Because it says in the directions that came with the temp gun to place a piece of tape on glass surfaces so that the infrared light doesn’t get reflected. I know for a fact that directly above my 93.5° UTH, the glass isn’t 77°. But inside the terrarium i didn’t want to put tape on the glass, so was just asking if others used tissue paper or something similar.
Maybe i’m missing something, because you do make a good point.
-Jason