Forehead temperature readings are a crude but useful surface temperature measurement because the forehead has less insulation between the skin and the brain compared to other areas.

“So when we're walking into restaurants and other places nowadays, and they're shining the laser at our forehead, that's probably giving a pretty crude readout of temperature. - It is, but there's much less insulation between your brain and your forehead skin than there is between your biceps and your arm skin. So if you're going to measure a surface temperature, that's where you would do it.”