Abstract: An apparatus is provided that includes a plurality of reflectors arranged over an area, and a plurality of transceivers. A first of the transceivers is configured to transmit radiation at each of the plurality of reflectors at a first wavelength and a second wavelength, and receive radiation reflected from each of the plurality of reflectors. A second of the plurality of transceivers is configured to transmit radiation at each of the plurality of reflectors at a third wavelength and a fourth wavelength and receive radiation reflected from each of the plurality of reflectors. The apparatus includes a processor configured to calculate a change in concentration of a gas within the area between a first time and a second time based upon signals received from the transceivers.

Abstract: Remote absorption spectroscopy uses coded electromagnetic transmission directed through a medium under investigation to one or more remote receivers. The coded transmission includes at least one wavelength coincident with an absorption band of interest and one wavelength in an off-line band and a predefined relationship between spectral components in and outside the absorption band is controlled. The relationship between spectral components may be evaluated at the receiver to determine whether deviation thereof from the controlled relationship at the transmitter exists at the receiver. The deviation of the received optical signal from the prescribed relationship is processed to indicate the absorption of the radiation in the absorption band.

Abstract: Remote absorption spectroscopy uses coded electromagnetic transmission directed through a medium under investigation to one or more remote receivers. The coded transmission includes at least one wavelength coincident with an absorption band of interest and one wavelength in an off-line band and a predefined relationship between spectral components in and outside the absorption band is controlled. The relationship between spectral components may be evaluated at the receiver to determine whether deviation thereof from the controlled relationship at the transmitter exists at the receiver. The deviation of the received optical signal from the prescribed relationship is processed to indicate the absorption of the radiation in the absorption band.