Thermal reflections are the bane of thermography, there’s no question about that. When inspecting surfaces that have a low emissivity, and hence a high reflectivity, temperature measurements can be much less reliable due to sources of energy, both warm and cool, that can be reflected from the surface and into the thermal imager’s detector. As we state in The Snell Group’sLevel I – Thermographic Applications training, the thermal imager can’t tell the difference between reflected energy from a surface and energy emitted by a surface, so we endeavor to tell it which is which. This is the essential function of the background temperature correction factor in your camera.
Whether your imager calls it “background” or “reflected temperature” or something else, this correction factor exists to identify energy levels that are being reflected from the surface being inspected. While this is an important function, it does have its limitations. The largest limitation of this feature is that it is a user selected value. That means simply that you indicate what the reflected energy level is. The camera doesn’t ask you to prove that you really know what that value is, it uses whatever you enter. It’s incumbent upon the thermographer to understand the factors that impact what that value is. That, however, is a discussion for another time. Right now let’s concentrate on what your camera does with the data you give it.
You choose a value for background temperature and input it to the imager. Let’s say that you determine your reflected background energy is 72°F, for example. You enter that value into the camera, and essentially the camera runs an internal algorithm wherein it tells the camera’s processor to ignore energy into the camera’s detector that is equal to the value of the input value for background. The degree to which the processor ignores that value depends upon what value is set as the emissivity correction factor. Another topic we can discuss in detail at another time. The second largest limiting factor in the effectiveness of background correction is how this algorithm functions. If you input the example value of 72°F, that’s the energy level that the processor will ignore. Not 71.9°F, not 72.1°F, just 72°F. So, if your estimation of the source or value of the reflected energy from the surface you’re inspecting is incorrect, the correction the camera tries to make will be incorrect. Also, if conditions change from when you made your determination of the reflected energy source, your attempt at correction will be skewed.
A once common misconception with regard to background correction is that pinpointing the value of the reflected source will result in reflections being absent in your image. It doesn't work that way. The background correction factor input impacts only the temperature calculation in the imager; it doesn't remove the source of background reflection from the image. Whatever energy is present on the surface being inspected remains visible to the camera as long as the level and span are optimized. Background comes into play as the camera attempts to calculate a temperature based on the level of radiation detected from the surface.
Hopefully this clears up some of the misconceptions about this important camera feature. Use it wisely to ensure the most reliable temperature measurements during your daily inspections.