Emissivity: Learn how it affects your Infrared thermal imager imagery

Emissivity describes how to quantify how efficiently a surface radiates energy at a defined wavelength band and at a given temperature. The reality is that any surface above absolute zero will always radiate some energy (greater than 0%), and no surface radiates well (100%).

Very shiny metal emitters are inefficient when considering infrared radiation. That means they don't tell us the real truth about themselves! Most non-metallic surfaces (such as paint, paper, and human skin) emit more efficiently, so it's easy to establish a direct relationship between how much they radiate and the surface temperature. Remember Hot Pot?

Emissivity values ​​can be determined or measured by engineers. Note, however, that they are very specific to the type of material, the condition of the surface, and (especially for metals) the temperature of the material. We can use these values ​​not only to help us understand how the surface behaves, but in some cases to correct our radiometric measurements.

It may be helpful to think of these values ​​as percentages. Human skin with a value of 0.98 emits thermal radiation 98% efficiently, while shiny aluminum (a value of 0.10) emits only 10% of the energy. When we input these values ​​into the imager, they automatically correct the raw data which assumes 100% radiation is emitted based on the surface temperature.

As you can imagine, measurements made with extreme corrections (often unreliable for bare metal) are therefore strongly recommended to be measured only on surfaces with values ​​greater than about 0.6. On metal, the easy way is to add a high-emissivity paint or tape "target".

Once corrections are entered, changes are made to the entire image. Obviously, due to this fact, a separate correction is required for each distinct point we want to measure. While some models allow these corrections to be made in the imager itself, the good news is that they can also be made in software on stored images. All changes can be undone or "adjusted" to better match reality.

I recommend that you practice taking measurements on various surfaces that are 10 o C (18 o F) warmer or cooler than your surroundings – windows, coffee mugs, skin, etc. Measurements were taken on high emissivity targets (use electrical tape with an emissivity correction of 0.94). Also, notice what happens when you adjust the emissivity correction for these surfaces - the image doesn't change, but the corrected temperature value does! Next week, we'll talk about what is being reflected from surfaces and how to correct that point.

Don't expect everything to be immediately clear, but you should quickly find that emissivity is not as confusing as it seems. Here are two good guidelines:

• Radiant temperatures on shiny metal surfaces will be unreliable. Use high emissivity targets whenever possible.

• In almost all cases, the radiant temperature of almost any other surface will be very reliable.