Phase-Locked Image Acquisition In Thermography

Abstract

Phase-locked detection is a very useful instrumentation technique. It can be used whenever the desired signal is phase-coherent with another reference signal. Very frequently, the reference signal is, or can be derived from, the periodic external stimulus which is responsible for the signal in the first place. Typically the use of a lock-in amplifier can improve the signal-to-noise ratio by several orders of magnitude. We describe a successful implementation of an infrared imaging system in which the images are phase-locked with the periodic thermal radiation used as source of illumination and we also report the application of this phase-locked infrared imaging technique to the detection of microcracks in Cu foils deposited on polyimide substrates.