An interferometric method of measuring small differences between weak optical absorptions of solutions has been developed using the thermooptic effect. To record the small changes in optical path length ~λ/200 due to heating, it was necessary to stabilize the fringe pattern with respect to slow thermal drift using a galvanometer-driven compensator plate controlled by a closed feedback loop. Fringe shifts from background absorptions were nulled out to better than 1 part in 400, permitting the measurement of differences in absorptions between two solutions that were l/100th of background. Using laser powers of 100 mW, absorptions ~5 × 10^-6 cm^-1 (base e) could be measured with CC1₄ solutions.

© 1982 Optical Society of America

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