Black body

A blackbody (also: black body, Planck radiator ) is an idealized thermal radiation source. Such a body absorbs incident electromagnetic radiation of any wavelength completely, and transmits the absorbed energy as electromagnetic radiation with a characteristic which is dependent only on the temperature range again. Such bodies serve as a basis for theoretical considerations as well as a reference for practical investigations of electromagnetic radiation. The term " black body " was coined in 1860 by Gustav Kirchhoff.

Properties

A blackbody absorbs incident electromagnetic radiation completely, thus light. He leaves no radiation to pass and reflects or scatters nothing. Except at the temperature of absolute zero point of the black body emits a designated as thermal radiation or black radiation is electromagnetic radiation. Intensity and spectral distribution of thermal radiation depend only on the temperature of the black body. In particular, its material properties have no effect, it is therefore for many theoretical and practical purposes the reference.

The ideal properties of a black body can be implemented only approximately, for example, in limited frequency ranges. According to the Kirchhoff's law of radiation is proportional to its absorption capacity for each real body at each wavelength and in each direction of the emissivity of thermal radiation. Since the absorptivity of the black body takes the maximum value at each wavelength, and its emissivity is at all wavelengths the widest possible. Any real bodies can therefore at any wavelength longer emit thermal radiation as a black body. It is the ideal thermal radiation source.

Intensity and frequency distribution of the light emitted by a black body will be described by means of the electromagnetic radiation be provided by the Max Planck 's radiation law. With increasing temperature shifts the maximum of the frequency distribution according to Wien's law of radiation at higher frequencies, ie to shorter wavelengths. The Stefan - Boltzmann law describes that the total radiated energy is proportional to the fourth power of the absolute temperature of the black body.

A black body emitting at a temperature of 300 K ( corresponding to a temperature of about 27 ° C) per square meter of surface of a radiation output of about 460 watts. For this temperature, the corresponding wavelength range, the eye is not sensitive and the black body appears dark. At a temperature of 5800 K ( temperature of the solar surface ), a black body emits a radiation power of 64 MW/m2. At this temperature a portion of the radiation in the visible spectral range, the body of the eye appears bright white. He is referred to as a black body radiator yet, because it absorbs all incident radiation.

The emissivity of the black body is independent of the direction of radiation ( Lambert radiator ). He radiates equally in all directions and sends completely diffuse radiation of the maximum possible strength.

Historical significance

The attempt to describe the black-body radiation theory, has contributed significantly to the birth of quantum physics. Thus, in a purely classical description diverges blackbody radiation in the UV region ( the so-called ultraviolet catastrophe ). It was not until the adoption of a quantized light field by Max Planck in 1900 was able to solve this puzzle.

Realization

An ideal black body can not be realized. There are no known materials which electromagnetic waves frequency independent fully absorb. Even the production of a body that comes close to the ideal of the black body is difficult. However, probably not at other wavelengths, - a while sooted surface has in the visible spectral range an absorption rate of about 0.96. Many non-metallic materials have in the Middle Infrared high emissivity, but may appear white in the visible (for example, wall color).

Usually only the absorption and emission properties of the radiation source is of interest, but not the form in practical applications. Therefore, the opening of a cavity radiator or just a long blind hole is used instead of a surface. Thus, the ideal characteristics of a blackbody radiator can better represent, even if the inner surfaces have a low emissivity.

Cavity radiation

In a warm cavity with walls made ​​of any non-transparent material, which are held at a constant temperature, giving the walls of the heat radiation against each other, and adjusts a radiation equilibrium. The electromagnetic radiation that meets the cavity called cavity radiation. The energy density and the frequency distribution of black body radiation does not depend on the nature of the walls from, but only on their temperature. In addition, the radiation is homogeneous, isotropic, unpolarized and the volume of the cavity independently.

One of introduced into the cavity body does not change the properties of the cavity radiation, as this reduced the radiation properties of the newly added surface and the cavity volume is independent. This is especially true for a fictitious introduced blackbody, it absorbs the radiation incident on it and entirely comes to thermal equilibrium. Thermal equilibrium of energy density, frequency distribution, homogeneity and isotropy of the blackbody radiation is retained. So the black body radiates as much energy at each frequency and in any direction from the own mission, as he absorbed from the cavity radiation. The black body radiation and the emission of the black body, therefore have the particular same energy density and the same spectrum - they are equivalent.

