  The epoch of the end of reionization and the Thomson optical depth to the
cosmic microwave background depend on the power spectrum amplitude on small
scales and on the ionizing photon emissivity per unit mass in collapsed halos.
In this paper is investigated the role of the radiative feedback effects for
the temporal evolution of the ionizing emissivity. It is shown that the
observational constrains on hydrogen photo-ionization rate based on
Ly-$\alpha$, Ly-$\beta$ and Ly-$\gamma$ Gunn-Peterson troughs and an electron
optical depth consistent with the latest CMB measurements requires an
emissivity of $\sim$10 ionizing photons per baryon and Hubble time at $z=6$.
Through E-mode CMB polarization power spectrum measurements, it is expected
that Planck experiment will have the sensitivity to distinguish between
different histories of stellar evolution.
