Dielectric relaxation studies are reported of several substances belonging to the homologous series of 4-isothiocyanatophenyl 4-(trans-4-alkylcyclohexyl) benzoates (nPCHB-NCS) which exhibit the nematic (N) and smectic A(SmA) polymorphism. From the relaxation times (tau) _(parallel) characterizing molecular reorientations around the short axes the activation enthalpies (Delta) H_(parallel) for both phases were calculated. It was found that (Delta) H_(parallel)(N) is markedly larger than (Delta) H_(parallel)(SmA). The effect is interpreted on the basis of existing models of smectic A phase.

Optical methods of detection of ferroelectric modes are described in detail and compared with the dielectric method. The advantages of both methods are discussed. It is concluded that the dielectric method is simple whereas the optical method is more sensitive. The optical response is free of the background steaming from nonferroelectric polarization mechanisms which are always present in the dielectric reponse. The realxation of the Goldstone- and soft modes in a room temperature ferroelectric liquid crystal has been investigated using dielectric and optical methods. Both methods have given concordant results.

Chiral 4-(2'-methylbutyl)-phenyl-4'-octylbiphenyl-4-carboxylate (CE8) has been studied by dielectric, densitometric, refractometric, and x-ray measurements. On the basis of these measurements the effective value of the molecular dipole moment ((mu) ), the angle between the vector of the molecular dipole moment and the long axis of molecule ((gamma) ) as well as the transverse ((alpha) _t) and longitudinal ((alpha) ₁) molecular polarizabilities have been calculated. In addition the sign and the magnitude of spontaneous polarization (P_s), the tilt angle ((Theta) ), and the helical pitch (gamma) have been measured for the same liquid crystal CE8. Knowing the molecular parameters (mu) , (gamma) , (alpha) _t, (alpha) ₁, and (Theta) from the experiment and the molecular structure from computer investigation, the spontaneous polarization P_s has been calculated as the vector sum of the components of molecular dipole moments (mu) perpendicular to the tilt direction. To do it the rotational potential given by B. Zeks has been used. According to this approximation, the measured spontaneous polarization P_s is induced by hindered rotation of the molecules around their long axes and is expressed by molecular parameters (mu) , (gamma) , (alpha) _t, (alpha) ₁, tilt angle (Theta) and the hindered rotation parameter (eta) . The hindered rotation parameter for CE8 shows that dipolar ordering in this smectic C^* phase is rather weak in comparison to the strong orientational order of long molecular axes of the same phase.

The low frequency dielectric relaxation has been studied for the SmC^* phase of the room-temperature ferroelectric liquid crystal mixture. The dielectric spectra have been measured on the planar oriented sample in the frequency range from 100 Hz to 100 kHz with the electrical measuring field being parallel to the layer planes. From experimental results presented in this work one can see that: 1) when the structure of sample for the SmC^* phase is helicoidal the dielectric relaxation of the studied material is shown to have the strong electric measuring field dependence, and 2) when the SmC^* structure is completely unwound a low frequency weak relaxation is detected; the relaxation frequency of this mode increases with increasing bias field.

Broadband dielectric spectroscopy (10² Hz-10⁹ Hz) is employed to study the molecular dynamics of the liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB) in cylindrical channels of Anopore membranes having a diameter between 0.2 micrometers and 0.02 micrometers , and thickness about 60 micrometers . The surfaces of channels are modified using the coupling agent C₉H₁₉COOH. In the bulk 5CB two relaxation processes are observed: the hindered rotation of the molecules around their short molecular axis ((delta) -relaxation, and tambling of the molecules around short molecular axis. In untreated Anopore membranes the first process is only observed, which is nearly uninfluenced by the presence of the inner surfaces. In contrast, in treated Anopore membranes the second process connected with tambling of the molecules around their short molecular axis becomes detectable. This phenomenon is connected with a changing of the planar axial to planar radial alignment of the molecules. The relaxation rate in the isotropic phase is nearly uninfluenced by the presence of the inner surfaces, it is in the nematic phase by two orders of magnitude faster for the porous materials with modified surface as compared to the bulk. The dielectric strength of the (delta) -relaxation is considerably weakened for the 5CB/Anopore system.

