Small colloidal particles in solution exhibit Brownian motion due to thermal forces. Typically, thermal motion is a nuisance and efforts are taken to minimize its effect. However, thermal noise imaging uses the naturally occurring thermal motion of probe particles to gain insights on the environment the probe particle is in. The photonic force microscope (PFM) allows for high spatial and temporal resolution particle tracking in three dimensions. Lower laser powers create larger trapping volumes, loosely confining a probe particle. Thermal noise images are created by defining surfaces of constant occupancy of the three-dimensional position histogram of the probe particle. Figure 1 shows a thermal image of an agar network created by a 200nm diameter probe particle. Since the particle motion is governed by Boltzmann statistics, the surface corresponds to an energy of a few kT.
Figure 1: Three dimensional thermal noise image of an agar network. The grid size is 100 nm x 100nm. For details see 
When the probe particle is free in solution it is straight forward to interpret the isosurface as a surface of constant potential. Likewise, when the probe particle interacts with a static energy landscape the isosurface represents a superposition of potentials. For example, if a particle were to interact with an immobile object, there would be a volume of steric depletion in the thermal noise image. This volume is simply the space that was inaccessible to the particle.
 Tischer, C., S. Altmann, S. Fisinger, E. H.K. Stelzer, J.K.H. Hörber, and E.-L. Florin, 2001, Three-dimensional thermal noise imaging, Applied Physics Letters, 79 (23): 3878-3880.