Black Body Emission

The presence of solid particles in a pyrotechnic flame can lead to a substantial loss of color purity due to a complex process known as black body radiation. Solid particles, as they are heated to high temperature, begin to radiate in the infrared region, and give a sensation of warmth if you bring such a heated object near your skin. As 

The development of color and light-producing compositions will be considered in more detail in Chapter 8. 

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Figure 9.33. Spectral black-body emissive powed45 ...

Equations similar to equation 9.158 may be obtained for each of the surfaces in an enclosure, 1 = 1,1 = 2, 1 = 3, 1 = n and the resulting set of simultaneous equations may then be solved for the unknown radiosities, qoi,qm- qun The radiation heat transfer is then obtained from equation 9.140. This approach requires data on the areas and view factors for all pairs of surfaces in the enclosure and the emissivity, reflectivity and the black body emissive power for each surface. Should any surface be well insulated, then, in this case, Qj — 0 and ... 

The intensity versus wavelength distribution according to the Planck equation for the black-body emission is used to calculate the temperature (see Fig. 12). This calculation is based on two severe approximations. The first concerns with the assumption that the system is an ideal black body, which corresponds to assuming that the emissivity e equal to 1. On the contrary, real systems are gray bodies that possess emissivity values less than 1. In addition, the e dependence on the wavelength and on the pressure is generally neglected. 

With good fuels (charcoal or active metals), potassium nitrate will burn well. Its use in colored flame compositions is limited, primarily due to low reaction temperatures. Magnesium may be added to these mixtures to raise the temperature (and hence the light intensity), but the color value is diminished by "black body" emission from solid MgO. 

The presence of incandescent solid or liquid particles in the flame will adversely affect color quality. The resulting "black body" emission of white light will enhance overall emission intensity, but the color quality will be lessened. A "washed out" color will be perceived by viewers. The use of magnesium or aluminum metal in color compositions will yield high flame temperatures and high overall intensity, but broad emission from incandescent magnesium oxide or aluminum oxide products may lower color purity. 

There are two categories of remote sensing, active and passive. Passive techniques utilise electromagnetic radiation emitted from or transmitted through the atmosphere, the radiation source being for example the black body emission from the earth s surface or solar and stellar irradiances. The most critical part of a passive remote sensing instrument is its detector. In contrast, active remote sensing systems have their own radiation source and a detector, for example, radar and lidar techniques. 

Millimeter waves can be used for both active and passive imaging systems. Active imaging systems primarily image the reflectivity of the person/scene including the effect of the object s shape and orientation. Passive systems measure the thermal (black-body) emission from the scene, which will include thermal emission from the environment that is reflected by objects in the scene (including the person). 

Passive FPAs are perhaps the most familiar imaging system architecture because this architecture is used for most optical and IR cameras. In this configuration, an array of small detectors is placed at the focal plane of a lens or reflector system. The received energy in this case is derived from thermal (black-body) emission or reflected radiation from the scene. Thermal emission near room temperature peaks in the long-wave IR... 

Reduction or elimination of fluorescence High resolution High throughput Good frequency accuracy Collect Stokes and anti-Stokes Raman simultaneously Both IR and Raman capabilities on same instrument. Absorptions in the NIR Black-body emissions in IR Lower scattering intensity due to v4 effect Difficult to detect low concentrations of impurities... 

For metallic surfaces (emissivity close to 0.1), heat transfer can be as low as 5% compared to that of black bodies (e=l). We also see that for materials such as silicon and WSix (e is about 0.6), heat transfer can be quite efficient and the radiation is about 50% of that of a black body (emissivity 1.0). 

At a difference with thermal detectors, the background noise of photoconducting detectors is frequency-dependent. If it is assumed that the photoconductor is used to detect radiation at a frequency just above its cut-off frequency z/c, the detectors with a cut-off in the near IR display a much smaller background noise than those with a cut-off at lower energies. This is because in the near IR, the black body emissivity contribution at room temperature and below is very small. 

Reconsider Ex. 8.3. For these radiation sources we wish to determine (a) the maximum monochromatic black-body emissive power and (b) the wavelength corresponding to the maximum emissive power,... 

Because the level of received black-body emission is comparable to that of the laser, special attention needs to be given to the elimination of this emission in optical diagnostics. For example, in scattering and extinction experiments, narrow-bandpass filters, combined with slits and shields and/or lock-in amplifiers with chopped signals, are common practices. ... 

At about 3700 °C the peak black body emissions are in the visible region and are more evenly distributed, thus appearing bright white. 

The form of the black-body emission spectmm cannot be explained by classical physics, and its... 

Figure 14.4 Black-body emission spectra the maxima move to shorter wavelengths as the temperature of the emitter increases...

Our consideration of the laws of radiation starts with the concept of a black body, defined as a solid that emits and absorbs (but does not reflect) radiation of all wavelengflis in certain proportions, the absolute and relative amounts governed by the Stefan-Boltzmann Law, Planck s Law, and Wien s Displacement Law, Even though the ideal black body is a theoretical concept, radiant emission from a hole in an otherwise enclosed heated cavity, such as a tube furnace, and from certain dark substances, such as carbon, oxidized iron, and the metal platinum, approximates the black body emission. 

Basically this wavelength range allows a unique probe into objects during their formation as it provides direct access to the peak of the lower temperature black-body emission from the dust which enshrouds them and to a wealth of atomic and molecular spectral lines critical in understanding their energy balance and chemical evolution. 

For a given temperature, the wavelength at which the black-body emissive power is a maximum can be determined by differentiating Eq. (4.11-1) with respect to A at constant T and setting the result equal to zero. The result is as follows and is known as... 

Here, Ebx is the black-body emissive power (Planck s law). 

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