The method of determination of the main components of electric permitivity tensor for the cholesteric liquid crystals was proposed. The method was used to interpret the results obtained for mixture of cyanobi phenyls and epoxy materials 4-[(S,S)-2,3 epoxyhexyloxy]- phenyl-4-(decyloxy)-benzoate.

Information on anisotropy of the local field of light wave in nematics is obtained mainly from refractivity measurements. Such investigations, supplemented by those concerning thermal dependence of density (rho) (T), allow for determining optical polarizability anisotropy of nematogenic molecules, as well as their orientational order parameter. On the basis of extensive experimental data, this paper discusses the following problems: applicability of approximate relations between refractive indices of nematic liquid crystals (NLCs) and their molecular parameters for describing optical properties of nematic mesophase has been estimated; anisotrpoy of the local field factor F_k has been investigated, as well as its influence on calculated anisotropy of molecular polarizability. Influence of various extrapolation procedures on calculated molecular parameters of the NLCs has been investigated.

The liquid crystal electro-optic effects which are linear in their coupling to an electric field are reviewed. These effects are found in materials which intrinsically, or in their configuration, lack mirror symmetry and may be divided into two main classes, in which the response- variable itself (for instance the induced tilt of the optic axis) is a linear or nonlinear function of the field. Whether the basic coupling is ferroelectric, paraelectric (dielectric) or flexoelectric in origin, the effects have essentially the same symmetry properties and show remarkable similarities in their device performance. While the first devices are beginning to emerge, these effects in particular constitute a considerable potential for future exploitation, based on their unique properties. Our examples discussed in this paper include para-, ferro-, antiferro-, and flexoelectric devices in chiral nematic and smectic (N^*, A^*, C^*) phases. Among these the ferroelectric and antiferroelectric devices stand out as being particularly powerful and promising.

A new electro-optic effect in an optically active nematic chiral structure beyond Mauguin's region has been studied. This structure rotates the polarization plane of incident light, and a liquid crystal with positive dielectric anisotropy sandwiched between crossed polarizers is switched from transparent into dark state in electric field. The characteristic features of this structure are: (a) the absence of oscillations in the transmission-voltage characteristics, (b) low demands on surface conditions (including nonrubbed cells), (c) the ability to function in any position between crossed polarizers, and (d) wide viewing angle.

Rotation of the light polarization plane in both pure and dye-doped chiral liquid crystals (CLC) was investigated close to the CLC selective reflection region. Spectral dependences of the angle of the polarization plane rotation and circular dichroism were obtained for a number of CLC-compounds. Selection of an appropriate dye-stuff and liquid crystal compound allows to design electrically controlled two-color filter with good color alliance and contrast. Gyrotropy of dye-doped CLC was studied using Nd-laser radiation with varying light intensity. Under the light intensities above 250 kW/sq. cm a polarization nonlinearity was observed as a variation of polarization plane rotation of the light passes through the cell.

The experimental results on thermo- and electro-optical properties of two liquid crystalline model dimers: disiloxane and disilazane and their blends with polyester resin (Polimal 192) are reported. The effects of temperature, voltage, and frequency of alternating electric field on electro-optical and switching results are discussed. Siloxane dimer shows a tendancy to planar orientation of molecules in (ac) electric field resulting from its negative dielectric anisotropy. Silazane dimer shows the trend toward homeotropic orientation of molecules, which is disturbed by the occurrence of an electrodynamic instability involving turbulent flow (at 2 kHz applied voltage.

