Owing to the health hazard of respirable airborne fibers, there is great interest in detectors able to monitor fibers online. This paper features such an optical fiber detector which is based on Fraunhofer theory for the estimation of fiber size. Because Fraunhofer theory is not an exact theory and does not take into account the three-dimensional shape of fibers and their material properties, comparative computations with an exact theory, the multiple multipole method (MMP), a variant of the generalized multipole technique, were performed. For small fiber diameters these simulations showed differences between diffraction patterns calculated via Fraunhofer theory and scattering patterns computed with MMR The differences were strongly dependent on the optical properties of the fiber material.