The dependence of the relative escape fraction from the absolute magnitude in the UV M₁₅₀₀. The black arrows are the upper limits to the relative escape fractions derived from the stacks of LBC data in this paper, summarized in Table 2. The observed trend of looser upper limits with fainter magnitudes is due to the limited depth of the data in the UV wavelengths, and plausibly it is not related to a physical property of the population of star-forming galaxies. The blue arrows summarized the values available in literature for individual lensed galaxies, by Amorin et al. (2014) and Bayliss et al. (2014), in the luminosity ranges not covered by our data. For the Bayliss et al. (2014) source, the limit of 1% has been plotted only for graphical reasons, but could be significantly lower. The different curves show analytic functions which have been adopted to infer the evolution of the relative escape fraction from luminosity. These functions have been used in order to check for the contribution of galaxies at different luminosities to the observed UVB.