The mixtures of liquid crystalline derivatives of (Beta) -naphtol esters forming glassy state above room temperature were studied. Nematic and chiral dopants were used to modify mixture properties. Several orienting layers were applied and the thickness of electro-optic cell was between 1.8 and 10 micrometers . Studies of thermo-electro-optical changes of molecular orientations are presented (viz. local cholesteric nematic phase transition and Freedericksz- effect). It is shown that the local change of orientation can be frozen into glassy state and long- time preserved without the orienting field. Relaxation processes appear after the temperature increases above T(subscript g$. The effects of changing molecular orientation stimulated by variations of electric field and temperature can be applied for reversible optical data storage and erasure. The temperature and thickness dependencies of cell parameters were also measured.

Excitation and propagation of surface guided electromagnetic waves (SGEW) of optical wave range in chiral liquid crystals (CLC) are theoretically examined in the framework of dynamical diffraction theory and an overview of the recent results in this field are presented. Unusual polarization properties and propagation diagram of SGEW are discussed. In the general case, the polarization properties of SGEW correspond to elliptical polarizations and are dependant on the SGEW propagation direction orientation relative to the director at the CLC surface, the SGEW frequency, and the film thickness. It is underlined that a qualitative difference related to the excitation and attenuation of SGEW in semi-infinite CLC and CLC films exists. The SGEW attenuation does not vanish even in nonabsorbing CLC films if total internal reflection at one of the film boundaries is absent. For CLC films, along with the conventional excitation of SGEW by means of attenuated total external reflection, it is possible to excite SGEW by a beam incident at the film from the side of a boundary where total internal reflection is absent. The above mentioned SGEW properties are illustrated by the results of calculations for typical experimental situations.

The analysis of a nonlinear directional coupler consisted of two planar waveguides separated by an asymmetrical nematic liquid crystal film is presented. The nonolinear effect is caused by an optically induced reorientational effect occurring in a hybrid aligned nematic geometry. The asymmetrical configuration cuases the nonreciprocity in switching characteristics of the directional coupler.

The orientational order of some nematogenic low molecular liquid crystals and three polymers with mesogenic side groups has been studied using 'guest-host' effect. The polarized absorption and fluorescence spectra of the fluorescent guest probe oriented in the liquid crystalline matrices have been measured as a function of the temperature in the nematic phase. From the experimental data, the order parameters <P₂> and <P₄> as well as the molecular distribution function have been determined. The small differences between the molecular orientation in the low molecular liquid crystals and the side chain polymers have been observed. It has been also found that the orientational order depends strongly on the molecular structure of the mesogenic molecules.

The elastic deformations induced by electric fields in nematic layers were simulated numerically by minimizing the free energy in the course of an iterative process. The director distribution and the developemnt of the deformation is presented for various nematic systems ranging from the Frederiks transition to the deformation of the assymetrically aligned and weakly anchored layer.

A simple method for determination of the orientational order parameter in nematic liquid crystals is described. To determine the orientational order parameter S, we exploited the de Gennes observation, that any anisotropic physical quantity may be a measure of orientational ordering in the nematic phase. We prove that in the case of optical anisotropy, the discussion on microscopic level in determination of approximate S value can be omitted and the order parameter can be calculated directly from the macroscopic quantity measured in the experiment, i.e. the birefringence. A simple and precise method for the determination of the optical birefringence is described. Similarily as in the case of microscopic quantities, the value of anisotropic physical quantity for ideally ordered nematic can be extrapolated from its temperature dependence. Results obtained using the proposed method agree well with literature data. The method can be used also for the determination of orientational order parameter in cholesteric and some smectic liquid crystals.

