Just cause everyone is getting all technical
. Getting in my way back machine from high school...... Infrared spectrum covers everything from near IR (just beyond human visiable range, many animals and insects see in this range) to far IR (heat, some snakes can see in this range). The IR spectrum is quite a big larger than the human visable range we see in.
Active IR is just a spotlight beyond visable spectum with a filter to the human eye to see it. Been used since the end of ww2.
Far IR is heat, yes the modern thermal sight. Been used since the 80's on some western tanks and ifv's.
Thank you for letting me play geek.
One last thing, dont get any of this confussed with light amplification stuff. However often times they will be able to pick up IR spotlights, and some have "IR" illuminators. Or you could play with you remote control from your tv.