Photothermal detection of trace optical absorption in water by use of visible light-emitting diodes

Visible-light-emitting diodes of three different colors have been used to detect an absorbing compound (potassium permanganate) in trace quantities in aqueous solution. Photothermal absorption in a closed cell caused deflection of a water meniscus held at a small pinhole. The displacement was monitored with optical-fiber interferometry. The technique was limited by LED emission intensities and environmental acoustic noise, giving minimum detectable absorption coefficients of 2 x [10.sup.-4] [cm.sup.-1] at 478 and 658 nm and 3 x [10.sup.-4] [cm.sup.-1] at 524 nm. The magnitude and form of meniscus deflection signals were shown to be in good agreement with theory.