Myths, Mysteries, and Misconceptions About Infrared Surveys Part 2 of 4

I was commenting back to the infrared camera salesman who had claimed that for qualitative thermography you can ignore emissivity and background radiation. I replied, "Very well, let's assume I have a well loaded three phase circuit where I see three warm wires, but only one has, say, a ten degree rise when compared to the others. How would you classify the problem using ISO assessment criteria, which calls the ten degree rise an Advisory level issue, would you be satisfied with that assessment without adjusting your emissivity?"

"Of course, that is the correct assessment for a ten degree rise," the camera salesman said.

"But what if the temperature of these three were approaching the maximum safe operating temperature for the specific wire? Then the ten degree rise puts the one wire over the safe operating temperature? Have you considered that? And, without correcting for emissivity, or adjusting for background radiation, perhaps all three wires were in unsafe operating conditions. A week later the wire insulation burns away, the wires short, and your factory burns down."

As shocked as I was to hear his first statement, I must admit he was more shocked to hear my reply, and well, he should have been. I don't know how many people in various industries and on ships he had sold cameras to; however, he then realized his culpability and that of the manufacture he represented in telling people the cameras were simply "point and shoot." He had forgotten to include the mind needed to interpret the results.

Emissivity is the surface condition of all solids which determines, along with its temperature, what percentage of the thermal energy that reaches its surface, emits from the surface and how much remains in the given mass.

The complementary number to emissivity is reflectivity, this also is a surface condition. At any given frequency of light, and any given temperature of the solid or liquid mass, the percent of that light incident onto the surface is reflected off that surface. The angle of incidence equals the angle of reflection. Therefore, if you as a camera operator are perpendicular to an object some of your energy will reflect back into the camera and appear as the hottest part of the image.

Since these two are additive to a thermal imager, your reflection will always look warmer because it adds to the heat radiated from the surface. Not a great problem if you recognize this reflective light you merely move out of the way. However, you have not altered the condition of the surface and made it less reflective, it is now merely reflecting something else, and that is the background radiation you will see and measure if you do not compensate for it. The question then is how do you measure background radiation? But that is another story altogether.

Perhaps hiring a thermographer is what is needed. This is no mystery at all.

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