For the chiral systems, the molecular dynamics were studied in view of the phase transitions. In this paper, we present the studies of the high frequency relaxation processes for two chiral substances by TDS method in the frequency range from 10 MHz to 10 GHz. Especially the SmA^*-SmC^* transition is of great interest as there are some theoretical models predicting distinct changes of the critical frequencies and dielectric increments. The high frequency results are in agreement with the experimental results for other systems which show clearly that the reorientation of molecules around the long axis takes place in ferroelectric SmC^* phases, and there is no distinct change of the dielectric parameters at the SmA^*-SmC^* transition. It was found, however, that the distribution parameter of relaxation times is enhanced at the SmA^*-SmC^* transition. The dielectric results obtained for nonchiral thioesters exhibiting the sequence of phases: N-SmA-SmC, show that the principal molecular motions (reorientations around the long and short axis) are not slowed down at the phase transitions.

Long-range orientational order of four nematogenic liquid crystals has been determined on the basis of the optical birefringence, the dielectric anisotropy, and the polarized absorption spectra of the dichroic dye dissolved in the liquid crystalline host. The values of the order parameters obtained by using various experimental methods have been compared with those obtained from the Maier-Saupe mean field theory.

Ferroelectric liquid crystals are molecular ferroelectrics showing up in the tilted liquid crystalline systems (SmC^*, SmI^*, SmF^*) composed of chiral molecules. In this work, we present the dielectric, electro-optic, and calorimetric studies of a single component system: 3-octyloxy-6[2-fluor-4-(2-fluoroctyloxy)phenyl]-pyridine showing interesting ferroelectric properties. The compound exhibits a first order N^*- SmC^* phase transition which leads to a qualitatively new behavior, for instance the relaxation frequency of the soft mode below T_C seems to be temperature independent. The high frequency relaxation process, connected with the reorientation around the long axis, is practically undisturbed at the N^*-SmC^* transition. Yet, it was found that in the SmC^* phase, the best fit was obatined with two Cole-Cole functions yielding two relaxation times to describe a biased reorientation of molecules in the SmC^* phase.

The nucleation in spherulitic crystallization from supercooled smectic phase to solid state for cholesteryl nonanoate was studied using classic transition-state theory of Turnbull and Fisher. In this paper we have used a unique method of the measurements and results interpretation instead of determining the nucleation rate. In the experiment, only the granularity and temperature were measured. It was found that the primary nuclei are 2D and the nucleation can be homogeneous or heterogeneous depending on the temperature range.

The behavior of the wide range of paramagnetic liquid crystalline complexes of Cu and VO was investigated by means of the EPR technique and magnetic susceptibility measurements in different kinds of mesophases. The investigation of EPR spectra shape made it possible to determine molecular orientation in external magnetic field. The temperature dependent magnetic susceptibility measurements were performed to evaluate overall susceptibility anisotropy of mesophase. To explain the observed ways of molecular orientation, the calculations of the total molecular susceptibility (paramagnetic and diamagnetic) were carried out. It was established that the ways of molecular orientation in magnetic field is predetermined by the amount of phenyl rings and enaminoketone cycles in molecular structure and the sign of paramagnetic anisotropy of central metal ion.

The purpose of this short review paper is to give an insight into the x-ray high-pressure camera construction and its applications to phase properties and crystallographic properties; investigations of crystals and especially liquid crystals. Our experience in this field is also presented here.

An influence of a flexible spacer length on a formation of mesomorphic phases in two classes of organosilicon polymers with different backbones but the same mesogenic rigid core has been investigated by x-ray diffraction measurements completed by differential scanning calorimetry and optical microscopy. Depending on the spacer length, these materials form nematic and/or smectic A₁ phases. The nematic phase with smectic C-like short range ordering was observed for the cyclosiloxanes. The dependence of the layer spacing in the smectic phase on the siloxane ring size was also investigated.

An x-ray analysis of stretched oriented fibers of polyurethanes TDI-CmCn, derived from various mesogenic alkylene di[4-(w-hydroxyalkyloxy-4-oxybenzoyl)oxybenzoate]s (CmCn) and 2,4-toluenediisocyanate, is reported. The structure of two sequential nematic mesophases, a cybotactic nematic, and a conventional nematic mesophase is evidenced.

