The light emission from the plasma induced by the interaction between laser beam and the keyhole provides important information concerning the keyhole phenomenon. In the present study, the light emission at different wavelengths was measured by using multi-photo sensor system, which consists of photo-diodes, and band pass of blue, green and red regions filters in CO2 laser welding at 20 kW power levels. The plume provided stronger DC emission in the blue region than in green and red regions, whereas the keyhole plasma, which is ejected cyclically at a frequency of approximately 100Hz from the keyhole by the fluctuation of the keyhole, is larger in the green and red regions than the blue region.

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