Fluorescence response of mineral oils: spectral yield vs absorption and decay time.

The fluorescent emission from film samples of representative oils has been investigated by means of two independent experiments: (i) time-resolved fluorosensing at a fixed excitation frequency, by which the spectral emission yield and the associated decay times, tau(lambda), were determined, and (ii) monitoring of the integrated fluorescence in a fixed spectral band as a function of film thickness, from which extinction coefficients alpha(lambda) were derived at a number of excitation frequencies. After checking the good mutual consistency of corresponding emission data obtained in the two types of experiments, a unified analysis was elaborated in terms of a renormalized conversion efficiency, i.e., correcting the measured spectral yields for the ratio (tau/alpha). This quantity can be shown by an ppropriate model to be typical of the emitting fluorophores but independent of absorption or quenching effects due to the complex oil medium. This was indeed found to be the case for a large variety of oils, in spite of extreme variations in their absolute emission yield.