G. Paul Montgomery

A Light Control Film Composed of Liquid Crystal Droplets Dispersed in a UV-Curable Polymer

Abstract This paper describes a new class of light control films consisting of submicron liquid crystal droplets dispersed in ultraviolct-cured polymer matrices. These films, which can respond optically to both applied electric fields and temperature changes, are potentially useful for displays and light shutters. The optical performance of these films depends on a variety of structural, electro-optical and thermal properties. This report describes scanning electron microscope studies of film structure, measurements of voltage dependent film transmittance and light scattering, and calorimetric studies which indicate that microdroplet formation in the films occurs as a result of phase separation which takes place during the cure process.

A Light Control Film Composed of Liquid Crystal Droplets Dispersed in an Epoxy Matrix

Abstract This paper describes light control films consisting of submicron liquid crystal droplets dispersed in epoxy matrices. These films, which can respond optically to both applied electric fields and temperature changes, are potentially useful for displays and light shutters. The optical performance of these films depends on a variety of structural, electrooptical and thermal properties. This paper describes scanning electron microscope studies of film structure, measurements of voltage dependent film transmittance, contrast ratio and light scattering, and calorimetric studies of the cure process which governs microdroplet formation in the films.

Dual frequency addressing of polymer-dispersed liquid-crystal films

We report the feasibility of using dual frequency addressing (DFA) to switch polymer‐dispersed liquid crystal (PDLC) films between their on‐ and off‐states. In this scheme, the on‐state is activated with an applied voltage of low frequency while the off‐state is activated with a high‐frequency voltage. We find that DFA increases the forward‐scattering efficiency of PDLC films in the off‐state without decreasing their on‐state transmittance. Consequently, DFA can be used to improve the contrast ratio of PDLC films in projection displays and similar devices. We also find that the addressing frequency required to activate the off‐state in a PDLC film has the same exponential temperature dependence observed in conventional liquid‐crystal devices not employing microdispersed liquid crystals; this limits the use of DFA in PDLC films to applications which do not require operation over a wide temperature range.

Light scattering from polymer‐dispersed liquid crystal films: Droplet size effects

The effects of droplet size on light scattering by polymer‐dispersed liquid crystal (PDLC) films have been studied theoretically and experimentally. Rayleigh–Gans calculations predict that, provided the droplets scatter independently, backscattering will be maximum at wavelength λ when the droplet radius is about λ/7. However, correlation effects become noticeable for droplet volume fractions η above about 0.2 and shift the radius for maximum backscattering to about λ/5 as η increases to 0.5. These calculations are consistent with hemispherical transmittance and reflectance measurements on PDLC films in which multiple scattering is known to be important. These measurements indicate that solar attenuation by PDLC films will probably be maximized by choosing droplet size to maximize backscattering of visible radiation. This choice also increases solar attenuation by absorption of radiation trapped inside a PDLC film by total internal reflection.

Refractive indices of polymer‐dispersed liquid‐crystal film materials: Epoxy‐based systems

Polymer‐dispersed liquid crystal (PDLC) films are potentially useful in applications requiring electrically controllable light transmission. In these applications, both a high on‐state transmittance and a strong off‐state attenuation are often needed over a wide operating temperature range. These transmittance characteristics depend strongly on the refractive indices of the materials in the PDLC films. We have measured the temperature dependent refractive indices of typical PDLC film materials and the temperature dependent electro‐optic transmittance of a PDLC film composed of liquid crystal microdroplets dispersed in an epoxy matrix. We show that our refractive index measurements can account for all the features in the measured transmittance characteristics and discuss several methods for controlling refractive indices to optimize electro‐optic transmittance over an extended temperature range. We have also measured the room temperature refractive indices of mixtures of epoxy resins and hardeners as a fun...

Contrast ratios of polymer-dispersed liquid crystal films.

Contrast ratio is an important measure of the performance of an electrooptic display. From measurements of film brightness and transmittance, we determined contrast ratios of thin polymer films containing microdroplets of liquid crystalline material. Contrast ratios based on brightness were typically ~3 for all samples studied, whereas contrast ratios based on transmittance varied from ~20 to 200 depending on the sample. We explain these differences by analyzing the relations between the illumination geometries in the two measurements and the voltage-dependent multiple scattering of light which controls the electrooptic performance of the films. Effects of the spectral content of the light source and the response of the human eye on contrast ratio were also determined. The results presented here demonstrate the importance of standardizing procedures for measuring contrast ratios of these new liquid crystal films.

