Sample radiance

BRDF is simply the ratio of sample radiance (w/cm sr) to sample irradiance (w/cm ). It is convenient to incorporate radiance in the definition or the scatter would depend on instrument parameters (detector aperture and distance to the detector). 

Sample radiance (L ) a differential quantity that is the reflected radiant flux per unit projected receiver solid angle per unit sample area. Note In practice is an average calculated from scattered power, P, collected by the projected receiver solid angle, 12 cos 0, from the illuminated area, A. The receiver aperture and distance from the sample determines Q and the angular resolution of the instrument ... 

BRDF the sample radiance divided by the sample irradiance. The procedures given in this practice are correct only if the field of view determined by the receiver field stop is sufficiently large to include the entire illuminated area ... 

The detection of Hquid crystal is based primarily on anisotropic optical properties. This means that a sample of this phase looks radiant when viewed against a light source placed between crossed polarizers. An isotropic solution is black under such conditions (Fig. 12). Optical microscopy may also detect the Hquid crystal in an emulsion. The Hquid crystal is conspicuous from its radiance in polarized light (Fig. 13). The stmcture of the Hquid crystalline phase is also most easily identified by optical microscopy. Lamellar Hquid crystals have a pattern of oil streaks and Maltese crosses (Fig. 14a), whereas ones with hexagonal arrays of cylinders give a different optical pattern (Fig. 14b). 

McQueen et al. (1982) demonstrated that by placing a series of high-impedance transparent fluids (called optical analyzers) over the sample at a series of thicknesses less than d in the target that the overtaking rarefaction (sound) velocity can be accurately obtained. Arrival of rarefaction waves rapidly reduce the shock pressure. These wave arrivals could be very readily detected by the change in light radiance caused by the onset of a decrease in shock amplitude when the rarefaction wave caught up to the shock front. The... 

Ideal matte and specular surfaces have a very simple BRDF. In general, however, the BRDF does not have a simple structure. For example, cloth or brushed metal has a BRDF which reflects most of the light along a preferred direction. Sample BRDFs for aluminum, magnesium oxide ceramic, and sandpaper can be found in (He et al. 1991). If we know the BRDF for a particular material, then we can compute the radiance given off by an infinitesimal small patch of this material. The radiance leaving the patch is given by... 

The Planck-Kirchhoff law allows a good approximation of the spectral radiance of any thermal radiator, the sources as well as the samples and detectors. Thermal radiators are characterized by a definite temperature as well as by their absorption coefficients f(i>) or a(i>), which describe the characteristic spectrum of the radiator ... 

Fig. 3.4-3 shows the infrared emission spectrum (the spectral radiance) of a sample of butadiene gas alp = 107 mbar in a cuvette v, th a thickness of 10 cm at 800 K, compared to the emi.ssion spectrum of a black body at the same temperature. It is interesting to note that - as theory predicts - the percentage emission of butadiene compared to that of a black body almost equals the percentage absorption of the same gas at room temperature as recorded with an ordinary spectrometer. 

If the vibrational temperature is determined by using the intensities of different bands, a distinct value is obtained for each band. These values do not represent the arithmetic mean of all temperatures. Due to the nonlinear increase of the spectral radiance by the black body radiator, the hot zones appear more pronounced than the cold ones. On the other hand, the influence of the more distant zones with respect to the observer is reduced by stronger self-absorption. The vibrational temperatures deduced from bands with high absorption coefficient are therefore lower than those derived from bands with smaller absorption coefficient. Nevertheless, all thus obtained temperature values are between the lowest and the highest temperature of the sample. The method of fitting calculated spectral profiles to the observed ones has been successfully applied in these cases, too. 

Figure 1. On the basis of this observation, and in the absence of skin (< 1 mm) temperature measurements, sea surface radiance was calculated from 2.5-m temperatures and used in the algorithm to correct satellite temperatures to sea surface temperatures. This approach worked well in this study because the relative distribution of 2.5-m temperatures was correlated with the relative distribution of skin temperatures. This conlcusion was supported in two ways (1) the mean difference between surface temperature measured in samples collected in a bucket and the thermistor at 2.5 m was 0.1 °C, and (2) when 2.5-m temperatures were compared with cor-...

More complex techniques for light measurements in tissues have been demonstrated experimentally or are under development. One example is the use of interstitial probes that have a deliberate directional dependence so that, at a fixed point in tissue, as the probe is rotated circumferentially it samples the local radiance distribution, rather than simply the local fluence rate that is the sum of the radiance over all directions. This gives more information about the light field, especially at... 

Spectral interferences may also result from combustion products that exhibit broad-band absorption or particulate matter that causes scattering of the incident radiation. As both reduce the spectral radiance of the Hght source, they may erroneously lead to an overestimation of the absorbance and, consequently, the concentration. When the combustion or the particulate products arise from the fuel and oxidant mixture, they may be determined by measuring the absorbance while a blank is aspirated into the flame. The situation is more compHcated if the absorption or scattering arises from a product associated with the sample or its matrix. 

Depending on the emittance of the sample and the wavelength of the pyrometer used, the lower onset temperature is typically limited to from around 1100 K to 1900 K. The upper temperature limit is once again set by the sample s radiance and its emittance at this temperature. 

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