Interferometric Spectrometer Research Articles

Some aspects of temperature sounding of the atmosphere

Based on measurements with the interferometer spectrometer SI-1 on board of Meteor satellites the following problems are investigated: the systematic effect of the spectral response function in deriving effective radiation temperatures; the selection of ‘optimal’ spectral channels within the 15 μm region, and effects of additional molecular absorbers on the determination of the temperature profile. It can be shown that using broad-band window channels (half width ⩾20cm −1) the derived surface temperature is significantly different from the surface temperature derived from narrow spectral band channels (e.g. for the HIRS window channel 8 this difference is about 1° C). Simulations of some combinations of spectral channels including one proposed as an optimal approach show no significant difference in the derived temperature profile. Neglecting the ozone absorption of the 14.1 μm band leads to a change in radiance of about 1.5 mW/m 2sr cm −1 and to a systematic error of the derived temperature of about −2° C in the troposphere.

The Cosmic Background Explorer Satellite

The COBE satellite, under study by NASA since 1976, is planned to make definitive measurements of the diffuse infrared and microwave radiation of the universe over the whole wavelength range from a few microns to 1.3 cm. It will include three instruments: a set of microwave isotropy radiometers at 23, 31, 53, and 90 GHz; an interferometer spectrometer from 1 to 100 cm-1; and a filter photometer from 1 to 300 microns. They will use the ideal space environment and a 1 year observing time to achieve orders of magnitude improvements in sensitivity and accuracy, providing a fundamental data base for cosmology. Launch is estimated for 1985. The scientific context and technical methods will both be discussed.

Seasonal and interannual variations in total ozone Revealed by the Nimbus 4 Backscattered Ultraviolet Experiment

The first 2 years of backscattered ultraviolet (BUV) ozone data from the Nimbus 4 spacecraft have been processed to a more refined level. The seasonal variations of total ozone for the period April 1970 to April 1972 are described using daily means for 10° latitude zones and a time‐latitude cross section. In addition, the BUV data are compared with analyzed Dobson data and with infrared interferometer spectrometer data also obtained from the Nimbus 4 spacecraft. A harmonic analysis was performed on the daily zonal means. Amplitudes, days of peak ozone values, and percentage of variance have been computed for the annual and semiannual waves and for higher harmonics of an annual period for the 2 years. Asymmetries are found in the annual waves in the two hemispheres, with a subtle interannual difference which may be due to changes in the general circulation. A significant semiannual component is detected in the tropics for the first year. This component appears to result from influences of the annual waves in the two hemispheres. A search for shorter periods using the harmonic analysis revealed no periodicity whose amplitude was higher than the noise level.

Predissociations in the B 3Π+ state of I35Cl and I37Cl

Profiles of lines in the 3–1 and 2–0 bands of the B 3Π0+←X 1Σ+ system of I35Cl and I37Cl vapor, as well as in several fragmentary P and R branches associated with an adiabatic B′ state, have been recorded using a Fabry–Perot interferometer spectrometer. The potential curves of the B state, and of the adiabatic B′ state formed by an avoided crossing near 3.6 Å and 18 140 cm−1 between the B state and a repulsive Y O+ state, have been determined using the intermediate coupling model of Child. Linewidths in v=2 of the B state are observed to be 0.034 cm−1, independent of J and of isotopic composition; however, linewidths in v=3 are broader, corresponding to predissociative lifetimes in the subnanosecond range, and are found to oscillate as a function of J. The major features of the observed linewidth pattern can be explained in terms of analytical approximations developed by Child and co-workers, and provide evidence for a second curve crossing, near 2.94 Å and 17 975 cm−1, of the B state and an Ω=1 state.

Selective Modulation Interferometric Spectrometer (SIMS) Technique Applied To Background Suppression

A method of using the SIMS (the Selective Modulation Interferometric Spectrometer) to measure the difference between the spectral content of two optical beams is given. The differencing is done optically; that is, the modulated detector signal is directly proportional to the difference between the two spectra being compared. This optical differencing minimizes the dynamic-range requirements of the electronics and requires only a simple modification of the basic cyclic SIMS spectrometer. This technique can be used to suppress background radiation for the enhancement of target detection and tracking. Laboratory measurements demonstrating the application of this technique are reported.

Beam splitter and reflection reducing coatings on ZnSe for 3–14 μm

Achromatic coatings to provide wideband amplitude division and reflection reduction at 45 degrees incidence have been developed for the 3-14-microm region on CVD ZnSe. The application is directed toward interferometer spectrometers operating in environments that include spaceflight as well as laboratory. Results in good agreement with theoretical designs have been obtained.

Simultaneous Observation Of Multiple Monochromatic Images With An Imaging Selective Modulation Interferometric Spectrometer

A method for simultaneously observing monochromatic images from a polychromatic source is presented that utilizes an imaging selective modulation interferometric spectrometer. With this technique, rapidly varying changes in many spectral components may be simultaneously monitored. After a brief review of the theory (mostly qualitative), the modifications and techniques necessary for producing more than one spectral image will be discussed.

