Standards black body

The problem is praticularly serious for the very weak metal-adsorbate bonds which are expected in the far-infrared region. Recently the IR radiation from a synchrotron has been used as a source for in-situ measurements at the electrode-solution interface [19]. The far-IR radiation from a synchrotron has an intensity between 100 and 1000 times higher than standard black body sources. 

The difficulty in setting up the initial system for color comparisons cannot be underestimated. The problem was enormous. Questions as to the suitability of various lamp sources, the nature of the filters to be used, and the exact nature of the primary colors to be defined occupied many years before the first attempts to specify color in terms of the standard observer were started. As we said previously, the Sun is a black-body radiator having a spectral temperature of about 10,000 °K (as viewed directly from space). Scattering and reflection... 

Specific solar radiation conditions are defined by the air mass (AM) value. The spectral distribution and total flux of radiation outside the Earth s atmosphere, similar to the radiation of a black body of 5,900 K, has been defined as AM-0. The AM-1 and AM-1.5 are defined as the path length of the solar light relative to a vertical position of the Sun above the terrestrial absorber, which is at the equator when the incidence of sunlight is vertical (90°) and 41.8°, respectively. The AM-1.5 conditions are achieved when the solar flux is 982 Wm2. However, for convenience purpose the flux of the standardized AM-1.5 spectrum has been corrected to 1,000 Wm2. 

The proper implementation of the CIE system requires use of a standard illumination source for calculation of the tristimulus values. Three standard sources were recommended in the 1931 CIE system, and these may be presented in terms of color temperatures (the temperature at which the color of a black-body radiator matches that of the illuminant). The, simplest source is an incandescent lamp, operating at a color temperature of 2856 K. The other two sources are combinations of lamps and solution filters designed to provide the equivalent of sunlight at noon, or the daylight associated with an overcast sky. The latter two sources are equivalent to color temperatures of 5000 K and 6800 K, respectively. 

There are few models with automatic test capability. Testing is usually limited to hand held devices only 2 meters (7 ft.) from the detector or directly on the lens test unit. It can be ineffective if ice forms on the lens. It is sensitive to modulated emissions from hot black body sources. Most of the detectors have fixed sensitivities. The standard being under five seconds to a petroleum fire of 0.1 square meter (1.08 sq. ft.) located 20 meters (66 ft.) from the device. Response times increase as the distance increases. It cannot be used in locations where the ambient temperatures could reach up to 75 °C (167 °F). It is resistant to contaminants that could affect a UV detector. Its response is dependent on fires possessing a flicker characteristic so that detection of high pressure gas flames may be difficult. 

The last point is worth considering in more detail. Most hydrocarbon diffusion flames are luminous, and this luminosity is due to carbon particulates that radiate strongly at the high combustion gas temperatures. As discussed in Chapter 6, most flames appear yellow when there is particulate formation. The solid-phase particulate cloud has a very high emissivity compared to a pure gaseous system thus, soot-laden flames appreciably increase the radiant heat transfer. In fact, some systems can approach black-body conditions. Thus, when the rate of heat transfer from the combustion gases to some surface, such as a melt, is important—as is the case in certain industrial furnaces—it is beneficial to operate the system in a particular diffusion flame mode to ensure formation of carbon particles. Such particles can later be burned off with additional air to meet emission standards. But some flames are not as luminous as others. Under certain conditions the very small particles that form are oxidized in the flame front and do not create a particulate cloud. 

The International Commission on Illumination (CIE) International Commission on Illumination (CIE) has defined a set of standard illuminants to be used for colorimetry (International Commission on Illumination 1996). Figure 3.21 shows the CIE illuminants A, C, Z>5o, Z>55, Z>65, and D15. Illuminant A represents the power spectrum of light from a black-body radiator at approximately 2856 K. If this type of light is required for experiments, a gas-filled tungsten filament lamp that operates at a temperature of 2856 K is to be used. Illuminant Z>65 represents a phase of daylight with a correlated color temperature of approximately 6500 K. The CIE recommends to use this illuminant wherever possible. 

