Hemispherical reflectance

Southern Hemisphere. The seasonal trends observed in both hemispheres reflect an anticorrelation with OH that removes CO from the atmosphere. CO in the NH, for example, peaks in March/April and is a minimum in July. 

For an opaque surface, the relationship between the spectral hemispherical reflectivity and the spectral hemispherical absorptivity is... 

For much of engineering practice, the reflectivity and the absorptivity, averaged over the entire wavelengths, is of relevance. When this is done, the resulting hemispherical reflectivity p and the hemispherical absorptivity a are defined as follows -... 

Fig. 6. Upper panel absorption bands of Ganymede which are attributed to solid (obtained as a ratio of Ganymede s trailing hemisphere reflectance to that of Callisto s). Lower panel pure O2 at 26 K (line with circles), codeposited O2 + H2O ice film at 26 K (solid curve). Same after warming to 100 K, (pluses) taken from Vidal et al. ...

Likewise, the spectral hemispherical absorptivity and. spectral hemispherical reflectivity of a surface arc defined as... 

Because of the reciprocity of the bidirectional reflectivity p", the hemispherical-directional spectral reflectivity for isotropic incident intensity pi, is equal to the directional-hemispherical reflectivity pi,. Here, a single prime ( ) is used to denote the directional nature of incident radiation. 

Hendricks and Howell [253] measured the spectral normal transmittance and normal hemispherical reflectance of three sample thicknesses each of reticulated partially stabilized zirconia and silicon carbide at pore sizes of 10, 20, and 65 ppi. The measurements covered a spectral range of 400-500 nm. They used an inverse discrete ordinates method to find the spectrally dependent absorption and scattering coefficients as well as the constants appropri-... 

Fig. 8.5 Successful compensatory vasodilation. The patient is a 57-year-old man who presented with 5 days of intermittent dizziness and dysarthria, and was found to have critical stenosis of the proximal left internal carotid artery on CT angiography. There is no evidence of infarction in the DWI image. CBV is elevated throughout the entire visualized left cerebral hemisphere, reflecting autoregulatory vasodilation that is successful in maintaining blood flow, as evidenced by the fact that there is...

When taking the total intensity of a light source where the intensity over a wavelength interval dX is given by l(X)dX, the total directional hemispherical reflectivity is given by... 

The cumulative deposition of anthropogenic radionuclides shows a typical latitudinal distribution, with high levels in the mid-latitude regions of the Northern Hemisphere, reflecting the stratosphere-troposphere exchange of air masses in the mid-latitude region (UNSCEAR 2000a). 

Clarke, F. J. J., and Compton, J. A. Correction methods for integrating sphere measurements of hemispherical reflectance. Color Res. Appl. 11(4) (1986). 

Fig. 2. 8° Hemispherical reflectance factor for packed barium sulfate and magnesium oxide powders.

Fig. 3. 6° Hemispherical reflectance for packed PTFE powder (Halon G-80) from Weidner and Hsia (1981).

Spectralon exhibits almost perfectly Lambertian reflectance over most of its usable range of 200-2500 nm. The total hemispherical reflectance is generally greater than 95% from 250 to 2500 nm and in the vicinity of 99% from 350 to 1500 nm. The material also shows no strong absorbance bands anywhere in its range of use. Spectralon is also stable to > 300°C and may be heated and cooled with no changes in reflectance properties. 

Fig. 10. 8 Hemispherical reflectance factor for a nine-step Spectralon gray scale. Nominal reflectances from bottom 2, 5, 10,20,40, 50,60,75, and 80%.

Hemispherical reflectance for mid-infrared gray scale (arc copper, zinc, and... 

Fig. 12. (a) 8° Hemispherical reflectance factor, mixed rare earth oxides in pseudo-spectralon, UV-VIS region, (b) 8° Hemispherical reflectance factor, mixed rare earth oxides in pseudo-spectralon, NIR region. 

In this notation, the first term refers to the geometry of illumination, whereas the second term refers to the geometry of light collection. 2ti refers to a hemispherical solid angle, whereas 0, 45, and 9 refer to small solid angles centered around the specified angle. Geometries 1 and 2 are used primarily for colorimetric measurements and will not be discussed here. The hemispherical/hemispherical reflectance is not easily measured directly however, it can still be an important quantity. For instance, the hemispherical/hemispherical... 

The directional/hemispherical reflectance is the fraction of flux incident within a small solid angle about the direction 6 that is reflected into the hemisphere above the surface and will be denoted by the symbol p. By the Helmholtz reciprocity principle (9,10), a measurement in the InfO geometry will yield the same reflectance value. In practice, actual measurement geometries only approximate the ideal geometry due to the presence of ports and other factors. The directional/hemispherical reflectance is composed of a diffuse and a specular or regular component. 

The term diffuse reflectance pfl will be used to refer to a directional/ hemispherical reflectance measurement in which the specular or regular component (p,) is not included. 

The use of hemispherical mirrors for measuring directional/hemispherical reflectance predates the use of integrating spheres for the same purpose by a few years. Paschen (11) placed lamp and platinum black-coated detectors at the center of a hemispherical mirror in order to enhance the detector s absorptance and used this arrangement to determine the constants in Wien s radiation law. Royds (12) positioned a sample and detector at the conjugate foci of a hemispherical mirror in order to measure the reflectance of blackened thermal detectors. In the acknowledgments of a 1911 paper... 

Here, we have used the same assumptions that were used to derive Eq. (8.7). In Beckett s original derivation, the distinction between directional/hemi-spherical and hemispherical/hemispherical reflectance was ignored. The derivation of Eq. (8.10) also assumes that the detector is fully illuminated and that the sample exchange factor is independent of which focus (sample or detector) is initially irradiated. This last assumption is not strictly correct because the spatial distribution of the irradiation on the detector will be different in the two cases. A raytracing analysis is needed to determine the size of the error related to this assumption. The ratio of the sample throughput result (Eq. 8.8) with Eq. (8.10) gives the apparent sample reflectance ... 

The mirror reflectance can be independently measured relative to a reflectance standard with an absolute V-W apparatus (60) or with the conic mirror reflectometer itself. The measurement of was discussed in Section V,D. An estimate of the hemispherical/hemispherical reflectance of the detector can be obtained by performing a series of measurements of the directional/hemispherical reflectance at various incidence angles. Beckett (15) did not suggest using the technique with 2n/d instruments however, the application is straightforward. 

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