Single LED based dual wavelength referenced optical fibre sensor system using intensity modulation

An intensity-modulated optical fibre sensor system is described which employs a single LED source to provide the measurand and the reference signals with two separate wavelength bands. The allocated wavelength bands are selected to minimize any differential intensity effects that may arise from LED thermal variations. The sensor system, comprising a transceiver unit connected to an optical displacement sensor using a single optical fibre, provides an output fully referenced for all major common-mode variations that are likely to occur. Performance characteristics for the prototype system are reported which show a linear displacement range of over 20 mm.     

Micellar Effects on Alkaline Hydrolysis of 3,5-Dinitro-2-chloro Benzotriflouride and 2,4-Dinitrochloro Benzene in Aqueous-Acetonitrile Mixtures

Reaction rate for alkaline hydrolysis of the substrates 3,5-dinitro-2-chloro benzotriflouride (DNCBTF) (1) at 30°C and 2,4-dinitrochloro benzene (DNCB) (2) at 50°C separetely with NaOH as nucleophile is carried out spectrophotometrically in mixed aqueous-acetonitrile solvents. In each system, cationic surfactant as dodecyltrimethyl ammonium bromide (DoTAB), or anionic one as sodium dodecyle sulfate (SDS) is used in wide range of concentrations to study the effect of micelle on the reaction rate. The micellar effect is explained in term of modified pseudo phase ion exchange model. Pseudo first order rate constant, k_obs is obtained for each of the nucleophile and for both substrates 1 and 2 at all range of X_AN · k_obs at given [OH^?] and in presence of any substrate is found to increase with the increase of DoTAB,while decrease with the increase of SDS as micellar phases. Critical micelle concentrations (CMCs) in similar trend are observed to increase in DoTAB while decrease in SDS systems by increasing acetonitrile (AN) content. Micellar binding constant (K_S) between any type of given substrate and the formed micelle, is found to decrease in presence of DoTAB and increase in SDS micellar phases by increasing AN content. Finally, the ratios between pseudo first order rate constants for hydrolysis in micellar phase k_M to that in the bulk phase k_w for DoTAB and SDS systems are found to be greater than and smaller than unity respectively at any given X_AN where the data indicated for (1) is always higher than those for (2). The results concluded that micelle DoTAB is working as a catalyst for the reaction rate, while that for SDS is considered as an inhibitor.     

Low-frequency scattering by rectangular dielectric particles

The field scattered by a homogeneous isotropic dielectric particle illuminated by a low-frequency plane electromagnetic wave is expressed in terms of a single polarizability tensor which is a function of only the geometry of the particle and a material parameter τ representing either the relative permittivity or permeability of the dielectric. The mathematical formulation is specialized to the case of a rectangular parallelepiped and numerical techniques are developed for computing the tensor elements. Specific data are presented for the tensor elements of rectangular parallelepipeds having square cross sections and are compared to the results obtained for spheroids and right circular cylinders of similar dimensions.     

On the validity of modeling rayleigh scatterers by spheroids

For single homogeneous lossy dielectric Rayleigh particles, the manner in which the particle shape affects the absorption and scattering is examined by computing the polarizability tensor elements using a general purpose program valid for any rotationally symmetric body. The results for a number of generic shapes are compared with those for the corresponding spheroids. When the dielectric constant is real and negative, the resonances attributable to bulk and to shape-dependent or surface polariton modes are tracked as functions of the length-to-width ratio of the particle and the bulk dielectric constant. The adequacy of using spheroid approximations for individual particles in a dispersion is also discussed.