Method for examination of a sample by means of the lock-in thermography

Abstract

Method for a non-destructive and image forming examination of a sample () by means of a heat flow thermography method where the examination consists of evaluating the presence of any gradients in heat flow velocity at respective depth distances from a surface of the sample (), comprising exciting the sample () by means of periodic heat pulses Pfrom at least one excitation source, and capturing thermal image sequences of a thermal flow originating from the heat pulses by at least one infrared camera (), implementing relative time delays Δt between a starting point of imaging of the respective image sequences and a starting point of the periodic excitation, combining all captured image sequences to a resulting image sequence in which all images are arranged in a correct time sequence, and extracting from the resulting image sequence an indication of the existence and depth distance of a heat flow velocity transition from a surface of the sample. Exciting the sample () comprises applying heat pulses to the sample with a lock-in frequency equal to or higher than one fourth of the imaging frequency of the camera for exciting the sample (), controlling an excitation period of the heat pulses during which the excitation of the sample () by means of the heat pulses takes place, and capturing thermal image sequences comprises capturing the plural image sequences during successive excitation periods of the heat pulses with the imaging frequency. The invention includes also a system for implementing the above method.