Lock-In Thermal Wave Imaging

Abstract

<p>Previously, some of the authors demonstrated a technique for parallel vector lock-in thermal wave infrared (IR) video imaging, using a patented technique. [1–3] The initial version [2] of the technique was restricted to the frame rate frequency of 15 Hz for the single-detector IR camera used for the experimental verification. In an application for imaging cracks in Cu microbridges, an alternative version of the technique [3] succeeded in achieving IR video lock-in imaging at frequencies up to 2 kHz. However, the latter implementation was seriously complicated by the complex timing pattern through which the pixels were acquired by the IR imaging system. The complication resulted from the fact that one needs to know the timing of each pixel, because the method assumes that the IR signal corresponding to each pixel of the image can be multiplied by the sine and cosine functions of the thermal modulation frequency, computed at the time at which the pixel datum was obtained. After multiplication, the results are accumulated, pixel-by-pixel, in two image buffers, which represent the two vector components of the lock-in image. If desired, these image buffers can then be transformed into magnitude (the square root of the sum of the squares of the components) and phase (the arctangent of the ratio of the two components) image buffers.</p>