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THERMOGRAPHY TECHNOLOGY

Line Scanning Thermography (LST™) is a dynamic thermography technique that has been successfully used for detecting regional alterations in the thermal properties of the material inspected. In the case of boiler tubes, LST has been observed to effectively detect thinning in carbon steel and in composites it can detect defects in thick layers.

Thermography Diagram

In LST, a line heat source of finite width moves across the sample' surface at a constant speed; an infrared camera is used to record the temperature changes in the surface during the heating protocol. The IR detector moves in tandem with the heat line source, and the camera's field of view is set to contain a region above and below the location where heat is deposited. Every image frame recorded by the thermal camera contains two main regions, one that where the heat source has passed through, and a region that has not been heated. Temperature recorded from the unheated region provides information related to the baseline or initial temperature distribution from the sample.

Heat intensity (q) and velocity (v) of the heat source govern energy deposition over the sample under study. On the other hand, thermal conductivity (k), heat capacity (Cp), density (p), and thermal diffusivity (a) control the propagation or diffusion of heat in the medium and are characteristic of each sample or plate under study The thermal signature detected with the IR imager depends on the transmitting medium and its temperature, the distance between the surface in consideration and the IR imager, as well as the optical properties of the surface such as emissivity and reflectivity. Depending on the magnitude of these quantities, the need for applying a coat of non-reflective paint might be necessary.

Advantages of LST
  • Lateral motion provides heating uniformity and allows image processing capabilities that improve distinction among defect region, image noise and sound area.

  • LST is capable of large scale inspection without loss of resolution.

  • The image reconstruction procedure provides an intentional in-plane variation in heat deposition that can be used to analyze material with directional properties.
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