Lockin thermography (or LT) is based on thermal waves generated inside the specimen under study by submitting it to a periodic (sinusoidal) thermal stimulation. In the case of a sinusoidal temperature stimulation of a specimen, highly attenuated and dispersive waves are found inside the material (in a near surface region). These waves are known as “thermal waves.” This is not a new concept since these thermal waves were first investigated by Fourier and Angström back in the XIX century.

In this case of thermography the increase of surface temperature is monitored during the application of a stepped heating pulse (a “long pulse”). The sample is continuously heated, at low power. Variations of surface temperature with time are related to specimen features as in pulse thermography. This technique of step heating (SH) is sometimes referred to as time-resolved infrared radiometry or TRIR. The time-resolved part means the temperature is monitored as it evolves during and after the heating process.