Check the "vision" parameter in the Unit Information Screen. It supersedes the scenario visibility (if non zero). If vision is 40 or above you're in the realm of Thermal Imagers and higher tech. Basic game implication is seeing through smoke.
Around 1958 you might find units with 5 or 10(??)vision. Probably multi reflectance multi lence + telescope effect "classical optics" systems.
Cats have excellent night vision and the don't use IR
Well at least not yet!
