https://www.flir.com/news-center/camera-cores--components/fl...
Thermal sensors of this type are huge and expensive. At work I also have a Flir with 1024x768 sensor, super expensive with a massive germanium (or something) lens. One of the problems for handheld applications is that you always need two separate optical paths since the lensing is totally incompatible. So you end up having to register the two images to each other, which can fail when it's darkish or the image doesn't have a lot of edges.
https://www.flir.com/products/lepton/
Scroll down for full specifications.
Key property is also thermal sensitivity, which for Lepton 3.5 is <50mK. Better sensitivity and you will be able to see finer gradients or need less temperature gradient to observe.
Lepton 3.5 professional grade camera 1440 x 1080 HD output with VividIR MSX linear overlay from visual cam Measurable range: -20°C to 400°C