Paint it Black?
No, this blog isn’t about The Rolling Stones – it’s about the practice of enhancing emissivity in the field. When a thermographer is tasked with inspecting thermally reflective surfaces there are two options. Either accept that the surface doesn’t emit well—and therefore reflects very well—and therefore can’t provide reliable qualitative or quantitative data, or enhance the emissivity of the surface by adding some sort of coating.
When there is a coating applied, the emissivity of the surface doesn’t actually change, it’s just adding a layer or a new surface. When inspecting a thermally reflective surface, and you apply electrical tape for example, which many people choose, the surface to which you affix the tape isn’t magically more emissive due to the presence of the tape. The tape acts as an additional layer, a solid material in direct contact with another solid material. There is conduction through the thickness of the tape and then the tape emits at its higher rate.
Paint is the same way. Paint used on a thermally reflective surface emits at its rate, which is fairly consistent regardless of the color. In training classes we often hear the question, “If I want to increase the emissivity of a surface, shouldn’t I paint it flat black?” Actually, the color doesn’t matter when inside a building or in the shade outside. Paint is paint, unless it has metallic components like silver or aluminum paint does or some paints that include metal flake. The answer to this question is always the same, painting a surface, regardless of the color, will enhance emissivity. Due to solar radiation outside, a different situation exists with colors outside in sunshine due to broadband solar absorption or when light energy is used to excite composite materials in NDT flaw detection.
So paint it black, or purple, or green. The thing to remember when enhancing emissivity in the field is to do it safely and with prior approval. Do NOT add a coating to energized electrical components. Wait until they are de-energized and placed in an electrically safe work condition and then add your paint or tape. Care must be taken to not affect the current path with any modifications. Always check with the electrical engineering group before modifying equipment. Good luck inspecting thermally reflective surfaces, and as always, do so safely.
Think Thermally, www.thesnellgroup.com The Snell Group, a Fluke Thermal Imaging Blog content partner