Emission reabsorption laser induced fluorescence (ERLIF) film thickness measurement

Abstract

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 µm with 1% accuracy are demonstrated.

Elastohydrodynamic film thickness of soft EHL contacts using optical interferometry

“…The dye then emits light at a lower frequency and the intensity of this emission provides a measure of film thickness [22]. In a more sophisticated approach, double-dye LIF is employed in which the emission from one dye excites a second fluorescent dye present and the ratio of the emission from the two dyes is used to calculate film thickness [33]. The LIF technique has proved to be effective for measuring and mapping film thickness in soft-EHL [34,35] but in practice is considerably less accurate than optical interferometry as used in hard-EHL.…”

Laser-induced fluorescence for film thickness mapping in pure sliding lubricated, compliant, contacts

“…For compliant contacts, where extensive film thickness investigations have yet to be carried out, no comparable calibration was possible. Hidrovo et al [23] and Poll et al [6] used an assumed geometry of a calibration wedge or cylinder, respectively. Hidrovo has remarked on the effect of reflectivity on emission intensity, and pointed out that any difference between the calibration piece and the test specimen will introduce an error when converting intensity to film thickness.…”

“…Measured fluorescence intensity is a function of the dye characteristics, the dye concentration, the exciting light intensity, and the scalar being measured. Once a particular dye and concentration are selected, the fluorescence dependence on these factors is constant [23].…”

Stability of a moving radial liquid sheet: experiments

“…Here, a non-linear Beer-Lambert law relating the incident light intensity, emission light intensity, film thickness, fluorescent dye concentration and extinction coefficient was used to determine the local variations in the film thickness. A more advanced technique proposed by Hidrovo & Douglas (2001) employs a dual emission laser induced fluorescence technique for the measurement of liquid film thickness where, the fluid contains two dyes such that the emission spectrum of one dye overlaps the absorption spectrum of another dye. The ratio of the emission intensity of the two dyes obtained from the Beer-Lambert law for the case of very thin liquid films is independent of the intensity of the incident light and becomes a function of thickness alone.…”