Optical Characterization of the Twisted Nematic Liquid Crystal Display
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Optical Characterization of the Twisted Nematic Liquid Crystal Display Carolina Rickenstorff, A. S. Ostrovsky Facultad de Ciencias Físico Matemáticas, Universidad Autónoma de Puebla, C. P. 72570 Puebla, Pue., México. ABSTRACT A simple optical characterization technique is developed for to find the amplitude and phase modulation modes for the twisted nematic liquid crystal display (TN-LCD). The developed technique is based on intensity and pixel shift measurements of the interference pattern obtained from the TN-LCD sandwiched between two polarizers in a modified Young’s experiment. Particularly, specimen LC2002 Holoeye is characterized and its amplitude and phase modulation curves as a function of image’s gray level, bright B and contrast C are presented. INTRODUCTION TN-LCD’s have found many applications in optical data processing and adaptative optics [1,2]. The TN-LCD can provide nearly independent amplitude and phase modulation of an optical signal. Usually the TN-LCD is sandwiched between two polarizers so the amplitude transmittance of the whole system can be described as T( D , , ;Ψ 1 ,Ψ 2 ) J P (Ψ 2 )J LCD ( D , , )J P (Ψ 1 ),
(1)
where JP and JLCD are the Jones matrices of polarizer and TN-LCD, respectively [3]. The parameters in Eq. (1) have the following meaning: D is the azimuth angle of the LCD front director; is the twist angle; β is the birefringence; Ψ 1 and Ψ 2 are the azimuth angles of extraordinary polarizer axes. Unfortunately, the system parameters, as a rule, are not provided by the manufacturers of commercially available TN-LCDs. Several methods have been proposed for measuring the TN-LCD’s parameters, however, these methods prove to be very complicated (and hence, rather expensive) in view of the ambiguity originated by the trigonometric functions being members of matrix (1) and the edge effects at the bounding surfaces of TN-LCDs [4,5]. Here we propose an alternative technique of TN-LCD characterization that permits to determine the optimum values of the controlled parameters Ψ 1 and Ψ 2 that allow to achieve the modes of phase and amplitude modulation of the wave front without necessity of measuring the TN-LCD internal parameters. This technique is based on easy measurements of the output intensity and fringe shift of the interference pattern obtained in a modified Young’s experiment. We illustrate the proposed technique by characterizing TN-LCD specimen Holoeye LC2002.
THEORY The transmitted field at the output plane x ( x, y ) of the TN-LCD sandwiched between two polarizers can be written in the complex form
T(x) T (x) exp i (x),
(2)
where T (x) and (x) are the amplitude and phase transmittances, respectively. From the user’s point of view, transmittance of the TN-LCD sandwiched between two polarizers is a function of Ψ 1 , Ψ 2 and image’s gray level Umod. Particularly, transmittance of TN-LDC LC2002 is modified by two additional parameters, bright B and contrast C. Following this reasoning we can define amplitude and phase modulation of the TN-LCD as T U mo
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