Emittance and Absorptance

Consider radiative exchange between a real surface of area at temperature T1 with black surroundings at temperature T2. The net radiant interchange is given by 

For a gray surface 6i = aw = Ex- A selective surface is one for which C, / .A exhibits a strong dependence on wavelength. If the wavelength dependence is monotonic, it follows from Eqs. (5-107) to (5-109) that c, and (X, 2 can differ markedly when T, and T2 are widely separated. For example, in solar energy applications, the nominal temperature of the earth is T, = 294 K, and the sun may be represented as a blackbody with radiation temperature T2 = 5800 K. For these temperature conditions, a white paint can exhibit tq = 0.9 and oti 2 = 0.1 to 0.2. In contrast, a thin layer of copper oxide on bright aluminum can exhibit E, as low as 0.12 and oq greater than 0.9. 

Total emittance is substantially proportional to absolute temperature, and at moderate temperatures e = 0.0587VrT, where T is measured in kelvins. 

5-10 Variation of absorptivity with temperature of radiation source. (1) Slate composition roofing. (2) Linoleum, red brown. (3) Asbestos slate. (4) Soft rubber, gray. (5) Concrete. (6) Porcelain. (7) Vitreous enamel, white. (8) Red brick. (9) Cork. (10) White dutch tile. (11) White chamotte. (12) MgO, evaporated. (13) Anodized aluminum. (14) Aluminum paint. (15) Polished aluminum. (16) Graphite. The two dashed lines bound the limits of data on gray paving brick, asbestos paper, wood, various cloths, plaster of paris, lithopone, and paper. To convert degrees Rankine to kelvins, multiply by (5.556)(10ty. 

5-11 Hemispherical and normal emissivities of metals and their ratio. Dashed lines monochromatic (spectral) values versus r/%. Solid lines total values versus rT. To convert ohm-centimeter-kelvins to ohm-meter-kelvins, multiply by 1(T2. 

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Although values of emittance and absorptance depend in very complex ways on the real and imaginaiy components of the refractive index and on the geometrical structure of the surface layer, the gener-ahzations that follow are possible. 

The emittance and absorptance increase with increase in grain size over a grain-size range of 1 to 200 [Lm. 

Surfaces are opaque and approach emittance and absorptance of unity. 

Refractory Materials For refractory materials, the dependence of emittance and absorptance on grain size and impurity concentrations is quite important. 

Emittance and Absorptance The ratio of the total radiating power of a real surface to that of a black surface at the same temperature is called the emittance of the surface (for a perfectly plane surface, the emissivity), designated by 8. Subscripts X, 0, and n may be assigned to differentiate monochromatic, directional, and surface-normal values respectively from the total hemispherical value. If radi-... 

TABLE 2.4. Total hemispheric emittances (and absorptances) of metals and their oxides, selected from references 42,51, and 70. Emittances of refractories and miscellaneous nonmetals are listed in chapter 4 of reference 51. 

M. Maksimovic, Z. Jaksic, Emittance and absorptance tailoring by negative re aetive index metamaterial-based Cantor multilayers. J. Opt A-Pure Appl. Opt 8(3), 355-362 (2006)... 

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