The optical properties of automatically darkening welding filters based on liquid crystal technology

Abstract: This thesis addresses the problem of the poor optical angular properties displayed by automatically darkening welding filters that are based on liquid crystal technology. It is shown that by reducing thetwist-angle present in the liquid crystal cell to below that of 90° together with employment of a novel polariser arrangement, an optical shutter design using a double-cell construction is obtained thatpossesses a reduced angular transmittance variation when in the activated phase. This produces anoptical filter with a wide central viewing cone whilst maintaining the remaining optical parameters at ahigh level.The loss of cell contrast upon twist-angle reduction in the cell can be eliminated by using retardationfilms that optically compensate for the remnant retardation present in a cell. Such techniques allow ahigh cell contrast to be obtained from low-twisting cells that is comparable to that available from thestandard 90° twisted-nematic device. Reduction of the product of the anisotropic index of refraction, δn, and the thickness of the cell, d, as well as a reduction of the twist-angle therefore enables automatically darkening welding filters to be developed that possess improved optical angular properties together with having a high cell contrast. A new mode of operation for a twisted-nematic liquid crystal cell is introduced whereby the unit is ina dark state in the absence of an applied voltage. A small electric field is required in order to attain the light state and further increment of the voltage beyond this point reverts the unit back to a lowtransmittance state. Such cells offer advantages over that of the normally white mode of operation in that a fail safe, dark phase is provided should the driving electronics malfunction, a property usually characteristic of the normally black mode, whilst the fast response time from the light to the dark state associated with the normally white mode of operation is maintained. The optical properties of such systems are analysed in some detail with the view of developing a fast automatically darkening welding filter based on this technology.