You’ve heard that the higher the emissivity, the more accurate the temperature reading, right? Right! Paint has an emissivity of around 0.95. Some surfaces are more specular and some are more diffuse. Glass, for example has a high emissivity, around 0.92, but it is specular. Sand paper has an emissivity of around 0.92, but it is diffuse. So, the question comes up about the angle at which we take images. Can the emissivity of a surface be affected just by the angle at which I take an image? A good guide to follow is to point your IR camera straight at the target (perpendicular to target) and move no further than 50° from perpendicular. As you move beyond the 50° from perpendicular the target begins to change from emissive to reflective. In other words, the emissivity decreases. When emissivity decreases what happens to reflectivity? You’re right. It increases. As reflectivity increases your temperature reading is more subject to corruption (inaccuracy). Look below and follow the “glass” line. At 0° (perpendicular) you are pointing your camera straight at the glass wall. The emissivity of glass is around 0.92. At 50° from perpendicular the emissivity is still around 0.92. But notice what happens as you move closer to the wall. At 70° from perpendicular (20° from the wall) the emissivity is around 0.75. At 80° from perpendicular the emissivity is around 0.40. Remember, glass is specular (smooth) and is affected more by this angle of incidence. Sand paper would not be affected as much due to the fact that it is a diffuse surface. So, it’s important to be aware of both the angle of incidence and the consequences of the direction that you point your thermal imaging system.
Remember, too, that when you are pointing your IR camera at the “edge” of a water tower (or anything round), you have increased your angle of incidence. The emissivity of the “edge” goes down as the reflectivity of the “edge” goes up, thus resulting in corrupted temperature readings.
How’s your comfort level in thermography?
(The above comments represent my opinion).
Rod Hoff / Restoration Consultants Inc
Thermography Instructor / IR camera sales
3284 Ramos Circle, Sacramento CA 95827
toll free 888-617-3266 ext 301
Provider of Fluke TiS, TiR, TiR1, Ti, Ti25, TiR27, Ti27, TiR29, Ti29, TiR32, Ti32, TiR3, and TiR4 infrared cameras.
See DEMO www.moistureview.com/demo.html