Lock-In Thermography

Active thermography, also known as “lock-in thermography,” is a method used to detect structural defects beneath the surface of an object. Unlike passive thermography, which focuses on measuring temperature, active thermography aims to identify hidden material defects such as cracks, fraying, inclusions, delamination, and other irregularities that are not visible to the naked eye or visual cameras.

The basic principle involves introducing a short pulse of high-energy electromagnetic radiation into the object. In an ideal, defect-free object, the resulting thermal energy flow inside the body is isotropic, meaning it is the same in all directions. However, if there is a hidden defect such as a crack, the energy flow is disrupted, causing the material near the defect to cool down at a different rate after the thermal stimulation.

Pulsed flashlights, microwaves, ultrasound, or eddy currents can be used to introduce energy into the object, and the resulting temperature variations are captured using a sensitive thermal imager. By adjusting parameters such as intensity, pulse interval, pulse width, and the phase between stimulation and measurement, it is possible to reveal hidden subsurface defects.