Is heat radiation only infrared radiation

Thermal radiation

Lexicon> Letter W> Thermal radiation

Definition: electromagnetic radiation emitted by warm or hot bodies

Alternative terms: infrared radiation, infrared light, radiant heat

English: heat radiation, thermal radiation

Categories: Basic Terms, Physical Basics, Heat and Cold

Author: Dr. Rüdiger Paschotta

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Original creation: May 15, 2010; last change: 03/14/2020

URL: https://www.energie-lexikon.info/waermestrahlung.html

A hot body can emit visible light; he glows. In addition to visible light, it also emits other electromagnetic radiation that is invisible to the human eye. As long as the temperature of a body does not get extremely high (well over 10,000 ° C), most of the radiated power is attributable to so-called Infrared light. When infrared light is generated by radiation from warm or hot bodies, it is also called Thermal radiation. In principle, radiation with longer wavelengths - e.g. B. Microwaves - contribute to energy transport, but only to a small extent; it is therefore sufficient to pay attention to the infrared light.

Even if a body is not hot enough to visibly glow, it can already give off considerable amounts of energy in the form of thermal radiation. The emitted power also depends on the size and nature of the surface; the more the material can absorb infrared radiation, the stronger the radiation. Therefore z. B. a radiator with a bare metal surface emit significantly less heat radiation than a painted one.

With the help of infrared cameras, heat radiation that is invisible to the eye can also be registered. Figure 3 shows an example. With the help of “false colors”, the intensity of the thermal radiation is displayed in the image, and this in turn allows conclusions to be drawn about the temperature. The color scale on the right enables the temperatures of various objects to be determined approximately. This is the principle of Thermography.

Simpler devices serve as infrared thermometers (see Figure 4, right): They determine the temperature of the object at which they are aimed by measuring the strength of the thermal radiation received.

Energy transport through thermal radiation

The thermal radiation given off by a warm or hot body transports energy. This then lacks the radiating body in the form of heat, i. H. its temperature will drop if heat is not supplied.

In a vacuum (e.g. between the sun and the earth), thermal radiation, like visible light, can propagate in a straight line without hindrance. Shadows are cast at obstacles. If bodies are partially permeable to visible light (e.g. clouds or layers of fabric), the permeability to thermal radiation is often even higher. In the air, thermal radiation spreads largely unhindered (like visible light), except that strong absorption occurs in certain wavelength ranges.

Where thermal radiation hits objects, it can be more or less absorbed, reflected or scattered, depending on the nature of the surface. (Quantitatively, this can be done by the Emissivity The absorbed energy becomes heat again. In this sense, heat can be transported by thermal radiation (which, strictly speaking, is not heat itself): Thermal energy is converted into thermal radiation, transported as such, and converted back into heat. The term is somewhat imprecise Radiant heat, since, strictly speaking, it is not about heat, but rather radiation that arises from (and transports) heat.

Some examples of energy transport through thermal radiation:

  • Most of the energy that reaches the earth from the sun is transported in the form of thermal radiation. The visible part of the sunlight, when it hits the body and is absorbed by it, also contributes to its warming, but the effect of thermal radiation dominates.
  • The principle of infrared heating is that a hot object emits thermal radiation, which then hits objects and people and thus heats them directly. (The room air plays a role in this effect no Role, although a certain additional part of the heating output is almost always transported by the circulation (convection) of warm air.) Ordinary radiators also give off heat radiation and are therefore sometimes also called Radiators (Emitter).
  • The warming effect of a campfire is based almost entirely on heat radiation. The fire also generates hot air, but this rises largely unused. The same applies to an open fireplace.
  • The filament of an incandescent lamp or halogen lamp emits approx. 5 to 10% of the electrical energy used as visible light, while the majority of the rest is radiated from heat. This is the reason for the low energy efficiency of incandescent lamps.
  • The human body exchanges a considerable amount of thermal radiation with its surroundings (see the next section).

The radiation is quantitatively due to the Stefan Boltzmann Law which applies to a fully absorbent body. The total radiated powerP. is after

Here σ = 5.67 · 10−8 W / (m2 K4) (Stefan-Boltzmann constant), A. the area of ​​the radiating body and T its absolute temperature (in Kelvin). Because of the fourth power, the radiation increases very sharply with increasing temperature.

The Stefan-Boltzmann law does not take into account the fact that every body emitting heat radiation can also absorb heat radiation, which reduces the net power emitted accordingly. For example, the walls in a room in which the temperatures are perfectly balanced do not radiate any net heat, as they absorb as much heat radiation as they emit. Similar considerations also apply to radiators (see below).

Real bodies are not entirely black bodies; they are described by an emissivity ε between 0 and 1, by which the above result must be multiplied. (If ε is strongly dependent on the wavelength, a much more complicated formula with an integral must be used.)

Example: radiation from a radiator

The from a radiator with a surface temperature of z. B. 50 ° C (= 323 K) radiated power per square meter of its surface can be calculated with the above formula to 618 W (assuming the emissivity as 1). (Of course, this does not include the release of heat into the air.) On the other hand, the entire environment also emits heat radiation, which is absorbed by the radiator. If the ambient temperature is 20 ° C, it will be 419 W per m2 absorbed, so that the net power output is reduced to 199 W. (With a lower emissivity, both the emitted and the absorbed power are correspondingly lower.) At a temperature of 60 ° C (= 333 K) a maximum of 698 W would be emitted, i.e. 279 W.

If the radiator has inwardly curved parts so that the radiation can be partially absorbed again on other parts of the radiator, only the area visible from the outside counts for the net radiation. So rib structures do not increase the net release of thermal radiation, but only increase the power released into the air by convection. If the proportion of radiant heat is to be as high as possible, ribs and tubes are dispensed with.

Thermal radiation and the human sensation of warmth

The radiation from the human body is also strong enough to play an essential role in its heat balance.

If a person stands naked in a room where the air, floor, ceiling and walls have a temperature of 20 ° C, his body loses heat just like a radiator not only through the air, but also through the emission of thermal radiation. On the other hand, the walls, floors, ceilings and all objects in the room also radiate heat, some of which hits the body again and is absorbed on it. In this way the net loss of heat is much lower. However, the radiated heat is higher because the surface of the body is warmer than the surface of the walls.

However, the net heat loss of the body increases when e.g. B. the walls are cooler - which can be in winter because uninsulated walls dissipate heat to the outside. Conversely, the body receives additional heat radiation if there are warm or hot objects in the room (such as stoves or radiant heaters) - even if the air temperature remains unchanged. In this way, depending on the strength of the thermal radiation in the room (also Radiant climate called) feel warmer or cooler. Clothing hinders the exchange of thermal radiation, but the radiation climate is still clearly noticeable.

Usually it is perceived as pleasant in winter when there is a relatively large amount of heat radiation in a room. Well-insulated outer walls contribute to this, but especially certain heating systems such as hot ovens and infrared heaters. Under certain circumstances, however, excessive infrared radiation in the vicinity of a stove can be unpleasant for the eyes; stinging of the eyes may occur.

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See also: heat, radiation, emissivity, thermography, infrared heating, radiation efficiency, radiant heater, heat wave heating, incandescent lamp, radiator exponent, vacuum insulation board
as well as other articles in the categories basic concepts, physical principles, heat and cold