Effects of refraction on axisymmetric flame temperatures measured by holographic interferometry.

Neglect of refraction can produce errors when temperature distributions in axisymmetric flames are reconstructed by Abel inversion of interferometric fringe data. This study quantifies these errors and their reduction by imaging during interferogram readout. Rays were traced through analytic temperature distributions characteristic of real flames at different equivalence ratios to determine the fringe patterns that would be observed interferometrically with and without imaging. The Abel inversion was applied to each computed fringe pattern to reconstruct the temperature distribution. Reconstructed and analytic distributions were compared to determine the error caused by using the Abel inversion. Our results indicate that proper imaging will generally be necessary to reduce reconstruction errors to below 5% in real axisymmetric flames.

Temperature dependence of infrared absorption by the water vapor continuum near 1200 cm −1

Infrared absorption by the water vapor continuum near 1200 cm(-1) has been measured with a lead-tin-telluride diode laser over a 40.5-m optical path. The measurements were made as a function of temperature from 333 K to 473 K; thus, they overlap and extend previous measurements made at temperatures between 293 K and 388 K. Over the entire temperature range studied here, the continuum extinction coefficient increases quadratically with water-vapor partial pressure as expected for the relatively high partial pressures used in these measurements. At temperatures below 398 K. our measured extinction coefficients agree well with previously reported data. At higher temperatures, however, the extinction coefficient is almost independent of temperature and is substantially larger than predicted by empirical formulas. Values of the self-broadening coefficient for water vapor have been extracted from the experimental data, and a possible interpretation of the results involving both dimer and line-broadening effects is presented.Infrared absorption by the water vapor continuum near 1200 cm−1 has been measured with a lead-tin-telluride diode laser over a 40.5-m optical path. The measurements were made as a function of temperature from 333 K to 473 K; thus, they overlap and extend previous measurements made at temperatures between 293 K and 388 K. Over the entire temperature range studied here, the continuum extinction coefficient increases quadratically with water-vapor partial pressure as expected for the relatively high partial pressures used in these measurements. At temperatures below 398 K, our measured extinction coefficients agree well with previously reported data. At higher temperatures, however, the extinction coefficient is almost independent of temperature and is substantially larger than predicted by empirical formulas. Values of the self-broadening coefficient for water vapor have been extracted from the experimental data, and a possible interpretation of the results involving both dimer and line-broadening effects is presented.

Polymer-Dispersed Liquid Crystal Films For Light Control Applications

Polymer-dispersed liquid crystal (PDLC) films, comprised of liquid crystal microdroplets dispersed in polymer matrices, are attractive for a variety of indoor and outdoor light control applications since they can be switched electrically from a light-scattering off-state to a transparent on-state. This paper reviews the electro-optic properties of PDLC films which govern their performance in such diverse applications as electronic information displays, signs, room dividers, and solar energy control in buildings and automobiles. Factors governing the operating temperature range of PDLC films will be identified and temperature-dependent transmittance and response-time characteristics of these films will be presented. Spectral transmittance characteristics will be discussed and used to determine contrast ratios of PDLC films. Dual frequency addressing of PDLC films will be demonstrated and shown to be a viable technique for increasing contrast ratio of PDLC displays. Solar attenuation properties of PDLC films will be reviewed.

High-resolution diode-laser spectroscopy of the 949.2 cm −1 band of ethylene

High-resolution absorption spectra within the central maximum of the ν7 band of C2H4 have been recorded using tunable semiconductor lasers. 169 individual transitions have been resolved in three nonoverlapping frequency regions covering approximately 97% of the spectral range from 947.1 to 951.9 cm−1. The frequency splittings between individual transitions are reproducible to ±0.008 cm−1 or better. The problems of scanning an extended frequency range with a multiplicity of diode lasers arise primarily from limited laser-tuning range and the lack of appropriate techniques for precise absolute frequency calibration. The strengths and weaknesses of diode lasers in high-resolution molecular spectroscopy are assessed.