Lock-in thermography: Like many of the other techniques, lock-in thermography requires that an external energy source (light, sound, heat, etc.) be applied to a component's surface to reveal abnormalities below the surface. It's important to know the depth, size and location of the abnormality, as well as the material makeup and properties for this technique to be accurate. While it takes longer than the techniques previously discussed, it can penetrate components with thicker walls.