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 the majority of automatically darkening welding filters currently on the market that are based on liquid crystal technology. It is shown that by reducing the twist-angle present in the liquid crystal cell to below that of 90 together with employment of a novel polariser arrangement, an optical shutter design based on a double-cell construction is obtained that boasts a reduced angular transmittance variation when in the activated phase. This gives an optical filter possessing a wide central viewing cone whilst maintaining the remaining optical parameters at a high level. The only point of compromise comes from the voltage increase requirement upon reduction of the twist- angle in order to maintain cell contrast. Although this inflates the power consumption of the system, beneficial effects upon the total light scattering provoked by the device are also observed. The final sections deal with a new mode of operation for a twisted-nematic liquid crystal cell when placed between crossed polarisers together with an interference filter possessing a high optical transmittance over the central part of the visible spectrum. This mode of operation means that the cell is in a dark state when inactivated. Application of a small stimulating voltage transmutes the unit into the light state, where upon further increment of the driving electronics beyond this point reverts the system back into a low transmittance phase. Such a mode of operation for a twisted-nematic cell offers several advantages over that of the normally white mode when considering the optical lens of an automatically darkening welding filter. In particular, a dark, fail-safe state is provided should the controlling electronics malfunction preventing the unit from holding in a potentially hazardous light phase, a property usually associated with the normally black mode of operation, whilst the fast switching speed from the light to the dark state associated with the normally white mode of operation is maintained. It is shown that there are only two cell types that display this phenomenon and the optical properties of these two systems are analysed in some detail with the view of developing an automatically darkening welding filter based on this technology. This thesis is submitted to the University of Durham for the degree of Master of Science. All work contained within this thesis was carried out by the author at Hornell Innovation AB in Sweden. No material contained within has been submitted for a previous degree and the copyright of this thesis rests with the author. No quotation from it should be published without his prior written consent and information derived from it should be acknowledged.