Radiometric calibration of IR Fourier transform spectrometers: solution to a problem with the High-Resolution Interferometer Sounder

Quantifying flare combustion efficiency using an imaging Fourier transform spectrometer

Mid-wavelength infrared (MWIR) imaging Fourier transform spectrometers (IFTSs) are a promising technology for measuring flare combustion efficiency (CE) and destruction removal efficiency (DRE). These devices generate spectrally resolved intensity images of the flare plume, which may then be used to infer column densities of relevant species along each pixel line-of-sight. In parallel, a 2D projected velocity field may be inferred from the apparent motion of flow features between successive images. Finally, the column densities and velocity field are combined to estimate the mass flow rates for the species needed to calculate the CE or DRE. Since the MWIR IFTS can measure key carbon-containing species in the flare plume, it is possible to measure CE without knowing the fuel flow rate, which is important for fenceline measurements. This work demonstrates this approach on a laboratory heated vent, and then deploys the technique on two working flares: a combustor burning natural gas at a known rate, and a steam-assisted flare at a petrochemical refinery. Analysis of the IFTS data highlights the potential of this approach, but also areas for future development to transform this approach into a reliable technique for quantifying flare emissions.Implications: Our research is motivated by the need to assess hydrocarbon emissions from flaring, which is a critical problem of global significance. For example, recent studies have shown that methane destruction efficiency of flaring from upstream oil may be significantly lower than the commonly assumed figure of 98%; work by Plant et al. , in particular, suggest that this discrepancy amounts to CO2 emissions from 2 to 8 million automobiles annually, considering the US alone. Similarly, the international energy agency (IEA) estimates a global flare efficiency of 92%, which translates in 8 million tons of CH4 emitted by flares in 2020. Highlighted by these studies and supported by the World Bank initiatives toward zero routine flaring emissions, there is an urgent need for oil and gas industry to assess their flare methane emission, and overall hydrocarbon emissions. At the very least, it is critical to identify problematic flare operating conditions and means to mitigate flare emissions. Focusing on remote quantification of plume species, the measurement technique and quantification method presented in this paper is a considerable step forward in that direction by computing combustion efficiency and key components for destruction efficiency.

Design optimization and implementation of a Fourier transform spectrometer with rotating motion for 0.1 cm-1 resolution spectroscopy

In this paper, we present the design optimization and implementation of a high-resolution near-infrared Fourier transform spectrometer (FTS) based on a rotating motion. The FTS system incorporates a rotating mirror-pair for scanning the optical path length (OPL). The design optimization process is performed to maximize the scanning range to obtain a resolution of 0.1 cm-1 while taking into account constraints on the volume of the system and the availability of commercial optics. By using a pattern search algorithm, we optimized the geometrical parameters of the rotating part, and found the best solution to satisfy the constraints. A data processing method is implemented to correct the nonlinear OPL scanning using a He-Ne laser. The performance of the implemented FTS is verified through spectral analysis within the spectral range of 1550 ± 25 nm. This spectral band corresponds to the wavelength range of the amplified spontaneous emission (ASE) obtained from an Er-doped fiber amplifier used in this study. Additionally, gas spectroscopy conducted using the FTS system successfully detects and analyzes the distinct absorption lines of hydrogen cyanide in 16.5 cm gas cell. The detection sensitivity of a single measurement is evaluated based on the noise equivalent absorption coefficient of 1.45 × 10-5 cm-1 Hz-1/2 calculated from 5-sec measurement time, 2000 spectral elements, and 208 signal-to-noise ratio with 0.2 scan/sec.

Solar Contamination on HIRAS Cold Calibration View and the Corrected Radiance Assessment

The deep-space (DS) view spectra are used as a cold reference to calibrate the Hyperspectral Infrared Atmospheric Sounder (HIRAS) Earth scene (ES) observations. The DS spectra stability in the moving average window is crucial to the calibration accuracy of ES radiances. While in the winter and spring seasons, the HIRAS detector-3 DS view is susceptible to solar stray light intrusion when the satellite flies towards the tail of every descending orbit, and as a result, the measured DS spectra are contaminated by the stray light pseudo spectra, especially in the short-wave infrared (SWIR) band. The solar light intrusion issue was addressed on 13 December 2019 when the DS view angle of the scene selection mirror (SSM) was adjusted from −77.4° to −87°. As for the historic contaminated data, a correction method is applied to detect the anomalous data by checking the continuity of the DS spectra and then replace them with the proximate normal ones. The historic ES observations are recalibrated after the contaminated DS spectra correction. The effect of the correction is assessed by comparing the recalibrated HIRAS radiances with those measured by the Cross-track Infrared Sounder onboard the Suomi National Polar-orbiting Partnership Satellite (SNPP/CrIS) via the extended simultaneous nadir overpasses (SNOx) technique and by checking the consistency among the radiance data from different HIRAS detectors. The results show that the large biases of the radiance brightness temperature (BT) caused by the contamination are ameliorated greatly to the levels observed in the normal conditions.

