Transmission normal-incidence

Figure 15.8. Light transmission of acrylic polymer (5 in thick moulded Diakon. Parallel light beam normally incident on surface). (Reproduced by permission of ICI)...

Observation of absorption bands due to LO phonons in RAIR spectra of thin, silica-like films deposited onto reflecting substrates demonstrates an important difference between RAIR and transmission spectra. Berreman has shown that absorption bands related to transverse optical (TO) phonons are observed in transmission infrared spectra of thin films obtained at normal incidence [17]. However, bands related to LO phonons are observed in transmission spectra of the same films obtained at non-normal incidence and in RAIR spectra. Thus, it is possible for RAIR and transmission spectra of thin films of some materials to appear very different for reasons that are purely optical in nature. For example, when the transmission infrared spectrum of a thin, silica-like film on a KBr disc was obtained at normal incidence, bands due to TO phonons were observed near 1060,790,and450cm [18]. 

Figure 5. Effect of the incidence angle on the spectral profile of a transmission coating. G normal incidence. R p-polarization. B s-polarization.

Normal incidence transmission IRLD measurements are used to study thin films (typically 100 pm thickness and less, depending on the molar extinction coefficient of the bands) with in-plane uniaxial orientation. Two spectra are recorded sequentially with the radiation polarized parallel (p) and perpendicular (s) to the principal (machine) direction of the sample. The order parameter of the transition moment of the studied vibration is calculated from either the dichroic ratio (R — Ap/As) or the dichroic difference (AA = Ap—As) as ... 

Figure 5 Polarized infrared spectra (s on top, p on bottom) recorded for an oriented 80/20 PS/PPO miscible blend using normal incidence transmission. Reproduced with permission from Lefebvre et al. [23]. Copyright Elsevier 1981.

Specular reflection IR spectroscopy has been used by Cole and coworkers to study the orientation and structure in PET films [36,37]. It has allowed characterizing directly very highly absorbing bands in thick samples, in particular the carbonyl band that can show saturation in transmission spectra for thickness as low as 2 pm. The orientation of different conformers could be determined independently. Specular reflection is normally limited to uniaxial samples because the near-normal incident light does not allow measuring Ay. However, it was shown that the orientation parameter along the ND can be indirectly determined for PET by using the ratio of specifically selected bands [38]. This approach was applied to the study of biaxially oriented PET bottles [39]. 

In the normal-incident transmission measurements of LB films deposited on transparent substrates, the electric vector of the infrared beam is parallel to the film surface (Figure 5A). Therefore, only absorption bands which have the transition moments parallel to the film surface can be detected by this method. On the other hand, in the above-mentioned RA measurements, in which the p-polarized infrared beam is incident upon the LB film prepared on Ag-evaporated substrates at a large angle of incidence, we have a strong electric field perpendicular to the film surface as shown in Figure 5B. Therefore, in this case, only absorption bands which have the transition moments perpendicular to the film surface can be detected with a large intensity enhancement. Thus, if the molecules are highly oriented in the LB films, the peak intensities of particular bands should be different between the transmission and RA spectra. 

Detonation, Shock Transmission from Explosive to Metal Plate. Accdg to Cook (Ref 3, p 1H)> R.W. Goranson is credited with suggesting that it is possible to determine the p(x), W(x) and p(x) distribution in the detonation wave by studying the characteristics of the shock wave transmitted from the explosive into a thin metal plate in shock loading of the plate by a detonation wave. In this theory, when a plane detonation wave strikes a metal plate at normal incidence, a shock wave is transmitted into the plate and another is reflected back into the incident wave such. as to give a pressure-distance profile like that illustrated in Fig 5.17 [reproduced by Cook from the paper of Walsh Christian (Ref 1)]. 

We now consider reflection and transmission of a wave Eiexp[iu(N2z/c — 0] normally incident on a plane-parallel slab of arbitrary material embedded in a nonabsorbing medium (Fig. 2.7). The reflected and transmitted waves are... 

The field amplitudes are written as scalars because reflection and transmission at normal incidence are independent of polarization. At the first boundary (z = 0), the amplitudes satisfy the usual boundary conditions ... 

Measurement of the transmittance and reflectance of a slab for light at near-normal incidence. The samples must be sufficiently transparent for measurable transmission in thin slabs, but not as transparent as required in method 1. 

In the following summary of contrast enhancement techniques, it is assumed that specimens are being observed in transmission, that they are not self-luminous, and that the light source is not imaged onto the specimen by the microscope condenser. All these assumptions describe typical conditions for LCP microscopy. Figures 5 and 6 show ray diagrams for a normally incident and obliquely incident beam of parallel rays, respectively. In both cases, the objective back focal plane contains the Fraunhofer diffraction pattern of the specimen. 

Recent developments have extended the ultrasonic techniques to the characterisation of thin layers of metals and polymers deposited on substances to obtain measurements of the thickness/density product. Using techniques where the film are immersed in a fluid, such as water, measurements have been made, by the low frequency normal incidence double through-transmission method, with film thickness ranging from 20 to 200 pm [112] a range which is of particular relevance to membrane systems. 

The method utiHzes the angular dependence of the dielectric filter on impacting photon direction, with its transmission spectral profile shifting to the blue, in fine with the increase in the deviation of photons away from normal incidence. This feature enables the filter to serve as a unidirectional mirror, passing a semi-colhmated laser beam through unhindered from one side while at the other side reflecting any photons emerging from the sample predominantly at random directions, back into the sample. 

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