Hemispherical/directional measurement geometry

Fig. 19. Diffuse/directional hemiellipsoidal geometry for reflectance measurements. Radiation beam illuminates sample at angle 6 radiation is scattered into hemisphere collected and detected. (Reprinted courtesy of Surface Optics Corp. 1997.)...

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 measurement geometry that provides directional incident flux and hemispherical collection is shown in Figure 16.4a. A sample that is illuminated hemi-spherically and viewed at an angle that is near normal (as shown in Figure 16.4h) is called the radiance factor. The selection of the optical geometry depends on the particular application being studied. 

A final geometry exists for directional illumination, diffuse collection integrating sphere measurements. The 30/T geometry (30° incidence, total hemispherical collection) is specified by the American Society for Testing and Materials (ASTM) for the measurement of acoustical ceiling tile reflectance, The author knows of no other applications that specify this integrating sphere geometry. 

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... 

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