A balloon-borne imaging Fourier transform spectrometer for atmospheric trace gas profiling

The upper troposphere and lower stratosphere (UTLS) region is a highly variable region of the atmosphere and critical for understanding climate. Yet, it remains undersampled in the observational satellite record. Due to recent advances in interferometer and infrared detection technologies, imaging Fourier transform spectrometer (FTS) technology has been identified as a feasible remote sensing approach to obtain the required precision and spatial resolution of atmospheric trace gas composition in the UTLS. Building on the success of instruments such as the Michelson Interferometer for Passive Atmospheric Sounding and gimbaled limb observer for radiance imaging of the atmosphere, the limb imaging Fourier transform spectrometer experiment (LIFE) instrument, of which this paper details the design and performance, is a balloon-borne infrared imaging FTS developed as an early prototype of a low earth orbit satellite instrument. LIFE is constructed primarily with commercially available off-the-shelf components, with a design emphasis on greatly reducing the complexity of the instrument, particularly the cooling requirements, with a minimal reduction in information gain on the target atmospheric greenhouse gases of water vapor, methane, ozone, and nitrous oxide. The developed instrument was characterized through a series of thermal and vacuum tests and validated through a successful demonstration balloon flight during the 2019 Strato-Science campaign in Canada. In the calibration of the data from the balloon flight, an issue was identified regarding a lack of knowledge in the emissivity of the on-board blackbody calibration sources. These systematic effects were minimized through the application of an emissivity ratio determined from the characterization tests where a wider range of known blackbody temperatures were available. Despite this identified calibration issue, the results demonstrate that the instrument is capable of meeting primary performance requirements for trace gas retrievals of the target atmospheric species.

Composite infrared spectrometer (CIRS) on Cassini

The Cassini spacecraft orbiting Saturn carries the composite infrared spectrometer (CIRS) designed to study thermal emission from Saturn and its rings and moons. CIRS, a Fourier transform spectrometer, is an indispensable part of the payload providing unique measurements and important synergies with the other instruments. It takes full advantage of Cassini's 13-year-long mission and surpasses the capabilities of previous spectrometers on Voyager 1 and 2. The instrument, consisting of two interferometers sharing a telescope and a scan mechanism, covers over a factor of 100 in wavelength in the mid and far infrared. It is used to study temperature, composition, structure, and dynamics of the atmospheres of Jupiter, Saturn, and Titan, the rings of Saturn, and surfaces of the icy moons. CIRS has returned a large volume of scientific results, the culmination of over 30 years of instrument development, operation, data calibration, and analysis. As Cassini and CIRS reach the end of their mission in 2017, we expect that archived spectra will be used by scientists for many years to come.

Airborne compact rotational Raman lidar for temperature measurement

We developed an airborne compact rotational Raman lidar (CRL) for use on the University of Wyoming King Air (UWKA) aircraft to obtain two-dimensional (2D) temperature disman tributions. It obtained fine-scale 2D temperature distributions within 3 km below the aircraft for the first time during the PECAN (Plains Elevated Convection At Night) campaign in 2015. The CRL provided nighttime temperature measurements with a random error of <0.5 K within 800 m below aircraft at 45 m vertical and 1000 m horizontal resolution. The temperatures obtained by the CRL and a radiosonde agreed. Along with water vapor and aerosol measurements, the CRL provides critical parameters on the state of the lower atmosphere for a wide range of atmospheric research.

Estimating index of refraction from polarimetric hyperspectral imaging measurements

Current material identification techniques rely on estimating reflectivity or emissivity which vary with viewing angle. As off-nadir remote sensing platforms become increasingly prevalent, techniques robust to changing viewing geometries are desired. A technique leveraging polarimetric hyperspectral imaging (P-HSI), to estimate complex index of refraction, N̂(ν̃), an inherent material property, is presented. The imaginary component of N̂(ν̃) is modeled using a small number of "knot" points and interpolation at in-between frequencies ν̃. The real component is derived via the Kramers-Kronig relationship. P-HSI measurements of blackbody radiation scattered off of a smooth quartz window show that N̂(ν̃) can be retrieved to within 0.08 RMS error between 875 cm^-1 ≤ ν̃ ≤ 1250 cm^-1. P-HSI emission measurements of a heated smooth Pyrex beaker also enable successful N̂(ν̃) estimates, which are also invariant to object temperature.

