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In the present study, laser-induced fluorescence (LIF) is used to investigate the mixing process of a droplet impacting onto a thin liquid film. A robust multidimensional calibration procedure is developed enabling the extraction of local instantaneous dye concentrations as well as film heights. A series of validation measurements are conducted confirming a low reconstruction error of 4.53%. The impact-induced mixing process is thoroughly investigated across various liquid film thicknesses to examine the propagation of the mixing zone and the instantaneous radial concentration gradients within it. It is shown that the maximum extent of the mixing zone scales inversely proportional with the thickness of the liquid film. Within our experiments, we discover the formation of wall-induced vortex ring instabilities subsequent to impact. The disintegration of vortex rings during droplet impact significantly enhances convection-driven mixing, as quantified by the coefficient of variation.