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Gas Correlation Spectroscopy

An imaging gas correlation camera operation was demonstrated under STTR Phase-I project funded by NASA.


Concept Demonstration Experiment Setup

The camera is intended to produce realtime 2-D optical density maps of target gas in a plume. In gas correlation spectroscopy, a cell containing the target gas is placed in front of the sensor. The scene is viewed twice, through the target gas cell and through an empty cell. The gas in the cell serves as a "matched filter" to the target gas in the plume. The "modulated" transmission of the system is very high at the spectral bands of the target gas while it practically blocks transmission at all other wavelengths. The system has a very high Etendue (throughput) and is simultaneously responsive to several spectral lines or bands -- increasing the overall sensitivity.

In the present application, a UV sensitive CCD camera is used to detect SO2 presence in plumes. SO2 exhibits a strong signature in the UVA range (~300 to 320 nm), as well as in the LWIR. It can be detected in emission or absorption against the sky scattered solar UV radiation.

In these experiments, a thinned, back-illuminated Hamamatsu CCD is used for a camera. The plume is simulated by a second set of cells that hold the target gas in the field of view of the camera (see setup image above).

The application will allow ground calibration of spaceborne UV instrument (e.g., TOMS); study of volcanic plumes; emission monitoring from industrial facilities, etc. A battery operated portable version of the system will be built for these applications.

Gas correlation spectroscopy can also be utilized in other portions of the spectrum, and IR instruments are also being explored for the detection of other compounds.

The performance of the UV system has been simulated using the Modtran model in which sensitivities, SNR values and other performance parameters were established.

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