Understanding Infrared Thermography Reading 3

In this case. however, the heat pulses and the detection intervals are
thousands of times faster. While the surface cools, the heat is conducted into
the material at a uniform rate until it reaches a thermal barrier or discontinuity,
such as a flaw. At this time the temperature at the surface is lower than that at
the discontinuity site, and a portion of the heat is conducted back to the
surface, simulating a thermal echo. The time it takes from the generation of
the pulse to the reheating at the surface, then, is an indication of the depth of
the discontinuity. The behavior of the thermal energy moving through the
material is similar in many ways to that of a wave of energy propagating
through the material and being eflected back to the surface. For this reason
the term thermal wave imaging has been adopted by some thermographers
to deseribe the process. By using diagnostic software to time-gate the return
thermal images, they can estimate the depths of flaws as well as their size
and location, often with excellent precision. The term time resolved infrared
radiometry is also used to describe the technique of selecting the image that
best indicates the detected discontinuity.
Charlie Chong/ Fion Zhang