Pulsed Thermography And Lock-in Thermography

With pulse thermography , the examined material is warmed up with a short energy pulse (light, eddy current, or ultrasonic pulse) and the heat response is recorded after a certain time. The result is an infrared image which indicates material defects in different depths.Since Pulsed thermography has its limitation in detecting faint fatigue, stress concentration and large scale testing, when using pulsed thermography, local variations of illumination and surface emissivity should also be strictly required All these factors limit the application of pulsed thermography, So we introduce lock-in thermography which can solve all these difficulties mentioned above.Remote material testing with monofrequent thermal waves by Lock-In Thermography uses sinusoidally modulated excitation signals in order to derive information from the observed phase and magnitude of this wave. The phase angle has the advantage that it is independent of local variations of illumination or of surface emissivity . However, if the thermal characteristics of the material are unknown or if defects in different depths have to be detected, then the phase images have to be taken at different frequencies which causes several measurements.This paper will emphases mainly on lock-in thermography. The paper gives basic principles of lock-in thermography,base on those principles, we invented our own lock-in system, with which we have tested different kind of materials successfully.Finally,advantages and disadvantages of both Pulsed thermography and Lockin thermography are presented.