Janez Pirs

Polymer Network Volume Stabilized Ferroelectric Liquid Crystal Displays

Abstract A ferroelectric liquid crystal (FLC) cell for flat panel displays with volume stabilized bistable states is described. Volume stabilization is achieved by the FLC/polymer gel dispersion which is prepared by polymerizing a FLC/polymer precursor solution in a suitably aligned FLC display cell. The volume stabilized LC cell gives mechanical stability to FLC displays with internal memory and provides for high contrast shuttering while keeping all other excellent properties of the surface stabilized FLC displays (SSFLCD).

Design of Automatic Eye Protective Welding Devices

LCD light shutters used as eye protective devices in welding environments have different requirements from LCD display devices typically used in consumer electronics. Their contrast must be many scales greater, while in the open state the shutter should be brighter. The light scattering should almost vanish and the switching time should be much shorter. This implies a different approach as typical solutions used in LCD displays don’t meet the required criteria. Although there are many solutions and concepts that are shared between both types of devices, the light shutters have a much different design. In order to find an optimal configuration many cells should be built and tested. However, this is a very time consuming task as there are many parameters that should be taken in consideration and each cell may take few days to build and test. This is where the computer simulation steps into. It takes only a few minutes to build an appropriate setup for a particular cell and to simulate it, leaving to the experimental tests only some fine tuning. Furthermore, the simulations give a deeper insight in what is really happening with the light polarization within the cell. Such a way a better understanding can be achieved. The simulator works with two different approaches. In the first approach it tries to give the best possible and exact numerical solution. It does so by minimizing the Frank elastic energy of a particular LC layer and by solving the Maxwell equations for the complete stack of optical elements. For the latter the Berreman method is used reducing a system of partial differential equations to 4 × 4 matrices manipulation (multiplication, inversion and eigenvalue problem). Although such an approach is very accurate and mimics the reality quite well it doesn’t give a deeper insight in the cell functionality. In such cases it is better to reduce the LC layer to a few simple uniaxial layers and follow the light polarization change by means of the transformations on the Poincar sphere. This makes it a very efficient tool in shutter design.

A novel ultra-low power liquid crystal display light shutter driving scheme

A novel liquid crystal display (LCD) driving concept for light shutters is shown here, based on driving signal polarity reversal controlled by the driving voltage integral across the LCD switching element electrodes, as opposed to plain periodic polarity reversal. This driving scheme optimizes the DC driving signal voltage balancing and guarantees compensation of the long-term (DC) component of the driving voltage within one polarity reversal cycle, irrespectively of driving voltage amplitude variations. The possibility of using low driving frequencies reduces the LCD switching elements power consumption significantly compared to standard driving techniques, which is important for portable devices such as welding light shutters.