The June 2006 Firehouse Magazine thermal imaging training article included a basic glossary of TI terms used on various specification sheets. Below, you will find additional terms listed and defined in an effort to help you understand what the manufacturers are trying to say. This will make you a more informed user and buyer of your thermal imaging equipment.
Background Noise: Background noise (infrared technology manufacturers use more complex terms) is naturally present in an infrared detector. It is generated naturally by the scene being viewed as well as the detector itself. Manufacturers work hard to mask this with software, but it may occasionally appear as image graininess in very bland scenes (scenes with little temperature difference). There is a balancing act in the software, as filtering too much background noise reduces sensitivity, while allowing too much background noise generates grainy images. Microbolometers adjust their gain levels when faced with dramatic changes in background noise.
Chopper Wheel: This is a small wheel that rotates approximately 60 times per second in front of a BST-based infrared detector. The wheel has a spiral cut out that partially blocks different portions of the detector momentarily as the wheel spins. This process generates fluctuation in the amount of infrared energy reaching the detector, allowing it to create an accurate thermal image. Very simplistically, this is the BST "reset" or "calibration" system, equivalent to the shutter on a microbolometer.
Distance to Spot Ratio: This term indicates the ratio of a measurement area to the distance from an object. Thermal imagers that have surface temperature measurement capabilities take temperature readings from an area that grows in diameter and size as the TI is moved further from the surface. The effect is similar to shining a flashlight on a wall; the closer the light is to the object, the tighter and more focused the area viewed.
A 10:1 ratio indicates the surface temperature measurement taken at 10 feet is averaging temperatures in an area of 1 square foot. Smaller ratios (10:1) indicate less "pinpoint" accuracy than larger ratios (30:1).
Emissivity: This term generally describes the ability of an object to absorb and then radiate infrared energy. Essentially, emissivity is listed as a percentage of how much heat the object will absorb and then radiate back to the TI. This analogy illustrates the basic effect: take a t-shirt and a brick and place them in the sun all day. Both objects absorb heat from the sun over the course of the day. As the sun sets, both objects begin to cool…but the brick retains its heat much longer than the t-shirt. Both items were exposed to the same amount of heat, but they both absorb and then release heat at different rates.
Good absorbers are poor reflectors of infrared heat, while good reflectors of infrared are poor absorbers. This notion can dramatically impact the accuracy of any temperature measurement device, as well as the overall thermal image. In general, manmade shiny materials are poor absorbers of heat energy and will show artificially cold thermal signatures on the TI.
Noise Equivalent Temperature Difference (NETD): This number expresses the sensitivity of an infrared detector. It defines the smallest temperature difference the detector can differentiate, normally in degrees milliKelvin (mK). An mK is 1/1000th of a degree Celsius. The lower the number, the more sensitive the unit is. Units with high levels of sensitivity (lower NETD) tend to produce better, more defined images, especially in bland scenes. This can be a complex term to understand as manufacturers measure it differently. What's more, the gain mode and scene dynamics create substantial fluctuations in the resulting NETD value.