In this paper, we developed a light emission measurement model and a plasma light intensity distribution estimation scheme to investigate 1) the light intensity distribution of plasma inside the keyhole along the keyhole depth direction; 2) the wavelength contents of keyhole plasma; 3) the effect of plasma plume above the keyhole on the keyhole plasma light emission measured by photodiodes arranged at different viewing angles into the keyhole. The results show that the peak plasma light intensity is located close to the keyhole opening and this peak value increases sharply as the welding speed becomes slower. In addition, the spectroscopic study suggests that wavelength contents of the keyhole plasma are the richest from 320 nm to 440 nm and secondly from 520nm to 580 nm for the test condition. The estimated results also suggest that the plume effect can attenuate more than 60% of the light emission signal measured by a photodiode arranged at 86° aimming angle. More studies are needed to determine more precisely the mechanism of the plume effect.

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