Enclosure spectral intensity

In order to derive these we will consider an adiabatic evacuated enclosure, like that shown in Fig. 5.19, with walls of any material. In this enclosure a state of thermodynamic equilibrium will be reached The walls assume the same temperature T overall and the enclosure is filled with radiation, which is known as hollow enclosure radiation. In the sense of quantum mechanics this can also be interpreted as a photon gas in equilibrium. This equilibrium radiation is fully homogeneous, isotropic and non-polarised. It is of equal strength at every point in the hollow enclosure and is independent of direction it is determined purely by the temperature T of the walls. Due to its isotropic nature, the spectral intensity L x of the hollow enclosure radiation does not depend on / and universal function of wavelength and temperature L x = L x X,T), which is also called Kirchhoff s function. As the enclosure is filled with the same diffuse radiation, the incident spectral intensity Kx for every element of any area that is oriented in any position, will, according... 

According to this, the spectral intensity of the black body is independent of direction and is the same as the spectral intensity of hollow enclosure radiation at the same temperature ... 

The enclosure schematically illustrated in Fig. 5.70 contains a homogeneous gas mixture with an absorbent component. The element dA on the surface of the gas volume, shown in Fig. 5.70, will be used for the definition and calculation of its directional spectral absorptivity a XG. The radiation emitted from dA, with the spectral intensity Lx, is weakened by absorption. Depending on the direction, the path through the gas is of different lengths, which according to (5.179) leads to varying reductions in Lx. 

Blackbody radiation is achieved in an isothermal enclosure or cavity under thermodynamic equilibrium, as shown in Figure 7.4a. A uniform and isotropic radiation field is formed inside the enclosure. The total or spectral irradiation on any surface inside the enclosure is diffuse and identical to that of the blackbody emissive power. The spectral intensity is the same in all directions and is a function of X and T given by Planck s law. If there is an aperture with an area much smaller compared with that of the cavity (see Figure 7.4b), X the radiation field may be assumed unchanged and the outgoing radiation approximates that of blackbody emission. All radiation incident on the aperture is completely absorbed as a consequence of reflection within the enclosure. Blackbody cavities are used for measurements of radiant power and radiative properties, and for calibration of radiation thermometers (RTs) traceable to the International Temperature Scale of 1990 (ITS-90) [5]. 

A 200 kW Argon arc lamp with a spectral filter (Vortek co, British Columbia, Canada) is used to irradiate the enclosure and closely simulate the entire UV-Vis ground-level solar spectra. The arc lamp is mounted on the far wall from the reactors at a minimum distance of 20 to provide uniform lighting within both reactors. Backup lighting is provided by banks of 80 1.22 m 115-W Sylvania 350BL blacklamps (peak intensity at 350 nm) moimted on the same wall of the enclosure. These provide a low-cost and efficient UV irradiation source... 

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