*2.3. Principle of Measurement*

The image analysis was carried out using a computer program built with LabView to obtain the change of the height of weld pool surface and the diameter of liquid bridge neck. Direct information about the weld pool oscillation from the high speed photography pictures was obtained by tracing the height of reference point on the weld pool surface as a function of time. The processing processes are as follows: (1) the contour of weld pool and wire silhouette in high speed photography was extracted by image processing system to obtain the pixel coordinates of contour, as shown in Figure 4b. (2) Direct information about the weld pool motion from the high speed video pictures was obtained by tracing the pool surface as a function of time. For this purpose, a reference point was defined on the weld pool surface, as depicted in Figure 4b. The distance between the reference point and the center of wire was

1.8 mm (1.5 times wire diameter). The Y-coordinate of reference point was measured as a function of time. In this way the change of the position of the reference point during welding can be outlined and the trend of the pool motion can be revealed [10]. (3) The shortest distance between the white line and the red line was calculated to obtain the diameter of liquid bridge necking of liquid bridge during short-circuit period. The process flow is shown in Figure 4.

**Figure 4.** Principle of measurement: (**a**) original images; (**b**) extraction of the outline of weld pool and the smallest diameter of liquid bridge.
