Thermal radiation

Thermal radiation or thermal radiation, rare temperature radiation is electromagnetic radiation that is at its source in thermal equilibrium with matter.

Thermal radiation emitted from solids, liquids, plasmas and sufficiently large gas masses. Planck's radiation law describes the wavelength dependency (spectrum ) of the beam density, which is the maximum achievable at a given temperature. This theoretical maximum would have a perfect black body.

Because at ordinary temperatures, the maximum radiation in the infrared range, is colloquially usually meant by thermal radiation only the invisible infrared radiation. However, shifts with increasing temperature, the maximum radiation of heat radiation to shorter wavelengths, the sunlight, for example, in the visible range with streamers to the ultraviolet.

Thermal radiation is in addition to convection and heat conduction path for transferring a thermal energy in the single vacuum.

Formation

Since this is a statistical phenomenon, the heat radiation, a plurality of particles and elementary excitations are needed in the development involved - one atom has no temperature can not radiate heat. On the nature of the participating elementary radiation processes, it does not matter. For each mechanism, the emitted spectrum can be thermally. The prerequisite is that the typical energies of the mechanism are not far above the characteristic of thermal excitation energy ( is the Boltzmann constant and the temperature of the radiating body ), otherwise this mechanism would either not involved or the non-thermal excitation.

Thermal balance between radiation and matter requires a high probability that a photon will be emitted again absorbed within the body. If this applies to photons each wavelength, the wavelength dependence of the radiation mechanism functions ( such as absorption of emission ) will not affect the spectrum of the radiation. For example, a single cubic meters from the photosphere of the sun would have a distinct line spectrum (and only short and weak light). Between the spectral lines of the material has a large optical depth, but is always less than the scale height of the photosphere of about 100 km so that the radiation field is still thermally.

Even if the radiation field is thermal in the source, its spectrum may differ significantly from the outside given by Planck 's radiation law ideal shape because the coupling is wavelength-dependent, eg by the jump of the refractive index at the surface, which causes especially metals shine. Which reflects not only external radiation, but also the thermal radiation from the inside. This would not influence the spectrum, if the external radiation would also be thermally treated at the same temperature. In order to measure the spectrum of thermal radiation, but the receiver has to be colder than the source of at least ( at BOOMERanG there were 0.27 K). To keep the disruptive effect of decoupling low radiation of a large internal surface area is through a small opening was observed ( cavity radiator ).

Examples of non -thermal radiation

• In the microwave energy in the form of radiation of a frequency (2.45 GHz) is radiated, which corresponds to the spectrum of a single line. Also the fact that you actually can heat so water does not change the fact that it is not to heat radiation.
• The same also applies to a high-performance carbon dioxide laser: Even though it is possible to melt the metals and rocks with the intense light, it generates no heat radiation, but very bright light of the wavelength of 10.6 microns, which is coincidentally in the IR range. That there is no simple relation between wavelength and temperature can already be seen in the comparison with a laser pointer: Although the wavelength of about a factor of 20 is smaller and therefore each photon carries the twenty times the energy, one can thus no melting metals.
• The spectrum of an X-ray tube consists of a bremsstrahlung, and additional distinctive spectral lines at specific wavelengths. The intensity of bremsstrahlung shows a similar "hump" as the heat radiation; its shape but differs significantly from the Planck's law of radiation from and coupled with (as opposed to heat radiation ) has an upper cutoff frequency. Therefore, this is no thermal bremsstrahlung radiation.
• The spectrum of fluorescent lamps of all types, and especially sodium vapor lamps does not have the slightest resemblance to the Planck's radiation law. The material of said light generator is selected so that as much power in the visible range is emitted, and as little as possible in the infrared region. Only in this way, the desired high efficiency can be achieved. Strong deviations from a white, thermal spectrum can distort the perceived color of illuminated objects. , Cold ' looking light is rarely thermally.

Practical distinction

By comparing measurements of neighboring wavelengths can decide whether a light source " thermal" or " non-thermal " and so infer the nature of the source. The result is referred to as signature of a light source.

