Spectral emissivity is the ratio of photonic emissive power of a surface at a given temperature to that of a black body at the same temperature and with the same surroundings. Therefore values range from zero, for no emissivity, to one for a black body. The quoted wavelength of emitted radiation in these tables is in the near infrared at 0.65∞m, but obviously, most compounds emit a range of wavelengths when heated, and the emissivity is slightly dependant upon temperature, increasing with temperature. Since, in these tables, the values for different elements are quoted at different temperatures, they are not directly comparable. Spectral emissivity is also highly dependant upon surface roughness, a highly polished metal surface can have an emissivity as low as 0.01. It will be noted that silver (1000 k) has the lowest emissivity quoted at 0.05 (1000 K), followed closely by gold at 0.15. Titanium has one of the highest emissivities at 0.6 (1000 K). Carbon black has an emissivity close to unity. The emissivity of certain metal oxides, notably calcium oxide, thoria, ceria, zirconia, and beryllia, have a high emissivity that decreases with temperature in a similar way to alumina. A mixture of 99% ceria and 1% thoria is used in gas mantles for tilley lamps, etc.
The total radiated power from a surface per unit area depends upon the emissivity and greatly upon the temperature, by the equation, Pr=σ≤Të. Thus a black body at 300 ░Kelvin radiates a total power (into absolute zero) of 462 Watts per square metre area, where ≤ is the Stefan-Boltzman constant equal to 5.7 x 10éì Watts per square metre per ░Kë, and σ is the emissivity.
