The remote detection of gases using coherence measurement
The remote detection of gases using coherence measurement
This thesis describes work related to a new technique for the passive remote sensing of gases. Passive sensors have advantages over active ones but are often limited by unpredictable spectral variations in the radiation source, which could be sky, terrain etc. Coherence measurements can reduce these errors, even when using simple gaseous spectra such as isolated lines and unresolved bands. The technique is based on similar principles to fourier Transform Spectroscopy (FTS). However, rather than scanning the interferometer over a range of path differences then taking the fourier transform of the resultant interferogram, only a single path difference is used. This corresponds to a null in the coherence function created by an optical band-pass filter, centred on the absorption or emission feature. Insensitivity to changes in the background spectrum is achieved using the real and imaginary components, which are found as quadrature terms in the interferogram. A digital algorithm was developed to measure these. Initial experiments, using a photodiode array, demonstrated the theory to within 2%. By using a scanning interferometer, a Noise Equivalent Concentration path Length (NECL) of 15 ppm m was obtained for NO2 (limited by uncertainty in the interferometric path difference). This was achieved in the visible band using both a quartz-halogen lamp and daylight as the source. This NECL was insuffient to measure the error caused by using very different background spectra. Calculations suggest that the NECL could be reduced to 1 ppm m, limited equally by shot noise, uncertainty in path difference and extreme variations in the spectrum.
University of Southampton
1990
Drum, Stephen Michael
(1990)
The remote detection of gases using coherence measurement.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
This thesis describes work related to a new technique for the passive remote sensing of gases. Passive sensors have advantages over active ones but are often limited by unpredictable spectral variations in the radiation source, which could be sky, terrain etc. Coherence measurements can reduce these errors, even when using simple gaseous spectra such as isolated lines and unresolved bands. The technique is based on similar principles to fourier Transform Spectroscopy (FTS). However, rather than scanning the interferometer over a range of path differences then taking the fourier transform of the resultant interferogram, only a single path difference is used. This corresponds to a null in the coherence function created by an optical band-pass filter, centred on the absorption or emission feature. Insensitivity to changes in the background spectrum is achieved using the real and imaginary components, which are found as quadrature terms in the interferogram. A digital algorithm was developed to measure these. Initial experiments, using a photodiode array, demonstrated the theory to within 2%. By using a scanning interferometer, a Noise Equivalent Concentration path Length (NECL) of 15 ppm m was obtained for NO2 (limited by uncertainty in the interferometric path difference). This was achieved in the visible band using both a quartz-halogen lamp and daylight as the source. This NECL was insuffient to measure the error caused by using very different background spectra. Calculations suggest that the NECL could be reduced to 1 ppm m, limited equally by shot noise, uncertainty in path difference and extreme variations in the spectrum.
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Published date: 1990
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Local EPrints ID: 462541
URI: http://eprints.soton.ac.uk/id/eprint/462541
PURE UUID: 90ce8dec-bd87-4b8e-b380-6d7f2ab43bc1
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Date deposited: 04 Jul 2022 19:18
Last modified: 04 Jul 2022 19:18
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Author:
Stephen Michael Drum
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