A method of adjusting plasma processing of a substrate in a plasma reactor having an electrode assembly. The method includes the steps of positioning the substrate in the plasma reactor, creating a plasma in the plasma reactor, monitoring optical emissions emanating from a plurality of different regions of the plasma in a direction substantially parallel to the surface of the substrate during plasma processing of the substrate, and determining an integrated power spectrum for each of the different plasma regions and comparing each of the integrated power spectra to a predetermined value. One aspect of the method includes utilizing an electrode assembly having a plurality of electrode segments and adjusting RF power delivered to the one or more electrode segments based on differences in the integrated power spectra from the predetermined value. Another aspect of the invention includes altering the flow of gas to different regions of the plasma in response to differences in the integrated power spectra detected by the fiber optic sensors. Several types of novel electrode assemblies suitable for carrying out the method of the invention are also disclosed.