We compare theory and experiments on relative stability of the two stable stationary states in optically bistable ZnSe interference filters. We examine the equistability predictions of the master equation, the Fokker-Planck equation, and a deterministic analysis. For this effectively one-variable system there exists a potential for each of these descriptions. We derive a master and Fokker-Planck equation for the fluctuations in temperature expected in our system and find that the three theoretical potentials differ significantly for homogeneous geometries of the system, but the stochastic predictions agree with the deterministic equistability point for inhomogeneous systems. We also examine the effect of external noise, which is independent of the state variable, and find that a very small amount of noise reduces the equistability prediction of the homogeneous master and Fokker-Planck equations to that of the deterministic potential. Noise in the constraints (such as light intensity) results in a new effective potential. © 1990 American Institute of Physics.