Blakeney, Susan Loma; (2001) Liquid crystal display as a polarisation filter. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis is concerned with researching novel applications for a commercially produced twisted nematic liquid crystal television display (TNLCTVD). TNLCTVDs work by changing the polarisation of polarised light. The polarisation change is dependent upon the voltage applied to the display, and can be converted to an intensity change by viewing the display through a polariser (analyser). The amount of polarisation change is experimentally quantified in this thesis, and is used in two main applications. Firstly the LCTVD can be used as a Stokes polarimeter. The (unknown) polarisation of light passing through the display can be determined by recording the intensity transmitted by a fixed analyser when four separate voltages are applied to the LCTVD. These intensities are used to compute the Stokes vector of the incident light. This Stokes polarimeter has no moving parts, and is easily calibrated to cope with different environmental conditions, such as temperature, as well as different wavelengths of light. The polarimeter is shown to be accurate in determining the full state of polarisation, and can distinguish between partially linearly polarised and elliptically polarised light. The second application is to use the LCTVD or a single pixel TNLC cell to read spatially varying patterns of polarisation. Some security marks have been patented which are produced using a spatially patterned birefringent material. These emit a pattern of polarisation that can be read using the LC security mark reader. This reader is shown to be accurate in distinguishing between complex polarisation varying marks which only differ in optical path lengths by 25nm, and it can detect a rotation of the mark of only 2°. The sensitivity of the system can be customised to meet the requirements of the end user.

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