Front-surface reflectance

A FRONT-SURFACE REFLECTION B DISCONTINUITY RESPONSE C BACK-SURFACE REFLECTION... 

In the past, extensive investigations were made to obtain better insight into the limitations of the diffuse reflectance measurement technique. Studies demonstrated that sample properties such as particle size and packing affect, in addition to the optical constants, the diffuse reflectance spectrum. The characteristics of the diffuse and specular components were studied for different particle sizes and dilution within a non-absorbing inert matrix. It was found that specularly and diffusely reflected radiation coexist in the measured diffuse reflectance spectrum, even in KCl-diluted samples. In addition, the specular component, which is certainly sample-dependent, is not necessarily the same as from the front-surface reflection.To prove this, a top layer of pure KCl powder was... 

The other two types of external reflection microspectroscopy are less well suited to the characterization of tissue samples. In the first type, which is variously called specular reflection, front-surface reflection or Kramers-Kronig reflection, the reflectance... 

The final type of measurement that can be made with the microscope in its reflection mode is diffuse reflection (DR) spectroscopy. Today, very few appHca-tions of mid-lR microspectroscopy of neat samples are available, because for mid-IR DR spectrometry the samples should be diluted to a concentration of between 0.5 and 5% with a nonabsorbing diluent (e.g., KBr powder) to preclude band saturation and severe distortion by reflection from the front surface of the particles. However, this mode has substantial application for NIR measurements, where sample dilution is not needed. Because the absorption of NIR radiation by most samples is rather weak, they must either be at least 1 mm thick or be mounted on a reflective or diffusing substrate, such as a ceramic or Teflon disk. In the latter case, the spectrum is caused by a combination of diffuse reflection, transflection and front-surface reflection (hopefully with diffuse reflection being the dominant process). 

Front-surface reflection IR spectroscopy is often used to characterise the structure of drawn samples. This approach provides the complete IR spectrum, including the highly absorbing bands that are often saturated in transmission spectra. ATR techniques are used to depth profile the changes in orientation. 

The majority of samples for mid-infrared investigations are presented as finely divided powders dispersed in an excess of a dry powdered nonabsorbing matrix, commonly KCl. This ensures that superimposed interferences from specular (front-surface) reflections are minimized in the recorded diffuse reflection spectrum. A useful approach for sampling intractable, composite, or gross objects is to abrade a fine powder from the article s surface with some SiC... 

Reflection from specimen front surface Reflection due to defect Reflection from specimen rear surface... 

By this means an optimized cell can have a total front surface reflection of <1% and a maximum CR (in a high f// projection system) of 100 1 or more. In comparison, a cell with no A/R coating and the wrong ITO thickness could have a front surface reflectivity of 12% and a maximum CR of only 8 1. 

A few points might be made about the assumption of isotropic scatter. If there is specular reflection in the scatter, there may be preferential directions of travel through the sample, and the assumption of diffuse radiation will be violated. Assumption 3 points out that the effect of front-surface reflection is ignored in their treatment. This assumption has often been interpreted as meaning that forward and backward scatter from a particle are assumed to be equal. As stated above, related to Equation (3.24) and Equation (3.25), the assumption of isotropic scatter of this kind is also built into their treatment. 

The other two types of external reflection microspectroscopy are less well suited to the characterization of tissue samples. In the first type, which is variously called specular reflection, front-surface reflection, or Kramers-Kronig reflection, the reflectance spectra of thick, nonscattering, bulk samples are measured and converted to the wavenumber-dependent optical constants, that is, the refractive index (v) and the absorption index k(v) by the Kramers-Kronig transform, as discussed by Griffiths and de Haseth [10]. As the requirement for thick nonscattering samples is essentially never met for tissue samples, this type of measurement is never used in medical diagnosis but has occasionally been used for the study of polymer blends. 

The chalcogenides are all insoluble in water and other common solvents. ZnSe and CdTe have excellent transmission characteristics. The only problem with these materials is their high refractive index, which leads to high front-surface reflectance (see Section 13.2.2), so that transmission spectra of liquids held in cells fabricated from these materials often give rise to interference fringes (see Section 11.1.3). These materials aU make excellent internal reflection elements. AMTIR (amorphous material that transmits infrared radiation) is a mixture of several chalcogenides. Many optical fibers used for mid-infrared spectrometry are made from this material (see Section 15.4). 

Accessories of this type have one major drawback. The particles in the top layer of the sample are often aligned so that one of their planes is parallel to the macroscopic plane of the sample (especially when the top of the sample is flattened with a spatula or razor blade). In this case, much of the front-surface reflection is collected along with the diffusely reflected reflection that has penetrated into the sample before reemerging from its top surface. The result is that bands become distorted and plots of/(/ oo) versus concentration become nonlinear at low concentration. Thus, DR accessories with an on-axis geometry are best used for qualitative measurements. 

TIRES and TIRTS have many of the same properties as photoacoustic spectrometry (see Chapter 20), in that they are largely insensitive to sample morphology and to the optical properties of the sample, such as scattering coefficient and front-surface reflection. Similarly, TIRES and TIRTS spectra are also largely unaffected by the sample backing (if any) and the surrounding atmosphere, although emission from hot water vapor and carbon dioxide must sometimes be subtracted from the spectrum after the measurement. 

Because the Raman effect is a scattering process, it does not have the problems associated with the requirement of light transmission. Raman front-surface reflection allows the examination of a sample of any size or shape. If the sample can be hit with the laser beam, a Raman spectrum can usually be obtained. 

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