ABSTRACT
Figure 3.1 Thermotropic switching due to phase separation or due to phase transition in permanent domains.The optical properties of thermotropic materials in the light scattering state can be calculated on the basis of the Mie theory for multiply scattering media [2]. These calculations were carried out with the Monte Carlo method and the N-flux method. The advantage of the Monte Carlo method is that it allows considering a specific measurement geometry. The reduction of transmitted light intensity (I/I0) caused by scattering of discrete spherical domains (SD) embedded in a transparent non-absorbing matrix (M) can be described by (3.2). I/I0 ≈ exp[(–3VSDx r SD3 /4l4) (nSD/nM – 1)] (3.2)In (3.2) VSD stands for the volume fraction of the scattering domains, x for the optical path-length, rSD for the radius of the scattering domains, l for the wavelength of the incident light and nSD, nM for the refractive indexes of scattering domains or matrix, respectively. As another outcome of the calculations, the back scattering depends on the domain size and the wavelength of the incident light. For solar radiation, continuous increase in the
total scattering with increasing domain size was found while the back scattering displayed a distinct maximum. The most efficient back scattering of solar radiation was reported to occur if the diameter of the scattering domains was between 200 and 400 nm. Decreasing domain sizes below 200 nm led to a significant decrease in the back scattering efficiency. Increasing domain sizes above 400 nm led also to a decrease in the back scattering efficiency, but this effect was much smaller. Accordingly, only slight changes of the optical properties occurred in the size range 200-1000 nm. Additionally, it was found that the optimal domain size is hardly affected by the refractive indexes of matrix and scattered domains. Thus the results of these calculations are applicable to all the various types of thermotropic systems.The characterization of the optical properties of light scattering materials is complex. The intensity of the scattered light depends on the angle between incident and scattered light as well as additionally on the wavelength of the incident light. In most publications about thermotropic materials, only transparency measurements in normal-normal geometry were published to characterize the light scattering state (see Fig. 3.2).
