Time of flight diffraction ultrasonic testing is excellent for detecting weld discontinuities and defects. To do this, at least two ultrasound probes are moved along the axis of weldment. One probe is known as the transmitter, which sends ultrasonic waves along the top of the weld joint and the backside of the weld joint. The transmitted waves are collected by the other probe, known as the receiver. If there is a discontinuity, such as a lack of fusion, the ultrasonic waves will diffract from the tips of an indication and will appear on a flaw detector. The technician will observe this and investigate further. Time of flight diffraction is excellent at detecting discontinuities and defects with very sharp geometrical features. Discontinuities such as weld cracks, lack of fusion and lack of penetration are excellent examples of discontinuities that are easily detected by the time of flight diffraction process. Discontinuities near the surface, such as weld undercut, are more difficult for time of flight diffraction to detect. Furthermore, discontinuities without sharp geometrical features, such as porosity, can be slightly more difficult to detect then weld cracks with the time of flight diffraction process.