  We calculate the intensity and photon spectrum of the intergalactic
background light (IBL) as a function of redshift using an approach based on
observational data obtained in many different wavelength bands from local to
deep galaxy surveys. This allows us to obtain an empirical determination of the
IBL and to quantify its observationally based uncertainties. Using our results
on the IBL, we then place 68% confidence upper and lower limits on the opacity
of the universe to gamma-rays, free of the theoretical assumptions that were
needed for past calculations. We compare our results with measurements of the
extragalactic background light and upper limits obtained from observations made
by the Fermi Gamma-ray Space Telescope.
