  We present a scheme for estimating the noise-equivalent temperature
difference (NETD) of frequency upconversion detectors (UCDs) that detect
mid-infrared (MIR) light. In particular, this letter investigates the frequency
upconversion of a periodically polarized crystal based on lithium niobate,
where a mid-infrared conversion bandwidth of 220 nm can be achieved in a single
poled period by a special design. Experimentally for a temperature target with
a central wavelength of 7.89 {\mu}m in mid-infrared radiation, we estimated the
NETD of the device to be 56 mK. Meanwhile, a direct measurement of the NETD was
performed utilizing conventional methods, which resulted in 48 mK. We also
compared the NETD of our UCD with commercially available direct mid-infrared
detectors. Here, we showed that the limiting factor for further NETD reduction
of our device is not primarily from the upconversion process and camera noise,
but from the limitations of the heat source performance. Our detectors have
good temperature measurement performance and can be used for a variety of
applications involving temperature object identification and material structure
detection.
