Noise Suppression in IR Thermal-Wave Video Images by Real-Time Processing inSynchronism with Active Stimulation of the Target

摘要

An infrared (IR) imaging system is described in which noise suppression and imageenhancement occur as a result of real-time processing techniques, synchronized with active time-dependent heating of the target. The system consists of an IR video camera and a real time image processor under the control of a computer workstation, together with various time-dependent heat sources. The heat sources are used to launch pulsed or periodic thermal waves into the target, and the camera is used to record the thermal waves scattered back to the surface by sub-surface thermal features (cracks, coating substrate boundaries, inter-ply delaminations, etc.). The technique involves applying either a sequence of short about 5 ms) heat pulses with relatively long time intervals between them, or a long train of square or sinusoidal heating pulses with frequencies of a few Hz to a few kHz. In the first (pulsed) case, the signal processor is operates a pixel-by-pixel box-car averager with images being taken in several gated time windows after each pulse. After averaging over multiple pulses, and suitable arithmetic manipulation of the gated images, a resultant image with increased contrast and a considerably higher signal-to-noise ratio is obtained. The result is as if a quarter of a million box-car averagers (one of each pixel of the image) had similar number of vector lock-in amplifiers, again producing an improved signal-to-noise ratio. Examples are shown of sub-surface features in graphite-epoxy laminates and copper printed circuit traces embedded in polymide. Reprints. (jhd)