The harmonic nature of the potential well for the small displacement of the probe from its equilibrium position allows us to classify the trap characteristics with three independent spring constants. These can be obtained from the spectral analysis of the thermal noise of the particle position. Probe position in all three dimensions is monitored with a single quadrant photodiode placed in the back focal plane of the microscope condenser. Experimental results of the trap stiffness measurements are presented.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.