Filip Bruyneel

Method for measuring the cell gap in liquid-crystal displays

A method for measuring the cell gap of liquid-crystal displays (LCDs) is presented. It is based on analyzing the interference spectrum of white light reflected by the LCD. It can be used before or after filling the LCD. The method is applicable for LCDs whose operation is based on the polarization or the scattering of light. Four methods for the calcu- lation of the cell gap are presented. The first method is based on the wavelengths corresponding with the peaks in the interference spectrum. Two other methods use analog and digital filtering techniques to filter out all the irrelevant information in the interference spectrum; the cell gap is then calculated with fast Fourier transform (FFT) or the inverse FFT. In the fourth method an empirical model is fitted to the measured interfer- ence spectrum.

Cell gap optimization and alignment effects in reflective PDLC microdisplays

In general this reduction of the cell gap improves the electro-optic properties of a polymer dispersed liquid crystal (PDLC) in reflective microdisplays. At the interface between the PDLC film and the silicon backplane or cover glass, the LC molecules have a different alignment from those in the droplets in the interior of the PDLC film. This is shown by microscopic observations and the temperature dependency of the brightness and capacitance of the displays. The influence of this alignment effect increases for smaller cell gaps and has an impact on the properties of the PDLC. During and after the filling of the displays, a compression and expansion of the cell gap takes place, respectively. If the curing of the PDLC takes place before the expansion of the cell gap has stopped, transparent areas in the PDLC film may occur some time after curing. This effect is caused by the expansion of the cell gap after curing resulting in the vertical alignment of LC molecules. This can be concluded from microscopic observations and from measurements of the refractive index and cell gap.

Comparison of reflective PNLCDs and reflective single-polarizer heilmeier guest-host displays

— In this paper, the applicability of a zero-polarizer reflective display (PNLC) and a single-polarizer reflective display (Heilmeier guest host) for direct-view applications is analyzed. A measurement set-up is designed to analyze the applicability of all types of reflective displays. Simulation of the different types of illumination caused by the environmental light is essential for this set-up. The measurements indicate that the contrast ratio and reflectance greatly depend on the type of illumination. It is demonstrated that the worst-case illumination for one display technology may be the best-case illumination for another one and vice versa.

Reduction of the switching time of polymer-dispersed liquid crystal using field oriented addressing

Polymer dispersed liquid crystal (PDLC) is a promising liquid crystal for reflective displays in low power mobile applications. However, it is difficult to obtain full color PDLC displays. Color sequential PDLC is one solution but this requires short PDLC switching times. It is demonstrated that the PDLC switching times can be reduced to color sequential levels by changing the orientation of the electric field (field-oriented addressing). By choosing the right pixel geometry, this can be done at relatively low switching voltages compared to the classic addressing method which is based on changing the magnitude of the electric field.

26.4: Assembly of Reflective PDLC Microdisplays

Different methods for the filling and the assembly of reflective PDLC (Polymer Dispersed Liquid Crystal) microdisplays with a 4 μm cell gap are evaluated. The filling of the vias in the microdisplay and the stability of the cell gap after the curing of the PDLC are important parameters for displays with a high brightness.

P‐86: Reflective Color PDLC Displays using Color Filters

Polymer Dispersed Liquid Crystal (PDLC) has a high reflectance for a wide range of viewing angles. This is an important advantage for low power reflective displays. So far there is no breakthrough technology for reflective color PDLC displays. This paper discusses reflective color PDLC displays using color filters.