Analysis of the spectral imaging capabilities of a moving grating interferometric spectrometer

A modified spectrometer capable of producing monochromatic images at selected wavelengths by-simple electrical filtering is described. The general equation is derived describing the dependence of the image on the partially coherent input. Two special cases with an incoherent input are calculated and qualitatively verified.

Precise submillimetre wave interferometric studies on atmospheric and stratospheric gases

A Fourier interferometric spectrometer system has been built for studies on atmospheric gases at temperatures between ambient and 220 K. An improved technique for reducing total observation time when background corrected spectra are needed has been tested using the ammonia spectrum between 60 and 170 cm −1. New measurements on the torsional band of chlorine nitrate near 120 cm −1 are reported for 300 and 250 K. The preliminary results of numerical calculations and group theoretic considerations on the torsional mode of this molecule are presented.

Relation between monthly variations of global ozone and solar activity

STUDIES of Dobson measurements have revealed long-term variations of total ozone of the order of 11 yr (refs 1–5). However, there is a lack of consensus as to whether these long-term variations are in response to changes in solar activity5–9. It has been suggested8 that shorter-term responses might be detected if a relationship between ozone and solar activity exists. In this study the shorter-term monthly averaged variations of global ozone, determined from infrared interferometer spectrometer (IRIS) measurements aboard Nimbus 4, are shown to be consistently in agreement with monthly averaged variations of solar activity. Such natural variations must be taken into account when trying to isolate the effects of anthropogenic species such as chlorofluoromethanes and nitrogen fertilisers on the ozone layer. The relationship between monthly changes of ozone and solar activity are found to be generally consistent with the longer-term ozone variations over the last solar cycle and can be accounted for by assuming monthly solar flux variations near 0.2 µm of about 2%.

The optical screw as a path difference measurement and control device: analysis of periodic errors

It is usual to measure the path difference in a Michelson interferometer in terms of the wavelength of a reference laser beam. One method is to use plane polarized light and to arrange that the plane of polarization of the output beam rotates with an angle which, in the ideal case, depends linearly on the path difference. A comparison of this angle with the axis of a polaroid analyser gives the required signal. However it is shown that departures from the ideal situation cause the emergent beam to be elliptically polarized and result in a periodic error in the orientation of the major axis of the ellipse. If the device is used in the servocontrol system of an interferometer spectrometer then, if the sampling interval is a non-integral multiple of the laser wavelength, this leads to a periodic error in the sample positions and to satellites on the instrumental profile. Formulae are presented from which the error can be calculated in any particular case.

Mars synthetic topographic mapping

Topographic contour maps of Mars are compiled by the synthesis of data acquired from various scientific experiments of the Mariner 9 mission, including S-band radio-occulation, the ultraviolet spectrometer (UVS), the infrared radiometer (IRR), the infrared interferometer spectrometer (IRIS) and television imagery, as well as Earth-based radar information collected at Goldstone, Haystack, and Arecibo Observatories. The entire planet is mapped at scales of 1:25,000,000 and 1:25,000,000 using Mercator, Lambert, and polar stereographic map projections. For the computation of map projections, a biaxial spheroid figure is adopted. The semimajor and semiminor axes are 3393.4 and 3375.7 km, respectively, with a polar flattening of 0.0052. For the computation of elevations, a topographic datum is defined by a gravity field described in terms of spherical harmonics of fourth order and fourth degree combined with a 6.1-mbar occulation pressure surface. This areoid can be approximated by a triaxial ellipsoid with semimajor axes of A = 3394.6 km and B = 3393.3 km and a semiminor axis of C = 3376.3 km. The semimajor axis A intersects the Martian surface at longitude 105°W. The dynamic flattening of Mars is 0.00525. The contour intercal of the maps is 1 km. For some prominent features where overlapping pictures from Mariner 9 are available, local contour maps at relatively larger scales were also compiled by photogrammetric methods on stereo plotters.

The Selective Modulation Interferometric Spectrometer

This paper reviews the theory and the practical implementation of the selective modulation interferometric spectrometer (the SIMS). This spectrometer has an extremely large optical throughput, and it can scan the spectrum in real time while requiring no more signal processing than a chopped radiometer. Equations are presented which describe the relationship between design parameters and spectrometer performance. Practical design problems are identified, and some solutions to these problems are given.

