LDRD 93-ERP-166 Final report Page: 4 of 13
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LDRD 93-ERP-166 FINAL REPORT
Charles L. Bennett, Michael R. Carter, David J. Fields, F. Dean Lee
Lawrence Livermore National Laboratory
P.O. Box 808, Livermore, CA. 94551
In this article, recent measurements made with LIFTIRS, the Livermore Imaging Fourier
Transform InfraRed Spectrometer, are presented. The experience gained with this instrument
has produced a variety of insights into the tradeoffs between signal to noise ratio (SNR), spectral
resolution and temporal resolution for time multiplexed Fourier transform imaging spectrometers.
This experience has also clarified the practical advantages and disadvantages of Fourier
transform hyperspectral imaging spectrometers regarding adaptation to varying measurement
requirements on SNR vs. spectral resolution, spatial resolution and temporal resolution.
Although a great deal of information about a scene can be deduced from shapes and
forms apparent in a two dimensional image, spectral measurements add a third dimension.
Multispectral imagers, characterized by having. only a few spectral channels, such as the
sensors on Landsat for example, enable qualitative identification of distinctive terrain features,
such as vegetation stress, or mineral species. Hyperspectral imagers, characterized by having a
large number of spectral channels, enable definitive identification and quantitative measurement
of the composition of objects in the field of view. Infrared hyperspectral imagers are particularly
useful for remote chemical analysis, since almost all molecules have characteristic rotation-
vibration spectra in the infrared, and a broad portion of the so-called "fingerprint" region of the
infrared spectrum lies within the thermal infrared water window between 8 to 12 m, or 750 to
1250 cm1. An example of the nature of such a hyperspectral data cube is illustrated in figure 1.
Until recently, very few infrared hyperspectral imaging instruments were available. We
have recently developed an infrared hyperspectral imager based on an imaging Fourier
transform spectrometer, and have begun to perform instrument development tests and field
measurements. We have found the technique of imaging FTIR to be quite flexible in terms of the
facility for tradeoff of spectral resolution for temporal resolution, with no effect on the degree of
spatial resolution. The very high optical throughput characteristic of FT spectrometers in general
enables high quality and high speed acquisition of imagery. Indeed, the interferometer need
produce little more than an average reduction by 50% (due to the beam splitter) in the intensity
of light which reaches the image focal plane. For this reason, it is possible to acquire thermal
infrared hyperspectral imagery with an ambient temperature interferometer, which is in sharp
contrast to the situation with dispersive imaging spectrometers. Furthermore, it is possible to use
the infrared imagery itself to aid in pointing and tracking a given subject.
In this article, first a brief history of the development of imaging FTIR spectrometers will be
given. Second, the choice of operating conditions is discussed, in particular the number of points
measured in an interferogram, which directly determines the instrumental resolution. Third a
series of measurements of gaseous plumes against both sky and hills will be discussed. Finally,
the impact of temporal fluctuations on the derived spectra will be addressed.
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Bennett, C.L.; Carter, M.R.; Fields, D.J. & Lee, F.D. LDRD 93-ERP-166 Final report, report, July 1, 1995; California. (digital.library.unt.edu/ark:/67531/metadc622320/m1/4/: accessed October 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.