Simple parametric model for intensity calibration of Cassini composite infrared spectrometer data

Accurate intensity calibration of a linear Fourier-transform spectrometer typically requires the unknown science target and the two calibration targets to be acquired under identical conditions. We present a simple model suitable for vector calibration that enables accurate calibration via adjustments of measured spectral amplitudes and phases when these three targets are recorded at different detector or optics temperatures. Our model makes calibration more accurate both by minimizing biases due to changing instrument temperatures that are always present at some level and by decreasing estimate variance through incorporating larger averages of science and calibration interferogram scans.

HYPERSPECTRAL ANALYSIS FOR STANDOFF DETECTION OF DIMETHYL METHYLPHOSPHONATE ON BUILDING MATERIALS

Detecting organophosphates in indoor settings can greatly benefit from more efficient and faster methods of surveying large surface areas than conventional approaches, which sample small surface areas followed by extraction and analysis. This study examined a standoff detection technique utilizing hyperspectral imaging for analysis of building materials in near-real time. In this proof-of-concept study, dimethyl methylphosphonate (DMMP) was applied to stainless steel and laminate coupons and spectra were collected during active illumination. Absorbance bands at approximately 1275 cm-1 and 1050 cm-1 were associated with phosphorus-oxygen double bond (P=O) and phosphorus-oxygen-carbon (P-O-C) bond stretches of DMMP, respectively. The magnitude of these bands increased linearly (r2 = 0.93) with DMMP across the full absorbance spectrum, between ν1 = 877 cm-1 to ν2 = 1262 cm-1. Comparisons between bare and contaminated surfaces on stainless steel using the spectral contrast angle technique indicated that the bare samples showed no sign of contamination, with large uniformly distributed contrast angles of 45°-55°, while the contaminated samples had smaller spectral contact angles of < 20° in the contaminated region and > 40° in the uncontaminated region. The laminate contaminated region exhibited contact angles of < 25°. To the best of our knowledge, this is the first report to demonstrate that hyperspectral imaging can be used to detect DMMP on building materials, with detection levels similar to concentrations expected for some organophosphate deposition scenarios.

Identifying sampling comb changes in Fourier transform spectrometers with significant self-emission and beam splitter absorption

For accurate calibration of Fourier transform spectrometers we must constrain or resample the interferogram data to an invariant sampling comb. This can become challenging when instrument self-emission is significant and beam splitter absorption is present. The originally-sampled interferogram center-burst position can move due not only to sampling comb changes, but also to an interaction between the strength of an external target and the so-called anomalous phase (the two ports of the interferometer contribute center-bursts at different locations, and the relative weighting of the two ports varies with the strength of the external target). We present a model of the anomalous phase to enable partitioning of changes in observed center-burst location between sampling comb changes and anomalous phase effects.

Mid-IR hyperspectral imaging of laminar flames for 2-D scalar values

This work presents a new emission-based measurement which permits quantification of two-dimensional scalar distributions in laminar flames. A Michelson-based Fourier-transform spectrometer coupled to a mid-infrared camera (1.5 μm to 5.5 μm) obtained 256 × 128pixel hyperspectral flame images at high spectral (δν̃ = 0.75cm(−1)) and spatial (0.52 mm) resolutions. The measurements revealed line and band emission from H2O, CO2, and CO. Measurements were collected from a well-characterized partially-premixed ethylene (C2H4) flame produced on a Hencken burner at equivalence ratios, Φ, of 0.8, 0.9, 1.1, and 1.3. After describing the instrument and novel calibration methodology, analysis of the flames is presented. A single-layer, line-by-line radiative transfer model is used to retrieve path-averaged temperature, H2O, CO2 and CO column densities from emission spectra between 2.3 μm to 5.1 μm. The radiative transfer model uses line intensities from the latest HITEMP and CDSD-4000 spectroscopic databases. For the Φ = 1.1 flame, the spectrally estimated temperature for a single pixel 10 mm above burner center was T = (2318 ± 19)K, and agrees favorably with recently reported laser absorption measurements, T = (2348 ± 115)K, and a NASA CEA equilibrium calculation, T = 2389K. Near the base of the flame, absolute concentrations can be estimated, and H2O, CO2, and CO concentrations of (12.5 ± 1.7) %, (10.1 ± 1.0) %, and (3.8 ± 0.3) %, respectively, compared favorably with the corresponding CEA values of 12.8%, 9.9% and 4.1%. Spectrally-estimated temperatures and concentrations at the other equivalence ratios were in similar agreement with measurements and equilibrium calculations. 2-D temperature and species column density maps underscore the Φ-dependent chemical composition of the flames. The reported uncertainties are 95% confidence intervals and include both statistical fit errors and the propagation of systematic calibration errors using a Monte Carlo approach. Systematic errors could warrant a factor of two increase in reported uncertainties. This work helps to establish IFTS as a valuable combustion diagnostic tool.

