Electromagnetic radiation can be divided into different spectral ranges depending on the wavelength. The spectrum ranges from hard gamma radiation with wavelengths of 10-14 m to long-wave broadcasting with wavelengths in the kilometer range. The visible spectral range has wavelengths from 380 nm to 780 nm.
The temperature of a body can be determined on the basis of its emitted radiation in the infrared spectral range. The maximum of the blackbody radiation emitted according to → Planck's Law of Radiation is at relevant temperatures in the infrared spectral range between 0.78 μm and 14 μm wavelength. For technical reasons – such as the absorption of infrared radiation in air – the different detectors for non-contact temperature measurement work only in filtered partial areas (so-called atmospheric windows) of the total infrared spectrum.