An experimental study was carried out to develop the seam tracking sensor using light emission instead of arc current in the GMA welding process. Welds were made on the inclined plane and V-groove using the argon and $CO_2$ gas with weaving motion, and signals from light emission and arc current were detected simultaneously for comparison. The metal transfer appeared to occur in the globular mode considering the current level of about 200 ampere and the amount of spatter. Light emission showed good coherence with arc current except the phase shift of 180 degrees. In general, light emission provides higher quality signals than arc current. The reason of generating high quality signal is speculated that the volume of the arc has more effects on the radiation than the arc current when using the argon gas. In the result using $CO_2$ gas, the hot electrode as well as the arc radiates the light emission.
