Diffuse transmittance

Figure 8.18. Intensities of forward and backward fluorescence of a scattering layer as a function of the layer thickness (ff = 1 cm-1, 5° = S = 100 cm-1). The corresponding curves of diffuse reflectance and diffuse transmittance at Xa are also given.

LP-CVD ZnO Total and diffuse transmittance (TT and DT) for boron-doped LP-CVD ZnO films are shown in Fig. 6.17 as a function of the film thickness. TT remains superior to 85% in the spectral range [500-900 nm] for ZnO layers with d < 1.5 pm. Because 15% of the incident light is reflected due to the change of refractive index at the air/ZnO and glass/air interfaces, this means that, for d < 1.5 pm, the absorption of the LP-CVD ZnO B itself is too low to be measured by the spectrometer. TT is reduced by only about 5% for a thickness d = 3 pm. This means that 3 pm-thick ZnO films still have a high transparency, in spite of their relatively high thickness. For A > 900 nm,... 

Fig. 6.17. Total and diffuse transmittance (TT and DT) of LP-CVD ZnO B films grown at 155° C and 0.5mbar, in function of their thickness d...

AP-CVD ZnO Figure 6.24 shows the diffuse transmittance (DT) of AP-CVD ZnO F deposited at various substrate temperatures. DT clearly increases with increasing substrate temperature, indicating that the light scattering capability of AP-CVD ZnO films is increased with temperature. This can be correlated with the increase of the grain size with temperature observed for AP-CVD ZnO films. 

LP-CVD ZnO Optical total and diffuse transmittance spectra (TT and DT spectra) of a temperature series of undoped LP-CVD ZnO films are shown in Fig. 6.25 TT does not vary strongly with substrate temperature. Indeed, as Fig. 6.25 is related to a series of undoped samples, the values of carrier density N are too low to produce an observable free carrier absorption effect... 

Fig. 6.24. Diffuse transmittance (DT) of AP-CVD ZnO F films deposited at vari ous substrate temperatures. Reprinted with permission from [15]...

Fig. 6.29. Variation of (a) the total and diffuse transmittance curves (TT and DT), (b) the haze factor measured at A = 600nm (DT/TT at 600nm), as a function of the H2O/DEZ ratio, for 2.5 j,rn-fhick LP-CVD ZnO B films deposited at 155°C, 0.5mbar and with B2H6/DEZ = 0.6...

Total and diffuse transmittance spectra (TT and DT, resp.) and haze factor (i.e., DT/TT measured at 600 nm) are presented in Fig. 6.29 as a function of the H2O/DEZ ratio. As the thickness d of the ZnO samples does not vary significantly within this series, we may assume that the trends observed hereafter are not due to a variation of d. TT does not vary within this series, with the single exception of the TT curve for the ZnO sample deposited with a H2O/DEZ ratio of 0.8, i.e., the only sample deposited with an excess of DEZ. The TT of this sample is systematically lower than the TT curves of the ZnO samples deposited with an excess of water. The reduction of TT in the NIR area is similar for all the curves. This indicates that free carrier... 

Modern NIR equipment is generally robust and precise and can be operated easily by unskilled personnel [51]. Commercial instruments which have been used for bioprocess analyses include the Nicolet 740 Fourier transform infrared spectrometer [52, 53] and NIRSystems, Inc. Biotech System [54, 55]. Off-line bioprocess analysis most often involves manually placing the sample in a cuvette with optical pathlengths of 0.5 mm to 2.0 mm, although automatic sampling and transport to the spectrometer by means of tubing pump has been used (Yano and Harata, 1994). A number of different spectral acquisition methods have been successfully applied, including reflectance [55], absorbance [56], and diffuse transmittance [51]. 

Adrenergic blockers inactive receptors by promoting the reuptake of the transmitter back to the neuron and degrading transmitters by enzymes, making the transmitters unable to attach to a receptor. Adrenergic blockers also diffuse transmitters away from receptors. 

Isaksson T, Kowalski BR, Piece-wise multiplicative scatter correction applied to near-infrared diffuse transmittance data from meat products, Applied Spectroscopy, 1993, 47, 702-709. 

