Radiation transmission medium

Microbending Loss. As a result of their small diameters, optical fibers bend very readily. This feature is advantageous in that the transmission medium is very flexible and easily routed. However, when the spatial period of the bending becomes small (approx. 1 mm or less), some of the light rays normally guided by the fiber are lost through radiation. Such small period distortions may occur when a fiber is wound on a spool under tension or when it is placed in cable structure. The phenomenon is termed microbending (11), and... 

Unlike conduction and convection, radiation does not require a transmission medium and may occur in a vacuum. The fastest form of heat transfer is by radiation, which occurs at the speed of light and does not suffer attenuation in a vacuum medium. 

Atmospheric attenuation is the consequence of absorption of radiation by the medium present between emitter and receiver. For thermal radiation, atmospheric absorption is primarily due to water vapor and, to a lesser extent, to carbon dioxide. Absorption also depends on radiation wavelength, and consequently, on hie temperature. Duiser approximates transmissivity as... 

From this equation it can be seen that the depth of penetration depends on the angle of incidence of the infrared radiation, the refractive indices of the ATR element and the sample, and the wavelength of the radiation. As a consequence of lower penetration at higher wavenumber (shorter wavelength), bands are relatively weaker compared to a transmission spectrum, but surface specificity is higher. It has to be kept in mind that the refractive index of a medium may change in the vicinity of an absorption band. This is especially the case for strong bands for which this variation (anomalous dispersion) can distort the band shape and shift the peak maxima, but mathematical models can be applied that correct for this effect, and these are made available as software commands by some instrument manufacturers. 

Radiation Heat The transmission of heat through the medium of heat rays. 

Specular reflection is encountered when the reflecting medium is a smooth polished surface. The angle of reflection is identical to the incident angle of the radiation beam. If the surface is IR absorbent, the relative intensity of reflection is less for wavelengths that are absorbed than for wavelengths that are not. Thus, the plot of reflectance R, defined as the fraction of reflected incident radiant energy versus the wavelength (or wavenumber) appears similar to a transmission spectrum for the sample. 

The extent of the interaction between the evanescent field and the absorbing medium is formally described by the effective thickness, effective thickness is the thickness of the absorbing phase that would have to be passed through by the incident beam in a transmission experiment to give the same energy loss as in the attenuated total reflection experiment. The exact expressions for effective thickness can be very complex. However, for a single attenuated total reflection of an incident beam of radiation of electric field E that occurs at an interface between two bulk phases (ie, phase 2 is not a thin film), d is given by... 

For nearly two hundred year s microscopy as a science depended on the use of visible and, rarely, near UV electromagnetic radiation. In the early part of this century developments in theoretical Physics opened other avenues of seeing objects. The following is not an exhaustive list but does illustrate the expansion in the science of microscopy which began earlier this century and which continues today. First came the use of electrons in the forms of transmission and scanning electron microscopies [TEM and SEM 3,4]. Then, relatively recently, came the use of sound as an imaging medium in the development of acoustic microscopy [5,6]. Most recently, near-field optical microscopy [7] and the family of scanning probe microscopies have been developed [8]. 

Diffuse reflectance IR spectroscopy has become an attractive alternative to mulls with the introduction of DRIFT cell by Griffiths,29 later modified by Yang.30 Since materials are dispersed in a nonabsorbing medium and not subjected to thermal or mechanical energy during sample preparation, DRIFT spectroscopy is especially suitable for the qualitative/quantitative analysis for polymorphs, which are prone to solid-state transformations. The Kubelka-Munk (K-M) equation,31 which is analogous to Beer s law for transmission measurements, is used to quantitatively describe diffusely-reflected radiation ... 

We now consider a simple extension of the presentations in Secs. 8-10 and 8-11 to analyze a medium where reflection, transmission, and absorption modes are all important. As in Sec. 8-10, we shall analyze a system consisting of two parallel diffuse planes with a medium in between which may absorb, transmit, and reflect radiation. For generality we assume that the surface of the transmitting medium may have both a specular and a diffuse component of reflection. The system is shown in Fig. 8-58. 

Gas Emissivities Radiant transfer in a gaseous medium is characterized by three quantities the gas emissivity, gas absorptivity, and gas transmissivity. Gas emissivity refers to radiation originating within a gas volume which is incident on some reference surface. Gas absorptivity and transmissivity, however, refer to the absorption and transmission of radiation from some external surface radiation source characterized by some radiation temperature 7. The sum of the gas absorptivity and transmissivity must, by definition, be unity. Gas absorptivity may be calculated from an appropriate gas emissivity. The gas emissivity is a function only of the gas temperature Tg while the absorptivity and transmissivity are functions ofboth Tg and Tt. 

SHG is a coherent process and in principle the experimental system needed to observe the response is very simple. The fundamental radiation from a laser source incident at an interface generates the harmonic beam via non-linear polarization of the medium. Typically, this beam is observed in reflection, but many studies have been undertaken in total internal reflection and transmission geometries. As the harmonic beam is well separated from the fundamental in frequency, it can be detected the difficulties arise due to the inherent inefficiency of the harmonic generation and the low intensities that need to be detected. The sensitivity and selectivity of SHG to the interfacial species in the presence of the same species in the bulk phase provides the driving force to overcome these experimental difficulties. 

---