Near-surface total light conditions

Anyhow, the LSC activity within each of the two or more plates is based on the same phenomenon of total reflection14,15 outside the critical cone. Once a light ray is totally reflected, it continues an indefinite number of total reflections on the three conditions of two absolutely planar parallel surfaces of the plate, and absence of (even very weak) absorption and of scattering due to turbidity, air bubbles, and so on. On the other hand, nearly all the luminescence irradiated inside the critical cone is lost through the surfaces of the plate16. The narrow air gap between the upper and the second plate seen on Fig. 2 is a necessary isolation preventing trapped luminescence in the upper plate to penetrate into the lower plate and getting absorbed. 

Recent studies of micro ATR have defined the best experimental conditions for establishing optical contact between the ATR crystal and the sample [25]. This experimental approach has been applied to the analysis of the failure surfaces of adhesively bonded joints. ATR-microscopic measurements have been used for direct measurement and identification of raw materials in textiles coated and impregnated substances on paper [26]. An ATR microscopic probe has been developed which allows one to examine the sample optically through the probe in the microscope. The hemispheric ATR crystal is mounted at the focus of the Cassegrain objective, below the secondary mirror. One can position the crystal in contact with the sample, and run the spectra [27]. In the survey mode, visible light at nearly normal incidence is selected to locate the area of measurement. In the contact mode, low incident angle visible is used to detect contact of the sample to the ATR crystal surface. In the measurement mode, the ATR crystal is slid into position and the incident beam is optimized for total internal reflection. In the Spectra Tech version, all of the available crystals, i.e. ZnSe, Diamond, Silicon, and Germanium can be used. However, Ge and Si are opaque and cannot be used in the survey or contact mode. An optical contact sensor can be used. 

Therefore, if e > 0, i is real, and the medium is transparent the electromagnetic wave propagates without any absorption. After Eq. (13.53), this is the case when to > tOp. For the high carrier densities met in metals, the plasma frequency is in the visible of near ultraviolet, which explains the transparency of alkali metals (Li, K,) in the UV. On the other hand, if to < tOpEq. (13.53) shows that is real but negative so that h (co) and q are pure imaginary. In that case, is an evanescent wave that is just needed to fulfill the boundary conditions (continuity of the fields) at the surface when the sample is illuminated by a beam at this frequency the light cannot penetrate in the sample, and is totally reflected at the surface. That is why well-polished metallic surfaces are mirrors. 

---