The triboluminescence of minerals has been studied visually (see the footnotes to Table I) but only a few minerals have been examined spectroscopically. There are a few clear examples of noncentric crystals, such as quartz, whose emission is lightning, sometimes with black body radiation. Most of the triboluminescent minerals appear to have activity and color which is dependent on impurities, as is the case for kunzite, fluorite, sphalerite and probably the alkali halides. Table I attempts to distinguish between fracto-luminescence and deformation luminescence, but the distinctions are not clear cut. A detailed analysis of the structural features of triboluminescent and nontriboluminescent minerals may make it possible to draw conclusions about the nature and concentration of trace impurities that are not obvious from the color or geological site of the crystals. Triboluminescence could be used as an additional method for characterizing minerals in the field, using only the standard rock hammer, with the sensitive human eye as a detector. 

Planck s constant was discovered as part of the solution to a nineteenth century conundrum in physics, known as the black-body problem. The challenge was to model the wavelength distribution of radiation emitted through the aperture in a closed cavity at various temperatures6. The standard equations of statistical thermodynamics failed to produce the observed spectrum, unless it was assumed that the energy of radiation with frequency v was an integral multiple of an elementary energy quantum hv. 

The major selling point of standard cosmology is the observed isotropic microwave background radiation, with black-body spectrum. In a closed universe it needs no explanation. Radiation, which accumulates in any closed cavity, tends, by definition, to an equilibrium wavelength distribution according to Planck s formula (Figure 2.5). 

The actual color appearance of light that comes from a "black body" source is called its color temperature or chromaticity. Correlated color temperature (CCT) is the terminology used when referring to the color of an artificial source. Because the color temperature of an artificial source will most likely not fall on the normal black body curve, it is the accepted practice to refer to their temperature as the temperature of the closest CIE daylight (D) temperature value. Hence, a CCT of 6500 K means that its color temperature is closest to that of CIE D6500 daylight standard. 

For a black body, <2 = 1. The emissive power is therefore E. The black body is a perfect radiator and is used as the comparative standard for other surfaces. The emissivity e of a surface is defined as the ratio of the emissive power E of the surface to the emissive power of a black body at the same temperature Eh, as shown by Eq. (37). 

Specific procedures for implementing Eqs. (10.1), (10.2), and (10.3) differ according to the nature of the standard source. The most common source is a black-body radiator, often approximated by a tungsten bulb. A less common alternative with some attractive advantages is a standard material which luminesces in response to laser excitation. These approaches will be addressed separately. 

Since the thermal emission curve from a black-botdy radiator may be calculated from first principles, it can serve as a primary standard for an intensity vs. wavelength curve. Petty et al. (16) described a procedure for intensity calibration of an FT-Raman spectrometer using such a source. Provided the source acts as a black body to the required accuracy, its temperature is known, and the laser wavelength is known accurately, the black-body provides a known i(AT>) curve to use in Eq. (10.3). Since most modern spectrometers measure photons/second rather than watts, the proper units for b fAv) are photons per second per wavenumber. ... 

Black-body sources have the attraction of being primary standards but are rather cumbersome. A quite hot furnace is required to produce sufficient intensity, particularly at visible wavelengths. In addition, the source is usually too large to be positioned near the sample region (assuming the spectrometer could tolerate the heat ), so coupling optics are required. These optics should attempt to position the source image at the normal laser and collection focus and may not introduce their own response function. At least for routine use, a black-body radiator is unlikely to be practical. 

The dashed curve on Fig. 1 is the standard spectrum of an opaque object23 traditionally called a black body at the absolute temperature T = 5750 K. This corresponds quite closely5 to the solar spectrum outside the terrestrial atmosphere in the wave-number range considered on Fig. 1. It was discussed23,25 that the maximum of such a standard spectrum as a function of the wave-length occurs at 4.9651 kT where the Boltzmann constant k = 0.695 cm-1. The constant 4.9651 is the root of the transcendent equation for C = 5 in... 

BLACKBODY RADIATION EMISSIVITY. As shown later [Eq. (14.10)], a black-body has the maximum attainable emissive power at any given temperature and is the standard to which all other radiators are referred. The ratio of the total emissive power IF of a body to that of a blackbody is by definition the emissivity s of the body. Thus,... 

Let us now summarize the results we have achieved. We have measured the luminosity response of the human eye, in terms of photopic and scotopic behavior. We also defined a "black-body" and its wavelength emission, stipulating its absolute temperature. We then defined Standard Sources. We next designed a Color Comparator and then determined the transmission characteristics of three (3) filters required to duplicate the response of the three color preceptors of the human eye. These we called the tristimulus response of the Standard Observer. 

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