• As carbon dioxide laser: If you filter different wavelengths such as 9 microns, 10.6 microns and 13 microns, to measure only at 10.6 microns appreciable light output. No thermal emitter can produce such a narrow range.
• Repeating the measurement on a light bulb, you will get three results that hardly differ because of the "hump" of the Planck radiation curve is relatively flat in this area. This is a strong indication for a thermal radiator, because you can build gas discharge lamps with such great linewidth hardly. Supplementary measurements must be performed at other wavelengths in doubt.

Such comparative measurements result from the infrared seeker heads of missiles to distinguish between hot engines of aircraft and decoys, the light of which have rather the non-thermal, so colorful signature from a firework.

In radio astronomy and SETI is a constant search for non-thermal signatures: The 21 -cm line of hydrogen and the 1.35 - cm line of the water molecule are working basis for most research. The radiant heat of neighboring planets like Jupiter can be measured at the exoplanets has not yet succeeded.

Calculation

The body of a radiated heat flow can be calculated using the Stefan- Boltzmann law as follows:

In which

Intensity

With increasing temperature of a body and the intensity of its heat radiation increases dramatically (see Stefan- Boltzmann law ), and the emission maximum shifts to shorter wavelengths (see Wien's displacement law ). To illustrate some examples of bodies whose temperatures gradually decrease by a factor of 10:

• First, a " white dwarf ", ie a star with the very high surface temperature of 57,000 K. It radiates per unit area of its surface 10,000 times as much power from such as our Sun, the intensity maximum is 50 nm, which is ultraviolet radiation. The Stefan - Boltzmann law provides a radiated power of 60 MW per square centimeter - the power of a small power plant.
• Sunlight is radiated from the hot surface of 5700 K to the sun. The intensity maximum is at 500 nm in the green region of the electromagnetic spectrum. The radiated power per square centimeter is 6 kW - equivalent to the heating power for a detached house in the winter.
• Every square inch of the black surface of 570 K ( 297 ° C) hot oven emits only 1/10.000 the benefit that would radiate an equal piece of the sun's surface (see Stefan- Boltzmann law ). The intensity maximum is 5 microns, ie in the infrared.
• Every square inch of the black surface of 57 K ( -216 ° C) cold body radiates electromagnetic waves whose power corresponds 1/10.000 of the same size piece oven surface. The intensity maximum is 50 microns in the far infrared.
• In principle, nothing changes when the body is frozen at 5.7 K ( -267 ° C). The radiated power again falls by a factor of 10,000, and the intensity maximum is 0.5 mm - almost in the radar range. With very sensitive receivers of radio astronomy can be a very weak noise, the cosmic background radiation are detected.

Of these five examples for heat radiation is just one in our strict environmental usual: the hot oven. And because sending most of its radiation in the IR range, it comes to the aforementioned erroneous equation. For certain galactic nuclei the maximum of the radiation is even in the X-ray region of the electromagnetic spectrum.

Influence of various body surfaces

A strong influence on the radiated intensity also has the surface condition of the body. This is characterized by the emissivity, which is at levels close to zero and reaches its maximum at matt black surfaces. If the temperature can be determined by non-contact thermography, can be a huge error caused by misjudgment of the emissivity, as shown here.

Since the emission maximum of the thermal radiation of the earth's surface is at a wavelength 8-10 microns and happens to coincide with the Absorbtionsminimum the air, the earth's surface cools on clear nights from radiated heat into space. Above all, clouds and water vapor, to a lesser extent so-called greenhouse gases such as carbon dioxide are transparent to this radiation; reduce or prevent this cooling by reflection or response ( see also global warming, greenhouse effect). The proportions of these gases affect the temperature regime of the earth.

Of particular importance in physics is the concept of the black body, an emitter and absorber of thermal radiation, which has an emission or absorption coefficient of one. Keeps to such absorber with a thermostat in thermal equilibrium with its surroundings, it is possible to determine the thermal and non-thermal radiation power radiation sources via the heat absorption.

Thermal radiation of humans

As any other material with similar temperature, the human body emits a large part of the energy absorbed by the food through thermal radiation, here substantially infrared light again. By infrared light energy but can also be included, as can be seen for example when approaching a campfire. The difference between emitted and absorbed thermal radiation:

By reason of the Stefan- Boltzmann law to a difference in temperature between the human body and the external radiation source:

The total surface area A of an adult is about 2 m2, the emissivity ε of human skin in the infrared range is approximately 1, regardless of the wavelength.

