Optical thin film measurement by interferometric fringe projection and fluorescence stimulated emission

The introduction of a new technique for metrology of thin liquid films to give both the profile of the exterior surface and information on the thickness of the film is the main focus of this research. The proposed approach is based on the use of fringe projection system with a narrow band laser illumination and a high concentration of fluorescent dye dissolved in the fluid in order to generate fluorescence emission from minimum thickness of the film (i.e. the top few microns). The method relies on calculation of an interference phase term and the modulation depth of the fringes created by means of a twin fibre configuration. The characterisation of candidate fluorescent dyes in terms of absorption, related to the depth of penetration of the incident light into the dye and their fluorescence emission efficiency is presented and their application in full field imaging experiments is evaluated. A strong focus of the technique proposed is its flexibility and versatility allowing its extension to phase stepping techniques applied to determine the (fringe) phase map from static and dynamic fluids. Some experiments are carried out using the best dye solution in terms of fluorescence emission and light depth penetration. On the basis of the phase-height relationship achieved during the calibration process, the proposed measurement system is applied for the shape measurement of some static fluids. The profile of the exterior surface of these fluids is investigated by means of phasestepping technique and the resolution of the measurements is estimated. Furthermore a flow rig set-up based on inclined system (gravity assisted) is presented in order to test the shape measurement system in presence of real liquid flows. Different liquid flow thicknesses are processed and analysed. Example data will be included from some fluid films of known geometry in order to validate the method.