IR thermal theory as it applies to thermal imagers and thermometers
An infrared (IR) thermometer works by measuring the infrared radiation emitted from objects. All objects emit electromagnetic radiation. Electromagnetic radiation ranges from radio waves at wavelengths of meters down to gamma rays with wavelengths smaller than atoms. The infrared range of the electromagnetic spectrum is from three-quarters of a micrometer to 100 micrometers, which is longer than visible light (0.380 to 0.750 micrometers) and shorter than terahertz radiation or T-rays (100 to 1000 micrometers). Infrared thermometers measure in a subset of the infrared range, usually 8 to 20 micrometers.
Most objects being measured are gray bodies. Gray bodies are objects that emit a spectrum similar to a black body but shifted down. This shift is because gray bodies do not emit the same amount as black bodies. Black bodies are pure emitters, their spectrum has a definite peak and this peak is used to determine the temperature. Gray bodies have this peak as well, but not as high, so a correction must be made to measure the temperature correctly. This correction is called the emissivity. Emissivity is the ratio of energy radiated by an object to the energy radiated by a black body at the same temperature. Objects will have an emissivity ranging from 0 to 1. Black-bodies have an emissivity of 1. Many IR thermometers allow the selecting of the emissivity because different objects will have different emissivities.
The IR thermometer measures the radiation spectrum of the object. By using this spectrum and the emissivity correction, it can give an accurate temperature for the object being measured. The temperature is determined by finding the peak of the spectrum and using the relation between the wavelength of the peak of the spectrum and the temperature.