Gas-phase plume from laser-irradiated fiberglass-reinforced polymers via imaging fourier transform spectroscopy

Emissive plumes from laser-irradiated fiberglass-reinforced polymers (FRP) were investigated using a mid-infrared imaging Fourier transform spectrometer, operating at fast framing rates (50 kHz imagery and 2.5 Hz hyperspectral imagery) with adequate spatial (0.81 mm(2) per pixel) and spectral resolution (2 cm(-1)). Fiberglass-reinforced polymer targets were irradiated with a 1064 nm continuous wave neodymium-doped yttrium aluminum garnet (Nd:YAG) laser for 60 s at 100 W in air. Strong emissions from H(2)O, CO, CO(2), and hydrocarbons were observed between 1800 and 5000 cm(-1). A single-layer radiative transfer model was developed for the spectral region from 2000 to 2400 cm(-1) to estimate spatial maps of temperature and column densities of CO and CO(2) from the hyperspectral imagery. The spectral model was used to compute the absorption cross sections of CO and CO(2) using spectral line parameters from the high-temperature extension of the HITRAN. The analysis of pre-combustion spectra yields effective temperatures rising from ambient to 1200 K and suddenly increasing to 1515 K upon combustion. The peak signal-to-noise ratio for a single spectrum exceeds 60:1, enabling temperature and column density determinations with low statistical error. For example, the spectral analysis for a single pixel within a single frame yields an effective temperature of 1019 ± 6 K, and CO and CO(2) column densities of 1.14 ± 0.05 and 1.11 ± 0.03 × 10(18) molec/cm(2), respectively. Systematic errors associated with the radiative transfer model dominate, yielding effective temperatures with uncertainties of >100 K and column densities to within a factor of 2-3. Hydrocarbon emission at 2800 to 3200 cm(-1) is well correlated with CO column density.

Recent advances in measurement of the water vapour continuum in the far-infrared spectral region

We present a new derivation of the foreign-broadened water vapour continuum in the far-infrared (far-IR) pure rotation band between 24 μm and 120 μm (85-420 cm(-1)) from field data collected in flight campaigns of the Continuum Absorption by Visible and IR radiation and Atmospheric Relevance (CAVIAR) project with Imperial College's Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) far-IR spectro-radiometer instrument onboard the Facility for Airborne Atmospheric Measurement (FAAM) BAe-146 research aircraft; and compare this new derivation with those recently published in the literature in this spectral band. This new dataset validates the current Mlawer-Tobin-Clough-Kneizys-Davies (MT-CKD) 2.5 model parametrization above 300 cm(-1), but indicates the need to strengthen the parametrization below 300 cm(-1), by up to 50 per cent at 100 cm(-1). Data recorded at a number of flight altitudes have allowed measurements within a wide range of column water vapour environments, greatly increasing the sensitivity of this analysis to the continuum strength.

Design and optimization of the calibration procedure for a miniaturized Fourier transform spectrometer

This paper presents a Fourier transform infrared (FT-IR) spectrometer calibration procedure based on an unusual source made from a spectrally selective surface. An alternative solution to the usual calibrators has been developed to cope with the tight mass budget of an instrument devoted to Mars surface exploration. The designed system has proved effective, in terms of achievable radiometric accuracy, despite the drawbacks due to the significant reflectivity of the sources. The proposed procedure is a standard "two-source" approach in which both cold and hot sources are thermally controlled surfaces, similar to an optical solar reflector, associated to a filament lamp. Such a system allows the required signal to be achieved in the 2-25 l m instrument wavelength range. Source optimization was performed using, as a cost function, the computed radiometric uncertainty, while the required absolute accuracy of the instrument was imposed as the optimization constraint.

Instrumental phase-based method for Fourier transform spectrometer measurements processing

Phase correction is a critical procedure for most space-borne Fourier transform spectrometers (FTSs) whose accuracy (owing to often poor signal-to-noise ratio, SNR) can be jeopardized from many uncontrollable environmental conditions. This work considers the phase correction in an FTS working under significant temperature change during the measurement and affected by mechanical disturbances. The implemented method is based on the identification of an instrumental phase that is dependent on the interferometer temperature and on the extraction of a linear phase component through a least-squares approach. The use of an instrumental phase parameterized with the interferometer temperature eases the determination of the linear phase that can be extracted using only a narrow spectral region selected to be immune from disturbances. The procedure, in this way, is made robust against phase errors arising from instrumental effects, a key feature to reduce the disturbances through spectra averaging. The method was specifically developed for the Mars IR Mapper spectrometer, that was designed for operation onboard a rover on the Mars surface; the validation was performed using ground and in-flight measurements of the Fourier transform IR spectrometer planetary Fourier spectrometer, onboard the MarsExpress mission. The symmetrization has been exploited also for the spectra calibration, highlighting the issues deriving from the cases of relevant beamsplitter emission. The applicability of this procedure to other instruments is conditional to the presence in the spectra of at least one spectral region with a large SNR along with a negligible (or known) beamsplitter emission. For the PFS instrument, the processing of data with relevant beamsplitter emission has been performed exploiting the absorption carbon dioxide bands present in Martian spectra.

