Sensors used in thermal remote sensing detect infrared radiation emitted from the Earth's surface considered as a blackbody. All materials at temperatures above Absolute zero (0 K, -273°C) continuously emit electromagnetic radiation. A blackbody spectrum is determined by the temperature alone. The Earth (surface temperature 300 K) has its emission peak in the infrared.
As optical systems, thermal systems may be distinguished according to the spectral sampling (multispectral and hyperspectral). The intensity of the thermal radiation emitted is lower than the intensity of the reflected solar radiation. The consequence is that the width of individual spectral bands is broader comparing with optical sensors.
Thermal infrared remote sensing is used for measurements of the Earth's land and sea surface temperature, for detection of forest fires and geological applications.