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We proposed in a previous paper [Opt. Commun. 228, 33 (2003)] a modifiedradiative transfer equation to describe radiative transfer in a medium with aspatially varying refractive index. The present paper is devoted to thedemonstration that this equation perfectly works in the non-absorbing /non-scattering limit, what was contested by L. Mart\'i-L\'opez and coworkers[Opt. Commun. 266, 44 (2006)]. The assertion that this equation would imply azero divergence of the rays is also commented.

Link between the laws of geometrical optics and the radiative transfer equation in media with a spatially varying refractive index