Temperature is related to energy emitted, the hotter the object the more energy that is emitted by the object. This allows for the use of a electromagnetic radiation measurement device to measure temperature.
Energy is emitted as radiation in the electromagnetic spectrum.
Frequency and wavelength are used to define the radiated energy. The intensity of the energy radiated from the body correlates to wavelength based on Plank s law. The energy absorbed by these components occurs between the sensor and the object and will show a lower temperature reading than what the target object really is. Another factor that these thermometers must be able to account for is the target objects emissivity. Emissivity is the ratio of energy radiated by the object in question to a perfect radiator known as a black box.