A brief overview of non-perturbing light diagnostics is followed by recent examples of process plasma properties measured via laser-induced fluorescence (LIF), optical emission, and absorption spectroscopy Examples include radical density measurement via absorption and emission spectroscopy. With LIF, examples show properties of ion beam etching sources and ion velocity angle variations in ICP sheaths near a process surface. Because of the wide range of process plasma parameters, appropriate choice of light diagnostics varies. (c) 2006 Published by Elsevier B.V.