Measurement of the IR spectra of an ultrathin film on a powder sample may be carried out using transmission, diffuse transmittance (DT), diffuse reflectance (DR), or ATR techniques. As mentioned in Section 1.10, calculations to model the IR spectra of ultrathin films on powders under a different set of experimental conditions have not yet been realized. Compared to the stratified systems considered in Sections 2.1-2.6, optimization of the measurements on powders is significantly more complicated. Moreover, this problem has not yet been studied in a systematic fashion. Below current knowledge concerning the optimization of such measurements will be presented, with emphasis on the requirements of the sample. The technical aspects (the production of IR spectra of powders) are discussed in Section 4.2. 

The method of diffuse transmittance (DT) is based on measurement of the radiation component 7dt (Fig. 1.22) that passes diffusely through an inhomogeneous layer. This method was first applied to the IR spectroscopic analysis of thin films on samples in powder form by Tolstoy in 1985 [116, 117], who obtained DT spectra of water adsorbed onto silica gel. When used in conjunction with a FTIR spectrometer, the method is called diffiise-transmittance infrared Fourier transform spectroscopy (DTIFTS). DTIFTS is the most recently developed IR spectroscopic methods for studying powder surfaces and has already found application in high-performance liquid chromatography (HPLC) and thin-layer chromatography (TLC) [118, 119]. Of increasing popularity are DTIFTS measurements of powders that use an IR microscope to collect radiation [112, 119] (Section 4.3). 

Figure 2.45. (a) Diffuse reflectance and (b) normalized diffuse transmittance of 1.3-mm-thick samples of Cab-O-Sil silica powders entirely covered by (4) DMB and (S) DMP.CN siloxy substituents, (a) Number of coadded scans 250, resolution 4 cm" interferometer scanning speed 0.5 cm s". (b) Photopyroelectric signals normalized to spectrum of empty sample holder/detector. Number of coadded scans 75, resolution 4 cm" interferometer scanning speed 0.03 cm s". Adapted, by permission, from F. Boroumand, J. E. Moser, and H. Vandenbergh, Appl. Spectrosc. 46, 1874 (1992), pp. 1883 (Fig. 7) and 1885 (Fig. 10). Copyright 1992 Society for Applied Spectroscopy.

Figure 2.46. Effect of depth of layer d on absolute diffuse reflectance and transmittance of derivatized LiChrosorb and Cab-O-Sil silica powder samples. Reported values are measured at maximum of absorption peak of CN group at v = 2247 cm. (a) Diffuse reflectance of various LiChrosorb silica layers treated up to surface saturation by DMP.CN (b) diffuse reflectance of various Cab-O-Sil powder layers treated by DMP.CN (c) diffuse transmittance of Cab-O-Sil powder under same conditions. Reprinted, by permission, from F. Boroumand, H. Vandenbergh, and J. E. Moser, Anal. Chem. 66, 2260 (1994), p. 2263, Fig. 2. Copyright 1994 by American Chemical Society.

Figure 4.21. Measurement of (a) total and (b) diffuse transmittance with integrating sphere, /rt and /dt are regularly and diffusively transmitted radiation, respectively.

Translucent or opaque solids diffuse reflectance or diffuse transmittance (for turbid samples)... 

Translucent or opaque liquids reflectance or diffuse transmittance High optical density (highly absorbing) tiny pathlengths in transmittance... 

Light incident on a substance can interact with the material in six ways (Fig. 1). It can be absorbed, transmitted either in a regular fashion or scattered, or reflected diffusely, specularly (also known as regular reflection), or retroreflected back to the source. This chapter will cover the techniques of measurement of reflectance and, to a small extent, diffuse transmittance where it is associated with diffuse reflectance in a technique called trans-flectance. Retroreflection, although a natural phenomenon—a cat s eye shining in the beam of a headlamp is an example of retroreflection in nature—is not commonly measured using spectrophotometers and those specialized instruments to do such measurements are beyond the scope of this chapter. 

The diffuse transmittance is defined as the total transmitted light passing through (and interacting with) a noninfinite thickness of a diffusely reflecting medium, with this medium being composed of multiple diffusely reflective surfaces. This measurement geometry is represented in Fig. 2. 

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