I use a Fluke 89A non-contact thermometer at work for checking motors and switchgear. (I'm in industrial electrician at a plastics manufacturer.) A small laser diode illumintes the spot you are reading. It then uses the infrared signal (every object gives off infrared radiation (except at Absolute Zero ) The sensor detects this radiation and translates it into degrees F or C. The hotter an object gets, the closer the infrared waves are. If you have ever seen the element of an electric stove or heater, you know the higher the temperature the more light (visible) it gives off and the higher the temp. Just because you cannot percieve the waves, doesn't mean they can be measured. Hope this answers your question.