IR Camera Measurement Tools and Parameters: What do They Mean?
Often, a newcomer to infrared thermography will power on their camera and the first time they open the menu they see a list of settings, tools, and parameters that can be adjusted, enabled, or set and it can be quite intimidating. A little knowledge can go a long way towards determining which of these tools and parameters can be useful to you, which ones to be careful of, and what kind of situation to use them in.
Most IR cameras have a group of adjustable settings on the camera that are usually referred to as measurement parameters. This is because these settings affect the temperature measurement in your thermal images, they do not affect the visual quality of the images. The most common measurement parameters found are emissivity, background (or reflected temperature correction RTC), and transmission. All three of these settings can be adjusted on the camera or on saved/stored images in the software.
Emissivity: To a thermographer, emissivity means two things. One is the actual definition of the term itself. Emissivity is a material property that describes a material’s ability to emit (or radiate) thermal energy in proportion to its surface temperature. It’s a value between 0 and 1, or if you’re considering it as an efficiency factor, between 0 and 100%. When emissivity values are assigned to a material, that value is a comparison to what is known as a “blackbody”, or a perfect thermal emitter. If I were to say that human skin for example has an emissivity of .98, that is compared to a black body at the same temperature and wavelength. The other meaning to thermographers when we hear the term “emissivity” is the correction factor in our imagers. Cameras that allow you to adjust emissivity will either let the operator pick a value between 0 and 1 or will provide the operator with preset emissivity tables that give manufacturer prescribed values for common materials. These emissivity tables are best used as guides only. A quick online search will yield a multitude of different emissivity tables, each with different values and material descriptions, so beware!
We advise Level 1 - Thermographic Applications attendees to resist the temptation to attempt to adjust this measurement parameter in the field. The reason for this is simple; it is nearly impossible to know the actual emittance of a surface, especially based only on the type of material in question. If you don’t know the precise emittance value for a material, why attempt to adjust your imager to account for it? Low emissivity surfaces, such as bare metals, emit very little energy in proportion to their surface temperature. We demonstrate this at infinitum in both Level 1 and Level 2 courses. Most of what we see on this type of surface thermally, are the thermal reflections of anything and everything around us making it very difficult to obtain reliable temperature measurements. Changing the emissivity parameter will have a significant effect on your temperature measurements. It’s generally a good idea during an inspection to leave the emissivity setting on your camera up high, .90 or better, and try to find high emissivity for qualitative comparison. Although the emissivity setting on your camera can be used to calculate the emissivity of a surface, the procedure is not something that can be done practically in the field. There is an ASTM standard describing the technique, ASTM C1371, and it can be purchased at astm.org. Emissivity can also be adjusted on a saved image in the software. Any changes made will affect the temperature measurement in the image.
Background (RTC, T Ambient, etc.): Much like emissivity, the term “background” has more than one meaning to thermographers. “Background” is the term we use to describe thermal energy sources that are reflected from the surfaces we’re attempting to inspect and measure. No surface is a perfect emitter, so even materials that are very emissive will have some reflection from them, even if the amount is so small it’s not readily apparent when viewing the surface through our imager. Background is also what many camera manufacturers call the correction factor that is intended to correct for the influence of reflected energy when attempting to measure surface temperature. This correction factor goes hand in glove with emissivity in the summing algorithm that imagers use to assign a value to detected surface radiation. Thermal reflections can come from virtually anywhere, above, below, or to either side of the object being inspected, so quantifying the value of reflected sources in the field is difficult at best. Rather than constantly adjusting the background setting on your camera, it’s usually easier to try to avoid extreme sources of hot or cold such as direct sun, hot lights, cold sky, etc.
Transmission: This setting is only adjusted if the thermographer is looking through another material, such as an IR window or a plastic bag, that is between themselves and the target. There are very few common materials that are transmissive to IR radiation in midwave or longwave, so the majority of the time we are looking at what we call thermally opaque surfaces. We cannot see through them with our IR cameras. Any time we do look through a material with our IR camera, there will be some attenuation occurring. We will not get reliable temperature measurements while inspecting through an IR window unless we adjust the transmission setting on our camera. Even then measurement accuracy can be influenced by the temperature of the transmissive material (secondary optics).
The measurement parameters discussed above will all change our temperature measurement in our thermal images, whether we change them on the camera or in the software. Measurement tools do not change our temperature measurements, but they provide us with different methods to report temperatures or visually illustrate anomalies in our images.