Thus, a substance used as radiation source cavity, the same radiative properties as a black body.

Cavity radiator

Bringing in the cavity wall has an opening, which is small enough not to disturb the thermal equilibrium remarkably, the hole absorbs the incident radiation close to ideal and the opening occurs only thermal radiation.

The radiation emanating from the opening then has the properties of a black body, when the opening is small compared to the inner volume. Here, the reflectivity of the internal cavity surface can be substantially larger than zero. From the outside incident into the cavity radiation is then reflected multiple times inside back and forth while absorbed for the most part, and only broadcast to a small residual back by reflections. Such openings appear almost completely black. To support the absorption of the cavity walls, if necessary, designed, if possible, black and rough. In practice blackbody radiators used are hollow spheres with a hole or blind hollow cylinder. Body in the blind holes can be introduced for measurement purposes. Blackbody radiators for high temperatures (eg up to 1800K ) exist inside of ceramic materials. For the determination of the thermal radiation power of the laser beams often used absorbent body in the form of hollow cones. Absorbing coatings depend on the wavelength to be measured.

Color temperature

There are basically for all body an equilibrium temperature, which occurs after some time, as long as absorption and emission remain constant.

The color temperature is a comparison value that describes after Planck's radiation law and Wien's displacement law, the intensity curve of a black body at the maximum. This intensity maximum shifts with increasing temperature to shorter wavelengths.

Incandescent lamps with a temperature of the filament of about 2700-2800 K, as the classic incandescent or halogen lamps 3100-3200 K are the maximum radiation in the near infrared. The spectral component in the visible region is a yellowish appearance. The color effect of the radiation of a thermal emitter as well as a black body can be used for the temperature determination.

At about 5500 Kelvin, the intensity maximum is located in the visible region and corresponds approximately to the bright sunlight in the clear sky. If the temperature, the maximum intensity in the ultraviolet, and reaches the area of ​​the X-ray radiation at further increased temperatures.

With increasing temperature shifts the maximum radiation intensity of a black body to shorter wavelengths, the color impression here changes from red to bluish white. The hue of a (heat) light source can be specified as a comparable temperature blackbody. This gives the color temperature of the light source. Accordingly this then also applies to other self- emitters. It is assumed that the properties not differing too much from a horror radiator.

For the visible range is valid at high temperatures, an approximation of Rayleigh and Jeans. The spectral radiance, which is the power per unit area per unit solid angle and per frequency interval is proportional to the square of the frequency.

Increasing the temperature over a given range does not affect the relative more radiation distribution in the visible impression of the color is "white". In the CIE chromaticity diagram, the "black -body- curve" ends in a point located in a very unsaturated purplish hue. This point corresponds to the color temperature "infinite".

Emissivities

The radiation of the black body depends only on its temperature - at each frequency and at the temperature in question, the largest physically possible thermal radiation power is emitted. Thus, the Blackbody radiation is suitable as a reference. The ratio of any of a surface of the thermally emitted from a black body radiation intensity is the emissivity of the surface. The emissivity is always between 0 and 1 and is wavelength-dependent in general - unless it is a horror radiator. The black body itself always has the emissivity of 1 and can therefore be used for calibration of pyrometers.

A real body usually has at different frequencies and possibly even in different Ausstrahlrichtungen different emissivities. For a complete characterization of the emissivity must be specified as a function of frequency and the beam angle.

A Lambert radiator is an independent body with directional emissivity, it radiates completely diffuse. A gray body is a body whose emissivity is the same for all frequencies. For both cases, simplifications for radiation calculations show, so that real bodies - if possible - approximately be considered as diffuse emitters and gray body.

According to Kirchhoff's radiation law is for every body the directional spectral emissivity is equal to the directional spectral absorptance. For the other integrated over the directions and frequencies emission and absorption coefficients, the equality holds only under additional conditions.

