  Fluorescence Correlation Spectroscopy (FCS) is widely used to detect and
quantify diffusion processes at the molecular level. The molecules of which
diffusion is studied are marked with fluorescent dyes. It is commonly
maintained that this technique only applies to systems where the concentration
of fluorescent molecules is low. Even if this is the optimal operational
condition, we show that FCS can be used also at high concentrations (up to
50$\mu$M) of fluorescent molecules: the detector blinding due to highly
fluorescent solutions of concentrated dyes can be avoided by using neutral
optic density (OD) filters, and the initial condition of very bad signal to
noise ratio (SNR) can be hampered by suitable statistical averaging, as usual
in other contexts of signal analysis.
