Film growth on glass fibers (40 &mgr;m diameter) is used to probe the distribution of SiH4decomposition products that producea‐Si:H films in silane rf glow discharges. The film thickness on fibers spanning the electrodes is measured versus position to map the spatial variation of the film‐precursor (radical) density. The optical emission from the discharge, which is shown to be essentially equivalent to the distributed source of SiH4decomposition products, is compared to the density maps. This comparison shows that the SiH3radical dominates deposition, that this SiH3is produced in the optically bright regions of the discharge, and that H atoms react rapidly with SiH4before diffusing significant distances in the discharge. The perturbative nature of the probes on the discharge environment is also addressed.