The use of ferrofluids to dope lyotropic liquid crystals has been discussed. In particular, the dynamics of those systems can be investigated using the doping experimental technique. The viscosities involved in different processes can be measured.

The methodologies used in simulation studies of liquid crystals with an emphasis on the collective phenomena are shortly outlined together with the attractive and orientational intermolecular interactions.

Phase diagrams of mono- and bifunctional acids and different derivatives of pyridine were investigated. By suitable selection of the proton donator and acceptor, a thermodynamically stable associate in the solid state and an induction or stabilization of liquid crystalline phases were observed. The reorganization of the hydrogen bondings and self organization of the aggregates were also proven by IR measurements.

A new family of liquid crystalline compound with the general formula C_nH_2n+1ON₂Br has been synthesized, for n equal to 1 until 12 (methyl until dodecyl groups). In this family starting from pentyl derivative smectic A mesophase has been identified. For pentyl, hexyl, and heptyl derivatives, an extra monotropic smectic E was found. For pentyl until dodecyl, except undecyl derivatives, interesting heat anomalies were found by DTA and DSC calorimetry. They are interpreted as a phase transition into unknown smectic X. The influence of the structure on the phase sequence is discussed and compared to similar families: 4-chloro-4'-alkoxyazobenzenes and 4-bromobenzylidene-4'- alkoxyanilines.

Three and four ring liquid crystalline compounds containing two flexible ethane groups or two carboxylic groups and benzene, biphenyl, cyclogexane and bicyclo (2,2,2) octane rings were synthesized and their phase transition temperatures and enthalpies were measured by a DSC calorimeter. These compounds were tested as components of mixtures for display applications. It was found that the diethane compounds, when mixed with two-ring isothiocyanates, form low viscosity mixtures with a broad range of the nematic phase suitable for the TN mode and the diesters form mixtures which are suitable for the STN mode.

Preparation of different types of 3,6-disubstituted cyclohex-2-en-1-ones was described and discussed. It was shown that they are very useful intermediates for synthesis of various known liquid crystals as well as new ones, especially those which contain lateral substituents in cyclohexane, cyclohexene, or benzene rings. The phase transition temperatures of many compounds were given.

Two homologous series of enaminoketone mesogens terminated with F₃C/OC_nH_2n+1 (perfluoromethyl series) and F₃CO/OC_nH_2n+1 groups (perfluoromethoxyl series) were synthesized and examined by optical and DSC methods. For the perfluoromethyl series, Sm-A and Cry-B smectic phases were observed and additional tilted phases appeared for short homologues (n < 7). For the perfluoromethoxyl series, the phase sequence of Sm-A, Cry-B and tilted smectic phases exist even for long examined homoglogues (to n equals 18). The occurrence of inverted phase sequence Cry-G - Sm-F - Cry- B and the presence of new orthogonal biaxial phase has also been observed for the last series.

General rules for choosing LC mixtures for PDLC devices are given. Several methods used for adjustment of LC properties are presented. An example of LC material optimization in case of PDLC information display is given.

Synthetic approach to novel groups of organsilicon liquid crystal polymers (and oligomers), i.e., polycarbosilanes, poly(cyclosiloxanes) and branched tetrahedral polysilanes (including their carbon centered analogues) is described as a method leading to products of well defined structure. These mesomorphic materials of comb- or star-shape types are made by hydrosilylation of mesogenic alkenes with the respective silicon-hydrogen bond containing polymers and oligomers. The effects of structural changes on the range and type of mesophase for these organometallic systems is shown to be not much different from those found for many organics. The synthetic results point to the ways of preparation of desired tailor-made organosilicon liquid crystals.

