Near-IR optical tomography is thwarted by the highly scattering nature of light propagation in tissue. We propose a weighted back-projection method to produce a spatial map of an optical parameter which characterized the investigated medium. We have studied the problem of the choice of the back-projection weight function for the absorption coefficient ((mu) a) and for the reduced scattering coefficient ((mu) s') of tissuelike phantoms. Working in frequency-domain optical imaging, we have initially approached the problem of quantifying the effect caused by a small absorbing defect embedded in the medium on the measured DC intensity, AC amplitude, and phase. The collection of DC, AC, and phase changes during a 1 mm step raster scan of the absorbing defect provides information on the photon path distributions and, in general, on the probed spatial region when DC, AC, and phase are, respectively, the measured parameters. We report experimentally determined weight functions for (mu) a and (mu) s'. They indicate that absorption and scattering maps can significantly differ in terms of resolution.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.