You may come across the expression or specification of “NETD” when you look at the technical details of a thermal camera. The expression stands for “Noise Equivalent Temperature Difference”. It is a measure for how well a thermal imaging detector is able to distinguish between very small differences in thermal radiation in the image. NETD is typically being expressed in milli-Kelvin (mK). It is also sometimes referred to as “Thermal Contrast”. When the noise is equivalent to the smallest measurable temperature difference, the detector has reached its limit of its ability to resolve a useful thermal signal. The more noise there is, the higher the NETD value of the detector.
How is NETD being measured?
In order to measure the noise equivalent temperature difference of a detector, the camera must be pointed at a temperature controlled black body. The black body needs to stabilize before starting the measurement. The noise equivalent temperature difference is then being measured at a specific temperature. It is not a single snapshot measurement, but rather a temporal measurement of noise.
How does NETD affect the measurement?
The images below show the same scene recorded by two different cameras. One camera has an NETD of 100 mK and the second has value of 200 mK. The areas in the image with very low temperature show significantly more noise in the image taken with the 200 mK camera.
NETD thermal image comparison
What affects NETD?
There are several factors that can affect NETD. Thermal cameras sometimes come with more than one calibrated temperature measurement range. The noise reading can vary based on the selected range and also the object temperature. As long as there is significant thermal contrast in the image and the temperature of interest is a lot higher than the background temperature, then this won’t affect the measurement accuracy much. The noise level can also be affected by the detector and/or camera temperature. If the camera is exposed to a high ambient temperature, the system noise may increase. This depends on how well the camera is internally stabilized. The effects of this internal temperature drift can be observed in between non-uniformity calibrations or ‘NUCs’, which can be several minutes apart.