The disclosure provides an optical calibration device for in-chamber calibration of optical signals associated with a processing chamber, a characterization system for plasma processing chambers, methods of characterizing plasma processing chambers, and a chamber characterizer. In one example, the optical calibration device includes: (1) an enclosure, (2) an optical source located within the enclosure and configured to provide a source light having a continuous spectrum, and (3) optical shaping elements located within the enclosure and configured to form the source light into a calibrating light that approximates a plasma emission during an operation within the processing chamber.