  We present the results of a detailed time resolved luminescence study carried
out on a very high quality InGaAs quantum well sample where the contributions
at the energy of the exciton and at the band edge can be clearly separated. We
perform this experiment with a spectral resolution and a sensitivity of the
set-up allowing to keep the observation of these two separate contributions
over a broad range of times and densities. This allows us to directly evidence
the exciton formation time, which depends on the density as expected from
theory. We also evidence the dominant contribution of a minority of excitons to
the luminescence signal, and the absence of thermodynamical equilibrium at low
densities.
