A method of characterizing a Plasma Processing Reactor (PPR) by measuring the electromagnetic (EM) emissions of a plasma inside the PPR using an Optical Plasma Monitoring Apparatus (OPMA) is described. The OPMA contains a plurality of photo-sensors that can measure EM emissions of narrow and/or broad spectral regions at various selected positions on the OPMA, and record them as a function of time. The OPMA can have substantially similar dimensions of a workpiece to facilitate loading and unloading into the PPR.