Lock-in thermography is a powerful technique used to explore sub-surface detail with high spatial and thermal resolution.

Quantitative thermal imaging (TI) temperature measurement applications are broadly defined as either a passive or active. In active TI measurement the object under investigation is 'actively' stimulated, for example using a modulated heat source on an object.

One of the most promising active TI measurement methodologies is lock-in thermography (LT). A modulated source, such as a chopped high-power lamp, is used to produce an oscillating temperature in an area under observation. By observing the temperature of the scene and monitoring the exact time dependence between the output signal and the reference input signal (modulated excitation source), both the phase and amplitude images become available. These images (phase, amplitude) provide depth information about the area under investigation, and discontinuities or abnormalities in the material appear as differences within the images. The technique is a powerful one; LT can improve thermal resolution by a factor of 100, providing microkelvin resolution.

Lock-in thermography has many uses; a primary application being non-destructive testing of composites and laminates for the aerospace sector. An example set of thermal images of a carbon fibre composite structure is shown, illustrating visual, passive thermal, and phase views.

Figure 1: Visual (left), thermal (centre) and phase (right) image of carbon fibre composite weave, with sub-surface anomalies (defects/discontinuities)
– showing as white/dark regions in the phase image.

Lock-in thermography also has broader application areas due to its versatility in operation and is used in the electronics sector (e.g. circuit boards, solar cells); healthcare sector (e.g. dermatological) and even the food sector (quality control). An example visual, passive thermal and phase thermal image of an apple is given below.

Figure 2: Visual (left), thermal (centre) and phase (right) image of apple, with sub-surface bruising showing as white regions in the phase image.

NPL's lock-in thermography facility is available to support measurement research and applied measurements. If you have a future measurement requirement or have a concept you would like to develop in partnership with NPL, please get in touch.

For more information, please contact Rob Simpson