The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is light, but for objects near room temperature this light is infrared and isn't visible to human eyes. The thermal radiation from very hot objects (see photograph) is easily visible to the eye. Quantitatively, emissivity is the ratio of the thermal radiation from a surface to the radiation from an ideal black surface at the same temperature. The ratio varies from 0 to 1. At room temperature, the surface of a black object emits thermal radiation at the rate of 418 watts per square meter; real objects with emissivities less than 1.0 emit radiation at correspondingly lower rates.
Emissivities are important in several contexts.
- insulated windows - Warm surfaces are usually cooled directly by air, but they also cool themselves by emitting thermal radiation. This second cooling mechanism is important for simple glass windows, which have emissivites close to the maximum possible value of 1.0. "Low-E windows" with transparent low emissivity coatings emit less thermal radiation than ordinary windows. In winter, these coatings can halve the rate at which a window loses heat compared to an uncoated glass window.
- solar heat collectors - Similarly, solar heat collectors lose heat by emitting thermal radiation. Advanced solar collectors incorporate selective surfaces that have very low emissivities. These collectors waste very little of the solar energy through emission of thermal radiation.
- planetary temperatures - The planets are solar thermal collectors on a vast scale. The temperature of a planet's surface is determined by the balance between the heat absorbed by the planet from sunlight and the thermal radiation emitted by the planet back into space. The emissivity of a planet is determined by the details of its surface and of its atmosphere.
- temperature measurements - Pyrometers and infrared cameras are instruments used to measure the temperature of an object by using its thermal radiation; no actual contact with the object is needed. The calibration of these instruments involves the emissivity of the surface that's being measured.
Emissivities of common surfaces
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. The apparatus compares the thermal radiation from a surface to be tested with the thermal radiation from a nearly ideal, black sample. The detectors are essentially black absorbers with very sensitive thermometers that record the detector's temperature rise when exposed to thermal radiation. For measuring room temperature emissivities, the detectors must absorb thermal radiation completely at infrared wavelengths near 10×10-6 meters. Visible light has a wavelength range of about 0.4 to 0.7×10-6 meters from violet to deep red.
|Glass, smooth (uncoated)||0.95|
|Marble (polished)||0.89 to 0.92|
|Paint (including white)||0.9|
|Paper, roofing or white||0.88 to 0.86|
|Snow||0.8 to 0.9|
- These emissivities are the "total hemispherical emissivities" from the surfaces. The term emissivity is also used for "directional spectral emissivities" that describe thermal radiation emitted near specific wavelengths and at specific angles to the surface.
- The values of the emissivities apply to materials that are optically thick. This means that the absorptivity at the wavelengths typical of thermal radiation doesn't depend on the thickness of the material. Very thin materials emit less thermal radiation than thicker materials.
Emissivity and absorptivity
There is a fundamental relationship (Gustav Kirchhoff's 1859 law of thermal radiation) that equates the emissivity of a surface with its absorption of incident light (the "absorptivity" of a surface). Kirchhoff's Law explains why emissivities cannot exceed 1, since the largest absorptivity - corresponding to complete absorption of all incident light by a truly black object - is also 1. Mirror-like, metallic surfaces that reflect light well thus have low emissivities, since the reflected light isn't absorbed. A polished silver surface has an emissivity of about 0.02 near room temperature. Black soot absorbs thermal radiation very well; it has an emissivity as large as 0.97, and hence soot is a fair approximation to an ideal black body.
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. Thus white paint absorbs very little visible light. However, at an infrared wavelength of 10x10-6 meters, paint absorbs light very well, and has a high emissivity. Similarly, pure water absorbs very little visible light, but water is nonetheless a strong infrared absorber and has a correspondingly high emissivity.
Directional spectral emissivity
In addition to the total hemispherical emissivities compiled in the table above, a more complex "directional spectral emissivity" can also be measured. This emissivity depends upon the wavelength and upon the angle of the outgoing thermal radiation. Kirchhoff's law actually applies exactly to this more complex emissivity: the emissivity for thermal radiation emerging in a particular direction and at a particular wavelength matches the absorptivity for incident light at the same wavelength and angle. The total hemispherical emissivity is a weighted average of this directional spectral emissivity; the average is described by textbooks on "radiative heat transfer".
Emissivity and emittance
The term emissivity is generally used to describe a simple, homogeneous surface such as silver. Similar terms, emittance and thermal emittance, are used to describe thermal radiation measurements on complex surfaces such as insulation products.
- The Stefan-Boltzmann law is that the rate of emission of thermal radiation is σT4, where σ=5.67×10-8 W/m2/K4, and the temperature T is in Kelvins. See Trefil, James S. (2003). The Nature of Science: An A-Z Guide to the Laws and Principles Governing Our Universe. Houghton Mifflin Harcourt. p. 377. ISBN 9780618319381.
- "The Low-E Window R&D Success Story". Windows and Building Envelope Research and Development: Roadmap for Emerging Technologies. U.S. Department of Energy. February 2014. p. 5.
- Fricke, Jochen; Borst, Walter L. (2013). Essentials of Energy Technology. Wiley-VCH. p. 37. ISBN 978-3527334162.
- Fricke, Jochen; Borst, Walter L. (2013). "9. Solar Space and Hot Water Heating". Essentials of Energy Technology. Wiley-VCH. p. 249. ISBN 978-3527334162.
- "Climate Sensitivity". American Chemical Society. Retrieved 2014-07-21.
- Siegel, Robert (2001). Thermal Radiation Heat Transfer, Fourth Edition. CRC Press. p. 41. ISBN 9781560328391.
- For a truly black object, the spectrum of its thermal radiation peaks at the wavelength given by Wien's Law: λmax=b/T, where the temperature T is in degrees Kelvin and the constant b≈2.90×10-3 meter-degrees. In Kelvins, room temperature is about 293 degrees. Sunlight itself is thermal radiation originating from the hot surface of the sun. The sun's surface temperature of about 5800 degrees Kelvin corresponds well to the peak wavelength of sunlight, which is at the green wavelength of about 0.5×10-6 meters. See Saha, Kshudiram (2008). The Earth's Atmosphere: Its Physics and Dynamics. Springer Science & Business Media. p. 84. ISBN 9783540784272.
- Brewster, M. Quinn (1992). Thermal Radiative Transfer and Properties. John Wiley & Sons. p. 56. ISBN 9780471539827.
- 2009 ASHRAE Handbook: Fundamentals - IP Edition. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers. 2009. ISBN 978-1-933742-56-4. "IP" refers to inch and pound units; a version of the handbook with metric units is also available. Emissivity is a simple number, and doesn't depend on the system of units.
- "Table of Total Emissivity". Table of emissivities provided by a company; no source for these data is provided.
- "Influencing factors". evitherm Society - Virtual Institute for Thermal Metrology. Retrieved 2014-07-19.
- "ASTM C835 - 06(2013)e1: Standard Test Method for Total Hemispherical Emittance of Surfaces up to 1400°C". ASTM International. Retrieved 2014-08-09.
- Kruger, Abe; Seville, Carl (2012). Green Building: Principles and Practices in Residential Construction. Cengage Learning. p. 198. ISBN 9781111135959.
- "Spectral emissivity and emittance". Southampton, PA: Temperatures.com, Inc. An open community-focused website & directory with resources related to spectral emissivity and emittance. On this site, the focus is on available data, references and links to resources related to spectral emissivity as it is measured & used in thermal radiation thermometry and thermography (thermal imaging).