One of the most difficult concepts for most new thermographers to grasp is the relationship between emissivity and reflectivity. Of the many technical inquiries posed to us at The Snell Group, I would wager the majority of them are on this topic. I know when I teach the Level I or Level II courses, I spend a great deal of time driving this concept home because it’s an essential precept one must grasp in order to understand the reliability of your temperature measurements in the field.
The questions I see most ask about emissivity and without any mention of reflectivity. I would assert one couldn’t discuss one concept without discussing the other because they go together like peas and carrots or Jenny and Forrest. In Level I, we discuss how thermographers can generally categorize all surfaces we inspect as being in some part reflective and part emissive. That is, assuming the surface in question is thermally opaque (meaning infrared radiation will not pass through it). However, what we inspect is in fact thermally opaque so these surfaces are part reflective and part emissive. For whatever reason, people fixate on E rather than R. However, you can’t do that because they go together.
If we were to talk about a particular surface that’s its emissivity is .85, we are saying that its reflectivity is .15. Remember the RAT formula we work with in our Level I infrared course. In most instances T=0, so we’re down to this relationship between E and R. You can’t discount R because it plays a part in temperature measurement (like E does). Emissivity is the ratio of a surface’s ability to radiate relative to its temperature compared to a perfect radiator. In the above example, we’re saying this surface is essentially an 85% efficient radiator. What we cannot ignore is the fact it is also a 15% efficient reflector. As important as the ability to radiate is to reliable temperature measurement, the ability of a surface to reflect has impact as well. These two factors are especially important when dealing with low emissivity (high reflectivity) surfaces.
A surface with low emissivity doesn’t radiate well. When we try to glean temperature data, it doesn’t tell us the truth. In addition to not telling us the truth, the low E (high R) surface will lie to us. No, those aren’t the same thing. If my wife is mad at me and I ask her, “Are you mad at me?” and her response is “No,” she’s not necessarily telling me the truth. If she’s mad at me and I ask her and her response is “Jim, I couldn’t possibly be more thrilled with you than I am at this moment,” Now that’s telling me a lie.
Low E or high R surfaces will do the same thing. Not telling me its true temperature because it can’t radiate is not telling me the truth. Reflecting an energy source from somewhere else is telling me a lie. Not only will a low E or high R surface not allow a reliable radiometric measurement of its temperature, but it will reflect other sources, hot or cold, in its thermal background leading my imager to believe the surface is a different temperature than it truly is. Therefore, not telling me the truth and lying to me.
Be cautious of Low E/High R surfaces you encounter in the field and remember when you’re considering emissivity you must also consider reflectivity. They go together and can both bite you in the field if you’re not careful.