Thermal radiation is the transfer of energy by electromagnetic radiation emitted because of the thermal state of matter. Unlike conduction and convection, thermal radiation does not require a material medium and can therefore travel through vacuum. All objects with a temperature above absolute zero emit electromagnetic radiation, although the wavelength distribution and intensity depend on temperature and material properties.
At ordinary terrestrial temperatures, much of the thermal radiation emitted by objects lies in the infrared part of the electromagnetic spectrum, which is why thermal imaging systems commonly detect infrared radiation. Hotter objects generally emit more total thermal radiation and shift their characteristic emission towards shorter wavelengths. Thermal radiation is therefore a fundamental heat transfer mechanism and a direct consequence of the electromagnetic behaviour of matter. It plays major roles in climate science, astronomy, engineering, thermometry, energy systems and everyday heating.
The idealised concept of a blackbody is central to thermal radiation theory. A blackbody is an ideal object that absorbs all incident electromagnetic radiation and emits the maximum possible thermal radiation for a given temperature. Its spectral distribution is described by Planck's law. The wavelength at which blackbody emission is greatest decreases as temperature increases, as described by Wien's displacement law.
The total power emitted per unit area is proportional to the fourth power of absolute temperature according to the Stefan Boltzmann law. These relationships explain why a room temperature object emits primarily infrared radiation, why a hot metal object can eventually glow visibly, and why very hot astronomical objects emit substantial amounts of shorter wavelength radiation. Real materials are not perfect blackbodies, so their emissivity modifies their emission relative to the blackbody ideal.
Emissivity describes how effectively a real surface emits thermal radiation compared with an ideal blackbody at the same temperature and wavelength under the relevant conditions. It can depend on wavelength, direction, surface condition, temperature and material composition. Shiny metals often have relatively low emissivity in certain infrared wavelength ranges, while many nonmetallic surfaces have higher emissivity. A surface can therefore appear visually dark or shiny while having very different infrared properties.
Thermal cameras must account for emissivity when converting detected infrared radiation into estimated temperature. Reflected environmental radiation can also affect measurements, particularly for low emissivity surfaces. Consequently, thermal radiation is not simply a direct measurement of temperature independent of material properties. Accurate radiometric temperature measurement requires consideration of emission, reflection, transmission, atmospheric absorption and instrument calibration.
Thermal radiation can propagate through vacuum because it consists of electromagnetic fields rather than mechanical disturbances in matter. This is why the Sun can transfer enormous quantities of energy to Earth across the vacuum of space. Solar radiation travels through space and is absorbed by Earth's atmosphere, oceans, land and living organisms. The absorbed energy is then redistributed through conduction, convection and other processes. Earth itself emits thermal infrared radiation back towards space.
The balance between incoming solar radiation and outgoing terrestrial radiation is fundamental to planetary climate. Greenhouse gases absorb and emit infrared radiation at particular wavelengths, altering the efficiency with which thermal radiation escapes to space. The resulting radiative processes are part of the physical basis of the greenhouse effect and planetary energy balance.
Thermal radiation differs from visible light mainly in wavelength and context rather than in fundamental physical category. Visible light is electromagnetic radiation, and infrared thermal radiation is also electromagnetic radiation. The distinction between thermal radiation and other electromagnetic radiation concerns the origin and spectral distribution rather than a separate kind of wave.
A hot object can emit visible radiation if its temperature is sufficiently high, while a cool object emits mostly longer wavelength infrared radiation that is invisible to human eyes. Thermal radiation can include a range of wavelengths depending on temperature and material properties. Consequently, saying that thermal radiation is always infrared is an oversimplification. At high temperatures, significant ultraviolet, visible and other shorter wavelength emissions can occur.
Radiation heat transfer depends on geometry and the temperatures and properties of exchanging surfaces. Two objects can exchange electromagnetic energy in both directions simultaneously, but if one is hotter, the net radiative heat transfer is generally from the hotter object towards the cooler one. The environment also emits radiation towards an object, so an object is not simply radiating energy into nothing. Radiative exchange can be altered by surface coatings, geometry, view factors and intervening materials.
In engineering, radiation can be particularly important at high temperatures because of the fourth power dependence in the Stefan Boltzmann relationship. Furnaces, spacecraft, thermal shields, infrared heaters and industrial processing systems therefore require careful radiation analysis. In low temperature environments, radiation can become a dominant heat transfer mechanism when conduction and convection are suppressed.
Thermal radiation is also an important scientific measurement tool. Infrared thermography detects emitted and reflected infrared radiation to create thermal images. Remote sensing satellites measure electromagnetic radiation from Earth's surface and atmosphere to estimate temperature, vegetation properties, atmospheric composition and other quantities. Astronomy relies heavily on thermal emission to determine temperatures and physical properties of stars, planets, dust clouds and other objects. Spectroscopy can reveal chemical composition because atoms and molecules interact with electromagnetic radiation at characteristic wavelengths.
Blackbody radiation theory was historically important in the development of quantum physics because classical physics could not correctly reproduce the observed high frequency spectrum of thermal emission. Planck's introduction of energy quantisation provided the correct spectral distribution and became one of the foundations of modern quantum theory.
Thermal radiation should be distinguished from heat itself. Heat is energy transferred between systems because of a temperature difference, whereas thermal radiation is one mechanism through which electromagnetic energy can be transferred. A body can simultaneously receive and emit radiation, conduct heat through contact and exchange heat through convection with a surrounding fluid. Radiation does not require contact and does not require an atmosphere. It is affected by wavelength, emissivity, temperature, geometry and intervening materials.
The physical laws of thermal radiation connect macroscopic temperature with microscopic electromagnetic emission, making the phenomenon important from household heating to stellar physics. Its ability to cross vacuum is also the key reason thermal energy from the Sun can reach Earth and why objects in space can lose energy by radiation even when conduction and convection through surrounding vacuum are negligible.
