Infrared radiation refers to the wavelength range of the electromagnetic spectrum between 0.78 µm and 1.000 µm – in other words beyond the red region of the visible spectrum up to terahertz radiation.
The astronomer →William Herschel discovered infrared radiation in 1800. He attempted to ascertain the temperatures of the various colors of sunlight which he had split using a prism. By moving a black-colored mercury thermometer through the colors of the spectrum, it became apparent that the temperature steadily increases from violet to red. The maximum is located in the dark region past the red end of the visible spectrum – the infrared range.
Infrared radiation is also regularly referred to as thermal radiation as in most cases, according to →Planck’s law, the highest levels of black body radiation lie within this range Measurements of infrared radiation are the basis for non-contact temperature measurement. Here, however, only the wavelength range up to around 14 µm is used.