The skin temperature T at 33 ° C, but at the surface of the clothing to measure only about 28 ° C. At an average ambient temperature of 20 ° C., a radiation loss of calculated

In addition to the thermal radiation of the body loses energy by convection and evaporation of water in the lungs and sweat on the skin. Estimates show that is emitted by thermal radiation " at rest ", ie without physical strain and with no wind, about 2/3 of the absorbed energy.

If one calculates using the Wien's displacement law, the central wavelength of the emitted IR radiation is obtained

Thermal imaging cameras for thermographic diagnosis in medicine should therefore be particularly sensitive in the range 7-14 microns.

Applications

Upon impact of thermal radiation on a body can

These three effects are quantified with the transmission, reflection, and absorption coefficients.

The absorption coefficient equals the emissivity, i.e., a light gray having a surface emission or absorption coefficient of 0.3 absorbed 30% of the incident radiation, but emitted at a given temperature with respect to a black body and only 30% of the thermal radiation.

The heat dissipation can be reduced by the use of bare metal surfaces (examples: metal layers on emergency blankets and insulation bag, mirror coatings of Dewar vessels as in thermos flasks and super insulation ).

To increase the heat radiation of a metallic body, you can provide it with a in the relevant wavelength range, " dark ", matt coating:

• Painting of radiators with color that emits good radiation in the infrared range.
• Black anodizing aluminum heatsink in the picture on the right.
• Enamelling furnace tubes and metal ovens
• Dark radiating at nuclear power sources from satellite

The color of such layers only applies to the relatively narrow range of visible light and is suitable for heat radiation of no significance.

With the help of thermal imaging cameras can detect unwanted heat losses of buildings; hidden in the masonry of hot or cold water lines can be localized quite accurately.

The body temperature of mammals is almost always higher than the ambient temperature (unless, for example, in the sauna ), which is why the thermal radiation of her body stands out clearly from the background radiation. Because some snakes have at least two pit organs with remarkably high temperature resolution of up to 0,003 ° C, they can locate their warm-blooded prey sufficiently accurate even at night. Homing systems self -directing missile weapons solve similar tasks.

History of Science

In the 18th and early 19th century was the question of how thermal energy is transferred, not yet fully understood. Among other things, there was the idea that not only heat, but also can be transmitted as cold radiation. Thus, Benjamin Thompson, who had a significant share in the development of thermodynamics, multiple experiments, the results seemed to suggest the existence of such radiation. For this purpose, he approached the ball a Thermoskops simultaneously and at equal distance from both sides of two cylinders, " one of which as much warmer, and the other was as much colder than the temperature of the ball, and when, there was no noticeable influence, Rumford believed herein the proof to find that the cold rays of equal intensity Seyen, as the heat rays. " In a presented on June 25, 1804 report, he described the repetition of the experiment, mainly as evidence, " that there is no caloric, but that this accompanying phenomena of heat and cold rays originate, although he did concede that they can bestehn in undulations of the ether which surrounds the molecules of the body and causes repulsion. "

Pierre Prévost, however, 1792 represented the view that " the more powerful heat rays hotter body overcome the weaker colder ". That is, there is only thermal radiation but no cold radiation. This proved to be correct in the next few decades out.

Gustav Kirchhoff formulated in 1859 with the Kirchhoff's radiation law, the relationship between thermal radiation and thermal equilibrium. The properties of thermal radiation were initially unknown. In particular, the Kirchhoff's law does not practical indication of how the heat radiation depends on the temperature. Finding a formula that fills this gap of knowledge, turned out to be fruitful for the progress of physics. From experiments and theoretical considerations were found with the Stefan- Boltzmann law and Wien's displacement law with certain properties of the desired formula. In 1900, an approximate formula for high temperature and a few years has been with the Rayleigh-Jeans law later found an approximate formula for low temperature with the Wien's radiation law first. Max Planck finally succeeded in uniting the statements of these laws to Planck 's radiation law for black body. In deriving this formula, Max Planck did to have intended without it, the first steps on the way to the development of quantum mechanics.