What I can say is the temp guns work off a reflection of the light, which it then reads that reflection, using electricity and a special lens. Now, there are some guns that take into account of ambient temperature, which is the temperature surrounding the guns, some do not. So if you are using a temp gun or even a gauge that reads temps via IR, then it should really only be measuring a small distance away, usually most accurate at about a 2 inch radius. Any further and ambient temperature or even your hand can cause it to read inaccurately.
So if you are using something like that to check the temperature just keep that in mind. But most IR temps are more accurate than any other kind of reader, but really cheap ones will cause the temps to not be read correctly because they won't account for ambient temperature, which is why most people will recommend not to use it any further than 2 inches away. But, you get what you pay for. I used to work in automotive so i had purchased a $300 temp gun a few years back, it's accurate up to 20 ft and i get it calibrated every 3 months. It could be hot as hell or cold as ice and I still get good readings. But if you buy a wal mart version or something like that for about $20 don't expect it to be able to do that. It's always safe to measure at 2 inches away from where you want to measure anyways.
