Emissivity

The surface of a perfect black body (with an emissivity of 1) emits thermal radiation at the rate of approximately 448 watts per square metre (W/m2) at a room temperature of 25 °C (298 K; 77 °F).

[12] Emissivities ε can be measured using simple devices such as Leslie's cube in conjunction with a thermal radiation detector such as a thermopile or a bolometer.

For measuring room temperature emissivities, the detectors must absorb thermal radiation completely at infrared wavelengths near 10×10−6 metre.

[21][22] With the exception of bare, polished metals, the appearance of a surface to the eye is not a good guide to emissivities near room temperature.

Following Planck's law, the total energy radiated increases with temperature while the peak of the emission spectrum shifts to shorter wavelengths.

In addition to these two commonly applied methods, inexpensive emission measurement technique based on the principle of two-color pyrometry.

The resulting radiative emissions to space typically function as the primary cooling mechanism for these otherwise isolated bodies.

[27] Emissivities for the atmosphere and surface components are often quantified separately, and validated against satellite- and terrestrial-based observations as well as laboratory measurements.

Emissivities of most surface regions are above 0.9 due to the dominant influence of water; including oceans, land vegetation, and snow/ice.

Clouds, carbon dioxide, and other components make substantial additional contributions, especially where there are gaps in the water vapor absorption spectrum.

[30] Nitrogen (N2) and oxygen (O2) - the primary atmospheric components - interact less significantly with thermal radiation in the infrared band.

Upper and lower limits have been measured and calculated for εa in accordance with extreme yet realistic local conditions.

At the upper limit, dense low cloud structures (consisting of liquid/ice aerosols and saturated water vapor) close the infrared transmission windows, yielding near to black body conditions with εa≈1.

[31] At a lower limit, clear sky (cloud-free) conditions promote the largest opening of transmission windows.

The more uniform concentration of long-lived trace greenhouse gases in combination with water vapor pressures of 0.25-20 mbar then yield minimum values in the range of εa=0.55-0.8 (with ε=0.35-0.75 for a simulated water-vapor-only atmosphere).

These can be based upon remote observations (from the ground or outer space) or defined according to the simplifications utilized by a particular model.

[38]: 934 The concepts of emissivity and absorptivity, as properties of matter and radiation, appeared in the late-eighteenth thru mid-nineteenth century writings of Pierre Prévost, John Leslie, Balfour Stewart and others.

[42] By 1884 the emissive power of a perfect blackbody was inferred by Josef Stefan using John Tyndall's experimental measurements, and derived by Ludwig Boltzmann from fundamental statistical principles.

[43] Emissivity, defined as a further proportionality factor to the Stefan-Boltzmann law, was thus implied and utilized in subsequent evaluations of the radiative behavior of grey bodies.

For example, Svante Arrhenius applied the recent theoretical developments to his 1896 investigation of Earth's surface temperatures as calculated from the planet's radiative equilibrium with all of space.

[44] By 1900 Max Planck empirically derived a generalized law of blackbody radiation, thus clarifying the emissivity and absorptivity concepts at individual wavelengths.

Blacksmiths work iron when it is hot enough to emit plainly visible thermal radiation .
Solar water heating system based on evacuated glass tube collectors . Sunlight is absorbed inside each tube by a selective surface. The surface absorbs sunlight nearly completely, but has a low thermal emissivity so that it loses very little heat. Ordinary black surfaces also absorb sunlight efficiently, but they emit thermal radiation copiously.
Due to differences in emissivity, this infrared picture of a cold beer can shows vastly different (and incorrect) temperature values depending on the surface material. Reflections (like on the blank end of the can and the countertop) make accurate measurements of reflective surfaces impossible.
Photographs of an aluminium Leslie's cube . The color photographs are taken using an infrared camera; the black and white photographs underneath are taken with an ordinary camera. All faces of the cube are at the same temperature of about 55 °C (131 °F). The face of the cube that has been painted (black or white paint has negligible impact) has a large emissivity, which is indicated by the reddish color in the infrared photograph. The polished face of the cube has a low emissivity indicated by the blue color, and the reflected image of the warm hand is clear.
A typical spectrum of Earth's total outgoing (upwelling) thermal radiation flux under clear-sky conditions, as simulated with MODTRAN . Planck curves are also shown for a range of Earth temperatures.
A typical spectrum of infrared radiation transmittance through Earth's atmosphere. A 'window' can be seen between 8 and 14 μm that enables direct transmission of the most intense thermal emissions from Earth's surface. The remaining portion of the upwelling energy, as well as downwelling radiation back to the surface, undergoes absorption and emission by the various atmospheric components as indicated.