Your THERMOGRAPHY COMFORT LEVEL – Emissivity? What is it? Is it important?
Every target surface that you point your infrared camera at has a surface characteristic of emissivity. How’s your comfort level with that subject? I talked about this a little in blog #015 (Radiosity! What is it?) As you might remember, there are three components to radiosity: (1) emissivity (2) reflectivity (3) transmissivity. All three have to equal one. So, if the emissivity is 0.95 and the transmissivity is 0.00 the reflectivity is what? Yes, 0.05.
How important is it to understand emissivity? Well, how important is the accuracy of your temperature reading? Your infrared camera, if it is a radiometric camera, gives you a surface temperature reading, right? Let’s say your IR camera is in perfect calibration and the reading is minus 3° F. Is that an accurate temperature? Maybe! Maybe not! The target surface characteristic called “emissivity” has a major influence on it. When you hear the word “emissivity”, what comes to mind? The thing that comes to my mind is the word “emit”. Target surfaces vary in the amount of infrared radiation that they “emit”. The higher the emissivity of the surface, the more infrared radiation it emits. The more it emits the more accurate the temperature reading from you camera. Please notice that I have emphasized the word “surface”. Why? Because an infrared camera reads only the infrared radiation that is emitted from the “surface”. For example, on a painted wall the camera reads the infrared radiation emitted from the last coat of paint, not anything below that. Polished chrome, for example has a very, very low emissivity of 0.05 (high reflectivity). Paint has a very, very high emissivity of 0.95 (low reflectivity). What, then, is the emissivity of painted chrome? It’s the same as the emissivity of “paint” because that is what constitutes the surface. Notice the same image I used in blog #015, the image of the car with a chrome surround on the windshield.
The ambient temperature is around 110° F. The painted door panel is 144.1 ° F. The polished chrome surface is reading -3° F. The paint, due to the high emissivity of paint (.95), is giving us a relatively accurate temperature surface temperature of 144.1° F. The polished chrome, with its very low emissivity (.05), is giving us a very inaccurate temperature of minus 3° F.
A good question to ask yourself when pointing an infrared camera at a surface is: “What is the emissivity of this material?” If you are involved in moisture investigations or building envelope investigations you are most often viewing high emissivity surfaces. Be careful about aluminum HVAC surfaces, though. If you are involved with industrial, manufacturing, mechanical investigations you will have to be more aware of the varying surface emissivities. In the next blog I will address how you can change the emissivity of a surface to achieve accuracy.