Flying Laser Spot Thermal Wave Ir Imaging of Horizontal and Vertical Cracks

As an extension of the work of Kuo et al[l], we have developed a "Flying-Spot" lasersource/IR-detector camera in which the focal point of an unmodulated heating laser is moved at constant velocity across the sample while the image point of an IR detector is scanned at the same speed at a point just behind the laser beam. The detector is thus looking at the "thermal wake" of the heated spot. The time delay between heating and detection is determined by the speed of the laser spot and the distance between it and the detector image. Since this distance can be made arbitrarily small (Actually it can be made negative; the detector can lead the heated spot.), the camera is capable of making thermal wave images of phenomena which occur on a very short time scale. In addition, because the heat source is a very small spot, the heat flow is fully three-dimensional. This makes the camera system sensitive to features like tightly closed vertical cracks which are invisible to imaging systems which employ full-field heating.