Time of Flight Diffraction (ToFD) method of ultrasonic testing is a accurate and sensitive method of non-destructive testing (NDT). Measuring the amplitude of reflected signal is a relatively unreliable method of sizing defects because the amplitude strongly depends on the orientation of the crack. Instead of amplitude, ToFD uses the time of flight of an ultrasonic pulse to determine the position and size of a reflector.

In a typical ToFD system, a pair of ultrasonic probes sits on opposite sides of the weld, one probe transmits a signal and the other receives the signal. first principles is then used with the measured time of flight of the ultrasonic pulse, distance can be calculated.