The structural characteristics are given for the polymer blend (PB) prepared by photopolymerization of the uniaxially oriented liquid crystal (LC)-phase of the hydroxypropylcellulose-acrylic acid (HPC-AA) system. The uniaxially oriented films of HPC- AA mesophase show a so-called banded structure under cross polarizers, characteristic for uniaxially oriented LC-polymer films. The state of order of HPC-AA mesophase was investigated by effect of dye orientation in LC-media. The influence of the length of the cholesteric pitch of HPC-AA on the order parameter of dye was found. The polymer blend as a product of the photopolymerization of AA in the lyotropic and uniaxially oriented medium HPC-AA is a birefringent solid system, was strongly affected by crosslinking processes. The PB has a LC-organization up to thermal decomposition at 230 degrees C, as shown by TOA and microscopic observations. The PB can be regarded as a new kind of the thermally resistant LC-polymer network.

New liquid crystalline polyesters were obtained, in which the mesogenic moiety consists of two aromatic rings connected with a cycloaliphatic ring of cyclohexanone. The polyesters have low temperatures of mesophase transitions.

Bicomponent mixtures of polar compounds with -CN terminal group, in which the induction of a smectic A_d phase occurs, were investigated by thermomicroscopy and x-ray method. It is shown that the induction phenomenon depends on the structure of rigid cores and the alkyl chain lengths of both compounds.

The advancement of the liquid crystal display (LCD) technology, and improvements of the optical and electro-optical properties, have enabled the broad expansion of LCDs application field. The rapid development of the multimedia techniques, new applications in automotive, office, medical domain, forced the demand for the color displays--for the information presentation with the color code. The necessity to fulfil many contradictory and extreme conditions caused the development of the optimization procedures of the color LC displays to be a big problem. Most of the LCDs used nowadays are the twisted nematic, super twisted nematic, and active matrix thin film transistor LCD. The characterization of the achromatic black/white LCDs is made by means of photometric measuring methods, and quantitative measures are used: luminance, reflectance, contrast, contrast ration; as a function of a driving voltage, viewing angle, temperature, etc. The characterization of the color LCD is based on the spectral distributions of the transmittance or reflectance. Quantitative measures are chromatic coordinates and luminance factors are defined according to the colorimetric systems--CIE 1931, CIE 1976, CIELUV, CIELAB. The color difference (Delta) E in the CIELUV system is applied as a optimization parameter for the color display module. The spectral properties of all optical elements of the display module are analyzed and their influence on the set of the optical factors of LCD is evaluated. The correlation between technological parameters and optical characteristics of the LCD has been investigated. The choice of the optimization criterion is discussed and the optimization algorithm is proposed. Results of the color displays evaluation for some examples with different preconditions are presented.

The paper discusses some aspects of the construction of multiplexing waveforms for ferroelectric liquid crystal devices. Conventional addressing methods are opposed to more recent ones, where time overlap of the selection waveforms and asymmetries between initial and final switching slopes are extensively schemes are discussed in detail. A new 'split writing' addressing scheme is proposed.

A universal modeling system of LC-electro-optics (MOUSE-LCD) is proposed. The system enables you to optimize the output characteristics of LCDs, i.e., B/W and color contrast, viewing angles, response times, multiplexing capability etc. for various LC physical parameters, display configurations, and driving modes.

The order parameters of some dichroic fluorescent dyes (naphthalene derivatives of bicarboxylic acid) in nematic liquid crystal matrices have been determined. The influence of the dye addition on the clearing temperature of the nematic host has been investigated. It has been found that the dyes studied can be utilized in the guest-host liquid crystal displays working both in passive and active modes.

The light controlling by electric field on the glass-NLC interface has been investigated. The electro-optical glass-NLC interface light modulator has been developed and tested. The modulation depth was equal 100% when the switching time was 0.2-0.5 ms.

New aspects for designing LC screens with cholesteric-nematic electro-optic effects is reviewed. Transition from planar oriented state with high selective reflection (mainly green region of spectrum) to high scattering confocal state (black) creates good contrast and remains a long time since the driving voltage switching off.

