the reliability of spectral responsivity results, especially in the critical ultra-violet and infrared range of the spectrum. This setup was constructed using a lamp-monochromator-based setup accompanied by a silicon trap detector and an Indium Gallium Arsenide (InGaAs) detector to extend the scale to the infrared (IR) spectral region. Both detectors are traceable and shown to be spatially uniform, linear, stable, and capable of disseminating an absolute spectral responsivity scale. The uncertainty contribution of the spectral responsivity for a silicon-based detector is less than 1–1.9% for the spectral range 300–1100 nm, and about 2–3.8 % (k = 2) for InGaAs-based detector in the wavelength range 800–1600 nm. Additionally, this paper gives a comprehensive overview of’ NIS’s spectral responsivity facilities, which are now fully automated and controlled by LabVIEW.

Method for Measuring the Absolute Spectral Response of Infrared Photodetector Arrays

Physical and technical aspects of the implementation of an alternative method for measuring the absolute spectral response of infrared photodetector arrays (IR PDA) (current sensitivity spectrum, voltage sensitivity, and quantum efficiency) without the use of spectral instruments are considered. The method is based on multiple measurements of the output signal of all IR array’s photosensitive elements (PSEs) generated by modulated black-body radiation at different temperatures of the black body (BB). The signal is measured against the sum of constant signals from the background radiation, the detector input optical window, the BB radiation modulator, the PSE dark current, and the constant signal of the LSI multiplexer. A system of Fredholm integral equations of the first kind is constructed based on the measured PSE signals. Its left side includes measured BB signals and, the right side, analytical expressions describing these signals. The solution of the system are the absolute values of the above-mentioned spectral components of all PSEs of PDAs. The block diagram of the measurement setup is considered, functional features of its operation are analyzed, and requirements for its blocks are substantiated. Additional advantages of the new method in comparison with the existing methods are presented.

Design and calibration of electrical substitution radiometer traced to absolute cryogenic radiometer

The quantification of remote sensing information requires spectral radiometric calibration technology support with high accuracy, and this technology can ensure the comparability, accuracy, and long-term stability of data acquisition for sensor. Now, the calibration technique traced to the absolute cryogenic radiometer (ACR) is the trend of development. To improve the accuracy of infrared absolute spectral responsivity, a gold-plated hemisphere reflector is added to a thermopile detector with electrical substitution pins. So an infrared electrical substitution radiometer (ESR) is developed as the standard transfer detector. The spectral response linearity, uniformity, and stability of ESR are tested by electric substitution technology. We also have established a radiometric calibration system based on ACR. The combined uncertainty in the spectral responsivity of this detector from 0.7 to 20 μm is about 0.99%. The application of infrared ESR to radiometric calibration can improve the calibration accuracy of detectors through its shorter calibration chain.

Measurement of the absolute spectral responsivity in the mid-infrared based on the cryogenic electrical substitution radiometer and an optimized thermopile detector

Abstract. The Physikalisch-Technische Bundesanstalt (PTB) expanded its capabilities of the absolute measurement of radiant power to the spectral range of the mid-infrared (MIR) by implementing additional MIR laser radiation sources at one of the PTB's cryogenic electrical substitution radiometer facilities. This extension enables absolute calibrations of the spectral responsivity of detectors in the MIR traceable to the International System of Units (SI). The thermopile detector TS-76 was characterized and calibrated in view of its spectral responsivity s(λ) in the wavelength range between 1.5 and 10.6 µm at the expanded cryogenic electrical substitution radiometer facility. The relative standard measurement uncertainty was significantly reduced to 1.4 % by developing an optimized and thermally stabilized detector housing design. The TS-76 was established as a mid-infrared transfer detector for the SI traceable measurement of radiant power and the dissemination of the spectral responsivity s(λ) in the MIR.

Climate Data Records from Meteosat First Generation Part II: Retrieval of the In-Flight Visible Spectral Response

How can the in-flight spectral response functions of a series of decades-old broad band radiometers in Space be retrieved post-flight? This question is the key to developing Climate Data Records from the Meteosat Visible and Infrared Imager on board the Meteosat First Generation (MFG) of geostationary satellites, which acquired Earth radiance images in the Visible (VIS) broad band from 1977 to 2017. This article presents a new metrologically sound method for retrieving the VIS spectral response from matchups of pseudo-invariant calibration site (PICS) pixels with datasets of simulated top-of-atmosphere spectral radiance used as reference. Calibration sites include bright desert, open ocean and deep convective cloud targets. The absolute instrument spectral response function is decomposed into generalised Bernstein basis polynomials and a degradation function that is based on plain physical considerations and able to represent typical chromatic ageing characteristics. Retrieval uncertainties are specified in terms of an error covariance matrix, which is projected from model parameter space into the spectral response function domain and range. The retrieval method considers target type-specific biases due to errors in, e.g., the selection of PICS target pixels and the spectral radiance simulation explicitly. It has been tested with artificial and well-comprehended observational data from the Spinning Enhanced Visible and Infrared Imager on-board Meteosat Second Generation and has retrieved meaningful results for all MFG satellites apart from Meteosat-1, which was not available for analysis.