A responsivity-based criterion for accurate calibration of FTIR emission spectra: identification of in-band low-responsivity wavenumbers

Spectra measured by remote-sensing Fourier transform infrared spectrometers are often calibrated using two calibration sources. At wavenumbers where the absorption coefficient is large, air within the optical path of the instrument can absorb most calibration-source signal, resulting in extreme errors. In this paper, a criterion in terms of the instrument responsivity is used to identify such wavenumbers within the instrument bandwidth of two remote-sensing Fourier transform infrared spectrometers. Wavenumbers identified by the criterion are found to be correlated with strong absorption line-centers of water vapor. Advantages of using a responsivity-based criterion are demonstrated.

Responsivity-based criterion for accurate calibration of FTIR emission spectra: theoretical development and bandwidth estimation

An analytical expression for the variance of the radiance measured by Fourier-transform infrared (FTIR) emission spectrometers exists only in the limit of low noise. Outside this limit, the variance needs to be calculated numerically. In addition, a criterion for low noise is needed to identify properly calibrated radiances and optimize the instrument bandwidth. In this work, the variance and the magnitude of a noise-dependent spectral bias are calculated as a function of the system responsivity (r) and the noise level in its estimate (σr). The criterion σr/r<0.3, applied to downwelling and upwelling FTIR emission spectra, shows that the instrument bandwidth is specified properly for one instrument but needs to be restricted for another.

Remote identification and quantification of industrial smokestack effluents via imaging Fourier-transform spectroscopy

Industrial smokestack plume emissions were remotely measured with a midwave infrared (1800-3000 cm(-1)) imaging Fourier-transform spectrometer operating at moderate spatial (128 × 64 with 19.4 × 19.4 cm(2) per pixel) and high spectral (0.25 cm(-1)) resolution over a 20 min period. Strong emissions from CO(2), H(2)O, SO(2), NO, HCl, and CO were observed. A single-layer plume radiative transfer model was used to estimate temperature T and effluent column densities q(i) for each pixel's spectrum immediately above the smokestack exit. Across the stack, temperature was uniform with T = 396.3 ± 1.3 K (mean ± stdev), and each q(i) varied in accordance with the plume path length defined by its cylindrical geometry. Estimated CO(2) and SO(2) volume fractions of 8.6 ± 0.4% and 380 ± 23 ppm(v), respectively, compared favorably with in situ measurements of 9.40 ± 0.03% and 383 ± 2 ppm(v). Total in situ NO(x) concentration (NO + NO(2)) was reported at 120 ± 1 ppm(v). While NO(2) was not spectrally detected, NO was remotely observed with a concentration of 104 ± 7 ppm(v). Concentration estimates for the unmonitored species CO, HCl, and H(2)O were 14.4 ± 0.3 ppm(v), 88 ± 1 ppm(v), and 4.7 ± 0.1%, respectively.

Simultaneous measurement of emittance, transmittance, and reflectance of semitransparent materials at elevated temperature

A two-substrate method is developed to simultaneously determine emissivity, transmittance, and reflectance of semitransparent materials with a single measurement under the same environment at elevated temperature. The three quantities can be obtained through the emissivities of substrates and the apparent emissivities resulting from the radiance of the sample heated by substrates. The two-substrate method is compared with the conventional method by measuring sapphire samples with various thicknesses, resulting in good agreements for all the samples. The present method will be useful to measure the temperature dependence of optical properties of porous ceramic materials.

Calibration algorithm for Fourier transform spectrometer with thermal instabilities

Complex domain calibration is an efficient method to correct the amplitude and phase of a spectrum obtained from a Fourier transform spectrometer. This method is, however, not directly applicable in the occurrence of a zero path difference (ZPD) shift between a scene interferogram and calibration blackbody interferograms. This situation is likely to happen for a system with thermal instabilities. It is found that a ZPD shift smaller than 1 sampling point can cause a large disagreement between the spectra evaluated from the two interferometer sweep directions. We have developed an algorithm for a complex calibration in the presence of ZPD shifts. The restricting aspect of the real-time capability is taken into account.

