are shown in relation with the timing of the two energy profiles or weld signatures of reflected and transmitted energy stage 41 shows a two piece lap joint before welding stage 42 shows 0 5 ms after the laser beam is initiated stage 43 shows a keyhole developing during time u03b4t1 stage 44 shows the keyhole penetrating through the work pieces during time u03b4t2 stage 45 shows keyhole closure at the start of u03b4t3 stage 46 shows the keyhole completely closed during time u03b4t3 and stage 47 shows the beam turned off and the welding cycle completed the signatures in fig4 shows that it took 1 15 ms u03b4t1 to penetrate both pieces the keyhole remained open for 1 41 ms u03b4t2 the keyhole closed in 0 83 ms u03b4 t3 and the total weld time was 3 39 ms the signatures shown in fig4 represent a 34 good acceptable 34 weld fig5 is a set of weld signatures of one form of 34 bad unacceptable 34 weld note that the reflected signal 51 is present but the transmitted signal 52 is indicative only of local heating or background radiation one can deduce that the weld is not a full penetration weld and lacks fusion fig6 is a representation of another 34 bad unacceptable 34 weld signatures this is.