Diffuse/directional integrating sphere

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 the diffuse reflectance technique (Fig. 14.2d), light scattered by a thick layer of particles is directed by the integrating sphere to a detector. Absorption... 

When light is directed onto a sample it may either be transmitted or reflected. Hence, one can obtain the spectra by either transmission or reflection. Since some of the light is absorbed and the remainder is reflected, study of the diffuse reflected light can be used to measure the amount absorbed. However, the low efficiency of this diffuse reflectance process makes it extremely difficult to measure 120) and it was speculated that infrared diffuse reflection measurements would be futile 120). Initially, an integrating sphere was used to capture all of the reflected light121) but more recently improved diffuse reflectance cells have been designed which allow the measurement of diffuse reflectance spectra using FT-IR instrumentation 122). 

For diffuse reflection analysis, it is necessary to fit the spectrometer with an integration sphere the function of which is to collect the light diffused in all directions. Although several designs have been proposed, the most commonly adopted is that of Harrick which enables... 

Fig. 18. A Schematic representation of a diffuse reflectance spectrometer. For (i) nondiffuse dispersed illumination the source at the left and the detector at the integrating sphere are used. For (ii) diffuse nondisperse illumination (in the case of fluorescent samples) a source directly attached to the sphere and the detectors on the left upper side are applied. B Temperature-regulated and evacuable diffuse reflectance sample cell according to [32]. Reprinted from [32] with permission of Academic Press, Inc...

In a typical UV-VIS diffuse/directional instrument, the source, generally a tungsten halogen lamp, is placed inside an integrating sphere. The diffuse illuminated sample is imaged onto a slit and then to a polychromator and diode array. By having two sets of sample holders, one with a wedge and one at 0°, both specular-included and specular-excluded measurements can be made. The 0° sample holder ensures that the sample is not illuminated at 0° incidence and is thus specular excluded. A typical accessory for diffuse/ directional measurement of reflectance is shown in Fig. 17. 

Analytical work has been published on the optical behavior of composite materials. Edwards et al. (1962) analyzed the spectral reflectance and transmittance of imperfectly diffuse samples in an integrating sphere and provided calculations of the measurement error resulting from the nonideal material. Scattering in composites has been predicted in recent works by Varadan (1991), White et al, and Lee et al. Individually, each fiber scatters the incident radiation in a forward-facing cone centered around the incident direction. 

Figure 6. Schematics of an experimental arrangement utilizing a double-integrating sphere geometry for simultaneous measurement of reflection, scattering and absorption. The detectors Dc, Dt, Dr at three different parts measure three quantities respectively (i) transmitted coherent light that passes through the tissue in the direction of light propagation, (ii) transmitted diffuse intensity, and (iii) reflected diffuse intensity.

The transmission haze is the ratio of the diffuse transmittance (DT) to the total transmittance (TT). Haze measurement documentary standards recommend using an integration sphere to get the summation of transmission light directly, that is, the summation of DT and TT. Since the transmittance depends strongly on the geometry of the incident beam, every documentary standard has a strict definition for the incident beam, such as shape, size, divergent angle, and so on [15]. 

NIR spectrometry has been used widely for the analysis of agricultural, food and pharmaceutical products. It is a rapid technique and may be adapted to the quality control of process streams as, using fiber optics, remote sampling in industrial environments is possible. One of the most useful NIR methods uses diffuse reflectance to analyze solid materials. The sample, usually as a powder, is placed in an integrating sphere and illuminated from an NIR source that directs radiation onto it. 

The first and second integrals have their coordinate systems centered on the catalytic C and noncatalytic N spheres, respectively. The local nonequilibrium average microscopic density field for species a is pa(r) = [Y = 5(r - ( )) The solution of the diffusion equation can be used to estimate this nonequilibrium density, and thus the velocity of the nanodimer can be computed. The simple model yields results in qualitative accord with the MPC dynamics simulations and shows how the nonequilibrium density field produced by reaction, in combination with the different interactions of the B particles with the noncatalytic sphere, leads to directed motion [117],... 

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