Integrating sphere geometries

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. 

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.

In this section the ideal case of vanishing reabsorption, Ke = 0, is discussed, where Fb + Ff= Ftot- A large area of the sample should be irradiated close to /to = 2/3, what is a very convenient geometry in most spectrometers, or diffusely via an integrating sphere, what is less convenient but guarantees homogeneous density of irradiation. Under these conditions Eqs. (8.27) and (8.28) are sufficiently accurate for quantitative evaluation. 

DR Diffuse reflection geometry, TRc transmission geometry, IS Integrating sphere, PM Praying Mantis... 

X-Ray Diffraction (XRD) patterns were recorded on a Philips HTK-KC diffractometer with a CuKa X-ray source, linked to Philips 386 computer. Thermo-Gravimetry (TG), was recorded on a Setaram TG-DTA 92 thermobalance. Diffuse Reflectance Spectra (DRS) were recorded on a Cary-5 spectrofotometer with a BaS04 integration-sphere in the UV-VIS-NIR region. Mossbauer spectra are recorded on a vertical constant acceleration drive in transmission geometry with a 28mCi Co(Rh) source. Isomer shift data are expressed relative to metallic Fe at 293 K which has an isomer shift of8 = -0.0888 mm/s relative to natural a-Fe. 

The geometry of the measurement, incident and refleeted angle of spectrophotometer beams, reference beams and use of integrating spheres are important considerations of these measurements especially when comparing optical properties measured using different configurations or instmments. 

Detector geometry is important to optimize the collection of diffuse reflectance energy in favor of specular reflectance energy that contains no chemical absorbance information. Two approaches are commonly used. One uses an integrating sphere that provides a double-beamlike instrument, whereby the arrangement of the detectors within the reflective sphere enables a reference and a sample beam to be employed. The reference spectrum is subtracted from... 

Specular surfaces can also be measured by using at integrating sphere at 8° incidence or collection geometry. The measurement technique is identical to the near-normal technique described previously. A reference mirror is used to establish the instrument baseline, and then the sample mirror replaces the reference and a scan is obtained. The reflectance of the sample is the product of the measured value for sample times the actual reflectance for the reference. This technique is quite convenient to use and is as accurate as the multiple mirror method. Care must be taken, however, to ensure that the incidence or collection angle is not 0°, because a specular-excluded measurement of a mirror will certainly give the incorrect answer for reflectance ... 

Another method of measurement of diffuse reflectance does not require an integrating sphere. In this method, the sample beam is focused onto the sample by means of ellipsoidal or spherical mirrors and collected by another ellipsoidal mirror, at either 180 or 90° from the incident beam. This method has been used for many years in the infrared, since proposed by Fuller and Griffiths (1978, 1980). The advantage is very high collection efficiency and the ability to measure very small samples. Such geometry is known by a number of names, including biconical, Praying Mantis (a trade-... 

The primary disadvantage is that the reflectance is highly dependent on the texture of the sample. Textiles, paper, and rough materials will show changes in the apparent reflectance just by rotation of the sample, which will not be observed using an integrating sphere. It is often said that directional/directional geometry is more a measurement of appearance than total reflectance, but the technique is still very useful if the types of samples measured are carefully controlled (Fig. 22). 

The absolute measurement of the quantum yield is very difficult because of the geometry of the emission and the presence of re-absorption. It is usually measured by direct comparison with samples with know quantum yields, or by using an integrating sphere. However, by considering negligible the temperature dependence of the absorption cross section (Demtrdder, 1996 Menzel, 2001) ... 

The accuracy of the experimental values of the photoluminescence quantum yield can be increased if the standard and the sample (either solid or film) are placed in an integrating sphere, which collects nearly the entire emitted photoluminescence intensity and is therefore less sensitive to the particular shape of the sample or the reference [182,183]. Recent comparative experiments with the circular cuvet geometry and also with an integrating sphere confirmed the quantum yield data presented in Table 30.1 within about the same experimental error [179,183]. 

Three types of devices have been described for the measurement of mid-infrared spectra on-axis accessories, off-axis accessories that are mounted in the sample compartment of the spectrometer, and integrating spheres. On-axis accessories for diffuse reflection spectrometry are very much like highly efficient specular reflection accessories. They usually have a higher optical efficiency than either of the other two designs. A typical design of a DR accessory with on-axis geometry is shown in Figure 16.2. 

Figure 16.4. Geometries for an integrating sphere used in configurations for measuring (a) reflectance factor and (b) radiance factor (c) center-mounted sample geometry.

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]. 

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