LCD light shutters have found a very interesting niche application in various optical applications related to light intensity and phase modulation. Typical mass produced devices of this kind are automatic protective LCD welding light filters. As optical personal protective devices, they have to comply with strict International Safety Regulations like EN 379 ⁽¹⁾, which among other require: ultra fast switching times (≈100 ms), very high and angular independent light attenuation (several 10.000) in the optically closed state, superior optical performance and ultra-low electric power consumption (battery powered, head mounted devices). The paper presents an essentially new technical solution for the LCD light modulators based on a novel LCD light shutter operational concept. This novel concept of a wide viewing angle LCD light shutter, makes advantage of the highly symmetric 180^o- twisted LC-configuration, single mode light propagation (preferably isotropic ordinary mode) and negative birefringent c-plate optical compensation layer. It can be viewed as a synergy of the well known “twisted nematic cell” and “pi cell” concepts^{2, 3}. Such a cell has extremely good optical self-compensating properties. In addition, the residual optical retardation of the boundary LC layers in the closed state can be optically approximated as an effective a-plate acting as a compensation layer for the angular dependence of the crossed polarizers. Poincare sphere presentation as well as the computer modeling based analysis of the angular dependence of the transmitted light in the optically closed state of the LCD light shutters is discussed in view of optimizing the device structure^{(5, 6)}. It is shown that the relative positioning of the negative c-plate birefringent compensating layer inside the stack of the high voltage (15 V) driven 180^o - twisted LC cell and crossed polarizers is not at all arbitrary as shown before⁽⁷⁾. Proper positioning as well as proper value of the negative c-plate retardation (< value of the positive retardation of the LC) can result in impressive compensation not only of the angular dependence of the high voltage driven 180^o - twisted LC cell but also of the crossed polarizers. Using the above described concept exceptionally good angular compensation of the light attenuation is achieved using only one single negative c-plate compensation layer. Such a simple, low cost technical solution can represent an extremely efficient solution for LCD light filters used for example in personal protection applications.

References: 1. European Standard EN379, CEN, Brussels. 2. T. Scheffer et al: Appl. Phys. Lett. 45, 1021, (1984) 3. P. Bos et al: JJAP 38, 2837, (1999) 4. Mori et al: Display and Imaging 5, p1 (1996) 5. J. Pirs et al: PCT/EP2004/003939 6. J. Pirs, M. Bazec, A. Vrecko, D. Ponikvar, 55.3, SID 03 Digest 7. T. Uchida, SID 20, p 387, (1989)

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^† Research supported by NATO Science for Peace Program (Grant SfP 971874 – Eye Protection), Slovenian Ministry for Science (Grant L1-6601) and Slovenian Ministry for Industry (Grant 2111-04-4010-511/2004-4286/BZ)