They may work by contact (the electronic sensor is placed in the location whose temperature is to be measured, and left long enough to reach equilibrium) or by remote sensing: an infrared sensor responds to the radiation spectrum emitted from the location.
They may work by contact (the electronic sensor is placed in the location) or by remote sensing. The location's temperature is to be measured, and left long enough to reach equilibrium. An infrared sensor responds to the radiation spectrum. This radiation spectrum is emitted from the location.