A New Sensitive Method for Measuring Photosensitive Gas Pollutants by Means of Concentration Modulation

Abstract

In order to discriminate thr IR spectroscopic absorption lines of a certain pollutant from the IR absorption lines of the background molecules(H2O,CO2,etc.), it is proposed here that a flash light is applied to the sampled air such that the concentration of the pollutant under measuring is subject to a step change. This step modulation of concentration can be discriminatively detected from the background noise by a differentiator connected to the output of a spectroscopic IR detector. The final output can be calibrated to read directly the concentration of the pollutant. The improvement of signal to noise ratio due to the background molecular noise has been calculated to be six-order increase in magnitude over the conventional IR spectroscopic schemes. The physical reason for this large improvement is due to the fact that the imposed concentration modulation proposed here is spatially coherent, while the fluctuation of background molecular concentrations is not, therefore the"signal" is additive spatislly while the background noise is not. A practical scheme is described and signal to noise ratios due to background molecular noise as well as due to circuit noise are calculated in detail. Research in the related fields has been surveyed and evaluated.