Uniformity measurements of large-area indium gallium arsenide and germanium photodetectors

The properties of six large area semiconductor photodetectors were investigated in the near infrared wavelength range. For potential use as transfer standard detectors in absolute spectral responsivity calibrations, the spatial uniformity and spectral responsivity of four InGaAs and two Ge photodiodes were characterized. Spatial uniformity measurements carried out at 1000 nm, 1550 nm, and 1650 nm show that photodiode spatial non-uniformity changes with wavelength for both InGaAs and Ge detectors. The photodiode characterization apparatus, results, and analysis are presented.

Realization of spectral irradiance responsivity at NIS-Egypt

Realization of the absolute spectral irradiance responsivity of photodetectors in A/W/cm2 is a fundamental requirement for many photometric, radiometric and colorimetric applications. This work aimed to develop a standardized radiometric setup for realizing the absolute spectral irradiance responsivity in the range of 500 nm–950 nm at the National Institute of Standards (NIS), Egypt. A silicon trap detector, calibrated against the primary standard cryogenic radiometer, is used as a transfer standard for the calibration of S2281 Hamamatsu silicon photodiode, as a working standard, using a lamp-double monochromator based system. The accompanied uncertainty components affecting the measurements were investigated.

Performance of Different Light Sources for the Absolute Calibration of Radiation Thermometers

The evolving mise en pratique for the definition of the kelvin (MeP-K) [1, 2] will, in its forthcoming edition, encourage the realization and dissemination of the thermodynamic temperature either directly (primary thermometry) or indirectly (relative primary thermometry) via fixed points with assigned reference thermodynamic temperatures. In the last years, the Centro Espanol de Metrologia (CEM), in collaboration with the Instituto de Optica of Consejo Superior de Investigaciones Cientificas (IO-CSIC), has developed several setups for absolute calibration of standard radiation thermometers using the radiance method to allow CEM the direct dissemination of the thermodynamic temperature and the assignment of the thermodynamic temperatures to several fixed points. Different calibration facilities based on a monochromator and/or a laser and an integrating sphere have been developed to calibrate CEM’s standard radiation thermometers (KE-LP2 and KE-LP4) and filter radiometer (FIRA2). This system is based on the one described in [3] placed in IO-CSIC. Different light sources have been tried and tested for measuring absolute spectral radiance responsivity: a Xe-Hg 500 W lamp, a supercontinuum laser NKT SuperK-EXR20 and a diode laser emitting at 6473 nm with a typical maximum power of 120 mW. Their advantages and disadvantages have been studied such as sensitivity to interferences generated by the laser inside the filter, flux stability generated by the radiant sources and so forth. This paper describes the setups used, the uncertainty budgets and the results obtained for the absolute temperatures of Cu, Co-C, Pt-C and Re-C fixed points, measured with the three thermometers with central wavelengths around 650 nm.

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).

基于长波红外探测器绝对光谱响应度测量的激光源

基于长波红外探测器绝对光谱响应度测量的激光源

Sounding Rocket Observations of Active Region Soft X-Ray Spectra Between 0.5 and 2.5 nm Using a Modified SDO/EVE Instrument

Spectrally resolved measurements of individual solar active regions (ARs) in the soft X-ray (SXR) range are important for studying dynamic processes in the solar corona and their associated effects on the Earth’s upper atmosphere. They are also a means of evaluating atomic data and elemental abundances used in physics-based solar spectral models. However, very few such measurements are available. We present spectral measurements of two individual ARs in the 0.5 to 2.5 nm range obtained on the NASA 36.290 sounding rocket flight of 21 October 2013 (at about 18:30 UT) using the Solar Aspect Monitor (SAM), a channel of the Extreme Ultaviolet Variability Experiment (EVE) payload designed for underflight calibrations of the orbital EVE on the Solar Dynamics Observatory (SDO). The EVE rocket instrument is a duplicate of the EVE on SDO, except the SAM channel on the rocket version was modified in 2012 to include a freestanding transmission grating to provide spectrally resolved images of the solar disk with the best signal to noise ratio for the brightest features, such as ARs. Calibrations of the EVE sounding rocket instrument at the National Institute of Standards and Technology Synchrotron Ultraviolet Radiation Facility (NIST/SURF) have provided a measurement of the SAM absolute spectral response function and a mapping of wavelength separation in the grating diffraction pattern. We discuss techniques (incorporating the NIST/SURF data) for determining SXR spectra from the dispersed AR images as well as the resulting spectra for NOAA ARs 11877 and 11875 observed on the 2013 rocket flight. In comparisons with physics-based spectral models using the CHIANTI v8 atomic database we find that both AR spectra are in good agreement with isothermal spectra (4 MK), as well as spectra based on an AR differential emission measure (DEM) included with the CHIANTI distribution, with the exception of the relative intensities of strong Fe xvii lines associated with $2p^{6}$ – $2p^{5}3{s}$ and $2p^{6}$ – $2p^{5}3{d}$ transitions at about 1.7 nm and 1.5 nm, respectively. The ratio of the Fe xvii lines suggests that the AR 11877 is hotter than the AR 11875. This result is confirmed with analysis of the active regions imaged by X-ray Telescope (XRT) onboard Hinode.