The application of liquid crystal displays (LCD) in correlation technics is presented. The optical processor for pattern recognition is described. Phase modulation properties of LCD are used to obtain limited intensity invariance of the optical correlator. Numerical and experimental results are presented.

The methods of creation of areas with different structural properties in one PDLC layer are described. It means that number, size, and shape of liquid crystal droplets, and also a director alignment, are different in particular areas of a polymer matrix. For this reason, optical and electro-optical properties of such a layer are unhomogeneous and an optical pattern may be obtained for the light passing through the layer. Possibilities of application of PDLC layers with structural pattern in optical devices are discussed.

Polymer network, which is used as a matrix for dipersing liquid crystal droplets, may be oriented in one direction by applying an oblique external electric field during photopolymerization induced phase separation. Liquid crystal molecules are aligned in the same direction as oriented polymer network wall. In this way, a PDLC film with fast dynamic response and gray scale for display application may be obtained. The results of studies of this problem are presented.

Optical binary computer may be as easy to operate as parallel system. For such configuration Boolean logic is not very convenient and therefore neural networks should be introduced. In works leading to the paper we used liquid crystal cells as a standard system of liquid crystalline layer between to conducting electrodes in 'sandwich' geometry. We have used 25 micrometers display cells filled with nematic 6CHBT working on 'twisted nematic' effect. Based on such elements a mode of a simple Hopfield network was set up. More advanced experiments were carried out on a model of neurone with supervised learning. The model consists of four laser diodes pigtailed to the multimode optical fibers with 50 micrometers core diameter. The directional couplers help to control the level of input optical power. Four liquid crystal cells allow to change the transmission level according to superivised learning requirements. All the signals were detected by one photodiode. The presented results of experiments are in excellent agreement with theoretical predictions. An additional study was done to check the possibility to build up a linear neural network with Grossberg layer, a neural network with Kohonen layer, and a counter propagation network with two layers of neurones. We have proved that such models may be set up based on simple liquid crystals cells and optical fiber networks.

Laboratory models of PDLC information displays have been developed. It is possible to replace conventional TN LCD diplays in certain applications by PDLC displays with a high unification of materials and technological instrumentation. Such PDLC displays may be useful especially for large dimension information boards.

Reverse-mode PDLC, i.e. transmitting in off-state and scattering in on-state, has been studied. It has been found that such a structure may be obtained without special treatment, however, it exhibits low contrast ratio and high haze in off-state. For improvement of electro-optical parameters, especially designed LC mixture should be used.

A permanent periodic structural grating has been formed in a PDLC layer. This structural pattern caused a diffraction of the incident light. The obtained structure has exhibited features of both amplitude and phase diffraction grating. Due to PDLC electro-optical properties, this grating is adjustable by electric field i.e., the grating type and the diffraction fringe number depends on applied field.

The particular diffraction on periodic orientation deformation of liquid crystal (LC) layer was investigated. The deformation was introduced by means of alternative electric field applied across the LC film using electrodes on both substrates plates, or along the one side of the sandwich, using interdigital electrodes patterned on one of the plates. Diffraction pattern was dependent on the frequency and amplitude of driving voltage and the thickness of the LC layer. For the first configuration, diffraction spots correspond to the diffraction period of electrodes. In the second configuration diffraction spots were observed as if the diffraction grating constant is two times smaller. Specific oscillations of intensity with their frequency minimum near 25 and 47 Hz of driving frequency were observed for the spots corresponding to greater period. Achieving this is possible with the increasing driving voltage. The theoretical description of the diffraction process based on earlier results has been shown. Presented formulae illustrate the possibility for twofold deformation maximum appearance in LC layer in dependence of the driving electric field frequency and such parameters as the LC viscosity, and the other mechanisms of dissipation. The computer simulation of the diffraction pattern is made herein.

