The next question is whether the lens is designed down to cover the 140 x 140 (or 160 x 120, both are 5mm diagonal which might not be a coincidence) or is capable of illuminating a 10mm image circle if required.
This is a good question, I've thought it before.
I think it could be checked by a similar method I used on visible light lens:
Find a material that is flat and "diffuse reflect" LWIR, similar to a paper for visible light;
Take the lens, and put it in front of the material;
Use another thermal imaging camera which is able to focus closely, to look at the material;
Adjust the distance between the lens and the reflect material, until you see a clear image on the material;
Now you can tell the imaging circle of the lens;
If the edge is not clearly cropped by the lens, you may point the lens to a wider/hotter scene, until you see the round image edge, which is the best the lens can do.
I don't have another TIC which can focus very near, so this job is leaving to you guys

edit: I guess the lens is for 140*140, maybe slightly larger than that for the sake of image quality, but no need for Flir to craft a lens covering 320*240 area.