  We study the thermophoretic motion of a micron sized single colloidal
particle in front of a flat wall by evanescent light scattering. To quantify
thermophoretic effects we analyse the nonequilibrium steady state (NESS) of the
particle in a constant temperature gradient perpendicular to the confining
walls. We propose to determine thermophoretic forces from a 'generalized
potential' associated with the probability distribution of the particle
position in the NESS. Experimentally we demonstrate, how this spatial
probability distribution is measured and how thermophoretic forces can be
extracted with 10 fN resolution. By varying temperature gradient and ambient
temperature, the temperature dependence of Soret coefficient $S_T(T)$ is
determined for $r = 2.5 \mu m$ polystyrene and $r = 1.35 \mu m$ melamine
particles. The functional form of $S_T(T)$ is in good agreement with findings
for smaller colloids. In addition, we measure and discuss hydrodynamic effects
in the confined geometry. The theoretical and experimental technique proposed
here extends thermophoresis measurements to so far inaccessible particle sizes
and particle solvent combinations.
