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Lighting either natural or not in the european medieval architecture (roman period) is studied and simulated around a special and famous example. "Notre-Dame de Paris" cathedral and its polychromy are studied for many years and, today the remains of polychromy are very reduced. This prestigious monument is then a unique opportunity to study the complex relationships between light, paint and gilt. Monumental polychromy of the religious buidings is now well established and some attempts exist for retrieving a kind of optical effects well controlled in the medieval era. An interdisciplinary study which derived from the french project TerraNumerica allowed to value a historical hypothesis about the possible optical role of gilts. Thanks to the simulation of a global illumination in spectral rendering operating with the photon mapping algorithm, a new interpretation of polychromy effects is proposed, including the lightguide effect produced by the gilts. Many measurements on formulated polychrome samples and spectrophotometric captures concerning natural lighting at sunset were made to validate that new hypothesis. The obtained results guided the interdisciplinary team on a new focus on the polychromy restitution of the medieval buildings and monuments. It became quite evident that the medieval sculptor brought a very high and accurate mastership in lighting. Playing with the symbolic and colored light for illuminating the Galerie des Rois (the kings of Judah and not the kings of France) he offered to the christian people a very "special effect" for the last minutes of the day time. Physically based simulations presented here show a brand new hypothesis concerning the optical role of gilts.