Dual Emission Laser Induced Fluorescence Technique (delif) for Oil Film Thickness and Temperature Measurement

This paper presents the development and implementation of a Dual Emission Laser Induced Fluorescence (DELIF) technique for the measurement of film thickness and temperature of tribological flows. The technique is based on a ratiometric principle that allows normalization of the fluorescence emission of one dye against the fluorescence emission of a second dye, eliminating undesirable effects of illumination intensity fluctuations in both space and time. Although oil film thickness and temperature measurements are based on the same two-dye ratiometric principle, the required spectral dye characteristics and optical conditions differ significantly. The effects of emission reabsorption and optical thickness are discussed for each technique. Finally, calibrations of the system for both techniques are presented along with their use in measuring the oil film thickness and two-dimensional temperature profile on the lubricating film of a rotating shaft seal. INTRODUCTION Laser Induced Fluorescence (LIF) is based on the use of a light source to excite a fluorescence substance (fluorophore or fluorescent dye) that subsequently emits light. The fluorescence substance is used as a tracer to determine characteristics of interest. LIF has gained popularity as a general purpose visualization tool for numerous 1-D, 2-D, and 3-D applications. It, however, has seen limited use as a quantitative tool. The reason for this stems primarily from the difficulty in separating variations in excitation illumination and vignetting effects from tracer emission. Presented herein is a two-dye ratiometric technique that allows measurement of temperature and film thickness while minimizing variations in excitation illumination and non-uniformities in optical imaging. Fluorescence is the result of a three-stage process that occurs in fluorophores or fluorescent dyes (Haugland, R. P., 1999). The three processes are (Fig. 1): 1: Excitation A photon of energy hvEX is supplied by an external source such as an incandescent lamp or a laser and absorbed by the fluorophore, creating an excited electronic singlet state (S1’). 2: Excited-State Lifetime The excited state exists for a finite time (typically 1–10 x 10 seconds). During this time, the fluorophore undergoes conformational changes and is also subject to a multitude of possible interactions with its molecular environment. These processes have two important consequences. First, the energy of S1' is partially dissipated, yielding a relaxed singlet excited state (S1) from which fluorescence emission originates. Second, not all the molecules initially excited by absorption (Stage 1) return to the ground state (S0) by fluorescence emission. Other processes such as collisional quenching, fluorescence energy transfer and intersystem crossing may also depopulate S1. The fluorescence quantum yield, which is the ratio of the number of fluorescence photons emitted (Stage 3) to the number of photons absorbed (Stage 1), is a measure of the relative extent to which these processes occur. 3: Fluorescence Emission A photon of energy hvEM is emitted, returning the fluorophore to its ground state S0. Due to energy dissipation during the excited-state lifetime, the energy of this photon is lower, and therefore of longer wavelength, than the excitation photon hvEX. The difference in energy or wavelength represented by (hvEX–hvEM) is called the Stokes shift. The Stokes shift is fundamental to the sensitivity of fluorescence techniques because it allows emission photons to be detected against a low background, isolated from excitation photons.