1. A method of monitoring a welding process, comprising: spatially and temporally detecting a selected portion of a light spectrum of plasma plume of a fusion welding process with a photo detector; processing the detected portion of the spectrum to determine a line intensity, an electron temperature, and relative quantities of iron and zinc in the plasma plume; comparing the detected portion of the spectrum with an expected spectra to determine whether a weld is acceptable; providing a signal to a welder control, the signal representing the presence of a weld defect; and adjusting the weld process based on the signal to correct the weld defect.