Applications

• Blackbody (here mostly cavity radiator ) and used as a radiation source or radiation standard for physical tests in interferometers ( ceramic emitters for the mid-infrared ).
• Laser power meter often use cavity absorbers for thermal or calorimetric determination of the laser beam power: raise such good absorber not only measurement accuracy but avoid dangerous scattered radiation. They are therefore used as " radiation trap ".
• In kilns, temperatures can be determined by looking through small viewing window pyrometers very accurate - the furnace chamber forms a black body radiator ( blackbody radiator ). The surface of bodies can be provided for emissivity independent temperature measurement using a pyrometer with a blind hole into which the pyrometer "looks".
• Many non-metallic materials have wavelengths which are larger than 5 microns ... about 3, a high emissivity in the range of 0.85 to 0.95. If the radiation behavior in not be determined at high temperatures ( at room temperature the thermal radiation maximum is 10 microns and thus in the relevant wavelength range ), so they often can in good approximation as a gray body, are considered at lower accuracy requirements even as a black body. That is, for example, important for measuring temperature with low temperature pyrometers or heat radiation from the heating or cooling elements: non-metallic coatings (! paint, anodizing - color any ) increase the low emissivity of bare metals in the mid-infrared to near unity, allowing accurate pyrometric temperature measurement and improve heat radiation.
• Is carbon black in a specific wavelength range, a good approximation of a black body. It reaches, however, depending on the consistency even one absorption and emissivity of about 0.96. Its emissivity, however, is almost independent of the wavelength, so that it represents a gray body in good approximation.
• Human skin has in the wavelength range 2-14 microns a relatively constant emissivity between about 0.97 and 0.98, it radiates at body temperature ( emission maximum 9.4 microns ) that is almost like a black body and absorbs virtually all the striking long-wave heat radiation from the environment ( the absorption properties in the visible spectral range, however, behave quite differently ). The pyrometric temperature measurement in the ear ( measurement wavelength mid-infrared ) finds a nearly ideal blackbody cavity radiator.
• Was the knowledge of the radiation behavior ( perceived color intensity) glowing metals is important for the processing and hardening of steel, especially at a time when they will not yet have pyrometric temperature measurement.
• The cosmic background radiation is a very good approximation a black-body radiation (more precisely, black body radiation ) with a temperature of 2.725 ± 0.002 Kelvin. Your detailed analysis is of interest for cosmology.
• According to the Stefan- Boltzmann law, the total thermal radiation energy of a black body is proportional to the fourth power of its absolute temperature. This regularity is used for radiation thermometers to determine the temperature of a body at a known emissivity. Usually only the radiation is evaluated in a certain wavelength range; Then, the emissivity has to be known in this field.
• The effective temperature of the Sun is 5777 K. The temperature that would have according to Stefan -Boltzmann law, a black body to emit the same radiation power per unit area as a given emitter is called effective temperature of this lamp. It differs from the actual temperature from the more, the less the radiator corresponding to a black body. The concept of effective temperature is therefore useful only for emitters whose radiative properties are not too different from those of a black body, ie for stars filaments. For fluorescent lamps, northern lights and other light sources with a strong line spectrum we use the term color temperature.
• The solar radiation heats the soil. The earth radiates the heat back in the deep infrared region on average at 228K with an average power of 235 W / m².
• In astronomy, stars are often approximated by black bodies, it is determined their effective surface temperature. The difference between the frequency distribution corresponding to the thermal emission and the real star spectrum gives us information about the chemical composition and properties such as the magnetic field of the star.

Color impression

The term "Black " body can lead to the erroneous assumption that all black -looking materials generally have a high absorption or emissivity in the infrared wavelength range. The " Black " in " Black Body " however, refers as generic term to the entire electromagnetic spectrum, not on a black impression in the field of human visible light. In practice this means:

• Everyone ( cold ) black body appears actually black because it absorbs all the radiation in the visible wavelength range.
• Not every black object is a black body in the sense of physical technical term, because, while it in the visible wavelength range radiation well, could absorb in the infrared but bad. Materials which have this property, for example, be used for coating of solar panels. Many black textiles appear in the near-infrared bright
• A non- black object may still absorb in the infrared wavelength range and emits radiation well, for example, white color, or window glass. Both materials have a high emissivity in the Middle infrared.