If the P-1 or the discrete ordinates radiation model is enabled, the emissivity of the fiber has to be specified. The fiber diameter can approach the order of magnitude of the wavelength of the irradiation. In this parameter range, additional effects like diffraction take place in addition to scattering and transmission. Because these properties are not well known for fibers, diffraction, scattering, and transmission are neglected. They have to be included only in the fiber emissivity.