Improvements in the data quality of the Interferometric Monitor for Greenhouse Gases

The Interferometric Monitor for Greenhouse Gases (IMG) operated aboard the polar-orbiting Advanced Earth Observing Satellite from October 1996 through June 1997. The IMG measured upwelling infrared radiance at fine spectral resolution. This paper identifies previously undocumented issues with IMG interferograms and describes procedures for correcting the majority of the affected data. In particular, single-sided interferograms should be used to avoid large noise bursts, and phase ambiguities must be resolved in uncalibrated spectra before radiometric calibration. The corrections are essential for studies that require accurately calibrated radiance spectra, including those that track atmospheric changes globally on decadal time scales.

Emissivity and reflection model for calculating unpolarized isotropic water surface-leaving radiance in the infrared. 2: validation using Fourier transform spectrometers

The surface-leaving radiance model developed in Part I [Appl. Opt.47,3701 (2008)] is validated against an exhaustive set of Fourier transform spectrometer field observations acquired at sea. Unlike prior limited studies, these data include varying all-sky atmospheric conditions (clear, cloudy, and dusty), with regional samples from the tropics, mid-latitudes, and high latitudes. Our analyses show the model to have reduced bias over standard models at emission angles > or = 45 degrees.

Interferometer for ground-based observations of emitted spectral radiance from the troposphere: evaluation and retrieval performance

We evaluate the spectral quality, radiometric noise, and retrieval performance of a Fourier transform infrared spectrometer, which has been developed for recording spectrally resolved observations in a region of the spectrum which is important both for the science of Earth's climate and applications, such as the remote sensing of temperature and atmospheric gas species. This spectral region extends from 100 to 1600 cm(-1) and encompasses the two fundamental, rotation and vibration, absorption bands of water vapor. The instrument is a customized version of a Bomem AERI (Atmospheric Emitted Radiance Interferometer) spectrometer, whose spectral coverage has been extended in the far infrared with the use of uncooled pyroelectric detectors. Retrieval examples for water vapor and temperature profiles are shown, which also allow us to intercompare the retrieval performance of both H(2)O vibration and rotation bands.

Phase determination for a Fourier transform infrared spectrometer in emission mode

Beam-splitter emission strongly influences the spectra measured with a Fourier transform spectrometer (FTS) as it affects the entire phase behavior, in particular in emission spectroscopy. The various radiation contributions of the scene and the FTS itself have different phases in the complex spectrum. As a specific feature, the radiation of the beam splitter is rotated by approximately pi/2 relative to the scene effective radiation. By classical methods of phase correction, the radiation components of different phases are mixed in the complex plane, which may lead to serious errors in the calibrated spectra. For this reason, the nature of the FTS phase has been studied, and a statistical phase determination method has been developed. It allows us to determine the phase function of the scene by minimizing the correlation between the imaginary and the real parts of the complex spectrum and by reducing the variance of the imaginary part. Thus phase accuracies of 10 to 30 mrad can be achieved. In addition, the remaining error of the phase can be calculated for each individual spectrum. The total phase error and its effect on the spectra are discussed.

Internal-energy measurements of angle-resolved product CO2 in catalytic CO oxidation by means of infrared chemiluminescence

Measurements of both vibrational and rotational energies of product CO(2) in CO oxidation on palladium surfaces have been successfully performed as a function of the desorption angle by means of infrared chemiluminescence. The remarkable angle dependences of both energies indicate facile energy partitioning in repulsive desorption and provide new dimensions in the study of surface reaction dynamics as well as additional insights into the product formation site. Details of the apparatus for energy analysis of angle-resolved products are described, especially on how to pick up extremely weak infrared emission signals.

Correction of instrument line shape in Fourier transform spectrometry using matrix inversion

A novel matrix inversion approach is proposed to correct several contributions to the instrument line shape (ILS) of a Fourier transform spectrometer. The matrix formalism for the ILS is first quickly reviewed. Formal inversion of the ILS matrix is next discussed, along with its limitations. The stability of the inversion process for large field-of view- (FOV-) limited and highly off-axis line shapes is investigated. The effect of inversion on the noise that is present in the spectrum is also presented. Use of classical iterative inversion methods, coupled with efficient synthesis algorithms, is proposed as a way to drastically speed up the inversion process. The method is applied to correct HBr spectra obtained from a laboratory spectrometer that has an adjustable field of view. ILSS from six FOVs are brought to the same spectral axis and to the same ideal sinc shape.

