Blackbody emission from

A serious problem of using an Nd YAG laser to excite FT-Raman is the difficulty of attempting to study samples at temperatures > 150°C. The thermal blackbody emission from the sample becomes more intense (broad background) than the Raman signal. The S/N ratio is lowered, and the detector becomes saturated. 

The stray light contribution is calculated as the product of the blackbody emission from the Sun multiplied by the scattering coefficient (user defined), transmitted... 

An additional surface arrangement of importance is a single-zone surface enclosing gas. With the gas assumed gray, the simplest derivation of GSi is to note that the emission from surface Ai per unit of its blackbody emissive power is Ai i, of which the fractions g and (1 - G)ei are absorbed Dy the gas and the surface, respectively, and the surface-reflected residue always repeats this distribution. Therefore,... 

Investigations on the emission properties of INSs started quite a long time ago, mainly in connection with the X-ray emission from PSRs. In the seventies it was a common wisdom that the radiation emitted by INSs comes directly from their solid crust and is very close to a blackbody. Lenzen and Trumper (1978) and Brinkmann (1980) were the first to address in detail the issue of the spectral distribution of INS surface emission. Their main result was that... 

The star in the numerical model has an inside and an outside. The outside is defined as the limit beyond which it becomes transparent. This boundary is called the photosphere, or sphere of light, for it is here that the light that comes to us is finally emitted. It is thus the visible surface of the star, located at a certain distance R from the centre, which defines the radius and hence the size of the star. The photosphere has a certain temperature with which it is a simple matter to associate a colour, since to the first approximation it radiates as a blackbody, or perfect radiator. Indeed, the emissions from such a body depend only on its temperature. The correspondence between temperature and colour is simple. In fact, the relation between temperature and predominant wavelength (which itself codifies colour) is given by Wien s law, viz. 

Another consideration in flames is radiatioiL The light that one sees in a flame is mostly fluorescence from the radiation of particular radical species formed in electronically excited states. (The blue color from CH4 flames is CH emission.) Gases also radiate blackbody radiation, primarily in the infiared. The glow from burning wood or coal is blackbody emission radiated from the surface. 

Recall that the emission observed from cracks in a burning wood surface is brighter because the emission from a cavity has the equilibrium blackbody distribution, which is independent of the emissvity of the surface. Also recall that most of the heat from a campfire... 

Clearly, 254 K is much colder than the typical temperatures around 288 K (15°C) found at the earth s surface. This difference between the calculated effective temperature and the true surface temperature is dramatically illustrated in Fig. 14.4, which shows the spectra of infrared radiation from earth measured from the Nimbus 4 satellite in three different locations, North Africa, Greenland, and Antarctica (Hanel et al., 1972). Also shown by the dotted lines are the calculated emissions from blackbodies at various temperatures. Over North Africa (Fig. 14.3a), in the window between 850 and 950 cm-1, where C02, O-, HzO, and other gases are not absorbing significantly, the temperature corresponds to blackbody emission at 320 K due to the infrared emissions from hot soil and vegetation. 

FIGURE 14-4 Infrared emission from earth measured from the Nimbus 4 satellite (a) over the Niger Valley, North Africa (14.8°N, 4.7°W) at 12 00 GMT (b) over Greenland (72.9 LN, 41.1°W) at 12 18 GMT, and (c) over Antarctica (74.6°S, 44.4°E) at 11 32 GMT. Emissions from blackbodies at various temperatures are shown by the dotted lines for comparison (adapted from Hanel et at., 1972). 

The emissivity is calculated from the measured emission by ratioing the measurement from a blackbody source at the same temperature as the sample l48). Since there is a background emission from instrumental surfaces, four measurements are often made 149,150), to remove the background emission. The measured intensity, S(v, T) at any temperature has several components... 

When an object is heated, it emits radiation—it glows. Even at room temperature, objects radiate at infrared frequencies. Imagine a hollow sphere whose inside surface is perfectly black. That is, the surface absorbs all radiation striking it. If the sphere is at constant temperature, it must emit as much radiation as it absorbs. If a small hole were made in the wall, we would observe that the escaping radiation has a continuous spectral distribution. The object is called a blackbody, and the radiation is called blackbody radiation. Emission from real objects such as the tungsten filament of a light bulb resembles that from an ideal blackbody. 

