  We use fluorescent nanospheres as scalar detectors for the electric-field
intensity in order to probe the near-field of an optical tip used in
aperture-type near-field scanning optical microscopy (NSOM). Surprisingly, the
recorded fluorescence images show two intensity lobes if the sphere diameter is
smaller that the aperture diameter, as expected only in the case of vector
detectors like single molecules. We present a simple but realistic, analytical
model for the electric field created by light emitted by a NSOM tip which is
quantitative agreement with the experimental data.
