The thermal nondestructive evaluation (NDE) technique termed "active thermo­ graphy" integrates infrared imaging with external heating to assess subsurface structure via the thermal response of the sample. It is important to distinguish this approach from that used in the predictive maintenance community where temperature distributions of operating machinery or electrical systems are imaged to locate hot spots indicative of operating problems. While this is a very important field on its own, it does not involve the use of an external heating source as is required in active thermography.