Blackbody sources

Unlike the typical laser source, the zero-point blackbody field is spectrally white , providing all colours, CO2, that seek out all co - CO2 = coj resonances available in a given sample. Thus all possible Raman lines can be seen with a single incident source at tOp Such multiplex capability is now found in the Class II spectroscopies where broadband excitation is obtained either by using modeless lasers, or a femtosecond pulse, which on first principles must be spectrally broad [32]. Another distinction between a coherent laser source and the blackbody radiation is that the zero-point field is spatially isotropic. By perfonuing the simple wavevector algebra for SR, we find that the scattered radiation is isotropic as well. This concept of spatial incoherence will be used to explain a certain stimulated Raman scattering event in a subsequent section. 

This is not the case for stimulated anti-Stokes radiation. There are two sources of polarization for anti-Stokes radiation [17]. The first is analogous to that in figure B1.3.3(b) where the action of the blackbody (- 2) is replaced by the action of a previously produced anti-Stokes wave, with frequency 03. This radiation actually experiences an attenuation since the value of Im x o3 ) is positive (leading to a negative gam coefficient). This is known as the stimulated Raman loss (SRL) spectroscopy [76]. Flowever the second source of anti-Stokes polarization relies on the presence of Stokes radiation [F7]. This anti-Stokes radiation will emerge from the sample in a direction given by the wavevector algebra = 2k - kg. Since the Stokes radiation is... 

For both aqueous and nonaqueous liquids, MBSL is caused by chemical reactions of high energy species formed duriag cavitation by bubble coUapse, and its principal source is most probably not blackbody radiation or electrical discharge. MBSL is predominandy a form of chemiluminescence. 

Because NEP is roughly proportional to D is more useful for comparing detectors of differing sizes. D depends on the wavelength distribution striking the detector (if it is quantum) and the frequency at which the radiation is modulated, so these measurement parameters need to be included for a D value to have meaning. Often detectivity is written as where Tis the temperature of the blackbody source of radiation or the wavelength of the... 

Real or gray bodies deviate from these ideal blackbody values by the A-dependent emissivity, but the color sequence remains essentially the same. This mechanism explains the color of incandescent light sources such as flames in a candle, tungsten filament light bulb, flash bulb, carbon arc, limelight, lightning in part, and the incandescent part of pyrotechnics (qv). 

Now that the parameters of typical prototype systems have been considered, it seems worthwhile to consider further variations that could be used to enhance the power and/or versatility of the basic apparatus. An obvious improvement would be to use a completely tunable IR probe source. As mentioned above, a globar source is completely tunable but with a lower than desired number of photons per unit bandwidth (60,73). There are a number of other means of producing completely tunable IR probe radiation with enough intensity to pull the detector out of the blackbody-noise regime. 

Variations in the temperature of a blackbody used as the source in a spectrometer. The energy density of blackbody radiation is given by the well-known formula ... 

The measurement target, tympanic membrane with surrounding tissue, can be treated as a blackbody radiation source with an emissivity of e(A) 0.98... 1. The emissivity is mainly independent of skin color, cemmen or hair in the ear canal. 

For the calibration of most infrared ear thermometers the sensitivities S0 and R0 and the temperature coefficients Sj and a for both sensors have to be determined. Typically a two-step calibration is performed. In the first step the ambient sensor is calibrated by immersing it into two different temperature controlled baths. In the second step the thermopile sensor is calibrated by measuring the output signal while placing it before two different blackbody radiation sources. 

Figures 7.4(a), (b) and (c) give the spectral characteristics of three sources (a) the sun at the earth s surface, (b) a JP-4 pool fire and (c) blackbody sources at typical room fire conditions (800-1100 K). The solar irradiance is mostly contained in about 0.3-2.4 fim while fire conditions span about 1-10 pm.

Figure 7.4 Spectral characteristics of sources (a) sun at Earth s surface [9], (b) JP-4 fuel fire [10], (c) blackbodies [11]...

The other forms of blackbody sources are adaptations of those used widely in conventional laboratory-style IR instruments, which feature exposed electrically heated elements. Various designs have been used, with metal filaments, made from Kanthral and Nichrome, being simple solutions in lower cost laboratory... 

Terahertz, or far infrared spectroscopy, covers the frequency range from 0.1 to lOTHz (300 to 3cm ) where torsional modes and lattice vibrations of molecules are detected. It is increasing in use in many application areas, including analysis of crystalline materials. Several dedicated conunercial instruments are available which use pulsed terahertz radiation which results in better signal to noise than those using blackbody sources for radiation (and associated with the terminology far infrared spectroscopy). Work using extended optics of FTIR instrumentation as weU as continuous-wave source THz has also been recently reported. ... 

The blackbody radiative mechanism proposed here is in fact very reminiscent of the model advanced by Dimbar in which CW infrared laser irradiation can be regarded as a blackbody source raising the effective temperature of the system. In this model, the population of ions achieves a truncated Boltzmann distribution, which resembles that of a normal Boltzmann distribution characteristic of the effective temperature but which abruptly ends near the dissociation energy of the ion of interest because once this energy is exceeded, the species rapidly imdergo... 

Example 10-2 Everyone has poked a campfire at night and watched the sparks fly up. They glow a bright red, which indicates that they are emitting blackbody radiation from a source with a surface temperature of -1200°C. You have also noticed (if you were thinking about scientific matters at the time) that their color does not change until they suddenly disappear. The time for this reaction varies, but they seem to glow for a few SdC and then suddenly disappear. 

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... 

One of the problems which must be solved for quantitative measurements by emission is the need for a blackbody source at the temperature of measurement. And a variety of blackbody references have been used including a V-shaped cavity of graphite 164), a metal plate covered with a flat black paint1S6 160) and a cone of black paper l53). However, none of these methods of producing a blackbody reference spectrum are adequate. In most cases the efficiency of the reference has not been established. The most recent recommendation 1S0) is an aluminium cup painted with an Epley-Parsons solar black lacquer which has an emittance of greater than 98% over the infrared spectral range. 

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