One of the many principles of successfully conducting thermal NDT on components is that the thermographer must consider—and sometimes control—the external environmental influences on the materials they are inspecting. These influencers may directly affect the thermal equilibrium of the part and/or its surface, and this may have an effect on the quality of the data collected for analysis.  

A few examples to consider include ambient temperatures, wind/convection, moisture, variable thermal backgrounds, internal exothermic or endothermic reactions, surface thermal emissivity and its optical absorptivity as well as thermal flux if the object is responding to the changing external and/or internal thermal variances.

Specific examples we have encountered include:

• Parts stored outside in the sun, rain or extreme ambient and then brought inside to be inspected immediately and before they have a chance to return to their new thermal equilibrium with the surroundings or are fully dry.
• Failure to shield materials from high-intensity heat sources, such as overhead lighting fixtures, steam lines or infrared heaters.
• Components being touched or handled by relatively warmer or cooler objects.  For example, a person handling the component or a fixture with significantly different thermal properties than the component.
• Failure to account for, and deflect if possible, air currents from hitting the inspected part’s surface.
• Allowing extraneous hot or cold objects brought into the inspection area by personnel, such as a hot cup of coffee or iced drink cups.
• Materials not being properly cleaned of dirt, contaminants, and scale which can cause uneven heating.
• Failure to properly offset or block the heating source, such as heat lamps or heat gun, which can reflect off the component.
• Components that have not had enough time to fully evaporate any moisture or cleaning solvents used on the parts.
• If conducting thermal NDT in a production area, not properly shielding hot/cold areas of the production process or even personnel from reflecting off the component.

Without taking these variables into account, or better yet controlling them when inspecting components, it may prove difficult to differentiate between actual flaws and false positives. If the thermographer is using a pulse thermography hood/flash lamp configuration with the infrared camera, the hood may shield the component from extraneous hot or cold sources, thus minimizing or eliminating many of these issues. Care must still be taken, especially on parts with curvature or challenging geometries or configurations.

If you have a portable infrared camera available to you, a quick scan around the entire work area before starting to inspect a component may show you possible external thermal sources that must be accounted for or shielded during inspection. Also remember that if the part orientation is changed during inspection, or the thermographer changes their inspection angles at all to look at the material, different sources of extraneous thermal radiation may be reflecting off the surface that did not appear earlier stages of the inspection.