  Fluorescence fluctuation spectroscopy is applied to study molecules, passing
through a small observation volume, usually subjected to diffusive or
convective motion in liquid phase. We suggest that such a technique could be
used to measure the areal absolute concentration of fluorophores deposited on a
substrate or imbedded in a thin film, with a resolution of a few micrometers.
The principle is to translate the solid substrate in front of a confocal
fluorescence microscope objective and to record the subsequent fluctuations of
the fluorescence intensity. The validity of this concept is investigated on
model substrates (fluorescent microspheres) and DNA biochips.