Measurements of the foreign-broadened continuum of water vapor in the 6.3 microm band at -30 degrees C

The foreign-broadened continuum of water vapor in the nu2 band (5-7.7 microm, 1300-2000 cm(-1)) is important for satellite-based retrievals of water vapor in the upper troposphere, where temperatures are below -25 degrees C. Continuum coefficients have previously been measured mostly at or above +23 degrees C. We present continuum coefficients in the nu(2) band retrieved from measurements made in Antarctica at temperatures near -30 degrees C: atmospheric transmission at South Pole Station and atmospheric emission at Dome C. The continuum coefficients derived from these measurements are generally in agreement with the widely used Mlawer, Tobin-Clough, Kneizys, Davies continuum. Differences are at most 30%, corresponding to a 6% relative error in retrieved upper-tropospheric humidity.

Correction of detector nonlinearity in Fourier transform spectroscopy with a low-temperature blackbody

The nonlinearity of a mercury cadmium telluride photoconductive detector, an integral part of a modified commercial interferometer used for airborne research, has been analyzed and evaluated against a number of correction schemes. A high-quality blackbody with accurate temperature control has been used as a stable and well-characterized radiation source. The detector nonlinearity was established as a function of scene temperature between 194 and 263 K. Second- and third-order corrections to the measured interferogram have been tested by analyzing the measured signal both within and outside the spectral response region of the detector. A combined correction scheme is proposed that best represents the real nonlinear response of the detector.

Breadboard of a Fourier-transform spectrometer for the radiation explorer in the far infrared atmospheric mission

In preparation for a possible space mission, a breadboard version named REFIR-BB of the Radiation Explorer in the Far Infrared (REFIR) instrument has been built. The REFIR is a Fourier-transform spectrometer with a new optical layout operating in the spectral range 100-1100 cm(-1) with a resolution of 0.5 cm(-1), a 7-s acquisition time, and a signal-to-noise ratio of better than 100. Its mission is the spectral measurement in the far infrared of the Earth's outgoing emission, with particular attention to the long-wavelength spectral region, which is not covered by either current or planned space missions. This measurement is of great importance for deriving an accurate estimate of the radiation budget in both clear and cloudy conditions. The REFIR-BB permits the trade-off among all instrument parameters to be studied, the optical layout to be tested, and the data-acquisition strategy to be optimized. The breadboard could be used for high-altitude ground-based campaigns or could be flown for test flights on aircraft or balloon stratospheric platforms. The breadboard's design and the experimental results are described, with particular attention to the acquisition strategy and characterization of the interferometer. Tests were performed both in laboratory conditions and in vacuum. Notwithstanding a loss of efficiency above 700 cm(-1) caused by the poor performance of the photolithographic polarizers used as beam splitters, the results demonstrate the feasibility of using the spectrometer for space applications.

Design and characterization of the balloon-borne Michelson Interferometer for Passive Atmospheric Sounding (MIPAS-B2)

MIPAS-B2 is a balloon-borne limb-emission sounder for atmospheric research. The heart of the instrument is a Fourier spectrometer that covers the mid-infrared spectral range (4-14 microns) and operates at cryogenic temperatures. Essential for this application is the sophisticated line-of-sight stabilization system, which is based on an inertial navigation system and is supplemented with an additional star reference system. The major scientific benefit of the instrument is the simultaneous detection of complete trace gas families in the stratosphere without restrictions concerning the time of day and viewing directions. The specifications, the design considerations, the actual realization of the instrument, and the results of characterization measurements that have been performed are described.

Preliminary measurements with an automated compact differential absorption lidar for the profiling of water vapor

The design and preliminary tests of an automated differential absorption lidar (DIAL) that profiles water vapor in the lower troposphere are presented. The instrument, named CODI (for compact DIAL), has been developed to be eye safe, low cost, weatherproof, and portable. The lidar design and its unattended operation are described. Nighttime intercomparisons with in situ sensors and a radiosonde are shown. Desired improvements to the lidar, including a more powerful laser, are also discussed.

Noise assessment of a Fourier transform infrared spectroradiometer subject to the stability of a conventional laboratory blackbody source

A method is described for the measurement of the noise-equivalent spectral radiance (NESR) of Fourier transform infrared (FTIR) spectroradiometers at all wave numbers of a selected range. The method requires minimal detailed knowledge of the sensor and no support equipment beyond a blackbody source. The NESRs of the FTIR spectroradiometer are determined at every wave-number increment in the 700-1300 cm(-1) range, for six resolutions, with a conventional blackbody source and ensembles of differential spectra. The NESRs are well behaved and consistent with the expected dependence on resolution; however, they depend on source temperature at the highest (1 cm(-1)) and lowest (32 cm(-1)) resolutions, with little or no statistical dependence at intermediate resolutions. Residual source drift is shown to be the likely cause of the dependence at 1 cm(-1); the dependence on the source at 32 cm(-1) resolution is shown to be most probably due to photon noise. At intermediate resolutions the sensor noise is dominant.

