Photothermal detection of trace optical absorption in water using 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 using optical fiber interferometry. The technique was limited by LED emission intensities and environmental acoustic noise, giving minimum detectable absorption coefficients of 210 cm at 478 nm and 658 nm, and 310 cm at 524 nm. The magnitude and form of meniscus deflection signals were shown to be in good agreement with theory.