Why Emissivity Matters with Infrared Temperature Measurements
Below is a slide illustrating how temperature is computed using infrared sensors and why the effective emissivity (ε) of the target (e.g., tyre) is so important. This is exactly why our sensors have an adjustable emissivity. Note that transmissivity (τ) increases with distance but can usually be ignored with our tyre temperature sensors provided the standoff distance (distance from tyre to sensor) is so short and τ ~ 1
A practical example is shown in the second slide, showing how non-uniformities in object emissivity results in significant temperature measurement errors. This temperature map of an Intel CPU die with local hotspot heating was captured using a $200,000 research-grade FLIR camera. The actual temperature of the CPU’s interposer (the large square with writing on it) is uniform BUT the measured temperature varies by as much as 10˚C because of non-uniformities in emissivity. Features like the die attach, labeling, and traces have higher emissivity’s, whereas the PCB substrate itself has a lower emissivity.
Fortunately, tyres have a relatively uniform emissivity but the effect on absolute temperature accuracy is still just as important. Our tyre temperature sensors are calibrated out-of-the-box for most general environments but the effective emissivity will have to be finely tuned for utmost accuracy (+/- 1˚C at best). This is necessary for ALL IR sensors, period. Engineers and companies get way too caught up in the importance of absolute accuracy, infrared radiometry is inherently inaccurate compared to contact techniques, BUT the lateral temperature distribution and time-response data these sensors provide is incredibly important when it comes to exploiting tyre grip… and there lies the importance of infrared sensors.
Emissivity, ε – Emitted infrared radiation of an object compared to that of a blackbody. Nondimensional number between 0 and 1.
Transmissivity, τ – Transmitted infrared radiation through a medium. Nondimensional number between 0 and 1.
W – Received radiation power