Figure 11.3 is a plot of the spectral blackbody emissive flux as a function of wavelength at various temperatures. From this figure, it is clear that at any given wavelength, the radiative energy emitted by a blackbody increases as the absolute temperature of the body increases. Each curve displays a peak, and the peaks shift toward smaller wavelengths as the temperature rises. The locus of the peaks calculated analytically by Wien s displacement rule is... 

Since the body is in radiative equilibrium, qx(T) also expresses the spectral radiative flux emitted by the body at the wavelength X. The incident radiation q (T) comes from the black walls of the enclosure at temperature T, and the emission by the walls is not influenced by the body regardless if it is a blackbody or not. Let qxb(T) be the spectral blackbody emissive flux at temperature T. Then,... 

Exponential integral of order n, where n = 1, 2,3,.. . Hemispherical emissive power, W/m2 Hemispherical blackbody emissive power, W/m2 Volumetric fraction of soot Blackbody fractional energy distribution Direct view factor from surface zone i to surface zonej Refractory augmented black view factor F-bar Total view factor from surface zone i to surface zonej Planck s constant, J s Heat-transfer coefficient, W/(m2 K)... 

The variation of the spectral blackbody emissive power with wavelength is plotted in Fig. 12-9 for selected temperatures. Several observations can be made from this figure ... 

Analysis (a) The tolal blackbody emissive power is determined from the Stefan-Boltzmann law to be... 

The gas molecules and the suspended partic e.s in the atmosphere emit radiation as well as absorbing it. The atmospheric emission is primarily due to the COj and H2O molecules and is concentrated in the regions front 5 to 8 p.m and above 13 p.m. Although this emission is far from resembling the distribution of radiation from a blackbody, it is found convenient in radiation calculations to treat the atmosphere as a blackbody at some lower fictitious temperature that emits an equivalent amount of radiation energy. This fictitious temperature is called the effective sky teniperatur Then the radiation emission from the atmosphere to the earth s surface is expressed as... 

A special case that occurs frequently in engineering practice involves radiation exchange between a small surface at Tg and a much larger, isothermal surface that completely surrounds the smaller one. The surroundings could be a furnace whose temperature Tgur differs from that of an enclosed surface ( sur — Tg). For such a condition, the irradiation may be approximated by emission from a blackbody at Tsud in which case G = crT. If the surface is assumed to be one for which a = e (a gray surface), the net rate of radiation heat transfer from the surface, is ... 

The color of a light source is typacally characterized in terms of its color temperature. If the x,y coordinates of an illumination source do not exactly sit on the blackbody locus, the color of a light source is characterized in terms of its CCT. The CCT is the temperature of a blackbody radiator that has a colour that most closely matches the emission from a non-blackbody radiator. For high quality white light illumination the CCT should between 2500K and 6500 K. There is an accepted method (Wyszelki et al 1982) to determine lines of constant correlated color temperature in x, y space. CIE, CCT and CRI for common white light sources are given in Table 1 for comparison purpose (Misra et al 2006). 

Transient infrared spectroscopy (TIRS) is a mid-infrared technique [82] that has been developed to obtain spectra of moving solids and viscous liquids. TIRS spectra are obtained from the generation of a thin, short-lived temperature differential that is introduced by means of either a hot or cold jet of gas. When a hot jet is used, an emission spectrum is obtained from the thin, heated surface layer. This technique is known as transient infrared emission spectroscopy (TIRES). When a cold jet is used, the blackbody-like thermal emission from the bulk of the sample is selectively absorbed as it passes through the thin, cooled surface layer. The result is a transmission spectrum convoluted with the observed thermal spectroscopy. This method is known as transient infrared transmission spectroscopy (TIRTS). TIRS is ideally suited for online analysis because it is a single-ended technique that requires no sample preparation. This technique has been applied to the lignin analysis of wood chips [83]. 

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