Semiactive infrared remote sensing: a practical prototype and field comparison

A semiactive method of Fourier-transform infrared (FTIR) remote sensing has been developed and field tested. The method replaces the sender telescope of an active technique with an extended, heated broadband source. The output of the extended source (a commercial griddle) is not collimated and thus facilitates alignment by having the detector optics simply point at the griddle. The present source fills the detector's field of view at 100 m and maintains a temperature approximately 80 K warmer than ambient. In field tests with live CO releases, the method was approximately 30 times less sensitive than active methods, but approximately 30 times more sensitive than passive methods, with far greater sensitivity in the midwave infrared.

Noise-equivalent change in radiance for misalignment noise in a double-sided interferogram

Engineers designing optical alignment servo systems for Michelson interferometers and Fourier-transform infrared spectrometers need to predict the amount of noise expected from the small and randomly varying amounts of misalignment that occur as the servo attempts to maintain alignment while taking data. A formula is derived for the noise-equivalent change in radiance due to this effect and the formula's accuracy is demonstrated by comparison of its predictions to the errors found in simulated interferometer measurements contaminated by misalignment noise.

Noise-equivalent change in radiance for sampling noise in a double-sided interferogram

The formula for the noise-equivalent change in radiance (NEdN) for sampling noise [Appl. Opt. 38, 139 (1999)] can work well when applied to the double-sided interferograms of radiance spectra dominated by isolated emission lines, but it does not work well when applied to broad, slowly varying radiance spectra such as a Planck blackbody curve. The modified formula for the sampling-noise NEdN works well when applied to the double-sided interferograms of both types of radiance spectra.

Radiometric errors in complex Fourier transform spectrometry

A complex spectrum arises from the Fourier transform of an asymmetric interferogram. A rigorous derivation shows that the rms noise in the real part of that spectrum is indeed given by the commonly used relation sigmaR = 2X x NEP/(etaAomega square root(tauN)), where NEP is the delay-independent and uncorrelated detector noise-equivalent power per unit bandwidth, +/- X is the delay range measured with N samples averaging for a time tau per sample, eta is the system optical efficiency, and Aomega is the system throughput. A real spectrum produced by complex calibration with two complex reference spectra [Appl. Opt. 27, 3210 (1988)] has a variance sigmaL2 = sigmaR2 + sigma(c)2 (Lh - Ls)2/(Lh - Lc)2 + sigma(h)2 (Ls - Lc)2/(Lh - Lc)2, valid for sigmaR, sigma(c), and sigma(h) small compared with Lh - Lc, where Ls, Lh, and Lc are scene, hot reference, and cold reference spectra, respectively, and sigma(c) and sigma(h) are the respective combined uncertainties in knowledge and measurement of the hot and cold reference spectra.

Balloonborne calibrated spectroradiometer for atmospheric nadir sounding

Laboratoire de Physique Moléculaire et Applications (LPMA)/Infrared Atmospheric Sounding interferometer (IASI) balloon is a calibrated infrared Fourier transform spectrometer that is used to measure the Earth's atmospheric emission (650-3000 cm(-1)). Operating under a stratospheric balloon, this spectroradiometer provides radiometrically calibrated spectra with an apodized spectral resolution of 0.1 cm(-1), which can be used to retrieve the concentration of atmospheric trace gases such as H2O, CO2, CO, O3, N2O, and CH4. The radiometric calibration is performed by use of two reference blackbodies. A reference cavity (LPMA blackbody) has been developed to validate the radiometric calibration procedure and to characterize the instrument performances. One goal of the LPMA/IASI balloon is the preparation of the IASI mission, which is a satellite instrument dedicated primarily to operational meteorology. A description of the LPMA/IASI balloon, its performances, and the results obtained during the first flight of the instrument are presented.

Phase anomalies in fourier-transform spectrometers: an absorbing beam splitter is neither sufficient nor necessary

Recently it has been a topic of some discussion that the phase associated with part of the self-emission of a Fourier-transform spectrometer may differ neither by 0 nor by pi rad from the phase of an external target if there is absorption in the beam splitter. The conventional interpretation of this has been to separate the self-emission into three terms: instrument emission from the input port, in phase with the external target; emission from the secondary input port, pi rad from the target; and emission from an absorbing beam splitter with an anomalous phase (neither 0 nor pi rad with respect to the target). There is another necessary condition that has not received much attention, that the instrument not be isothermal. For polarized radiation there is an additional condition that suppresses the anomalous phase, and for unpolarized radiation there is a way to produce an anomalous phase without beam-splitter absorption.