Radiant flux density

Variously called, in the literature, emittance, total hemispherical intensity, or radiant flux density. 

Some specific studies on the measurement of heat losses and indoor temperatures in buildings deserve attention. In his review of the relative importance of thermal comfort in buildings, McIntyre considered that the mean radiant temperature was the most important parameter, followed closely by the "radiation vector," which is defined as the net radiant flux density vector at a given point and measures the asymmetry of the thermal radiation field in a room (97). Benzinger et al. characterized the mean radiant temperature, and asymmetric radiation fields, using a scanning plane radiometer, which maps the plane radiant temperature in a given space indoors (98). 

The hemispherical emissive power E is defined as the radiant flux density (W/m2) associated with emission from an element of surface area dA into a surrounding unit hemisphere whose base is copla-nar with dA. If the monochromatic intensity ( 2, X) of emission from the surface is isotropic (independent of the angle of emission, 2), Eq. (5-101) may be integrated over the 2k sr of the surrounding unit hemisphere to yield the simple relation Ex = nix, where >, = Ex(X) is defined as the monochromatic or spectral hemispherical emissive power. 

Note that Eq. (5-136b) is identical to the expression for the gas emissivity for a column of path length ft. In Eqs. (5-136) the gas absorption coefficient is a function of gas temperature, composition, and wavelength, that is, Kb = Kb(T,X). The net monochromatic radiant flux density at dAi due to irradiation from the gas volume is then given by... 

Radiant flux density is the radiant flux per unit area (W m-2). 

Essentially all of the energy for life originates in the form of electromagnetic radiation from the sun. In radiometric units the radiant flux density of solar irradiation (irradiance) perpendicularly incident on the earth s atmosphere—the solar constant — is about 1366 W m-2. The solar constant varies by up to 3.4% from the average due to the earth s elliptical orbit. The value given is for the mean distance between the earth and the sun (the earth is closest to the sun on January 3, at 1.471 x 10s km, and furthest away on July 4, at 1.521 x 108 km). There are additional variations in solar irradiation based on changes in solar activity, such as occur for sun spots, which lead to the 11-year solar cycle (Pap and Frolich, 1999). In Chapter 6 (Section 6.5) we will consider the solar constant in terms of the annual photosynthetic yield and in Chapter 7 (Section 7.1) in terms of the energy balance of a leaf. 

Absolute determinations of radiant flux densities or radiant fluxes need frequent and tedious recalibration. 

A units can be of interest) to obtain good precision and a low limit of detection. Since the detector actually measures the radiant flux density (intensity) in the presence or absence of analyte, the direct readout woxUd be in transmittance (P/Po) or in percent transmission (% T = lOOP/Po) or in some early models % absorption (100 - %T). Since linear relationships are easier to handle than logarithmic relationships, %T is converted into absorbance units automatically by the instrument ... 

Consider a beam of radiation of radiance L crossing a surface dS with the beam axis making an angle 0 to the normal to dS (Figure 4.6). dS projects as dS cos 0 perpendicular to the beam axis of the radiation, and the radiant flux density dF on dS is... 

The radiant flux density, or irradiance, on the surface dS is obtained by integrating the radiance over all angles ... 

FIGURE 4.6 Relation between radiance and radiant flux density. 

The spectral radiant flux density, F(X), is the radiant flux density per unit wavelength interval, expressed in watts per square meter per nanometer (W m 2 nm ). Equivalently, when considering radiation incident upon a surface, the spectral irradiance, E(X), is expressed as W m-2 nm 1. 

Sometimes the spectral radiant flux density is expressed as a function of frequency v, that is, F(v). Because the frequency v of radiation is related to its wavelength by (4.1), v = c/X, F(X) and F(v) can be interrelated. Since the flux of energy in a small interval of... 

If Foo( ) is the spectral radiant flux density at the top of the atmosphere (TOA), that at ground level on a plane perpendicular to the solar beam can be determined from an extension of (3.11),... 

Any absolute determination of the radiant flux density or radiant flux needs frequent and troublesome re-calibration due to changing sensitivity of the detector surface. 

The total radiant flux received on a unit area of a given real or imaginary surface. Also called the radiant flux density, isopynic... 

Thermal equilibrium means that a body (molecule, particle, surface) receives from its environment as much energy as it emits. The higher the temperature of the body, the higher is the radiant flux density of emitted (thermal) radiation. The percentage of absorbed radiation is called absorptivity (or absorbance) degree e (and emissivity, respectively), complementary with the reflectivity (1 - e). For each temperature the emitted radiation is linear with the absorption degree to obtain the thermal... 

The lack of interaction between radiant flux density and salinity up to 130 mol.m" NaCl (Fig. 3) also suggest that the effect was not due to chloroplast activity. 

Fig. 3 CO2 fixation rates of mesophyll cells from plants of different salinity levels as a function of radiant flux densities...

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