Emission reabsorption laser induced fluorescence (ERLIF) film thickness measurement

This paper presents a unique optical technique that utilizes the reabsorption and emission of two fluorescent dyes to accurately measure film thickness while minimizing errors caused by variations in illumination intensity and surface reflectivity. Combinations of dyes are selected that exhibit a high degree of emission reabsorption and each dye concentration is adjusted to create an optically thick system where emission reabsorption is intrinsic to the fluorescence of the film being measured. Film thickness information as well as excitation and dye response characteristics are all imbedded in the emission intensities of the dyes. Errors normally associated with laser induced fluorescence based film thickness measurements, including those due to optical distortion, variations in surface reflectivity and excitation non-uniformities, are minimized by observing the ratio of the dye emissions. The principle and constitutive equations characterizing emission reabsorption laser induced fluorescence (ERLIF) film thickness measurement are presented. In addition, film thickness measurements from 5 to 400 mum with 1% accuracy are demonstrated.