I should probably go ahead and throw out this spoiler alert: this tech tip is not going to suggest methods for estimating or correcting emissivities in the field. We’re going to discuss in depth why this shouldn’t be attempted. For those reading who have only been to Level I, and are anticipating Level II or other training courses, there’s no cheat code for emissivity estimation or correction. It is important however to understand the dynamic nature of surface emissivity and why correction and estimation is so hard to pinpoint.
To refresh your collective memory, the ‘Ɛ’ Word, emissivity, is the ratio of a surface’s ability to radiate energy in proportion to its temperature, compared to a blackbody at the same temperature. A blackbody is a theoretical surface that is a perfect emitter. The greater the level of surface emissivity, the more a surface radiates, and hence the more likely it would be to get reliable temperature measurements. We discuss in Level I that temperature measurements of surfaces with low emissivities (lower than 0.6 is what we specify) are quite unreliable. The question from that point is often why the capability of adjusting emissivity in the IR imager is not advisable.
Surface emissivity is very dynamic and can be impacted by many factors. The emissivity of a blackbody is assumed to be constant across wavelengths and temperatures, and when we assign emissivity values to actual surfaces, we’re treating them the same way. The term “Gray Body” is the name given to a surface that has an emissivity below 1.0 (that of a blackbody) and is assumed like a blackbody to have an even value over a spectrum of wavelengths or temperatures. Actual surfaces are what in physics we refer to as “real bodies” or “spectral bodies”, and the emissivity of a real body varies with wavelength and temperature. See the graphic above.
Surface condition can also influence emissivity, even from one spot on a surface to an adjacent spot fractions of an inch away. Even if one were able to perform an emissivity test, as outlined in the ASTM E1933, for a particular surface, that measurement would not be the same across the possible operating temperatures for that surface, nor at every wavelength to which an imager might be tuned to detect. Additionally, not every model of thermal imager is tuned to the same spectral band, even ones made by the same manufacturer. One longwave detector may have its “sweet spot” at something between 8.0 and 14.0 microns and the detector in a similar imager is its most sensitive from 7.5 to 12 microns.
Another concept to consider is the impact of reflectivity. No surface is a perfect emitter, so there’s always some amount of reflected radiant energy from every surface we may try to measure. For example, a painted surface. With the exception of metallic spray paints, most paint, regardless of color, has an emissivity of around 0.90.* Remember R+A+T=1, paint has no T, so it’s R would be 0.10. That means that 10% of the radiant energy leaving a painted surface is reflected off it from some other source. As emissivity for thermally opaque surfaces decreases, reflectivity increases, so the amount of reflected radiant energy we see with our imager increases, skewing our measurement.
It bears mention here that radiometric temperature measurement should not be dismissed out of hand due to its limitations. In some cases, there may be no other choice for measuring a surface. The key then becomes focusing on the conditions that give the best advantage for the collection of quality data. Measuring on high emissivity spots on a surface, for example where there’s dirt, grease, paint or some other coating that emits well. Where there are holes, voids or some other cavity in the surface where emission is increased, measure there.
For the majority of field inspection scenarios, a relatively high emissivity setting can be relied upon to provide temperature measurements that could be considered minimums. At settings of 0.95 or higher, we’re essentially telling the imager that the surface we’re measuring is a better emitter than it is, so the measurements will in almost any situation be lower than actual surface temperature. There are obviously exceptions to this, but few. Adjustments to your emissivity and background correction settings in your imager can be made but should only be done when the emissivity of the inspected surface and the source of background reflection can be quantified, and then only on surfaces with emissivities above 0.60.
So, there you have it. Emissivity correction is best left to inspection scenarios where all the potential variables can be known and controlled. Go do good work and Think Thermally®.
*Revised on June 7, 2019