Black-body radiation source

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... 

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. 

Detectors with complete black body rejection capability are usually less sensitive to fires than a single frequency infrared optical detector. Because it s discrimination of fire and non-fire sources depend upon an analysis of the ratio between fire and reference frequencies, there is a variation in the amount of black body rejection achieved. A detector s degree of black body radiation rejection is in inversely proportion to its ability to sense a fire. The detectors are limited to applications that involve hydrocarbon materials. 

Black body radiators are used as sources of infrared radiation in the range 2-15 yum, e.g. the Nemst glower, which consists of a hollow rod made of the fused oxides of zirconium, yttrium and thorium. For use it is preheated and, when a voltage is applied, it emits intense continuous infrared radiation with very little visible radiation. 

A color correction may also be achieved by using filters. Table 3.1 shows the type of filter used by professional photographers to achieve accurate color reproduction. The required filter depends on the type of illuminant and also on the type of film. The type of light source can be described using the temperature of a black-body radiator. A black-body radiator is a light source whose spectral power distribution depends only on its temperature (Jacobsen et al. 2000). The color temperature of a fight source is the temperature of a black-body radiator, which essentially has the same spectral distribution in the visible region. The concept of a black-body radiator is formally introduced in Section 3.5. 

The equation of the black-body radiator can be simplified by noting that the temperature T used to approximate many light sources is usually no larger than 10000 K. Also, the visible spectrum ranges from 400 nm to 700 nm. Therefore, we have... 

The radiant flux

thermal radiation source through a spectrometer is calculated by multiplying the spectral radiance by the spectral optical conductance, the square of the bandwidth of the spectrometer, and the transmission factor of the entire system (Eq, 3.1-9). Fig. 3.3-1 shows the Planck function according to Eq. 3.3-3. The absorption properties of non-black body radiators can be described by the Bouguer-Lambert-Beer law ... 

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. 

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. 

Figure 10.13 Typical emission spectrum of a mid-IR source along with a basic design of a source. In dotted lines is represented the theoretical emission curve of a perfect source ( black body radiation at 1000 °C). According to the model of source, of the beam-splitter and of the detector being used, an experimental curve is registered which appreciably differs from the theoretical curve. It is noticed in particular that between 4000 and 400 cm f the signal intensity is divided by a factor ten approximately. Current sources are robust and have a many years hfe expectancy.

A luminescent lighting lamp has to emit white light, so that the sun, our natural lighting source, is imitated. The sun is a black body radiator, so that its emission spectrum obeys Planck s equation ... 

Apart from the color point, there is another important lamp characteristic, viz. the color rendition. This property depends on the spectral enei distribution of the emitted light. It is characterized by comparing the color points of a set of test colors under illumination with the lamp to be tested and with a black body radiator. The color rendering index (CRl) equals 100 if the color points are the same under illumination with both sources. Under illumination with a lamp with low CRI, an object does not appear natural to the human eye. 

The most common source in MIR spectrometers is a glowing ceramic bar, a so-called glowbar (or globar). More intense emission is provided by the Nernst glower due to its higher operation temperature (black body radiator). A thermocouple or a thermopile is commonly used as detector. The response behaviour of such detectors is slow, which prevents rapid scanning by dispersive MIR spectrometers. 

Schematics of the calorimetric portions of the experimental apparatus used are included in Figs, la and lb. Figure 2 includes a complete flow diagram of the experimental setup. The calorimetric portion of the apparatus consists of an outer guard, an inner guard, and receiver reservoirs. The guard reservoirs and associated thermal shorting links ensure that the only significant thermal energy transfer to the receiver will be from the infrared black-body radiator or the mercury-xenon source. The absorptance of the receiver surface can then be determined by measuring the vaporization rate of the receiver cryogen.

The color temperature is used to characterize the color of a light source. Its unit is Kelvin, which is defined as the temperature a black body must have to emit light with the desired color. It is directly related to Planck s law of black body radiation. Sunlight, for example, has a color temperature of 5,500 K. Light with a lower color temperature appears more red, while light with a higher color temperature appears blue. 

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