An Evaluation of the Practical Performance of a Digilab FTS-14 Fourier Transform Infrared Interferometer Working in the Region of 4000 to 400 cm−1

The performance of a Digilab FTS-14 Fourier transform infrared interferometric spectrometer has been evaluated in terms of (1) sensitivity and signal/noise ratio (in comparison with a high quality infrared grating instrument), (2) the attainment of very high absolute signal/noise ratios by use of double-precision arithmetic, (3) photometric accuracy, (4) wavenumber accuracy, (5) resolution, (6a) water vapor cancellation and double-beam operation, (6b) ratioing facilities against stored reference spectra, (7) the measurement of very high optical densities, and (8) miscellaneous other operational considerations. It is shown that, compared with a diffraction grating spectrometer of good performance, over most of the spectral region the interferometer enables the attainment of more than an order of magnitude improvement in signal/noise ratio (S/N) for a given time of scan and resolution. This improvement in S/N was found to reach a maximum of about 40 times at 2000 cm−1 when the full wavenumber range (3600 cm−1) is measured with the highest resolution (0.5 cm−1). This corresponds to the measurement of 7200 spectral elements, n. The S/N improvement is proportional to n and is correspondingly reduced for more limited ranges of spectral measurement or for more limited resolutions. The enhanced S/N can alternatively be traded for rapid scanning. The performance of the interferometer is shown to be very good in relation to the other operational parameters listed above. Substantial additional advantages – particularly in relation to automated operation, ratioing of related spectra, and flexibility in modes of plotting wavenumber or intensities – occur from the use of a dedicated minicomputer with, or as part of, a spectrometer.

Temperature sounding from the absorption spectrum of CO_2 at 43 μm

A new method is described for obtaining the temperature profile in the stratosphere and lower mesosphere from observations of the absorption spectrum of the high J lines of carbon dioxide at 4.3 microm. This concept is based upon the measurement of the integrated absorption of individual CO(2) lines whose strengths depend strongly on temperature and that the absorption of these lines are obtained from measurements of the solar or stellar spectrum through an atmospheric path. The technique involves a rapidly converging iterative process in which the equivalent widths of the individual vibration-rotation lines of CO(2) are used. Theoretical calculations are presented for balloon and satellite observations using a model atmosphere. Experimental results are given from spectra obtained with a balloon-borne Fourier interferometer spectrometer in which the sun was observed at low zenith angles. The experimental results are compared to rocketsonde data.

A Theoretical Discussion of the Comparative Performances of Dispersion and Fourier-Transform Interferometric Spectrometers for the Infrared Region

A summary is given of the theoretical factors that determine the performances of dispersion and Fourier-transform (interferometric) infrared spectrometers in terms of the signal/noise ratio obtainable in relation to the time spent in signal measurement for a given spectral resolution. It is assumed that, as is normal in the infrared region of the spectrum, the performances of the spectrometers are limited by detector noise. “Trading rules” are given relating the spectral resolution to the time of spectral measurement or the signal/noise ratio for each type of instrument. It is shown that, for a given pair of dispersion and interferometric spectrometers, the latter operating with matched-throughput, the relative performance of the interferometer improves by a factor of √2 in signal/noise ratio (or 2 in time) when the resolution interval, Δ, is halved.

Far infrared rotational and torsional absorption spectra of chlorine nitrate

The rotational absorption spectrum of chlorine nitrate has been measured with a Fourier interferometric spectrometer in the region 5–15 cm −1 at 0.07 cm −1 resolution. Peak positions of 66 absorption lines are listed. The spectrum indicates that there are two molecular conformations present in the gas phase at ambient temperature. A torsional band exhibiting at least 7 peaks has been observed near 120 cm −1 and is consistent with a four-fold torsional barrier of about 2 kcal/mole.

Spacing ratios of Fabry-Perot etalons in a purely interferometric spectrometer. II. Case of two Fabry-Perot in series with filter

For pt.I see ibid., vol.5, p.231 (1974). The use of the 'forbidden zones' method is extended to the case of chains of two Fabry-Perots and an interference filter.

Analytical capabilities of the selectively modulated interferometric dispersive spectrometer

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTAnalytical capabilities of the selectively modulated interferometric dispersive spectrometerT. L. Chester and J. D. WinefordnerCite this: Anal. Chem. 1977, 49, 1, 113–118Publication Date (Print):January 1, 1977Publication History Published online1 May 2002Published inissue 1 January 1977https://doi.org/10.1021/ac50009a036RIGHTS & PERMISSIONSArticle Views26Altmetric-Citations2LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (665 KB) Get e-Alerts Get e-Alerts

Gas phase i.r. absorption spectrometry at extremely long wavelengths (714 μm–1667 μm)

Rotational absorption spectra at extremely long wavelengths (714 gmm–1667 μm) have been measured using a Fourier interferometric spectrometer equipped with a liquid helium cooled InSb detector. The spectral resolution was 0.07 cm −1. The observations on CO, N 2O and NO indicate that the wavenumber accuracy was ±0.003 cm −1 under ideal conditions. Rotational spectra of H 2S, SO 2 and NO 2 were also recorded under the same conditions.