A new optical technique has been developed for low level optical extinction measurements in gaseous media. It is a variation on direct light transmission where the sample gas density in a cell is modulated acoustically by a loudspeaker. The amplitude variation of the light transmission is proportional to the gas density and is insensitive to window contamination and detector instabilities. Preliminary analysis and experiments indicate tat the instrument promises to measure light absorption to < 1%/m. The technique has been demonstrated through the use of an absorbing gas, namely, 200 ppm of NO(2) in N(2) which produces 2.5% absorption over the observed path length of 10 cm.
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