Measurement of the fuel aerosol droplet diameter is very important in the course of construction of new type sprayers (burners, carburettors, injectors), a diagnostic process. Diffraction method is one of the most useful measuring procedures in this case. The droplet size of monodispersion medium is easy to recognize (Airy rings). Analysis of a diffraction pattern of polydispersion medium is more difficult, however. After recording the diffraction pattern I(r) of an aerosol stream, one of the numerical procedures is used. Registration process and data processing are usually separated. The final result of investigation--average droplet diameter D_rho of distribution (rho) (D) is obtained after some time. The most useful procedure in a diagnostic is real-time measuring. Diffraction light intensity curve I(r) depends on the type of droplet diameter statistical distribution (rho) (D) and relative standard deviation d equals (sigma) /D_rho as well. Precise analysis in nondimensional units (z equals (pi) Dr/(lambda) f and I_w(z) equals I(z)/I_max) shows the point (z₀ equals 1.47, I_w(z₀) equals 0.57) which may be used to display average droplet diameter D_rho. The position of the point [z₀,I(z₀)] is independent on relative standard deviation d and type of distribution function (rho) (D) as well. After finding the value of radius-vector r_0.57 equal I (r_0.57) equals 0.57, it is possible to evaluate the average droplet diameter from the relation D_(rho ) equals (0.47(lambda) f)/r_0.57. After using image converter with a rectangular optical response OR(I), the Airy picture I(r) becomes a ring. The radius of that ring depends on the threshold light intensity I_p of characteristic OR(I). The radius of converted Airy picture will be r equals r_0.57 if I_p equals 0.57I_max. After introducing a nonlinear scale, D equals f(r_0.57) image converter becomes an analogue measuring instrument. This idea has been tested with TV and CCD cameras. Using threshold effects of liquid crystal displays it is possible to reduce volume of measuring stand and its price.

The paper presents further development of a novel fiber optic low and high hydrostatic pressure sensing technique utilizing new classes of chiral nematic liquid crystals with a significantly reduced thermal sensitivity. The low-pressure sensor is based on polarization effects and employs pressure-induced deformations occurring in a twisted nematic cell. This approach is particularly suitable for measurement of hydrostatic pressure in order of 1 MPa (10 bar) and utilizes strong rotatory power occurring in chiral nematics. In the theoretical part, by using the Berreman 4 X 4-matrix method we present calculations of transmission of a twisted nematic cell. The high pressure sensor is based on intensity phenomena occurring in novel classes of chiral nematics with induced smectic A_d phase and exploits the effect of pressure induced changes in the wavelength of selective Bragg light reflection. The experiment was conducted in a high pressure environment up to 100 MPa (1kbar) and the preliminary data demonstrates a potential of the induced liquid crystal systems for applications in high pressure sensing.

An electrically controllable optical power attenuator, operating on a light scattering effect in a liquid crystalline cell was constructed for optoelectronic applications and examined. Its optical parameters and performance are presented and discussed.

The application of liquid crystal contact thermography for point skin tests used in allergology diagnostic has been studied. The effect of a concentration of histamine, adopted as the etalon substance, on observed temperature fields is presented. Obtained results have been confirmed by thermovision measurements. A correlation between studied method and visual estimation used until now is the best for temperature range observed as a blue color.

Optical displays with polymer dispersed liquid crystals have a diffused interphase region between LC droplets and polymer, composed mostly from LC swelling polymer matrix. This interphase region has a different optical characteristic comparing to the bulk material inside LC droplets. For the purpose of reducing volume occupied by the interphase region, we have adopted a procedure used in the liposome reserach. LC with dichroic dye was encapsulated in the vesicles formed by the polymerizable amphiphilic compound Dimethyldioctadecylammonium Methacrylate. After encapsulation, vesicles with LC/dye system inside were UV polymerized and dispersed in the poly(vinyl alcohol) PVA matrix. Changes of the optical transmittances of obtained thin films versus applied electrical field showed symmetrical s-shaped curves but low transmittance changes.