Real time detection of defects in laser welds has been the subject of substantial research and development efforts. In this review, the state of progress in developing so called emission based monitoring systems is described. These monitors collect and analyse various forms of energy radiated from the laser–material interaction site as the weld is being formed. The most common emissions that have been used for monitoring purposes include optical and near optical wavelength electromagnetic radiation, audible and ultrasonic airborne acoustic waves, and integrated charged particle flux. The various system configurations and the weld quality characteristics that have been measured with these emissions are summarised. As researchers continue to refine these systems, there has been increasing interest in the mechanisms by which the signals are produced and their relationship to the various physical processes occurring in the laser weld keyhole. A review of the current literature pertinent to the sources of electromagnetic, acoustic, and plasma charge emissions is given. Finally, results illustrating the relationships between the various emissions signals are presented.
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