Total internal reflection microscopy (TIRM) is an optical technique for monitoring Brownian fluctuations in the separation between a single microscopic sphere and a flat plate in aqueous media. The sphere is levitated above the plate by colloidal forces such as double-layer or steric repulsion. Changes in elevation as small as 1 nm can be detected by measuring the light scattered by a single sphere when illuminated by an evanescent wave. From the Boltzmann distribution of elevations sampled by the sphere over a long time, the potential energy (PE) profile can be determined with a resolution of about 0.1kT. By corrupting clean data (having a mean scattering intensity of (I) over bar (s) and a standard deviation of sigma(b)) obtained by Brownian dynamics simulations with various levels of additive background noise (having a mean background intensity of (I) over bar (b) and a standard deviation of sigma(b)), we show how the PE profiles obtained from TIRM are distorted. Increasing (I) over bar (b) narrows the profile and shifts it toward smaller elevations; conversely, increasing ab broadens the PE profile. Subtracting (I) over bar (b) from the measured total intensities before analysis removes much of the distortion. The extent of remaining distortion (broadening) depends primarily on the ratio sigma(b)/sigma(s) and can be neglected when this ratio is less than 0.15.