Alongside the optical resolution of an thermal optic, its thermal resolution or thermal sensitivity also plays an important role in temperature measurement.
In English, the level of thermal sensitivity is referred to as Noise Equivalent Temperature Difference (NETD).
This value indicates the minimum temperature difference that is still resolvable by the IR camera. The NETD value, which is usually indicated in mK, is a result of spotify promotion the noise of the infrared detector, as the English term suggests. The lower the noise (and thus the NETD value) of the detector, the smaller the temperature differences that can be detected by the infrared camera.
In our industry a thermal resolution of <50 mK is sufficient for most applications and commonly found in optics from Pard, Guide and Burris. The better performers such as Conotech and Sytong have a NETD of <35 Mk and <30Mk respectively.
Put simply a lower NETD enables a higher resolution thermal image.