Fresnel reflection-transmission techniques

In spite of these problems, the Fresnel reflection-transmission technique is relatively easy to use, and, because of that, it has been employed by various researchers to determine the coal refractive index (see Refs. 227 and 228 for reviews). 

Complex Index of Refraction of Soot. Soot refractive index has been measured by several researchers. The experimental techniques used can be broadly categorized as in situ and ex situ techniques. In the former, the measurements are performed nonintrusively in a flame environment. The necessary information is retrieved either from spectral transmission data or both the transmission and scattering information, as in Refs. 215-224. The ex situ measurements involve the reflection/transmission of incident spectral radiation on planar pellets of soot, and the optical properties are determined using the Fresnel relations [225]. An alternative ex situ technique was used by Janzen [226], who dispersed the soot particles in a KBr matrix and used transmission measurements to extract the required optical properties. 

Such lenses are cheaply constructed but suffer from the absorption and scattering. Many other techniques for the formation of intense X-ray microbeams are available on the basis of the use of various types of X-ray optics based on refraction, diffraction, or reflection bent mirrors, crystals and multi-layers, linear and tapered glass mono-capillaries, complex polycapillary lens systems, transmission Fresnel zone... 

External reflection. This is not as well developed a technique as internal reflection the physics of reflection of light from surfaces is less accommodating to the infrared spectroscopist. Smooth or shiny surfaces are particular problems. Specular reflection from the surface itself is governed by Fresnel s equations—the reflectance depends on a complicated combination of refractive index, sample absorbance and polarisation. Consequently, samples where the reflectance is mainly from the surface give rise to spectra which bear little relation to conventional transmission spectra. A transformation known as the Kramers-Kronig transformation does exist which attempts to convert a specular reflectance spectrum into a conventional-looking one. It is not 100% successful, and also very computer-intensive. For these reasons, specular reflectance is not commonly used by the analyst. 

---