When compared with other classical NDT techniques such as ultrasonic or radiographic testing, thermographic inspection is safe, nonintrusive, and usually noncontact, allowing the detection of relatively shallow subsurface defects (a few millimeters in depth) under large surfaces (typically covering an area of 30 by 30 cm (12 by 12 in) at once, although inspection of larger surfaces is possible) and quickly (from a fraction of a second to a few minutes depending on the configuration).