We report a laser-induced fluorescence (LIF) experimental method that is able to quantify dye concentration in a laminar three-dimensional flow. The geometry of interest is a stirred tank reactor agitated by three impellers. The method is employed to reveal the spatial structure of the evolving mixing patterns and its statistical properties. For chaotic flow conditions, the statistics of the passive scalar reveals a non-Gaussian concentration probability density function with an exponential tail. We find that the concentration patterns, once established, do not change with time; i.e., they are self-similar. This LIF method also provides detailed flow structures that are of importance to the study of mixing. Flow structure varies considerably as a function of Reynolds number (Re). However, we find that the mixing patterns are structurally invariant and they slowly decay as a result of a nonequilibrium state between stretching and diffusion.