We have used scanning tunneling microscopy simultaneously as a minority charge injection source for local cathodoluminescence measurements and as a topographic characterization technique to investigate porous silicon films on the nanometer scale. Photon spectra stimulated by field emission electrons show close similarities to photoluminescence spectra recorded on similarly prepared samples. Photon maps of the integral photon intensity show sharp contrast on the nanometer scale, and the surface topography reveals structural features also on that scale. We discuss our observations in terms of the proposed mechanism for the optical properties of this modification of Si. In particular, our results are found to be consistent with models involving quantum-sized effects which increase the band gap and shift it to a direct band gap at k = 0.