Vladimir P. Dick

Extinction of light in dispersive media with high particle concentrations: applicability limits of the interference approximation

We investigated the limits of applicability of the interference approximation (ITA), which is often used to calculate light fields in dispersive media with high particle concentrations. We analyzed the conditions under which the use of the ITA leads to an error in calculation of the dispersive-medium extinction coefficient ∊ equal to 10%, 15%, and 25%. The analysis was carried out by comparing the values of ∊ calculated in the ITA with the results of the calculation of ∊ in a much more exact quasi-crystalline approximation (QCA). The validity of the QCA in estimating the error of the ITA is shown by comparing the values of ∊ calculated in the ITA and the QCA with experimental data [J. Opt. Soc. Am.72, 1317 (1982)]. Our investigation shows that the ITA can be used only at relatively small values of the particle size parameter x. When the relative refractive index of the particles is n≳1.8, the applicability of the ITA is limited to the case in which the particle size parameter x⩽xmax≈1.5. A decrease in n leads to an increase in xmax, and for nonabsorbing particles at n≲1.3,xmax is approximately proportional: 1/(n-1). An increase in the imaginary part of the relative refractive index of the particles κ leads to a decrease in xmax. The smaller the n, the greater the dependence of xmax on κ.

Model for coherent transmittance calculation for polymer dispersed liquid crystal films

This paper analyses the methods of calculating the coherent (direct) transmittance of polymer dispersed liquid crystal (PDLC) films which are polymer-based films with embedded liquid crystal droplets. By comparison with experimental data it is shown that at a high concentration of LC droplets, Beers law, which is frequently used, leads to large errors in calculations of the transmittance of PDLC films. To calculate coherent transmittance, it is expedient to use the interference approximation which takes into account the interference of waves scattered by individual LC droplets and leads to much more accurate results.

Passage of light through a dispersion medium with a high concentration of discrete inhomogeneities: experiment

The passage of light through dispersion layers consisting of large weakly absorbing scatterers has been investigated. Measurements of the transmittance and the angular structure of scattered radiation have been made. The dependence of these characteristics on the thickness and the concentration of scatterers has been analyzed.

Features in coherent transmittance of a monolayer of particles

Coherent and incoherent transmittance values of a monolayer of particles are considered. Such a monolayer is a set of particles whose centers are located in the same plane. We set forth the conditions for the effect of coherent-transmittance quenching, which takes place as a result of the interference between incident and forward-scattered waves. Using the single-scattering approximation we determined size parameters and particle refractive indexes for this interference effect in the case of identical isotropic spherical particles. The influence of polydispersity and the fine structure of light-scattering characteristics on the quenching effect has been estimated. It is shown that the polydispersity destroys this interference effect only at large widths of particle-size distribution functions. The influence of multiple scattering on this effect is considered in the quasi-crystalline approximation. Multiple scattering results in increasing size parameters and decreasing particle concentration at which coherent transmittance quenching takes place in comparison with the case of single scattering. Our theoretical results for suspensions of latex particles in water are in fairly good agreement with the experimental results.

LIGHT TRANSMISSION BY A MONOLAYER OF PARTICLES : COMPARISON OF EXPERIMENTAL DATA WITH CALCULATION AS A SINGLE-SCATTERING APPROXIMATION

The laws of light transmission by particle monolayers are investigated. The results of measurement of coherent transmission for monolayers formed by large nonabsorbing particles are presented. The obtained results are compared to the calculation in the single-scattering approximation. Good agreement between experimental and calculated data is shown.

Angular structure of radiation scattered by monolayers of particles: experimental study

An experimental study has been made of the angular structure of radiation scattered by monolayers of large nonabsorbing particles. We have investigated three media exhibiting different light-scattering properties: monodisperse polyvinyl toluene latex particles of 3.75-mum diameter suspended in water, polydisperse rice starch particles with a mean diameter of 6.4 mum suspended in water, and a mixture of ethyl and benzoic alcohols. It is shown that for large particles the structure of scattered radiation can be described in a single-scattering approximation. Analysis of the influence of polydispersity and the relative refractive index of particles on the intensity scattered by the monolayer has been performed.