Optically excited Lockin Thermography (OLT)

Lockin thermography is a dynamic measurement method (thermal wave analysis). The wave is excited sinusoidally with halogen lamps, laser, or hot-air pistols on the surface of the tested component. It propagates wavelike into the testing object and is reflected at boundaries due to sudden changes in temperature conductivity and thermal impedance of the material.
In difference to common thermography, temperature changes are analysed only at the excitation frequency. The phase image shows colour-coded the time delays due to heat transportation.
A practical advantage of phase images is the suppression of optical or infrared surface structures of the test object, so that almost only the thermal structures become visible. The depth range, which is greater than it would be using amplitude, depends on the modulation frequency. The method is therefore adjustable to different materials and testing circumstances. With the help of frequency variations one can find out the depth of thermal structures (thermal tomography).