Emissivity
The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation that may include both visible radiation (light) and infrared radiation, which is not 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 as given by the Stefan–Boltzmann law. The ratio varies from 0 to 1. 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 at room temperature (, ); all real objects have emissivities less than 1.0, and emit radiation at correspondingly lower rates. Emissivities are important in several contexts: Mathematical definitions. Hemispherical emissivity. Hemispherical emissivity of a surface, denoted "ε", is defined as where Spectral hemispherical emissivity. Spectral hemispherical emissivity in frequency and spectral hemispherical emissivity in wavelength of a surface, denoted "ε" and "ε", respectively, are defined as where Directional emissivity. Directional emissivity of a surface, denoted "ε", is defined as where Spectral directional emissivity. Spectral  directional emissivity in frequency and spectral directional emissivity in wavelength of a surface, denoted "ε" and "ε", respectively, are defined as where 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 metre. Visible light has a wavelength range of about 0.4 to 0.7×10 metre from violet to deep red. Emissivity measurements for many surfaces are compiled in many handbooks and texts. Some of these are listed in the following table. Notes: 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 radiation (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 will 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 metre, 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". Emittance. Emittance (or emissive power) is the total amount of thermal energy emitted per unit area per unit time for all possible wavelengths. Emissivity of a body at a given temperature is the ratio of the total emissive power of a body to the total emissive power of a perfectly black  body at that temperature. Following Planck's law, the total energy radiated increases with temperature while the peak of the emission spectrum shifts to shorter wavelengths. The energy emitted at shorter wavelengths increases more rapidly with temperature. For example, an ideal blackbody in thermal equilibrium at 1,273 K, will emit 97% of its energy at wavelengths below . 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.