Thermal emission of a semi-transparent slab with variable spatial refractive index

The directional emission intensities emerging from a parallel plane slab or semi-transparent media in which is imposed a variable spatial refractive index, have been determined by a curved ray-tracing method determining the trajectories of radiation propagation and integrating the radiative transfer equation on each of these trajectories. Cases of linear and sinusoidal refractive indexes are examined, for both isothermal and non-isothermal situations, which display significant differences from the case of a constant refractive index.