Methods and systems for detecting at least one chemical species including obtaining a first image from a first electromagnetic radiation detector and obtaining a second image from a second electromagnetic radiation detector. The first image includes a first plurality of pixels and the second image includes a second plurality of pixels, each pixel having an associated intensity value. A first resultant image is generated. The first resultant image includes a plurality of resultant pixels, each pixel having an associated intensity value. One or more regions of interest are determined. The correlation between the first image, the second image, and the first resultant image is determined for the one or more regions of interest using a correlation coefficient algorithm to calculate a first correlation coefficient and a second correlation coefficient. The presence of the chemical species is determined based, at least in part, on the first correlation coefficient and the second correlation coefficient.

The concentration of one or more gases in a surrounding fluid is measured using apparatus having a gas-permeable membrane separating an enclosed internal space within the apparatus from the surrounding fluid while allowing gases to diffuse from the surrounding fluid into the internal space. One or more sources sends infrared radiation along optical paths to one or more infrared detectors measuring intensity of radiation after absorption by gas(es) in the internal space and concentrations are determined from measured absorptions. The apparatus has more than one optical path through the internal space, enabling measurements of concentration over a greater range or enabling measurements of more than one gas present in very different concentrations or having very different infrared absorptivities.