Rubens Technology,Translucent layer Square Root of Refractive Index of Color Dyes

In the Rubens’s Holy Women at the Sepulchre (Fig. 1) the light appears to come from the sepulchre in a dark background. The artist obtained this effect deposing on the painting a translucent layer
with a refractive index intermediate between the refractive indexes of air and of paint.The layer act as a wave-guide, leading more light to penetrate into the painting and enhancing the intensity of the observed colors. Analogous result can be obtained by covering a casein paint or a tempera colors paint with a transparent varnish layer.A simple model of transmitted light into the paint layer evidence that the maximum efficiency occurs when the varnish refractive index is equal to the square root of the refractive index of the paint.​