Traceability of solar UV measurements using the Qasume reference spectroradiometer.

One major objective of the European Joint Research Project "Traceability for surface spectral solar ultraviolet (UV) radiation" was to reduce the uncertainty of spectral UV measurements. The measurement instrument used for this work was the portable UV European reference spectroradiometer Qasume. The calibration uncertainty of this instrument was decreased and validated by a comparison of direct calibrations against a primary standard for spectral irradiance, a high temperature blackbody radiator, and against a reference detector using a spectrally tunable laser as a monochromatic source. The spectral irradiance responsivity of the reference detector is traceable to the primary standard of optical power, realized through a cryogenic radiometer, and to the SI unit of meter. The measuring technique was improved by the construction of a new reference spectroradiometer, QasumeII. An improved input optics removes the dependences of the measured solar irradiance on the angle of incident for solar zenith angle smaller than 75deg. Moreover, a hybrid photon detection system enables continuous tracking of the instrument's responsivity changes. For both spectroradiometer systems an uncertainty budget was calculated. The improvements have reduced the measurement uncertainties of solar spectral UV irradiance measurements from 4.8% in 2005 to 2.0% (k=2) in the spectral region above 310nm. The largest sources of uncertainty were the absolute spectral irradiance responsivity calibration, the angular response uncertainty, and the instrument stability using the hybrid detector, which were reduced from 3.6% to 1.1%, from 1.2% to 0.6%, and from 0.65% to 0.4%, with respect to the situation prior to the project. The new instrument was validated during a four month intercomparison relative to the Qasume reference. The mean ratio of the solar irradiance scans between the two reference spectroradiometers has an offset of +0.7% and a standard deviation of ±1.5% for a wavelength greater than 305nm, which is well within the combined uncertainty of 3.7% calculated from the uncertainties of the two systems.

Mid-infrared absolute spectral responsivity scale based on an absolute cryogenic radiometer and an optical parametric oscillator laser

We are reporting on a laser-based absolute spectral responsivity scale in the mid-infrared spectral range. By using a mid-infrared tunable optical parametric oscillator as the laser source, the absolute responsivity scale has been established by calibrating thin-film thermopile detectors against an absolute cryogenic radiometer. The thin-film thermopile detectors can be then used as transfer standard detectors. The extended uncertainty of the absolute spectral responsivity measurement has been analyzed to be 0.58%–0.68% (k = 2).

用于探测器中红外绝对光谱响应度测量的激光源

用于探测器中红外绝对光谱响应度测量的激光源

Comparison of two methodologies for calibrating satellite instruments in the visible and near-infrared.

Traditionally, satellite instruments that measure Earth-reflected solar radiation in the visible and near infrared wavelength regions have been calibrated for radiance responsivity in a two-step method. In the first step, the relative spectral response (RSR) of the instrument is determined using a nearly monochromatic light source such as a lamp-illuminated monochromator. These sources do not typically fill the field of view of the instrument nor act as calibrated sources of light. Consequently, they only provide a relative (not absolute) spectral response for the instrument. In the second step, the instrument views a calibrated source of broadband light, such as a lamp-illuminated integrating sphere. The RSR and the sphere's absolute spectral radiance are combined to determine the absolute spectral radiance responsivity (ASR) of the instrument. More recently, a full-aperture absolute calibration approach using widely tunable monochromatic lasers has been developed. Using these sources, the ASR of an instrument can be determined in a single step on a wavelength-by-wavelength basis. From these monochromatic ASRs, the responses of the instrument bands to broadband radiance sources can be calculated directly, eliminating the need for calibrated broadband light sources such as lamp-illuminated integrating spheres. In this work, the traditional broadband source-based calibration of the Suomi National Preparatory Project Visible Infrared Imaging Radiometer Suite sensor is compared with the laser-based calibration of the sensor. Finally, the impact of the new full-aperture laser-based calibration approach on the on-orbit performance of the sensor is considered.